Physiology of the Peanut Plant 1032201045, 9781032201047

Peanut is an important crop in the semi-arid regions of the world. Both, irrigation and well water can provide the water

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Physiology of the Peanut Plant
 1032201045, 9781032201047

Table of contents :
Cover
Title Page
Copyright Page
Dedication
Preface
Table of Contents
1. Introduction
2. Seed Dormancy and Germination
2.1. Seed Dormancy
2.1.1. Influence of Temperature
2.1.2. Longevity of Seed
2.2. Seed Components
2.3. Seed Germination
2.3.1. Seed Size
2.3.2. Salinity
2.3.3. Temperature
2.4. Viability and Vigour
2.5. Seed Deterioration
2.6. Seed Priming
3. Vegetative Growth
3.1. Vegetative Growth Stages
3.2. Water
3.3. Temperature
3.4. Photosynthesis
3.4.1. Light Intensity
3.4.2. Carbon Dioxide
3.5. Characteristics of Peanut Plant Parts
3.6. Nodulation
3.7. Drought Tolerant
3.8. Temperature × Carbon Dioxide
3.9. Temperature × Photoperiod
3.10. Defoliation
3.11. Salinity
4. Reproductive Development
4.1. Flower Development, Pollination and Fertilization
4.1.1. Intensity of Flowering
4.1.2. Pollination
4.2. Reproductive Stages
4.3. Drought
4.4. Temperature
4.5. Peg Development
5. Pod Growth and Yield
5.1. Pod Growth
5.2. Yield
6. Plant Nutrition
6.1. Nitrogen
6.2. Phosphorus
6.3. Potassium
6.4. Calcium
6.5. Magnesium
6.6. Sulphur
6.7. Iron
6.8. Zinc
6.9. Manganese
6.10. Boron
6.11. Copper
6.12. Molybdenum
7. Photosynthesis
7.1. Chlorophyll
7.2. Photosynthetic Rate
7.3. Light
7.4. Water
7.5. Temperature
7.6. Carbondioxide
7.7. Nutrition
7.8. Intercropping
8. Respiration
8.1. Dormancy and Seed Germination
8.2. Respiration in Peanut Plant
8.3. Temperature
8.4. Heat Stress
8.5. Salt Stress
8.6. Temperature × CO2
8.7. Water
8.8. Nutrition
8.9. Root Nodules
8.10. Other Aspects
9. Nitrogen Metabolism and Biological Nitrogen Fixation
9.1. Ammonia Assimilation
9.2. Nitrogen Fixation
9.3. Biological Nitrogen Fixation
9.4. Rhizobium Strains
9.5. Abiotic Stress
10. Lipid Metabolism
10.1. Seed
10.2. Plant
10.3. Synthesis of Plastid Lipids
10.3.1. Synthesis of Monogalactosyl Diacylglycerol (MGDG)
10.3.2 Synthesis of the DGDG
10.3.3 Synthesis of Sulfoquinovosyl-diacylglycerol (SQDG)
10.3.4 Synthesis of phosphatidylglycerol (PG)
10.4. Synthesis of Glycerophospholipids in the Endoplasmic Reticulum
10.5. Seed Development
11. Plant Growth Regulators
11.1. Dormancy and Germination
11.2. Plant Seedlings
11.2.1. Auxin
11.2.2. Cytokinin
11.2.3. Gibberellins
11.2.4. Ethylene
11.2.5. Abscisic Acid
11.3. Physiological Aspects
11.3.1. Drought
11.3.2. Organogenesis
12. Abiotic Stresses
12.1. Drought
12.2. Salinity
12.3. Heat Stress
12.4. Iron Deficiency
13. Source–Sink Relationships
13.1. Source–Sink Concept
13.2. Photosynthetic Rate
13.3. Harvest Index
13.4. Translocation from Source to Sink
13.5. Source–Sink Interaction
Index

Citation preview

Physiology of the

Peanut Plant

P. Basuchaudhuri

Formerly Senior Scientist

Indian Council of Agricultural Research

New Delhi, India

p,

A SCIENCE PUBLISHERS BOOK

First edition published 2022 by CRC Press 6000 Broken Sound Parkway NW, Suite 300, Boca Raton, FL 33487-2742 and by CRC Press 4 Park Square, Milton Park, Abingdon, Oxon, OX14 4RN © 2022 P. Basuchaudhuri

CRC Press is an imprint of Taylor & Francis Group, LLC Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, access www.copyright.com or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978­ 750-8400. For works that are not available on CCC please contact [email protected]

Trademark notice: Product or corporate names may be trademarks or registered trademarks and are used only for identification and explanation without intent to infringe. Library of Congress Cataloging-in-Publication Data (applied for) ISBN: 978-1-032-20104-7 (hbk) ISBN: 978-1-032-20105-4 (pbk) ISBN: 978-1-003-26222-0 (ebk) DOI: 10.1201/9781003262220 Typeset in Times New Roman by Innovative Processors

To My Loving Youngest Son

Mr. Priyanko Basuchaudhuri, B. Tech

Preface

Peanuts are rich in protein, fat, and fiber. While peanuts may have a large amount of fat, most of the fats they contain are known as “good fats.” These kinds of fats actually help lower your cholesterol levels. Peanuts are a low-glycemic food, which means that eating them won’t cause a spike in your blood sugar levels. Studies have shown that eating peanuts can lower the risk of type 2 diabetes in women. Research has demonstrated that for older people, eating peanut butter may help lower the risk of developing a certain type of stomach cancer called gastric non cardia adenocarcinoma. The peanut (Arachis hypogaea) also known as the groundnut, goober (US), pindar (US) or monkey nut (UK), is a legume crop grown mainly for its edible seeds. It is widely grown in the tropics and subtropics, being important to both small and large commercial producers. The environmental, soil moisture and climatic resilience of a plant depends on the interaction between abiotic stresses and several physiological traits which finally affect the survival and reproduction of crop plants. Some of these traits include D13C, harvest index (HI), haulm weight (HLMWT), leaf dry weight (LDWT), leaf area (LA), leaf length (LLN), leaf weight (LWT), leaf width (LWD), root/shoot ratio (RSR), rate of water loss (RWL), root length (RTL), root volume (RTVOL), root weight (RWT), shelling percentage (ShP), shoot length (SLN), shoot weight (SWT), specific leaf area (SLA), total leaf area (TLA), total leaf weight (TLWT), SPAD chlorophyll meter reading (SCMR), total dry matter (TDM), TDM/LA, days to flowering (DF), days to maturity (DM), emergence (EMR) and first flowering (FFL). In view of making peanut crop more resilient to stresses with high pod and oil yield and improved oil and nutritional quality, the study is the timeliest and most comprehensive marker-trait association study conducted so far in peanut using thousands of markers and multiple season phenotyping data generated on wide range of economically important traits. Thus, several MTAs detected for many disease resistance, oil content and quality, drought tolerance related (physiological) traits, yield components and yield in the present study based on multiple season phenotyping data will facilitate their improvement through GAB. To achieve this, these MTAs upon validation may be deployed in marker-assisted improvement of peanut leading to development of improved cultivars with higher resilience to drought tolerance and disease resistance, increased yield and, improved oil and nutritional quality. Such improved cultivars will ensure sustainable livelihood to the farmers of SAT regions of Africa and Asia, and better nutritional supply to the consumers’ world wide.

vi

Preface

In this context it is imperative to understand the physiological aspects of peanut growth, development and yield under abiotic and biotic stress conditions so that improvement of suitable varieties and agronomic techniques can be achieved for high yield and sustainability under adverse conditions. Here in this book an attempt has been made to describe systematically, comprehensively and lucidly the different aspects of peanut physiology in thirteen chapters beginning from seed dormancy and germination to source-sink relationships with several illustrations. I hope the book will be very useful for postgraduate students and researchers of Universities. I would appreciate if book is liked by them and share their comments. Kolkata, 2022

P. Basuchaudhuri

Contents

Preface

iii

1. Introduction 2. Seed Dormancy and Germination 2.1. Seed Dormancy 2.1.1. Influence of Temperature 2.1.2. Longevity of Seed 2.2. Seed Components 2.3. Seed Germination 2.3.1. Seed Size 2.3.2. Salinity 2.3.3. Temperature 2.4. Viability and Vigour 2.5. Seed Deterioration 2.6. Seed Priming 3. Vegetative Growth 3.1. 3.2. 3.3. 3.4. 3.5. 3.6. 3.7. 3.8. 3.9. 3.10. 3.11.

Vegetative Growth Stages Water Temperature Photosynthesis 3.4.1. Light Intensity 3.4.2. Carbon Dioxide Characteristics of Peanut Plant Parts Nodulation Drought Tolerant Temperature × Carbon Dioxide Temperature × Photoperiod Defoliation Salinity

4. Reproductive Development 4.1. Flower Development, Pollination and Fertilization

1

12

13 15 16 17 20 21 23 26 32 34 35 46

46 50 54 57 57 58 61 66 67 71 74 75 77 84

87

viii

Contents

4.2. 4.3. 4.4. 4.5.

4.1.1. Intensity of Flowering 4.1.2. Pollination Reproductive Stages Drought Temperature Peg Development

89 93 96 98 100 103

5. Pod Growth and Yield

115

5.1. Pod Growth 5.2. Yield

120 130

6. Plant Nutrition 6.1. 6.2. 6.3. 6.4. 6.5. 6.6. 6.7. 6.8. 6.9. 6.10. 6.11. 6.12.

Nitrogen Phosphorus Potassium Calcium Magnesium Sulphur Iron Zinc Manganese Boron Copper Molybdenum

7. Photosynthesis 7.1. 7.2. 7.3. 7.4. 7.5. 7.6. 7.7. 7.8.

Chlorophyll Photosynthetic Rate Light Water Temperature Carbondioxide Nutrition Intercropping

8. Respiration 8.1. 8.2. 8.3. 8.4. 8.5. 8.6. 8.7. 8.8. 8.9. 8.10.

Dormancy and Seed Germination Respiration in Peanut Plant Temperature Heat Stress Salt Stress Temperature × CO2 Water Nutrition Root Nodules Other Aspects

149

151 154 157 159 161 163 166 169 172 175 177 178 188

188 192 196 201 207 209 212 214 221

222 229 233 235 237 238 238 240 241 245

ix

Contents 9. Nitrogen Metabolism and Biological Nitrogen Fixation 9.1. 9.2. 9.3. 9.4. 9.5.

Ammonia Assimilation Nitrogen Fixation Biological Nitrogen Fixation Rhizobium Strains Abiotic Stress

10. Lipid Metabolism 10.1. Seed 10.2. Plant 10.3. Synthesis of Plastid Lipids 10.3.1. Synthesis of Monogalactosyl Diacylglycerol (MGDG) 10.3.2 Synthesis of the DGDG 10.3.3 Synthesis of Sulfoquinovosyl-diacylglycerol (SQDG) 10.3.4 Synthesis of phosphatidylglycerol (PG) 10.4. Synthesis of Glycerophospholipids in the Endoplasmic Reticulum 10.5. Seed Development 11. Plant Growth Regulators 11.1. Dormancy and Germination 11.2. Plant Seedlings 11.2.1. Auxin 11.2.2. Cytokinin 11.2.3. Gibberellins 11.2.4. Ethylene 11.2.5. Abscisic Acid 11.3. Physiological Aspects 11.3.1. Drought 11.3.2. Organogenesis 12. Abiotic Stresses 12.1. 12.2. 12.3. 12.4.

Drought Salinity Heat Stress Iron Deficiency

13. Source–Sink Relationships 13.1. 13.2. 13.3. 13.4. 13.5. Index

Source–Sink Concept Photosynthetic Rate Harvest Index Translocation from Source to Sink Source–Sink Interaction

256

263 270 270 276 278 289

289 294 302 303 303 303 303 304 308 322

322 328 330 330 331 331 332 332 339 342 351

354 360 370 377 383

386 388 391 395 402 417

CHAPTER

1

Introduction Peanut, also commonly known as groundnut (Arachis hypogaea), is a major food crop, grown throughout the tropics and sub-tropics. World annual production is about 38 million tonnes. Like so many other crops, it has assumed importance in regions of the world far from its original home. Peanut is particularly important in Asia, which accounts for 64% of the world production, where it provides number of calories similar to soya. In Africa, which accounts for 26% of the world production, peanut has a key role in providing protein, energy and iron; amazingly, in this continent, its production exceeds that of all other grain legumes put together. In the USA, largely due to the research efforts of Dr George Washington Carver, peanut became an important crop in the South. The USA now accounts for some 6% of the world production. South America currently accounts for only 3% of the world production, but the genus Arachis is endemic, and originally cultivated peanut production increased (production Statistics from 2008 (FAOSTAT, 2008)). Peanut follows this production pattern, and considering its recent origin, it exhibits a remarkable amount of morphological variability. Based on this, two subspecies were recognized, hypogaea and fastigiata. These, in turn, have two (hypogaea and hirsuta) and four (fastigiata, vulgaris, aequatoriana and peruviana) botanical varieties, respectively. This type variety (A. hypogaea subsp. hypogaea var. hypogaea) has a long cycle with no flowers on the central stem, and regularly alternating vegetative and reproductive side stems. It is widely present as landraces along the tributaries to the South of the Amazon River in Brazil and Bolivia. The modern agricultural types ‘Virginia’ or ‘Runner’ exemplify it. Also classified within subsp. hypogaea, but with more hirsute leaflets and even longer cycle, is the variety hirsute KÖhler (Peruvian Runner). Nowadays, this variety is concentrated in the coastal regions of Peru, from where it extends to Central America and Mexico, Asia and Madagascar. The variability of this variety found in the Old World even suggests the possibility of preColombian contacts. The subspecies fastigiata Waldron has a shorter cycle, flowers on the central stem and reproductive and vegetative stems distributed in a disorganized way. The variety vulgaris C. Harz has its distribution centred on the basin of the river Uruguay. Usually, the fruits are two seeded, and the varieties correspond to the agricultural type known as ‘Spanish’. The variety fastigiata has fruits with more than two seeds and a smooth pericarp; this variety corresponds to the agricultural type ‘Valencia’; centres of diversity are in Paraguay, and Central and North-Eastern Brazil extending to Peru. The other two varieties aequatoriana Krapov. and W.C. Gregory (Ecuador and North of Peru) and peruviana Krapov. and W.C. Gregory (Peru, North East of Bolivia and the Brazilian State of Acre) have fruits with more than two seeds,

2

Physiology of the Peanut Plant

heavy reticulation of the pericarp and very restricted distributions. Initially, the very limited DNA polymorphism present in A. hypogaea limited the information that could be gained from molecular studies. The initial studies were based on isozymes and proteins, followed by restricted fragment length polymorphism – RFLPs, RAPDs and AFLPs. None of these marker systems were very informative in cultivated germplasm. Higher levels of polymorphism were observed with microsatellites, in particular with longer TC repetitive motifs . Over the last few years, many new microsatellite markers have been developed, and this has enabled the detection of moderate levels of genetic variation in A. hypogaea accessions and even intravariety polymorphism. These studies have shown the grouping of accessions according to the varieties they belong to. In general, two main groups were observed, joining accessions of Hypoagea ssp. fastigiata ‘fastigiata’(Valencia type) and fastigiata ‘vulgaris’ (Spanish type) in one group, and hypogaea ‘hypogaea’ (Virginia and Runner types) and hypogaea ‘hirsuta’ (Peruvian runner) in a second group. These results corroborated the current taxonomic status of these subspecies and varieties. Exceptions to these results may be explained by the erroneous use of modern cultivars or breeding lines to represent these varieties (Fig. 1.1 and Table 1.1).

Fig. 1.1. Botanical flowchart of peanut species Arachis hypogaea

The morphology, anatomy, and reproductive development of peanut has been described numerous times and will only be briefly described here. Inflorescences are borne in the axils of the leaves on both primary and secondary branches. They are simple or compound and each has up to five flowers. Only one flower per inflorescence usually opens on any given day. Flowers are papilionaceous and sessile, but appear to be stalked because of an elongated tubular hypanthium or calyx tube. Styles are contained within the calyx tube, and both the style and calyx tube rapidly elongate during the 12 to 24 hrs prior to anthesis and can reach a length of 5 cm or more. The hypanthium is attached to the base of the ovary, which is superior. The corolla consists of standard, wing and keel petals, and the calyx has five sepals that are borne on the distal end of the elongated hypanthium. The standard ranges in colour are from deep orange to light yellow, and in rare cases it may be white. A central crescent area exists on the face of the standard that is usually darker in colour, or in some cases a different colour than the remainder of the standard. Wings are usually the same colour as the standard. Flowers contain ten androeciums, with five anthers being elongated and the remaining five being small and more globular. One or more anthers are usually sterile and difficult to observe. It was observed that sterility is more common in Spanish and Valencia types than in Virginia types. Microsporogenesis in peanut was described by

3

Introduction Table 1.1. Arachis hypogaea subspecific and varietal classification Botanical variety

Market type

Hypogaea

Hirsuta

Virginia Runner Peruvian runner

Location

Traits

Bolivia, Amazon

No flowers on the mainstem; alternating pairs of floral and reproductive nodes on lateral branches; branches short; relatively few trichomes Large seeds; less hairy Small seeds; less hairy More hairy

Peru

Fastigiata Valencia

Aequatoriana

Peru, N.W. Bolivia Ecuador

Vulgaris

Spanish

Brazil— Guaranian Goias Minas Gerais

Valencia

Brazil Paraguay Uruguay

Peruviana

a

Brazil— Guaranian Goias Minas Gerais Paraguay Peru Uruguay

Flowers on the main stem; sequential pairs of floral and vegetative axes on branches Little branched; curved brench as

Less hairy, deep pod reticulation Very hairy, deep pod reticulation; purple stems, more branched, erect More branched; upright branches

Little branched, curved branches

After Stalker and Simpson (1995)

Xi. Pollen is usually mature 6 to 8 hrs prior to anthesis and, at anthesis the pollen is two-celled and each cell has two nuclei. Stigmas are generally as long as or slightly shorter than the anthers. Both the stigma and anthers are enclosed by the keel, which induces self-fertilization. However, bees may be needed to trip flowers in some species, and these insects also carry pollen to other plants resulting in up to 8 per cent out crossing. Pollination occurs at approximately the same time as anthesis, which occurs a few hours after sunrise. The stigmatic surface contains enzymes that promote pollen adhesion, and within 8 hrs after anthesis these enzymes apparently degrade. Thus, the most successful crossing in artificial hybridization programs occurs during the early morning hours. In contrast to the large stigmatic surface of A. hypogaea and other annual species in the genus, perennials have small stigmas which are surrounded by hairs, that greatly limit the number of pollen grains that can adhere to the receptive surface. This may account for annuals being much better seed parents than perennials

4

Physiology of the Peanut Plant

in crossing programs, as well as generally producing greater numbers of seeds in germplasm nurseries. The ovary of peanut is unilocular and usually has one to three ovules. The embryo sac has a prominent egg, and starch grains surround the endosperm nucleus. After fertilization, the starch grains disappear, and a multicellular proembryo develops. The flower petals then wither within 24 hours, but the hypanthium and style usually remain attached to the base of the ovary for up to five days. The proembryo divides three to four times (resulting in an 8 to 16 nucleate egg) and then becomes quiescent at the time when a meristem located adjacent to the basal ovule becomes active. A carpophore (or gynophore, commonly called a “peg”) begins to elongate with positive geotropism into the soil. After the peg enters the soil, it becomes diageotropic (i.e., begins to grow horizontally), ceases to elongate, and at the same time swells, and the embryos resume cell division. Pods usually develop in the absence of light, but aerial pods can occur. In A. hypogaea, pod development generally begins 16 to 17 days after pollination, but in other species the process may be delayed until 23 to 25 days. Pegs of the domesticated species are relatively short and do not break easily, but pegs of the Arachis species may be a metre or more in length and are fragile. The seed has two cotyledons, a hypocotyl, epicotyl, and radicle. The cotyledons comprise nearly 96 per cent of the seed weight and are then a storage tissue for the developing seedlings. The seeds contain about 20 per cent carbohydrates, 45 per cent oil, and 25 per cent protein; but a considerable range in oil and protein percentage exists among genotypes. Most peanuts are deficient in lysine and tryptophan, and allergens are associated with seed storage proteins. • Worldwide 44,041,913 tonnes of peanut is produced per year. • China is the largest peanut producer in the world with 16,685,915 tonnes production volume per year. • India comes second with 6,857,000 tonnes yearly production. • China and India produce more than 50% of the total “World” production. • Peanuts are grown in 13 states, across the southern United States and in many countries around the world. World production of peanuts was approximately 45 million metric tons in 2017, with China being the world’s largest producer. • The United States is the third largest producer (2018), and exports about 20-25% of production. In 2018, about 60% of the peanuts grown were made into peanut butter.

Fig. 1.2. Production of peanut by different countries in per cent

Introduction

5

• Peanuts, or “groundnuts” as they are known in some parts of the world, are the edible seeds of a legume, Arachis hypogaea and they are high in protein, oil and fiber. Peanuts produced in the U.S. are mostly used in food and confectionary products, but more than 50 per cent of the worldwide production is crushed for its oil (Fig. 1.2). Ghana, a country located in West Africa, is the land of groundnuts. Their annual production is far more than its occupied land area. Its farmers earn a great fortune by growing peanuts on massive land farms. The country annually produces around 0.4 million metric tons of peanuts. The warm and temperate climate of the state supports peanut production significantly. The northern area of Ghana is seen to produce more peanuts than any other region of the country. Senegal is a country not known by many of us but its peanut production stands in the list of the top 10 producers. Annually the country produces 0.6 metric tons of peanuts approximately. This land is mineral rich with conducive climatic conditions for groundnut production making big companies establish their peanut businesses in the country. Today, peanut production accounts for 40 per cent of the total agricultural output of the country; hence it has a major share in the country’s economy and revenue generation. Peanut is one of the most important crops in the country, and even after the failure of many commercial scale projects, the country still manages to produce about 0.8 million metric tons of groundnuts. Throughout the history of Chad, its agricultural foundation lies greatly in peanut production. The local farmers earn handsome amounts of money by selling these through international trade routes to other countries. Argentina is famous for its amazing Golden peanuts which are exported in large amounts to other nations. More than 35,000 hectares of land is used for farming peanuts in the country. Many companies including Olega, Sol Argentina and Olam Argentina have expanded their business empire entirely through good quality Argentinian peanuts. The annual production of peanuts in the country is approximately about 1.1 million metric tons. Indonesia is one of the major peanut producing Asian countries . Recent figures revealed that the production of groundnuts in the country has now reached 1.9 million metric tons per year. Indonesia produces almost 4% of the total peanut world production. In a country like Indonesia, peanuts are most widely used to extract peanut oil which is also exported around the world. In spite of the instability of the country’s political and economic conditions, Burma stands 5th in groundnut production because of the heavy production it has had for a number of years. According to recent estimates, it produces 2 million metric tons of peanuts each year. The whole country is said to be powered by peanut production. The country is still yet to explore and exploit many of its land areas to put them to best use for peanut production. Northern Nigeria has had a significant groundnut production in recent years. Its temperate climate is most suitable for it. The country has attained a production rate of 3.8 million metric tons per year. Kano, Taraba, Bauchi and Bornu are the major areas which account for 85% of the total peanut production of the country (Fig. 1.3). Groundnut yields in sub-Saharan Africa (SSA) are generally low (964 kg/ha) which is far less than potential yields of up to 3500 kg/ha reported elsewhere (African Institute of Corporate Citizenship 2016). The low yield levels of groundnut in SSA is attributed to various stresses such as abiotic (drought and low soil fertility), biotic [pests such as aphids (Aphis craccivora Koch), leaf-miner (Aproarema modicella Deventer), thrips (Thripspalmi Karny, Frankiniella schultzie Trybom, Scirtothripsdorsalis Hood and Caliothrips indicus) and termites (Iso-ptera)], and

6

Physiology of the Peanut Plant

Fig. 1.3. Peanut producing countries of world

diseases (i.e. groundnut rosette disease, leaf spot, rust). Further, farmers in the region are cultivating unimproved varieties using poor agronomic practices with limited access to extension and advisory services. For example, in Senegal, water stress occurring during flowering and the seed filling period reduced the groundnut shelled yields by 33 and 50%, respectively. Groundnut rosette disease causes more severe yield losses than any of the groundnut viral diseases in the region. Early and late leaf spots caused 100% yield loss in Ghana. In SSA, efforts are being made to improve groundnut yield levels which aided in the release of few genetically superior and improved groundnut varieties. Reports showed that introduced groundnut varieties had considerable resistance to both biotic and abiotic stresses. In addition, groundnut varieties with some desirable quality attributes such as high oil content and larger seed size for confectionery purposes have also been recently popularised. Consumer concerns about food quality of peanut has become increasingly important. Since, peanuts are susceptible to Aspergillus infection which results in aflatoxin production, all seed lots are tested at buying points by processors to eliminate toxins from the food chain. Peanuts also have proteins that result in allergic reactions in about 0.6 per cent of the population. Trace amounts of peanut protein can lead to fatal anaphylactic reactions in individuals allergic to peanuts, and this is a great concern for the industry. In the United States, many peanuts are dry roasted, which apparently increases the allergic properties of the proteins. Refined peanut oil does not contain proteins and thus the oil is allergen free. However, when the seed is cold pressed, as is done in many parts of the world, proteins remain in the oil used for cooking and allergic reactions can occur. Properties of peanut oil are determined by the fatty acid composition. Two fatty acids, oleic (O) and linoleic (L), comprise over 80 per cent of the oil content of peanut. Standard peanut cultivars average 55 per cent oleic acid and 25 per cent linoleic acid. Linoleic acid is less saturated and less stable than oleic acid, and the oxidative stability and shelf life of peanut and peanut products can be enhanced by increasing the O/L ratio. Norden et al. (1987) examined the fatty acid composition

Introduction

7

of 494 genotypes and identified two breeding lines with 80 per cent oleic acid and 2 per cent linoleic acid. This was a major deviation from previously known levels of fatty acid composition in peanut. It found that inheritance of the high-oleate trait was controlled by duplicate recessive genes, ol1and ol2. F435 differed at both loci from a Virginia-type line but at only one locus from a runner line. Monogenic inheritance was reported in crosses of the runner market-type cultivars and breeding lines. A cross with a Virginia market-type segregated in a 15:1 ratio typical of recessive digenic inheritance. It was concluded that one of the recessive alleles occurs with high frequency in peanut breeding populations in the United States whereas the other allele is rarer. Isleib et al. (1996); Lopez et al. (2001); Gorbet and Knauft (1997) examined five different cultivars of Virginia-type peanut cultivars and found that four were either Ol1Ol1ol2ol2 or ol1ol1Ol2Ol2 and one was Ol1Ol1Ol2Ol2. When only one gene transfer is required, Isleib et al. were able to identify heterozygotes based on linoleate levels. This will allow breeders to identify carriers of the recessive allele in successive cycles of back crossing without intervening generations of selfing and decrease the time required to achieve the desired number of back crosses. Lopez et al. examined the inheritance of high oleic acid in six Spanish market-type peanut cultivars. Segregation patterns indicated that two major genes were involved. However, the presence of low-intermediate O/L ratio genotypes indicated that other genetic modifiers might be involved in the expression of the O/L ratio in these genotypes. Isleib et al. also observed an effect of other loci on fatty acid concentrations. Studies of peanut lines without the high oleic characteristic have indicated that oleic content can be influenced by additive gene effects, and by additive × additive epistasis. Gorbet and Knauft found that ‘Sun Oleic 95R’, a high oleic runner cultivar, had a much longer shelf life than the traditional runner-type peanut cultivars. Peanuts with high levels of oleic acid also show some promise for beneficial health effects in humans and animals that consume them. Groundnut is an energy rich crop but it is grown under energy starved conditions. The nutritional needs of the groundnut must be satisfied to attain maximum yields. An average crop of groundnut yielding 1900 kg/ ha removes about 112 kg N, 27 kg P2O5 and 34 kg K2O from one hectare of land. It is reported that the groundnut plant has a universal ability to utilize soil nutrients that are relatively unavailable to other crops and is very effective in extracting nutrients from sandy soils with a low nutrient supply. Fertilizer requirements for each field should be determined on the basis of laboratory soil analyses. A balanced fertility program, with emphasis on available levels of phosphorous, potassium, magnesium and nitrogen, is essential for high yields. Following fertilizer recommendations based on the laboratory analysis for each field and yield goal is important. Heavy rates of potassium fertilizers applied to the pegging zone can interfere with calcium uptake for developing pegs and pods. Certain micronutrients, including zinc, iron, manganese, copper, boron and molybdenum, also are essential to peanut production. Boron deficiency impairs normal seed development and causes a hollow heart. A hollow heart is an irregularly shaped blackened cavity on the inner face of the peanut seed. The condition is classified as concealed damage. Evidence from field studies indicates that the symptoms of boron deficiency are more likely to occur at high yield levels. Soils testing less than 1 pounds per acre should be considered for boron application. Apply boron when there is a deficiency because too much boron can be harmful to yield and quality. Zincdeficient soils also can reduce crop yields. Soils are deficient if the DPTA extractable zinc concentration is less than 0.4 ppm. Zinc deficiencies can occur in alkaline soils

8

Physiology of the Peanut Plant

that are low in organic matter and high in available phosphorous. In addition to adding zinc fertilizers, adding large amounts of organic materials, such as barnyard manure, and incorporating plant residues is also helpful. The most common recommendation for inorganic zinc fertilizer application is 6 to 10 pounds per acre. Zinc remains available in the soil for several years, although it may be tied up with phosphorus. Soils should be tested before adding zinc. Symptoms of zinc deficiency can occur concurrently with those of iron deficiency. The chlorotic strips of zinc deficiency are usually wider than those of iron deficiency on the portion nearest the petiole and may not run the entire length of the leaflets. High temperatures can cause zinc deficiency to appear as leaflet bronzing. Zinc-deficient plants also may be stunted. Alkaline soils also may be deficient in copper, manganese, and available iron. Nutrient deficiencies, with the exception of iron, can be corrected by applying the required elements before or at planting time. Iron deficiency in peanuts can be corrected with foliar applications of iron chelates or iron sulphates. Applying iron through irrigation systems is not as efficient as foliar applications of chelates or sulphates. Soil applications of iron may cause it to be chemically bound and unavailable. Although peanuts are legumes and can provide some of their own nitrogen (Table 1.2), soil-available nitrogen is required by young plant seedling. Healthy, vigorous seedlings can be ensured with 10 to 20 pounds per acre of starter nitrogen. If soil tests indicate this high nitrogen, carry-over is available from the previous crop and additional nitrogen may not be necessary. Organic manures are important primarily as humus fertilizers. Humus is decomposed by soil organisms providing them with nutrients and energy. Humus also improves soil fertility by improving physical and biological characteristics. Farmyard manure (FYM) and compost are mainly used as organic manures in groundnut crop. In addition to serving as a source of supply of nutrients, bulky organic manures like FYM serve as a source of organic matter which influences nutrient supply to plants in many ways. Organic matter improves the structure and reduces compaction and crusting of the soil. It is also required as a source of energy for nodulation and nitrogen fixation by microorganisms. It is the most important and commonly used organic manure, which is produced by the cattle, pigs and poultry. Low moisture holding capacity of the soil, quick drying and crust formations are the problems in the lateritic soils. The hardness of soil physically restricts the full development of pods. Addition of FYM considerably Table 1.2. Nutrient uptake of peanuts with different production levels Pods (t/ha)

H

P

K

Ca

Mg

S

Fe

Mn

Zn

B

1

58

5

18

11

9

4

2

0.09

0.08

0.05

2

117

10

36

23

18

9

4

0.19

0.16

0.11

3

174

15

54

34

27

13

6

0.29

0.24

0.16

4

232

20

73

45

36

18

8

0.38

0.32

0.22

5

290

25

91

56

45

22

10

0.48

0.41

0.27

6

348

30

109

68

54

26

12

0.58

0.49

0.33

The present recommended dose of 17:35:54kg NPK/ha. can be applied as below for betteryield recovery. a. Entire P 33.3% N&K as basal. b. 33.3% N & K as top dressing at flowering. c. 33.3% N & K as top dressing at pod initiation stage.

9

Introduction

reduces these problems. Usually 12.5 t/ha of FYM is essential because its application improves the porosity and structure of the soil and makes it less sticky. It also provides most micronutrients. Addition of FYM should be applied well in advance (15 to 30 days before sowing) and should be incorporated into the soil by a country plough or blade harrow. FYM, besides cow dung, contains stubbles, stalks and other crop residues. Compost is a product of the decomposition of plant and animal wastes with various additives. Well-decomposed compost should be applied about one month before sowing. It may be broadcasted and incorporated into the soil with the help of a country plough or blade harrow (Table 1.3). Rhizobium, soil bacteria have the ability to fix atmospheric nitrogen in symbiotic association with host legumes. These bacteria enter the plants through the root hairs, multiply there, form nodules and fix biological nitrogen in the nodules. To ensure effective nodules, the crop has to be provided with highly efficient Rhizobium in the vicinity of its root system. This can be achieved by artificially inoculating pre-selected effective and efficient Rhizobium. It is estimated that a well-nodulated groundnut under normal growth conditions is capable of fixing about 180 kg N/ha. Rhizobium inoculation is a cheaper and usually more effective agronomic practice for ensuring adequate nitrogen supply. A higher rate of Rhizobium inoculation is required to obtain sufficient nodulation of the inoculant strain to overcome the competition by the native Rhizobium. Inoculum containing 105 cells/ seed is the minimum requirement. Of the agronomic factors known to augment crop production, water management stands second, next to fertilizers, contributing 27% in crop production. Groundnut is generally grown as kharif/autumn crop in rainfed areas, but the productivity of kharif groundnut is lower than rabi/summer crop. Among the various constraints for low yield, the principal one is erratic, insufficient and unevenly distributed rainfall during kharif. Success will be assured in the kharif season if irrigation is considered as a basic input and rain as a supplement. Since irrigated groundnut is not subjected to vagaries of monsoon and is less exposed to a pest and disease complex, possibilities of increasing its productivity and stabilizing production are immense (Table 1.3). Regulate irrigation based on physiological growth phases as given in the table. Table 1.3. Tentative irrigation requirements in peanut Crop phase Pre-flowering

Days From 0

To 25

No. of Irrigation 1

Sowing/pre-sowing

1

Life irrigation (4-5 DAS)

1

Vegetative (20 DAS) Flowering

26

45

Stray flowering (26-30 DAS) Peak flowering (35-40 DAS) Post-flowering Pegging (46-65 DAS) Pod development (66-105 DAS)

1 1

45

105

2 2-3

10

Physiology of the Peanut Plant

The legume Arachis hypogaea, commonly known as peanut or groundnut, is a very important food crop throughout the tropics and sub-tropics. The genus is endemic to South America being mostly associated with the savannah-like Cerrado. All species in the genus are unusual among legumes in that they produce their fruit below the ground. This profoundly influences their biology and natural distributions. The species occur in diverse habitats including grasslands, open patches of forest and even in temporarily flooded areas. Based on a number of criteria, including morphology and sexual compatibilities, the 80 described species are arranged in nine infrageneric taxonomic sections. While most wild species are diploid, cultivated peanut is a tetraploid. It is of recent origin and has an AABB-type genome. The most probable ancestral species are Arachis duranensis and Arachis ipaetensis, which contributed the A and B genome components, respectively. Although cultivated peanut is tetraploid, genetically it behaves as a diploid, the A and B chromosomes only rarely pairing during meiosis. Although morphologically variable, cultivated peanut has a very narrow genetic base. For some traits, such as disease and pest resistance, this has been a fundamental limitation to crop improvement using only cultivated germplasm. Transfer of some wild resistance genes to cultivated peanut has been achieved, for instance, the gene for resistance to root-knot nematode. However, a wider use of wild species in breeding has been hampered by ploidy and sexual incompatibility barriers, by linkage drag, and historically, by a lack of the tools needed to conveniently confirm hybrid identities and track introgressed chromosomal segments. In recent years, improved knowledge of species relationships has been gained by more detailed cytogenetic studies and molecular phylogenies. Osmotic adjustment, accumulation and remobilization of stem reserves, superior photosynthesis, heat- and desiccation-tolerant enzymes, and so on are important physiological traits (PTs) in a breeding programme either by direct selection or in the course of a substitute such as molecular markers. Several pieces of information on important PTs may be collected on potential parental lines that engross screening of whole crossing block, or a set of commonly used parents, thus bringing into being an index of useful PTs which have to imperatively establish their heritability and genetic correlation with yield in target environments that can be used advantageously in designing crosses, which bring together desirable traits through increasing transgressive segregation events. It is vital that the application of the trait as a selection criterion be definite when significant genetic diversity for a physiological trait in a germplasm collection for the given species is established. Subsequently, breeding strategies are effective only when these traits are rightly defined in terms of the stage of crop development so as to using specific attributes of the target environment and their potential contribution to yield. Genetic Engineering of groundnut is one of the potential options for improving abiotic stress tolerance and food safety. While groundnut transformation is reproducible, it still is low in efficiency and more research to improve the efficiency would be justified, particularly as a functional genomics tool. Kanamycin was used as the selection marker in transformation, but groundnut appears to have some resistance to it, which leads to a large number of escapers in the regeneration process. Because of the resistance to kanamycin, transgenic groundnut cannot be screened directly at the seedling stage in kanamycin media, instead transgenic groundnut plants have to be screened by DNA blot, PCR or RNA blot analysis, which makes characterization of transgenic groundnut plants labour intensive and time-consuming. While hygromycin resistance is a very effective selection method

Introduction

11

for groundnut, there is a trend to eliminate antibiotic resistance genes from the plant transformation toolbox. Therefore, it is necessary to develop a more efficient system for groundnut transformation and alternative selectable marker genes have been proposed and tested in other plants, but their effectiveness in groundnut remains to be examined. Groundnut can be more efficiently engineered with the signalling components and transcription factors (TF). For example, over expression of a transcription factor AtDREB1A in groundnut under the control of a stress responsive promoter resulted in enhanced drought tolerance and produced 24% higher seed yield under field drought stress condition. Hence, transcriptome engineering seems to be promising for the development of abiotic stress-tolerant groundnut varieties. Transcription factors implicated in more than one type of stress might also be identified. Appropriate promoters need to be selected, depending upon the gene used, to obtain desirable transgenic plants with high yield stability under stress conditions. However, the key concern about TF transgenics is whether they will perform consistently under drought and/or heat stress conditions in the field. Recently, with the advantage of GATEWAY technology employing multigene expression cassettes with pathway engineered genes appears to be the most promising strategy. The conceptual strategies to pyramid traits by the transgenic approach require initial identification of genotypes with superior water relations and using them as recipient genotypes to express validated upstream regulatory genes that improve cellular level tolerance. However, the limitations could be the issues related to bio safety, which need to be addressed carefully. The reproductive stage is the most critical stage for productivity, in the majority of studies, stress tolerance has been assessed at the initial growth stages, that is, germination and seedling stages, using survival rate as the main trait to represent tolerance to stress. Under field conditions, genetically engineered plants have to cope with multiple stresses (such as water deficit and heat) for longer periods. Hence, more emphasis should be given to the study of the responses of these plants to a combination of environmental stresses at the reproductive stage, under field conditions. The major breeding objectives in this crop are the development of high yielding cultivars of suitable duration to escape moisture stress with resistance to various biotic stresses (foliar diseases like rust and early and late leaf spots and aflatoxin contamination by Aspergillus flavans, pod and stem rot, and more) and tolerance to different abiotic stresses (moisture stress). Continuous efforts have yielded genetic resistance for these diseases. Short and medium duration and confectionery type varieties with multiple tolerance/resistance have been developed by ICARISAT as well as NARS in India. Significant progress has been seen in understanding the underlying mechanism of drought tolerance in groundnut. As it has been established that yield under water limited conditions is a function of transpiration (T), transpiration use efficiency (TE) and harvest index (HI), large exploitable genetic variation has been observed in the germplasm of groundnuts for these traits. There is a need to develop a selection index integrating T, TE and HI with appropriate weights for use as selection criteria in a breeding programme. In addition to resistance/tolerance to the prevailing biotic and abiotic stresses, a variety for achieving success should be in harmony with the edaphic and climatic factors of the ecosystem. The duration of the variety, irrespective of its growth habits, should match with the period of soil moisture availability particularly under rainfed situations. Novel techniques such as genetic transformation, molecular markers, adding selection and gene transfer from alien sources need to be exploited more for making an impact on groundnut research.

12

Physiology of the Peanut Plant

REFERENCES FAOSTAT.2008.http://faostat.fao.org/faostat/. Gorbet, D.W. and D.A. Knauft. 1997. Registration of Sun Oleic 95R peanut. Crop Sci., 37: 1392. Isleib, T.G., C.T. Young and D.A. Knauft. 1996. Fatty acid genotypes of five Virginia-type cultivars. Crop Sci., 36: 556-558. Lopez, Y., O.D. Smith, S.A. Senscman and W.L. Rooncy. 2001. Genetic factors influencing high oleic acid content in Spanish market – type peanut cultivars. Crop Sci., 41: 5156. Norden, A.J., D.W. Gorbet, D.A. Kauft and C.T. Young. 1987. Variability in oil quality among peanut genotypes in the Florida Breeding Program. Peanut Sci., 14: 7-11. Stalker, H.T. and C.E. Simpson. 1995. Germplasm resources in Arachis. pp. 556-558. In: H.E. Pattee and H.T. Stalker (eds.). Advances in Peanut Science. American Peanut Research and Education Society, Stillwater, Oklahoma.

CHAPTER

2

Seed Dormancy and Germination Seed dormancy is defined as a state in which seeds are prevented from germinating even under environmental conditions normally favourable for germination. Seed dormancy in groundnut is regulated mainly by testa in Spanish type and by cotyledons and embryonic axis (both zygotic tissues ) as well as testa in Virginia type. Issues with Groundnut Seed Chain • • • •

High seed rate, low seed multiplication ratio. Groundnut seeds belonging to Spanish and Valencia group bear no seed dormancy. Lacking of short term fresh seed dormancy causes in situ sprouting. Seeds produced during rabi and summer seasons lose about 50% viability within 4-5 months (Nautial et al. 2004). • Improper seed storage conditions lead to a drastic drop in germination rate and seedling vigour. • Patchy crop stand resulted by one or a combination of the above listed factors reducing crop yield.

2.1.

Seed Dormancy

The inability of certain seeds to germinate readily even when they are provided with all conditions required for germination is known as seed dormancy. A dormant seed is not a ‘failure’. It may be due to conditions associated with either the seeds or with existing environmental factors such as temperature and moisture. In general, in groundnut, bunch types are non-dormant while spreading and semi spreading types have varying periods of dormancy. Mature peanut kernels are dormant to some degree. Interestingly, seeds that develop at the peg end of the pod have a longer dormant period than those at the opposite end. The period of dormancy depends on variety and storage conditions. Spanish types have virtually no dormancy (5–50 days), whereas Virginia types can be dormant for 100–120 days. If sufficient moisture is available, seeds with little or no dormancy period can sprout in the field before harvest. Currently grown Spanish varieties can have this problem, but pre-harvest sprouting is generally not a problem with Virginia or Runner types.

14

Physiology of the Peanut Plant

Mechanisms of Seed Dormancy There are basically two types of dormancies which involve different mechanisms. a. Embryo dormancy: Where the control of dormancy resides within the embryo itself. b. Coat imposed dormancy: In which the dormancy is maintained by the structures enclosing the embryo, viz. seed coat.

Control of Embryo Dormancy in Groundnut Different parts of the groundnut seed including the seed coat, cotyledons and embryo are involved in imposing dormancy (Nautiyal, 2004). The inhibitory effects of different parts of the dormant seeds on the growth of the embryonic axes are as follows: seed coats > cotyledons > embryonic axes with seed coat Sreeramulu, (1974). Dormancy in groundnut is regulated mainly by testa in the Spanish type, but by cotyledons, and embryonic axis as well as testa in Virginia types. (Bandyopadhyay et al., 1999).

Coat Imposed Dormancy Seed dormancy in the majority of species is imposed by the structure surrounding the embryo. This is often referred to as the seed coat imposed dormancy. The mechanism of seed coat imposed dormancy is poorly understood.

Control of Seed Dormancy in Groundnut 1. 2. 3. 4.

Genetic factors Environmental factors Hormones Seed coat

Genetic Factors The dispersal units generally consist of three genetically different tissues: 1. A diploid embryo produced by fertilization of the ovum 2. A triploid endosperm containing one set of paternal genes and two sets of maternal genes

Seed Dormancy and Germination

15

3. The diploid testa, all pericarp of maternal genetic constitution. Dormancy can be inherent within the embryo or can be imposed by these extra embryonic tissues.

Inheritance of Seed Dormancy Dormancy is a quantitatively inherited trait and additive, dominance and digenic epistasis effects are involved in its genetic control (Khalfaoui, 1991). The characteristic may be quantitatively inherited (Nautiyal et al., 1994). Kumar et al. (1991) noted that there is additive dominance gene action. Beyond additive and dominance effects, there is duplicate epistasis in the control of fresh seed dormancy (Ndoye, 2001). Upadhyay and Nigam (1999); Asibuo et al. (2008); Yaw et al. (2008) noted that monogenic inheritance with dormancy is dominant over nondormancy. The lack of seed dormancy in groundnut causes huge losses due to in situ germination in Spanish bunch groundnut. Groundnut seed dormancy seems to be a complex adaptive trait reported to be controlled by maternal and zygotic tissues as well as the environment. It is believed to be induced by physiological and biochemical phenomenon. The mechanisms of dormancy have been understood partially. Considerable genetic variability has been observed for seed dormancy in groundnut. Efforts have been made to induce dormancy in bunch type groundnut.

2.1.1. Influence of Temperature Temperature is one of the most important environmental factors that affects germination and dormancy in seeds (Bewley et al., 2013). Moreover, the response of seed germination to temperature in crop production influences planting dates and growing seasons (Probert, 2000; Bewley and Black, 1994) thus influencing yields directly. Peanut seed survival in the field is also affected by high temperatures as is common in semi-arid and arid areas where it is grown (ICRISAT, 1992). As an adaptation mechanism, some plant seed species enter a state of dormancy. It is therefore important to identify a temperature regime that provides for thermotolerance in peanut seeds to overcome the effects of high temperatures encountered prior to seed germination. Crop heat tolerance can be enhanced by preconditioning of plants under different environmental stresses or by exogenous application of osmoprotectants such as glycinebetaine and proline (Wahid et al., 2007). An environmental stress in the form of high temperature has been implicated in bringing about regulation of changes in seed dormancy (Baskin and Baskin, 1998; Benech-Arnold et al., 2000). Dormancy may be influenced through exposure of dry or imbibed seeds to certain temperature regimes before germination. Seeds were kept in sealed laminated aluminium foil bags and exposed to heat stress for 48 h at four temperatures; 27±2°C, 70±5% RH (control: room temperature), 40°C, 50°C, and 60°C. Seed germination and seed vigour decreased significantly as the heat stress temperature increased. The seed moisture content and total abnormal seedlings did not vary among treatments whereas occurrence of deformed seedlings was significant at temperatures greater than 40±1°C. However, 40°C significantly increased dormancy by 31% compared to the control seeds, while heat stress beyond 50°C had a detrimental effect on peanut seeds in the form of embryonic death. Highest seed dormancy was shown by seeds exposed to 40°C. Several authors have explained how heat stress influences dormancy in seeds and findings seemed to be in agreement with their results. Toh et al. (2008) reported that high temperatures

16

Physiology of the Peanut Plant

induce accumulation of reactive oxygen species (ROS) and abscisic acid (ABA) which are central ideas in dormancy and germination control in Arabidopsis seeds. On the other hand, Piskurewicz et al. (2009) stressed that the heat shock proteins (HSPs) produced as a result of heat stress on seeds is linked to dormancy such that endosperm rupture during germination is inhibited. Furthermore, HSPs produced after exposure to high temperatures maintain the proteins in the seed in a folding-competent manner and this limits germination until favourable conditions occur (Smy´kal et al., 2000). From the investigation it was clarified that all six genotypes (Kaushal, Utkarsh, Prakash, Chitra, Amber and TG37A) of groundnut breaked their dormancy between three to five months of ambient storage, because observations of germination were taken in tri-monthly intervals i.e., zero, three, six, nine, twelve, fifteen months. All the genotypes showed less germination in the 0 month but germination percent increased in the third month but when germination per cent was taken in the sixth month it decreased again (Table 2.1). Table 2.1. Effect of heat stress for 48 h on germination of peanut seeds* Heat stress (°C)

Ungerminated seeds (%)

Germination (%)

Abnormal seedlings (%)

Decayed (%)

Viable dead

Ambient

75.3a

4.7ab

2.0c

7.5b

10.5b

40

58.0b

9.5a

4.8c

20.0a

7.5b

50

12.8c

6.2a

52.5b

5.0b

23.5a

60

0.2c

0b

99.8a

0c

0c

*Means (n = 4) within a column with different lower case letters indicate that values are significantly different at P≤0.05, DMRT.

2.1.2.

Longevity of Seed

Longevity of seeds differed from species to species and variety to variety. It is also dependent on the stored food material in the seed and affected by the temperature and humidity. Longevity is taken here as long as germination is retained above the IMSCS. Highest longevity (up to IMSCS) of 12 months was exhibited by Chitra, Utkarsh and TG37A followed by Kaushal, Prakash and Amber i.e., 9, 6 and 3 months respectively. However, genotype Amber maintained germination of 69.7 per cent in six months. So the longevity of Amber can be six months in place of three months. In this month it maintained germination (73.0%) (Table 2.2). This suggests that the seed coat is possibly imposing a physical effect on the sprouting of embryo. Total removal of seed coats also failed to break complete seed dormancy in all the varieties. In freshly harvested seeds it caused 30 to 60% germination in TMV-2, TMV-3, TMV-10 and 0-17% in C-148, M-13 and M-145. The germination percentage increased to about 80 to 100% at about 20 days when compared to the intact coat seeds where the complete removal of dormancy was noticed by about 50 days. Application of GA during imbibition showed that there was no promoting effect on seed germination until the seeds were 15 days old. However, it enhanced germination percentage in most of the dormant varieties when applied in 15 to 40 days old seeds. The effect of GA was less in M-13 which showed a longer period of seed dormancy.

17

Seed Dormancy and Germination Table 2.2. Mean germination (%) of different genotypes of peanut during ambient storage Variety

0 month

3 months

6 months

9 months

12 months

15 months

Mean

Kausha1

71.0 (57.42)

85.3 (67.61)

82.0 (64.95)

77.6 (61.89)

68.3 (55.77)

58.6 (44.99)

73.8 (59.60)

Utkarsh

79.6 (63.22)

91.3 (72.92)

82.3 (65.82)

76.0 (60.82)

70.0 (56.79)

54.6 (47.68)

75.6 (60.90)

Chitra

74.0 (59.38)

80.0 (63.55)

76.3 (60.94)

65.0 (53.76)

62.6 (52.34)

51.6 (45.96)

66.9 (55.01)

Amber

61.0 (51.36)

73.0 (58.71)

69.7 (56.59)

64.3 (53.35)

62.3 (52.15)

49.0 (44.43)

63.2 (52.76)

Prakash

66.3 (54.54)

80.0 (63.55)

76.3 (60.94)

65.0 (53.76)

62.6 (52.34)

51.6 (45.96)

66.9 (55.01)

TG37A

81.6 (64.68)

86.6 (68.62)

81.0 (64.18)

72.3 (58.28)

70.0 (56.79)

57.3 (49.22)

74.8 (60.29)

Mean

72.3 (59.09)

82.7 (64.99)

77.9 (62.19)

71.6 (57.98)

67.8 (55.20)

55.1 (47.94)

Mean angular value in parenthesis

Thus, the data suggest that seed dormancy in groundnut is not controlled by any single factor and the period of seed dormancy varies in different varieties. In the nondormant varieties there was no seed coat dormancy as they are loosely held by the cotyledons. On the other hand, the tightly held seed coat in dormant varieties inhibited the embryos from sprouting and probably partially responsible for leaching of inhibitors in some varieties (Amen, 1964; Rao and Rao, 1972; Sreeramulu and Rao, 1968, 1971). However, in some varieties (M-145 and M-13) it was noticed that the promotion of germination did not occur until about two weeks even after total seed coat removal. Application of GA was also proved to be ineffective in enhancing germination during that period. Thus, there might be the presence of endogenous substances in the seeds which blocked the germination and thus it was not removed either by seed coat removal or negated by application of GA. After about two weeks the block to germination imposed by this inhibitor was removed possibly through metabolism and permitted gibberellin mediated reactions or seed coat removal enhancement of germination (Khan and Waters, 1969; Khan et al., 1971). Thus, it appears that groundnut seeds have two phases of seed dormancy and GA is effective in enhancing germination only when the first phase is over.

2.2.

Seed Components

The seeds of cultivated peanut, Arachis hypogaea, store proteins, lipids and starch required for energy and growth upon germination. Lipid (oil) is the predominant macro component, and generally increases as the peanut seed matures (Patee et al., 1974). For mature peanuts, total oil was reported to average about 50% on a fresh weight basis (Patee et al., 1983). Oilseeds, such as peanuts, store most of their lipids in small, intracellular organelles commonly called oil bodies (Young and Schadel, 1990; Huang, 1992). Triglycerides form the majority of these oil bodies, and the interior

18

Physiology of the Peanut Plant

triglycerides are encapsulated by a phospholipid bilayer and embedded oleosin protein. The oil bodies provide a stable energy reserve that can be accessed upon germination. Phytosterols are a special class of structural lipids that provide stability and fluidity in cell membranes including specialized encapsulation of lipids, such as oil bodies (Dufourc, 2008). The primary function of phytosterols is as membrane reinforcers and precursors for brassinosteroids, important phytohormones in plants. Physiological maturity impacts seed quality through a variety of mechanisms including desiccation tolerance, preparation of storage reserves and establishment of dormancy (Ventura et al., 2012). Thus, the impact of seed maturity on germination efficiency is of primary importance to peanut producers. To best assess physiological maturation, a method of classification based on colour and morphological differences of the mesocarp was described for determining the developmental stages of fresh peanut (Williams and Drexter, 1981). Seed maturity proceeds with the thickening of cell walls and addition of oil bodies (Young et al., 2004). For mature peanuts of the highest classification (black mesocarp), the cytoplasm of the parenchyma cells is essentially full of oil bodies. Table 2.3. Macronutrient composition of Arachis hypogaea seeds of different maturation classes Fat (%)

Protein (%)

Sugar (%)

Orange

Pod colour

50.95

20.90

3.43

Brown

51.49

19.90

3.63

Black

51.96

18.00

4.42

Method error – 0.33% fat, 0.648–0.798% protein, and 0.33–0.52% sugar.

The water, minerals, aspartic acid, methionine, proline, folic acid, thiamine and total phenolics contents increased dramatically in peanut cotyledons and sprouts after germination, while the fat, riboflavin and ascorbic acid contents decreased markedly. The total amino acid content, moreover, showed no obvious decrease, and the relative amounts of some limiting and essential amino acids clearly increased after germination in the peanut seed. Fatty acid composition of peanut seed oil in four varieties cultivated in Tunisia showed that linoleic (C18:2), oleic (C18:1) and palmitic (C16) acids account for more than 84% of Chounfakhi and Massriya and for more than 85% of the total fatty acids of Trabilsia and Sinya seed oils respectively. Seed oil contents were significantly different (P ≤ 0.05) and did not exceed 48% (Table 2.4). The study of total phenolics revealed that Chounfakhi contained more total phenolics (2.1 mg GAE/g DW), followed by the Massriya and Sinya cultivars (1.35 mg GAE/g DW for each); Trabilsia presented the lowest total phenolic content with 1 mg GAE/g DW. Considerable antiradical ability was found, especially in the Trabilsia peanut seed cultivar (IC50 = 1550 μg/ ml), and the Massriya and Sinya cultivars had 720 and 820 mg/ml IC50, respectively. In the Massriya variety the sterol fraction showed antibacterial activity against Listeria ivanovii, Listeria inocua, Pseudomonas aeruginosa, Staphylococus aureus, Enterococcus hirae and Bacillus cereus. During the maturation process, triglycerides are stored in oil bodies as an energy resource during germination and seedling development. The stability of oil body

19

Seed Dormancy and Germination Table 2.4. Fatty acid composition of peanut varieties Fatty acid

Chounfakhi

Trabilsia

Sinya

Massriya

C14

1.19

0.77

0.79

1.56

C16

12.11

17.45

13.34

11.89

C16:1

3.43

1.93

2.15

3.17

C18

4.01

4.12

4.1

4.59

C18:1

32.63

27.16

30.31

32.12

C18:2

39.65

41.38

41.85

40.06

C18:3(w )

1.63

1.27

1.31

1.41

C22

0.58

1.1

0.98

0.39

C22:1

2.73

2.22

2.35

2.57

C24

2.04

2.60

2.82

2.24

SFA

19.93

26.04

22.03

20.07

MUFA

38.79

31.31

34.81

37.86

PUFA

41.28

42.65

43.16

41.47

3

membranes is essential for nutrient mobilization during germination. This study focused on evaluating the phytosterol composition in seed components including the kernel, embryo (heart), and seed coat or skin. Samples of different maturity classes were analysed for macronutrients and phytosterol content. The three biosynthetic end products in the phytosterol pathway, β-sitosterol, campesterol and stigmasterol, comprised 82.29%, 86.39% and 94.25% of seed hearts, kernels and seed coats, respectively. Stigmasterol concentration was highest in the seed kernel, providing an excellent source of this sterol known to have beneficial effects on human health. Peanut hearts contained the highest concentration of sterols by mass, potentially providing protection and resources for the developing seedling. The amount of α-tocopherol increases in peanut hearts during the maturation process, providing protection from temperature stress, as well as stability required for seedling vigour. These results suggest that phytosterols may play a significant role in the performance of seeds, and provide a possible explanation for the poor germination efficiency of immature seeds. The breakdown of seed storage reserves, transport of reserve material to the embryonic axis and synthesis of new materials from the breakdown products are the three main chemical changes occurring in rehydrated imbibed seeds. In groundnuts as the metabolic reserves are largely lipids (40-54%) and proteins (20-30%), the lipids breakdown takes place through glyoxylate cycle and enzymes malate synthatase and isocitriclyase are essential during conversion of fats to carbohydrates. The abscisic acid (ABA) inhibits germination and synthesis of isocitriclyase and ethylene reverse the inhibitory effects of ABA on germination (Ketring, 1975). The gibberellic acid and ethylene promote the germination and isocitriclyase enzymes control the balance between fats and carbohydrates. The protein degradation occurs during the 4 to 9th days of germination. The chemical compounds after breakdown are translocated to the growing points of the embryonic axis and utilized for seedling growth. During germination the dry weight of developing seedlings decreases (cotyledons dry wt. decreases by 60% and proteins depleted by 70%). Misra et al. (1992), however, reported that sucrose, indigenously present in cotyledons, is translocated to the

20

Physiology of the Peanut Plant

growing axis for the first three days, and subsequently the catabolism of oil begins; the proteolysis begins soon after imbibitions and free amino acids increased, but a rapid degradation of proteins was observed after the 4th day of imbibition. Profiles of total seed proteins isolated from mature seeds of four peanut cultivars, New Mexico Valencia C (NM Valencia C), Tamspan 90, Georgia Green, and NC-7, were studied using two-dimensional gel electrophoresis coupled with nano-electrospray ionization liquid chromatography tandem mass spectrometry (nESI-LC–MS/MS). Two dimensional gels stained with silver nitrate revealed a total of 457, 516, 556, and 530 protein spots in NM Valencia C, Tamspan 90, Georgia Green, and NC-7, respectively. Twenty abundant protein spots showing differences in relative abundance among these cultivars were analysed by nESI-LC–MS/MS, resulting in identification of 14 nonredundant proteins. The majority of these proteins belonged to the globulin fraction consisting of arachin (glycinin and Arah3/4) and conarachin seed storage proteins as well as other allergen proteins. The expression of some of these identified protein spots was cultivar-specific. For example, allergen Arah3/Arah4 and conarachin protein spots were only detected in Tamspan 90 and NC-7, whereas the Gly1 protein spot was detected only in NM Valencia C and NC-7. Moreover, a galactose-binding lectin protein spot with anti-nutritive properties was only present in Tamspan 90. Other proteins showing differences in relative abundance among the four cultivars included 13 lipoxygenase, fructose biphosphate aldolase, and glyceraldehyde 3-phosphate dehydrogenase. Together, these results suggest that identified proteins might serve as potential markers for cultivar differentiation and may be associated with underlying sensory and nutritional traits of peanut cultivars.

2.3.

Seed Germination

Germination of non-dormant peanut seeds is characterized by a “climacteric-like” pattern in ethylene production. Treatment with ethylene resulted in the autocatalytic stimulation of ethylene synthesis, accompanied by an acceleration in the rate of germination. It appears that the availability of ACC could be limiting during the initial phases of the germination process. An increase in the conversion of ACC to ethylene during germination suggested an increase in EFE activity of the seeds. Continuous application of 100 µM octanoic acid during the germination period resulted in the inhibition of seed germination. However, a brief treatment with 1 to 100 µM octanoic acid resulted in the stimulation of germination due to an increase in the sensitivity of the seed tissue to ethylene. Groundnut (Arachis hypogaea) seeds lose their germinability faster in seed lots harvested in the summer season than those harvested in the rainy season. The study was initiated to understand the possible causes of rapid loss of germinability in two contrasting seasons. The study was conducted with five groundnut cultivars, varying in several physical, morphological and chemical characteristics of seed. Seeds before storage and after 12 months were shelled and categorized based on the “shell-inside” colour, i.e. over mature (OVM), optimally mature (OPM) and immature (IMM), and a natural seed lot (NTL) was considered as control. The trend of loss of germinability and seedling vigour in seeds of different maturity stages was similar in both the seasons, except that germinability and seed vigour decreased more rapidly in the seed lots obtained in summer than in rainy season. Among various maturity stages, the overall germinability and seedling vigour was higher in OPM than in NTL

21

Seed Dormancy and Germination

Table 2.5. Influence of seed maturity stages on seed reducing sugars, total sugars and calcium (Ca) content in five groundnut cultivars Description GAUG10 Kadiri 3 ICGS 11 Girnar 1 GG2 CD (0.05) Storage period (m) 0 12 CD (0.05) Seed maturity stage 1 OVM OPM IMM NTL CD (0.05)

Reducing sugar (%) 6.18 4.77 4.04 4.97 5.88 1.0

Total sugars(%) 14.29 10.08 9.22 8.76 13.14 1.07

5.59 5.11 0.33

14.0 10.2 0.57

4.89 5.22 5.95 0.4

10.5 12.2 13.5 0.7

Ca content (%) 0.038 0.040 0.040 0.038 0.034 0.002 0.040 0.039 0.035 0.038 0.002

OVM = Over mature; OPM = Optimally mature; IMM = Immature; NTL = Natural seed lot.

and IMM seeds. The cultivars Kadiri 3 and ICGS 11 showed least influence of seed maturity stages and storage period on seed vigour, whereas GG 2 showed highest influence, indicating wide genetic variability in seedling vigour. However, IMM seed percentage was higher in Virginia (15%) as compared to the Spanish market types (10%). In addition, analysis of total sugars in seeds of different maturity stages showed that higher sugars in IMM seeds may be responsible for imbibitional injury due to absorption of excessive water resulting in poor germination. On the other hand, seed calcium (Ca) content was higher in OVM (0.040 ppm) and OPM (0.039 ppm) seeds. The cultivars Kadiri 3 and ICGS 11 (both 0.040 ppm Ca) showed higher Ca content and GG2 (0.034 ppm) least. In addition, relationships between seed Ca and germination percentage (r = 0.79**) and seed Ca and seedling vigour (r = 0.84**) were positive. Thus, Kadiri 3 and ICGS 11 were identified with higher seed Ca and GG2 with lower seed Ca content. If these results are confirmed in a large number of genotypes, seed Ca content could be used as an index of seed and seedling vigour in groundnut (Table 2.5 and Fig. 2.1).

2.3.1.

Seed Size

Seed size is an important physical indicator of seed quality that affects the emergence, plant growth and performance of the crop in the field (Adebisi et al., 2013). Indeed, the sowing of mixed seeds of a species may result in establishing non uniformity, which may lead to heterogeneity in the plant vigour and size (Mishra et al., 2010). Distinct seed sizes have different levels of starch and other energy reserves which may be an important factor in improving the expression of germination and initial growth of seedlings (Shahi et al., 2015). Germination depends on the ability of the seed to use reserves more efficiently (Bewley et al., 2013) by their mobilization for germination traits (Sikder et al., 2009). A wide array of different effects of seed size in non-stressful

22

Physiology of the Peanut Plant

Fig. 2.1. Seed germination (a) and seedling vigour index (b) of peanut varieties of different maturity

and stressful conditions has been reported for seed germination, seedling emergence and establishment in many crop species (Mut and Akay, 2010; Shahi et al., 2015; Soares et al., 2015). However, these results vary widely between the crop species and the germination and growth environment. In general, large seeds have a higher seedling survival rate, higher growth and better field performance than small seeds, under non-stressful environments (Ambika et al., 2014). In groundnut, seedling vigour grown from shrivelled and small seeds was less than that from large seeds, but more abnormal seedlings were formed from large seeds (34.9%) than from small and shrivelled seeds (10.6%) (Sulochanamma Reddy, 2007). In many situations, crop sowing is performed under inappropriate soil moisture conditions to support seed germination or in areas with an excess of salts in the soil or in the irrigation water (Munns and Tester, 2008). Water shortage and salt excess in the soil at the sowing time cause delayed and reduced seed germination, unequal seedling emergence and unsatisfactory stand establishment (Steiner et al., 2017), which results in crop yield reductions (Lawles et al., 2012). Drought and salinity affect germination by creating highly negative water potentials, thus preventing the seed water uptake. Salinity may also cause direct phytotoxic effects of Na+ and Cl– ions (Acosta-Motos et

Seed Dormancy and Germination

23

al., 2017). However, the negative effects of drought and salinity on seed germination and plant establishment can be alleviated or potentiated with the use of different seed sizes. Research results have reported that the effects of seed size on germination and seedling growth under water and saline stress conditions are still controversial and inconclusive. The use of increased seed size resulted in higher germination and seedling growth rates in naked oat (Mut and Akay, 2010) and triticale (Kaydan and Yagmur, 2008). On the contrary, Pereira et al. (2013) obtained more vigorous growth in seedlings from small soybean seeds under water stress conditions, whereas large seeds produced seedlings with a higher growth rate than small seeds under optimal soil moisture conditions. On the other hand, Soares et al. (2015) reported that the size of soybean seeds does not affect the germination and initial seedling growth under water and saline stress conditions. Limede et al. (2018) also verified that the seed size does not affect the emergence of soybean seedlings, although large seeds produce plants with the highest dry shoot matter. However, the effects of seed size on the germination and growth of peanut seedlings under stressful environments are still unknown. Thus, this study aimed to investigate the effects of seed size on the seed germination and initial growth of peanut (Arachis hypogaea L.) seedlings under salinity and water stress conditions. Seed size is an important indicator of physiological quality, since it may affect seed germination and seedling growth, especially under stress conditions. This study aimed to investigate the effects of seed size on germination and initial seedling growth, under salinity and water stress conditions. The treatments were arranged in a completely randomized design, in a 3 × 3 factorial scheme: three seed size classes (small, medium and large) and three stress treatments (control, saline or water stress), with four replicates. Water and salt stresses do not reduce the germination rate of medium and large seeds; however, the germination rate of small seeds is reduced under salt stress conditions. Drought stress drastically reduces the shoot growth of seedlings regardless of seed size, whereas root growth is higher in seedlings from medium and large seeds under water stress conditions. Under non-stressful environments, the use of large seeds is preferable, resulting in more vigorous seedlings with a higher dry matter accumulation. Medium-size seeds are more adapted to adverse environmental conditions and, therefore, should be used under conditions of water shortage and salt excess in the soil at sowing time. Seedlings are more tolerant to salinity than to water stress during the germination stage and initial growth under laboratory conditions (Fig. 2.2).

2.3.2.

Salinity

The active growth of quiescent seeds is resumed after water uptake imbibitions. The metabolism in groundnut seed is very low at seed moisture levels below 10% but increases rapidly during water absorption and hydration of cell walls and protoplast. Generally, groundnut seed requires more than 35% seed moisture for germination. Besides water, the external conditions required for seed germination are availability of oxygen and a suitable temperature. During germination of groundnut seeds, C2H4 production rises prior to any visible signs of growth and peaks twice, at the emergence of the hypocotyl-radical and when the radical emerges from the hypocotyl (Morgan et al., 1970). Apart from germination, average root length also decreased with increasing salinity levels and ranged between 6.3 cm in control (seeds) and 0.4 cm in T4(seeds). The magnitude of variations in root length in T4 was between 0.0 in cultivars Co 3, Jawan, DH 3-30, VG 9521 and ICG (FDRS) 10 and 1.97 cm in GG4.

24

Physiology of the Peanut Plant

The average hypocotyls length under salinity stress decreased in the following order: 1.64 cm (control) > 0.83 cm (T1) > 0.40 cm (T2) > 0.13 cm (T3) and > 0.03 cm (T4). However, the hypocotyls length in T4 was higher in cultivars TG 37-A, Kopergaon 3, ICGS 44, ICGV 86590 and GG4 and lesser in GG3, ICG (FDRS) 10, OG 521, VG 9521 and DH 3-30. Hypocotyl length reduced to half at each increase in the salinity level and was affected more than root length; in addition, about 18 cultivars did not show any hypocotyl growth in T4. Growth and initiation of secondary roots were affected adversely due to salinity and only a few cultivars were able to develop secondary roots. For example, among the cultivars, the number of secondary roots ranged between 39 and 1 in control and 6.3 and 0.0 in T2 and completely inhibited in T3 and T4. Thus, higher salinity levels were found detrimental to the growth of secondary roots more than any of the parameters studied in this experiment. Based on the number of secondary roots, cultivars Kopergaon 3, MH 2, Gangapuri, VRI 4 and MH 4 were found relatively tolerant, whereas Co 3, ICG (FDRS) 10, Tirupati 4, Table 2.6. Salinity tolerance index (STI) of groundnut cultivars Cultivar

Botanical group

Kopergaon3

Valencia

STI 90

GG4

Spanish

71

MH2

Valancia

67

ICGV86590

Spanish

57

Gangapuri

Valencia

56

ICGS44

Spanish

42

TG37A

Spanish

42

TMV12

Spanish

39

VRI4

Spanish

39

SBXI

Spanish

33

GAUG1

Spanish

28

MH4

Valencia

25

GG7

Spanish

25

Tirupati4

Spanish

24

JL220

Spanish

23

OG 52-1

Spanish

22

TPG 41

Spanish

18

DH8

Spanish

17

Jyoti

Spanish

16

BSR1

Spanish

14

TMV 7

Spanish

12

GG3

Spanish

08

DH3-30

Spanish

01

Co 3

Spanish

01

VG9521

Spanish

00

Jawan

Spanish

00

ICG(FDRS) 10

Spanish

00

25

Seed Dormancy and Germination

GG3 and VG 9521 were susceptible. All the vigour parameters such as GR, GS, GC, StG and CVG were affected adversely with increasing salinity levels. The ranking of cultivars based on STI, ranged between 90 and 0.0; however, among the top five, i.e., Kopergaon 3, GG4, MH 2, ICGV 86590 and Gangapuri, three belong to Valencia botanical group (Table 2.6). These results showed that the effect of salinity was more on seedling vigour and not on the initial germination process per se. Such toxicity due to salinity during seed germination is usually associated with a significant decrease in the seed K+ content, which could reduce metabolic functions and ultimately reduce germination and seedling growth (Rehman et al., 1996) and osmotic effects due to declining solute potential or toxic effects due to uptake and/or accumulation of some ions in the seed (Tobe et al. 2001). The germination rate of groundnut (BARI Badam-8) seeds is presented in Table 2.7. The germination rate and seedling length decreased with the increase of salt concentration. The highest germination rate and seedling length (100% and 1.5 cm, respectively) were obtained from control (0 mM), and was followed by 50 mM, 100 mM, 150 mM, 200 mM and 250 mM (Table 2.7). The ability of a seed to germinate and emerge under salt stress indicates that it has genetic potential for salt tolerance (Tejovathi et al., 1988). Increases in NaCl concentration progressively inhibited seed germination and seedling growth. An experiment was carried out to study the effect of seed priming on germination behaviour, solute accumulation and antioxidative enzyme activities in germinating seeds of groundnut under salinity stress. Seeds of groundnut cv.TG-51 were treated with three different concentrations each of various priming agents for 14 hours and were subjected to 200mM NaCl salinity stress. Results indicated that the priming with gibberellic acid 50 ppm, hydrogen peroxide 60 mM, ascorbic acid 100 ppm, salicylic acid 25 ppm, mannitol 2.5% and sodium chloride 50 mM showed significant improvement in different germination parameters studied along with the length of the embryonic axis at 72 hours of germination over the water-soaked unprimed ones under salinity treatment (Table 2.8). Further physiological studies with these selected concentrations of priming agents revealed that in general, the cotyledon showed a much higher extent of membrane damage under salinity than the embryonic axis with water-soaked unprimed seeds registering the maximum damage. Seed treatment with hydrogen peroxide 60 mM caused minimum lipid peroxidation of the membrane in both the embryonic axis and the cotyledon at different hours of germination and this might be attributed to a much higher guaiacol peroxidase (GPOX) activity induced by this priming treatment. Seed priming with salicylic acid 25 ppm recorded the Table 2.7. Germination test of groundnut seeds at different concentrations of salt solution Salt conc. (mM)

Germination rate (%)

Mean seedling length (cm)

1 day

2 day

3 day

0

67

100

100

1.5

st

nd

rd

50

50

84

100

1.1

100

34

67

84

0.8

150

17

34

67

0.7

200

17

34

50

0.5

250

00

17

34

0.3

26

Physiology of the Peanut Plant

Table 2.8. Effect of seed priming on germination behaviour and growth of embryonic axis under salinity stress in groundnut cv TG 51 Treatment

Speed of Germigermi­ nation nation time

Peak value

Mean daily germination

Germination value

Embryonic axis length (mm) at 72 hrs

Control (no salinity)

6.94

26.67

5.56

4.17

23.15

52.33

Control (salinity)

4.49

45.00

2.08

1.67

3.47

19.33

H2O2 20 mM

4.75

46.15

2.56

2.08

5.34

28.68

H2O2 40 mM

5.07

43.08

3.21

2.78

8.90

28.33

H2O2 60 mM

6.52

30.77

5.13

2.78

14.25

35.33

GA3 25 ppm

5.24

40.00

3.21

2.78

8.90

25.67

GA3 50 ppm

7.20

25.45

6.06

4.17

25.25

34.00

GA3 100 ppm

4.06

47.69

1.50

2.78

4.16

17.33

SA 25 ppm

6.38

29.40

4.42

4.17

18.40

26.33

SA 50 ppm

5.30

38.18

1.52

2.78

4.21

22.67

SA 100 ppm

5.14

44.00

3.33

2.08

6.97

17.00

Mannitol 0.5%

6.31

30.91

4.55

2.78

12.63

19.33

Mannitol 1%

5.13

38.46

1.92

2.78

5.34

14.33

Mannitol 2.5%

6.67

28.00

5.00

4.17

20.83

26.00

AA 25 ppm

4.31

48.00

1.67

2.08

3.47

28.33

AA 50 ppm

4.72

40.00

2.50

2.78

6.94

22.00

AA 100 ppm

5.74

33.40

2.21

2.78

6.13

32.67

NaCl 25 mM

6.06

30.91

2.27

4.17

9.47

20.67

NaCl 50 mM

7.20

25.45

6.06

4.17

25.25

26.33

NaCl 100 mM

4.23

47.27

1.52

2.08

3.16

15.00

CD (0.05)

0.22

1.52

0.13

0.14

0.48

1.19

highest mean activity of catalase (CAT) enzyme, total phenol content in embryonic axis and cotyledon. However, NaCl 50 mM and mannitol 2.5% induced much higher accumulation of proline in the cotyledon, especially, at 48 and 72 hours of germination under salinity stress. In the embryonic axis, the effect of hydrogen peroxide 60 mM was much higher in inducing proline accumulation than any other priming agent. Overall mean comparison indicated higher activities of GPOX and CAT enzymes along with higher proline and phenol content in the cotyledon than the embryonic axis in the germinating seeds of groundnut. The priming treatments showed enhanced accumulation of proline along with higher activities of antioxidant enzymes GPOX and CAT and phenol content which might regulate osmotic adjustment and mitigate oxidative stress under salinity stress during seed germination.

2.3.3. Temperature Mohamed (1984) reported cardinal temperatures for seed germination in 14 contrasting genotypes of groundnut, which are shown in Table 2.9. These values showed that

27

Seed Dormancy and Germination

Tb is not varying much across genotypes (ranges from 8-11.5ºC), whereas optimum temperatures (29-36.5ºC) and maximum temperatures (41-47ºC) are varying more. Table 2.9. Base (Tb), optimum (TO) and maximum (Tm) temperatures of 14 groundnut cultivars for seed germination Cultivar

Base temp. (Tb)

Optimum temp. (TO)

Max. temp. (Tm)

Valencia R2

8

35

43

Flammings

8

34.5

42

Makulu Red

8.5

29

42

8

36

44

ICG30 EGRET

9

29

43

ICG47

9

36.5

47

Robut 33-1

10

36.5

46

TMV2

10

36

42

MK374

10

36

44

Plover

10.5

34

42

ICG21

11

35.5

45

M13

11

34

45

Swallow N. Common Range

11

29

42

11.5

29

41

8-11.5

29-36.5

41-47

Source: Mohamed, 1984

During November 1976, freshly dug, high-moisture (30-40%) peanuts drying in the windrow in North Texas were exposed to subfreezing overnight temperatures for six days. The effects of that exposure on germination of the seeds were studied. Samples of the subsequently cured and hand-shelled peanut seeds were tested for germination, seedling emergence, ethylene and carbon dioxide production, and certain enzyme activities. Laboratory germination was 4.2%, greenhouse seedling emergence 32%, and most of the freeze-damaged seeds that germinated grew at a slow rate. Germination and greenhouse seedling emergence of controls were 96 and 100%, respectively. At their maximum rates, ethylene and carbon dioxide production by freeze-damaged seeds were reduced to 83 and 36%, respectively. Mean enzyme activities measured from protein extracts of the freeze-damaged seeds were reduced, but they were not always significantly different from the control. However, isocitriclyase activity, which depends on de novo protein synthesis, was significantly lower for freeze-damaged than for control seeds, particularly during initial stages of germination. Thus, low-temperature exposure of high-moisture peanut seeds interfered with the initial biochemical and developmental processes, such as synthesis of new proteins, that determine seedling growth. The data suggests that exposure of highmoisture peanut seeds to freezing injures the protein synthesizing system. This system is partially a membrane-bound sequence of biochemical reactions and membranes have been found to be damaged by freezing (Anon., 1966). Carbon dioxide production from mitochondrial activity also could have been reduced by damage to these

28

Physiology of the Peanut Plant

membranes. The results were reduced ethylene production, germination, and seedling growth or complete loss of most of these functions by a majority of the seeds. Two factors known to influence the preservation of peanut seed are temperature and relative humidity (r.h.) (Hafterkamp et al., 1953; Roberts, 1972; Smith and Davidson, 1982). Molds that affect the germination power of seeds are also influenced by the ambient humidity and temperatures in storage (Christensen, 1972). However, literature on the comparative preservation of shelled and in-shell peanut seeds is limited. According to Gelmond (1971), to preserve peanut seeds for one year at 21 °C, an m.c. of 5% or less is necessary. Boswell et al. (1940) reported on peanut seed preservation at different temperatures and relative humidities (Figs. 2.3, 2.4 and 2.5). Peanut (Arachis hypogaea L.) germplasm accessions in ICRISAT gene bank are conserved as pods under medium-term conditions (4°C and 30% RH). Pod storage

Fig. 2.2. Effects of seed size on the first germination count test (a) and germination percentage (b) of peanut (Arachis hypogaea L.; IAC-Tatu ST cultivar) seeds under salinity and water stress conditions. Bars followed by the same lower-case letter between the seed sizes or the same upper-case letter for the stress treatments are not significantly different for the t-test at a confidence level of 0.05. Data refer to mean values (n = 4) ± mean standard error

Seed Dormancy and Germination

29

Fig. 2.3. Percent germination of Hanoch peanut, shelled and in-shell under varying temperature and moisture

requires far greater space than seed storage and is more likely to be expensive, especially in a controlled environment. With the objective to evolve cost effective strategies or conservation, the survival of in-shell and shelled seeds of two peanut cultivars, ICGS 76 (Virginia bunch) and JL24 (Spanish), was studied under three different storage conditions-ambient (20-40°C and 30-80% RH), short term (23-25°C and 40-50% RH), and medium term (4°C and 30% RH). In shell seeds had marginally greater longevity than shelled seed in all storage conditions. The differences in time for regeneration of in-shell and shelled seeds stored under medium term conditions were estimated to be less than 4 months for both the cultivars. Because of the much-reduced volume required for storage and the insignificant differences in regeneration intervals, conservation of shelled seeds would be highly cost effective under the controlled environmental conditions, as compared to in-shell seeds. Since storage at very low moisture contents was suggested as a simple and low-cost option for conservation of seed lots required for short-term use, the longevity of peanut seeds (cv.ICGS76) hermetically sealed with 3.6% moisture content was studied in comparison with seeds held at 5.8% moisture. The studies showed that peanut seeds hermetically stored at room temperature (23-25°C) with low moisture content (below 4%) could retain high germination (>85%) for up to 8 yrs (Table 2.10).

30

Physiology of the Peanut Plant

Fig. 2.4. Per cent germination of Congo peanut, shelled and in-shell under varying temperature and moisture Table 2.10. Estimates of half-viability period (P50’ in years) of in-shell and shelled peanut seeds stored under different conditons Storage condition

ICS 76 In-shell

JL 24

Shelled

..............yr ............. Ambient storage (20-40 C/30-80% RH)

1.94

1.84

In-shell

Shelled

..............yr ............. 2.88

2.28

Short-term storage (23-25 C/40-50% RH)

6.10

5.59

7.45

6.16

Medium-term storage (4 C/30% RH)

16.24

14.15

18.75

16.99

LSD (0.05) for comparison of means = 3.04

Germination rate and seedling length decreased with the increase of salt concentration. The highest germination rate and seedling length (100% and 1.5 cm, respectively) were obtained from control (0 mM), and was followed by 50 mM, 100 mM, 150 mM, 200 mM and 250 mM. The ability of a seed to germinate and emerge under salt stress indicates that it has genetic potential for salt tolerance (Tejovathi et al., 1988). Increases in NaCl concentration progressively inhibited seed germination and seedling growth.

Seed Dormancy and Germination

31

Fig. 2.5. Per cent germination of peanuts in relation to temperature and moisture

Seed imbibition with SA leads to an activation of germination and seedling growth (Shakirova et al., 2003; Singh et al., 2010). Several workers reported that stimulating effects of SA on germination are concentration dependent (Rajjou et al., 2006; Singh et al., 2010). SA significantly stimulated the activities of enzymes involved in germination such as transkelolase, enolase, malate dehydrogenase, phosphoglycerate kinase, glyceraldehyde 3-phosphate, dehydrogenase, fructose 1,6-diphosphatase, and pyruvate decarboxylase. In addition to it, seeds germinated in SA supplemented media showed abundant levels of isocitratelyase and malate synthase (key enzymes of glyoxylate cycle) (Eastmond and Graham, 2001; Rajjou et al., 2006). Rajjou et al. (2006) hypothesized that the detoxification mechanism in germinating seeds counteracted by exogenous SA treatments. The present investigation has been carried out on the influence of various concentrations of salicylic acid (SA) on the germination performance of groundnut seeds of cv. W-55, W-44, TAG and SB-11. The groundnut cultivars W-44, TAG and SB-11 showed significant germination by SA application over control. In cv. W-55 all the imposed SA concentrations increased germination except 10 ppm SA particularly after 48 hrs of germination. In general, salicylic acid with 50 ppm concentration showed significant germination in all groundnut cultivars. SA also showed a positive impact on the root and shoot growth in W-44, TAG and SB-11 cultivars whereas an opposite trend was noticed in W-55. These findings clearly indicated that the cv. SB-11 was the best performer with salicylic acid among the studied cultivars of groundnut. Groundnut varieties, JL-24, GG-2, GAUG-10 (tolerant group) and GG-7, GG-13, GG-20 (susceptible group) were germinated under sulphate dominant salinity ranging from 0, 20, 40, 80 m. eq/L. Sulphate salinity decreased the seedling vigour index of all groundnut varieties, and the decrease was found more in susceptible varieties at 1st and 4th days after germination (DAG). With increasing salinity regimes, various metabolites like free amino acid, protein, total phenol and free proline contents were deposited at higher rates in seedlings of tolerant varieties compared to susceptible ones for better osmotic adjustment. However, sulphate salinity decreased the accumulation of total sugars, starch and free fatty acid contents in the seedlings of all groundnut

32

Physiology of the Peanut Plant

varieties during the 1st and 4th DAG. The decrease in sugar content was found more in susceptible varieties than tolerant ones. Activities of alpha-amylase decreased but that of protease and peroxidase increased under salt stress at 1st and 4th DAG in all varieties of groundnut. Groundnut possesses high oil content (44-56%) and protein (22-30%) (Holaday and Pearson, 1974) and is also a valuable source of vitamins E, K and B. It is the richest plant source of thiamine and niacin, which is low in cereals.

2.4.

Viability and Vigour

The groundnut seed has a short life and loses viability quickly under ambient conditions. Several factors affect the self-life of the seed; among them infections by seed-borne fungi are one of the factors for quick loss of viability of a seed (Urosevic, 1964). Ageing in groundnut seeds leads to increased lipid peroxidation and decreased activities of several free radical and peroxide scavenging enzymes (Rao et al., 2006). Groundnut seeds are more sensitive to storage conditions like high temperature; high seed moisture content and light exposure. The qualitative loss of seeds can be attributed to biochemical changes in protein, carbohydrates, fatty acids and vitamins (Girish et al., 1972). In recent years a rapid ageing method where seeds are subjected continuously to high temperature in saturating humidity has been recognized as an important technique in estimating the rate of deterioration in seed storage. The rate of ageing mainly depends on genotype, moisture and temperature. In rapid and slow ageing (natural ageing), the pattern of deterioration preceding death is the same whether the seed survives for a few hours or decades. Accelerated ageing (AA) is an important procedure for understanding the events that lead to the loss of seed viability. AA damages DNA and mRNA, causes biochemical deterioration of the stored material and reduces the vigour of the seedling and early plantlet development shortly after germination. The consequences of ageing on cooking properties, digestibility and formation of resistant starch in seeds have also been investigated. However, little is known about the cellular alterations, storage mobilization and vigour during early plantlet growth. At a cellular level, aged seeds show a significant increase in number and a decrease in the size of starch grains, as well as rupture of the cell walls and membrane-bound organelles, including protein bodies. However, the information related to sub cellular changes due to ageing under the natural ageing or accelerated ageing condition is more scanty. The metabolic defects that occur due to these changes can be rectified to the extent possible by the technique of seed priming; pre-soaking seeds in osmotic solutions has been demonstrated to improve the viability and vigour of aged seeds in various crops (Bhanuprakash et al., 2010). Study of disinfestations of stored seeds using modified atmosphere storage (MA) involves the alteration of the natural storage gases such as carbon dioxide (CO2), oxygen (O2) and nitrogen (N2) to render the atmosphere in the storage as lethal to pests. The MA includes neither the alteration of the storage atmosphere by addition of toxic gases such as phosphine or methyl bromide nor atmospheric water content. The MA may be achieved in several ways by adding gaseous or solid CO2, by adding a gas of low O2 content (e.g., pure N2 or output from a hydrocarbon burner) or by allowing metabolic processes within an airtight storage to remove O2, usually with an associated release of CO2. Such an atmosphere is referred to as high-CO2, low-O2 and hermetic storage atmospheres, respectively, they are collectively known as ‘modified atmospheres’ (Abdul-Baki and Anderson, 1973). Modified atmosphere storage of

Seed Dormancy and Germination

33

seeds is a suitable alternative to the use of chemical fumigants and contact insecticides that are known to leave carcinogenic residues in the treated product (Alagusundarum et al., 1995; Almedia et al., 1997). The most important component in MA is CO2 which is a nonflammable, colourless gas which is about 1.5 times as heavy as air. Carbon dioxide can be supplied from an external source to a silo using either the gas produced from the liquid supplied in pressurized cylinders or from solid-dry ice. Solid-dry ice is a useful source of CO2 because it changes directly from solid to a gas. It can be supplied as blocks, crushed ice or pellets. Blocks are useful to make up gas loss during treatment due to their slower release. Crushed ice or pellets rapidly change to a gas and are best for initial gas addition. Jayas and Jeyamkondan reported that there are some differing methods for introducing CO2 as dry ice in the seeds mass in silo, which are as follows: (i) introducing dry ice under the perforated floor or in the perforated duct, (ii) introducing dry ice on the top surface of the seeds covered with a CO2 impermeable sheet, (iii) introduction of equal amounts of dry ice on the top surface under the sheet and in the perforated duct, (iv) introducing dry ice through a 10 cm diameter perforated tube installed vertically in the centre of the seeds bulk, and (v) introduction of one-quarter of the dry ice on the top surface under the sheet and the remaining three-quarter in an insulated box placed under the sheet. The fourth method gave the most uniform CO2 concentration in the seeds mass and used the least amount of CO2 to maintain the desired CO2 concentration. However, installation of a 10 cm diameter perforated tube would be very difficult, therefore the last method was recommended for practical use. In general, ageing is manifested by the decrease of metabolic activity and an increase of catabolic processes (Gorecki et al., 1996). In particular, an oxidative stress might be reduced in O2-free storage atmospheres (Justice and Bass, 1978; Wilson and McDonald, 1986; Benson, 1990). It should be noted that seed deterioration during storage could result in marked changes in the content and activity of enzymes capable of degrading the stored reserves (Prestley, 1986; Smith and Berjak, 1995). In the present investigation it was observed that the dehydrogenise activity in the seed kernels was maximum and hence, better maintenance of seed quality occurred in modified atmospheric storage conditions compared to control. Another reason for seed ageing may be the accumulation of deleterious effects on membranes due to oxidative damages to fatty acids and proteins denaturation as a result of Maillard reactions (Narayana Murthy and Sun, 2000). The advantage of higher seed reserve utilization efficiency in seeds stored in a vacuum, is that it provides energy for a faster seedlings growth rate. In the present study also the maximum speed of germination (15.68 and 15.61) was noticed in the treatment T8 (60% N2 + 0% O2 + 40% CO2) and T12 vacuum respectively. Pods and kernels of two genotypes (GPBD-5 and DH-86) of summer groundnut were subjected to accelerated ageing at 40°C and 90% RH for various intervals (0, 2, 4, 6, 8 days). Accelerated ageing results in progressive loss of seed viability and vigour in both genotypes. A marked genotypic difference in storage of both genotypes in the form of pods and kernels on ageing had been recorded. Germination tests and biochemical analysis were carried out for control and aged pod and kernels. The initial germination percentage was 97.86% and after accelerated ageing it declined to 49.32%. During the ageing process a decline in oil percentage and total soluble protein, whereas an increase in total soluble sugars and phenols content was recorded. A gradual decrease in the activity of β-amylase and an increase in the activity of

34

Physiology of the Peanut Plant

lipase was recorded during accelerated ageing. Hence, there was a decrease in the biochemical content and the activity of enzymes involved in the degradation of seed reserves.

2.5.

Seed Deterioration

Seed deterioration is an undesirable event associated with various cellular, metabolic and chemical alterations including chromosomal aberrations and damage to DNA, impairment of RNA and protein synthesis, changes in the enzymes and food reserves and loss of membrane integrity (Kibinza et al., 2006). In groundnut, several biochemical and physiological changes occur during storage due to the presence of high fat and protein content, and alternation in the transcription and translation processes (Shelar et al., 2008; Walters et al., 2010). Accumulation of reactive oxygen species (ROS) in seed tissues plays an important role in the loss of seed viability during storage. Halogens are compounds that help to prevent seed deterioration and dry halogenation treatment was shown to exhibit a beneficial effect of prolonging the shelf life of groundnut pods (Murugan, 1981; Bindu Mathew, 1996). Groundnut kernels deteriorate rapidly mainly due to chemical composition and moisture content. In this study, the activities of hydrolytic enzymes and antioxidant enzymes was assessed in kernels of contrasting groundnut genotypes differing in their dormancy, pre-storage treatments and storage conditions. Kernels of groundnut genotypes stored under cold conditions registered significantly lower electrical conductivity than the ones stored under ambient conditions irrespective of prestorage treatments. Kernels of dormant groundnut variety VRI 7 treated with halogen impregnated powder and stored under cold conditions showed the lowest electrical conductivity as compared to other treatments. Dormant genotype VRI 7 was found to possess significantly reduced activity of proteases than the non-dormant CO 7. Lipase activity and lipid peroxidation were found to be less in dormant ground nut genotype VRI 7 (0.440 and 1.635) and halo polymer treatment was found to exhibit a significant effect on this (0.041 and 1.650) (Fig. 2.6).

Fig. 2.6. Effect of seed dormancy, halogenation and storage condition on lipid peroxidation (OD value) of groundnut seeds during storage

Seed Dormancy and Germination

35

One of the approaches for improving the performance of partially-deteriorated seeds is through priming. Seed priming is a controlled hydration technique in which seeds are soaked in water or a low osmotic potential solution to a point where germination related metabolic activities begin in the seeds but radicle emergence does not occur. Priming allows some of the metabolic processes necessary for germination to occur without germination actually taking place. Seed priming is an effective technology to enhance rapid and uniform emergence and to achieve high vigour, leading to better stand establishment and yield (Hussein et al., 2007; Farooq et al., 2008; Rehman et al., 2011). It is a simple and low cost hydration technique, which can easily be adopted by farmers for improving the performance of their seeds. Various studies have shown better seedling performance, crop establishment, and ultimately, increased yield due to seed priming in several crops, including groundnut. Harris (2007) reported that seed priming led to better establishment and growth, earlier flowering, increased tolerance to adverse environments and greater maize yields. Rehman (2011) reported that seed priming is a cost effective technology that can enhance early crop growth leading to earlier and more uniform stand with yield associated benefits in many field crops, including oilseeds. Priming treatments have been reported to show better seedling performance and crop establishment in groundnut crops (Dhedhi et al., 2007).

2.6.

Seed Priming

An investigation was undertaken in the Department of Seed Science and Technology, OUAT, Bhubaneswar to study the effect of a few priming treatments on the enhancement of seed quality and improvement of subsequent performance of groundnut. Rabi 2016-17 harvested seeds of groundnut cv. ICGV 91114 with 92.0% germination were packed in HDPE containers and stored under ambient conditions for five months. At the end of the storage period the seeds had 69.0% germination. The partially-deteriorated seeds were subjected to priming treatments, viz. hydropriming of kernels for 2, 3, 4 and 5 hours and moist sand conditioning (MSC) of kernels for 24, 36, 48, 60 and 72 hours. An unprimed control was maintained for comparison. After hydration, the seeds were dried back to the original moisture content. Performance of the primed seeds was studied in a field trial during Rabi 2017-18. Priming of seeds significantly increased the germination percentage. MSC-24 hours gave the highest germination (82.5%), followed by MSC-36 hours (79.50%), an increase of 20.0% and 15.6%, respectively, over the unprimed control. Seeds without priming treatment gave the lowest germination of 68.75%. Priming treatments also resulted in higher SVI-I and lower EC of seed leachate. Highest field emergence of 75.60% was recorded in MSC-24 hours, followed by 74.4% in MSC-36 hours, as against 59.2% in unprimed seeds. The primed seeds also took a smaller number of days to flowering initiation and maturity, with minimum number of days recorded in MSC-24 hours, followed by MSC-36 hours. The two treatments also recorded higher plant height, number of mature pods per plant, pod yield per plant and pod yield per hectare. MSC-24 hours produced the highest pod yield of 1557.14 kg/ha followed by MSC-36 hours, hydropriming-3 hours and MSC-48 hours. The crop raised from unprimed seed (T1) recorded the lowest pod yield of 1101.42 kg/ha. The yield advantage of moist sand conditioning of kernels for 24 hours was 41.4% compared to the unprimed control. Priming treatments had no significant influence on the pod and seed characteristics of the produced seeds viz., mean pod length, 100-pod weight, number of kernels per pod,

36

Physiology of the Peanut Plant

shelling percentage, oil content and protein content. Similarly, no significant influence of the priming treatments was observed on germination of the produced seed and accelerated ageing test. However, lowest EC of seed leachate was recorded in seeds produced from MSC-24 hours, while it was highest in unprimed control. Therefore, moist sand conditioning of kernels for 24 to 36 hours (@ 1 part seed:3 parts sand moistened with water equivalent to 10% of its weight), followed by drying to the original moisture content, proved to be superior to other treatments and can be taken up as a low cost technique for improving the quality and performance of partiallydeteriorated groundnut seeds (Table 2.11). Table 2.11. Seed quality parameters of groundnut ICGV91114 subjected to various priming treatments Treatment Moisture Germination Seedling Seed Seedling Seed EC of Germination (%) (%) length vigour dry wt. vigour seed (%) (cm) index I (mg) index leachate after AA II (dS/m)

T1

8.33

68.75

14.28

T2 T3 T4 T5 T6 T7 T8 T9 T10 CD5%

8.36 8.39 8.36 8.38 8.32 8.33 8.36 8.39 8.32 NS

76.25 76.75 76.50 73.50 82.50 79.50 77.00 76.25 73.75 1.115

15.14 921.04 241.93 14.72 15.38 940.67 245.62 15.02 15.14 923.68 241.93 14.76 14.66 865.49 237.78 14.03 16.72 1091.76 251.38 16.40 15.56 981.59 249.51 15.74 15.38 943.78 245.62 15.07 15.14 921.75 241.93 14.73 14.66 867.39 237.78 14.73 NS 87.015 NS NS

799.93 229.22 12.84

0.66

55.75

0.61 0.57 0.61 0.63 0.42 0.48 0.57 0.61 0.61 0.041

58.25 58.50 58.25 57.75 60.50 59.75 58.50 58.25 58.25 1.421

T1: Control (No priming), T2: Hydro priming for 2 hours, T3: Hydropriming for 3 hours, T4: Hydropriming for 4 hours, T5: Hydropriming for 5 hours, T6: Moist sand conditioning for 24 hours, T7: Moist sand conditioning for 36 hours, T8: Moist sand conditioning for 48 hours, T9: Moist sand conditioning for 60 hours, T10: Moist sand conditioning for 72 hours.

Peanut seeds deteriorate rapidly during storage resulting in a loss of seed vigour. The research was established: (a) to study the process of deterioration in whole seeds, cotyledons, and embryonic axes of peanut seeds subjected to natural and accelerated aging, and (b) to analyse changes occurring in membranes under such conditions. The biochemical changes which take place in peanut seed membranes, aged both naturally and artificially, may be detected best in the embryonic axes, either through changes in leakage of electrolytes or in malondialdehyde (MDA) content. Results also indicate that there were no changes in the relative proportion of oleic/linoleic acids in the axes after exposure to accelerated aging for various lengths of time. Germination percentage was not a sensitive assay for detecting the degree of deterioration in peanuts. These results demonstrate that changes in membrane integrity associated with seed deterioration occur first in the embryonic axes and can best be monitored by the conductivity seed vigour test. Rapid loss of peanut seed quality during storage is a common problem in seed production for which little information concerning the physiological mechanism(s) is

Seed Dormancy and Germination

37

known. This study examined changes in esterase activity of unimbibed and imbibed peanut seeds previously exposed to ambient storage conditions. A high-quality seed lot was divided into three subsamples. One was immediately placed into cool storage and classified high quality. The remaining two were exposed to ambient conditions until their seed quality declined to a medium or low-quality level at which time they were also placed in cool storage. Total specific esterase activity of unimbibed seeds declined from 100% to 91% to 84% in the cotyledon and 100% to 92% to 87% in the axes of high, medium, and low-quality seeds, respectively. After three days imbibition, total specific esterase activity for the low-quality seed cotyledons and axes was 94% of the high-quality seeds. Examination of the isoesterase profiles using isoelectric focussing demonstrated that these changes in esterase activity were not uniform among all isoesterase species. These changes were not associated with damaged isoesterase species found in unimbibed seeds following seed storage. It was concluded that specific isoesterases are prone to deterioration during ageing of peanut seeds and that other isoesterases associated with germination are preferentially synthesised in deteriorated peanut seeds during imbibition. Early stages of the deterioration process can be detected by assessing the activity of enzymes associated with membrane degradation, oxidation of storage compounds, and cell respiration, such as esterase, glutamate dehydrogenase, malate dehydrogenase, superoxide dismutase, acid phosphatase, malic enzyme and peroxidase, among others. Thus, the use of molecular techniques and the study of aging mechanisms can contribute to preserve the genetic biodiversity of species in germplasm banks. They are also the starting point to the rational use of plant species, the main factor for the success of seedling production programs for several purposes. A study was aimed at standardizing a protein extraction protocol and evaluating its suitability for peanut (Arachis hypogaea L.) seeds aged several times. Examination of the hypocotyl-radicle axes of peanut seeds variety Runner IAC 886 harvested in 2007-2008 and 2011-2012 were pre-sprouted and stored in a climate chamber at 25°C. Seeds were subjected to different treatments followed by artificial aging, and evaluated at time intervals of 0, 6, 12, 24, 36, 48, 60, 72, 96, and 120 hours. Seeds were then analysed by vertical electrophoresis using 5% (stacking) and 10% (separating) polyacrylamide gels performed at 4°C, with normal and constant polarity of 25 mA at 200 V. Detection systems were used for the following enzymes: esterase, malate dehydrogenase, glutamate dehydrogenase, and superoxide dismutase. The extraction buffer consisted of 0.025 M Tris pH 8.3 + 0.019 M glycine + 0.001 EDTA producing the most distinct bands in seeds from the two harvests in all enzymatic systems. In the artificial aging treatment, the activity of esterase decreased after 48 h, while that of glutamate dehydrogenase increased after 96 h, and those of malate dehydrogenase and superoxide dismutase remained unaltered. Accelerated aging is known to reduce seed viability and vigour in many crop species. The phenomenon is due in part to aging‐induced lipid peroxidation, which has the potential to damage membranes of the seed tissues. This study was undertaken to evaluate the effect of accelerated aging on germinability and several physiological characteristics related to peroxidation in the seed of two peanut cultivars. Accelerated aging was achieved by incubating seeds at 45°C and 79% relative humidity in a closed chamber for 3, 6, or 9 days. The results indicate that accelerated aging inhibited seed germination and seedling growth. Enhanced lipid peroxidation and increased peroxide accumulation were observed in the axis and cotyledons of aged seeds. Accelerated aging also inhibited the activity of

38

Physiology of the Peanut Plant

superoxide dismutase, peroxidase, ascorbate peroxidase, and lipoxygenase. Seed axes appeared to be more susceptible to aging than cotyledons. The changes in germination and physiological activities, expressed as a function of aging duration, were similar in the two cultivars, despite differences in their seed weight. Seed priming treatment is done before sowing seeds, which involves hydration of plenty of seeds enough to enable metabolic events before germination to take place, although preventing radicle emergence to occur (Nascimento et al., 2004; Rehman et al., 2011). Priming is an approach that involves treating seeds with different organic or inorganic chemicals and/or with high or low temperatures (Kamithi et al., 2016). It entails imbibition of seeds in different solutions for a specified duration under controlled conditions, then drying them back to their original moisture content, so that radicles do not emerge before sowing. This stimulates various metabolic processes that improve germination and emergence of several seed species, particularly seeds of vegetables, small seeded grasses and ornamental species (Tavili et al., 2011) whereas also reverses the detrimental effects of seed deterioration (Ghassemi-Golezani et al., 2012). Seed priming is considered to be an easy, highly effective, low cost and low risk technique. Primed seeds are more useful because of numerous advantages such as uniformity, early and faster appearance (Musa et al., 1999) of germination. A field study was conducted at a farmer’s field in Kodukkanpalayam village in Cuddalore district, Tamilnadu during Rabi 2016, to assess the effect of hydro priming on growth components of groundnut. The experiment was adopted in Randomized Block Design (RBD) with seven treatment and three replications. The treatments consist of seed hydro priming of 0 hours (control), 6 hours, 12 hours, 24 hours, 36 hours, 48 hours and 72 hours. The results indicated that all traits such as percentage of seed germination, seedling emergence, plant height, leaf area, number of leaves and leaf area index (LAI) were significantly influenced by the duration of hydro priming. The seeds soaked for 24 hours duration gave the highest percentage of seed germination (97.1%), no damages to testa (outer seed coat) and seedling establishment was within 3-5 days, whereas in the seed soaked for 72 hours they were the least (Table 2.12). Therefore, the results suggested that hydro priming is a useful method of improving the percentage of seed germination, seedling emergence, stand establishment and productivity of groundnut. Table 2.12. Influence of hydro priming on growth components of groundnut Hydro priming (hrs.)

Emergence (%)

Plant height (cm) at 30 DAS

No. of leaves at 30 DAS

LAI at 30 DAS

3 DAS

7 DAS

0

35.7

50.1

21.40

31

0.98

6

54.2

68.9

24.57

33

1.28

12

66.5

78.3

27.33

36

1.54

24

91.7

97.1

28.53

39

1.85

36

51.9

65.8

22.04

33

1.25

48

5.7

8.3

9.78

18

0.31

72

0

0

4.7

5.5

CD (0.05)

0

0

0

1.71

2.6

0.1

Seed Dormancy and Germination

39

A field experiment was conducted to study the effect of priming treatments on seedling vigour, growth and yield contributing characters in groundnuts under rainfed conditions. The maximum seed yield (2255 kg/ha) was recorded due to seeds primed with CaCl2 2% followed by CaCl2 1% (2036 kg/ha). The seed primed with CaCl2 2% recorded the higher field emergence percentage (89.67 %), plant height (39.87 cm), number of pods per plant (27), and 100 seed weight (38 g) followed by CaCl2 (1% ). In case of flowering and maturity, the seeds hydrated with CaCl2 2% had earlier flowering and maturity than control. Regarding seed quality parameters viz., germination percentage (94.17%), root length (12.28 cm), shoot length (18.41 cm), total seedling length (29.10 cm) and vigour index I (2739.41), were enhanced by seeds primed with CaCl2 2% followed by CaCl2 1% . In case of electrical conductivity of seed leachate, the seeds hydrated with CaCl2 2% recorded a lower electrical conductivity (0.41 dsm-1) than unprimed seeds. The present experiment was envisaged to evaluate the effect of different seed priming treatments on the germination behaviour of groundnut cultivar (TG-51) under salinity stress. For this purpose, seeds of groundnut cv. TG 51 were soaked for 14 hours with solutions of GA3 50 ppm, hydrogen peroxide 60 mM, ascorbic acid 100 ppm, salicylic acid (SA) 25 ppm; mannitol 2.5% and NaCl 50 mM and were subjected to germination under salinity stress induced by 200 mM NaCl. The results indicated beneficial effects of seed priming in respect of seed germination percentage, germination speed, reserve mobilization and uniformity of seedling growth and development and anti-oxidative enzyme activity for scavenging ROS over unprimed seeds under the salinity stress. Seeds primed with 60 mM H2O2, 2.5% mannitol and SA 25 ppm reached 100% germination at 24 hours of germination under salinity stress and recorded high germination speeds whereas treatment with 50 ppm GA3 registered high rates of reserve depletion and translocation of reserve food material towards sites of active growth. The lipid peroxidation in unprimed seeds recorded high mean values at all stages of study indicating a higher rate of membrane damage under stress conditions. On the contrary, seed priming with H2 O2 (60 mM), GA3 (50 ppm) and mannitol (2.5%) showed the minimum change of TBARS content in the embryonic axis indicating the least damage under oxidative stress. The seeds soaked with different priming agents also registered a higher means for catalase activity over unprimed seeds in all the cases. The present investigation was carried out to study the influence of various organic seed priming treatments on seed quality parameters in groundnut. The groundnut cv. TMV 7 was imposed with various seed priming treatments viz. panchagavya 1%, vermiwash 25%, starter solution 10%, coconut water 50%, ginger (Zingiber officinale) rhizome extract 5%, garlic (Allium sativum) bulb extract 5 per cent and turmeric (Curcuma longa.) rhizome extract 5%. The treated seeds along with control were evaluated for their seed quality parameters under laboratory conditions. The study revealed that the seeds primed with coconut water 50% recorded higher seed germination (91%), root length (20.20 cm), shoot length (16.30 cm), dry matter production (3.68 g seedling-10) and vigour index 3322 as compared to other treatments and control. Coconut water 50 per cent induced the maximum germination per cent, shoot length, dry matter production of seedlings and vigour index. So, we can conclude that the 50 per cent coconut water can be effectively used for orgopriming and is the ideal one followed by the starter solution (10%) and the ginger extract (5%).

40

Physiology of the Peanut Plant

Two seed lots of groundnut (Arachis hypogaea L.) viz., fresh seed having high germination vigour and revalidated seed (low vigour) were subjected to pre-sowing seed treatments and their efficacy was evaluated during the summer seasons of 2005, 2006 and 2007. Pre-sowing seed invigoration by hydration for 16 h and air drying at room temperature followed by dressing with Thiram (75% DS) @ 0.25 per cent registered consistently and significantly higher pod yields than the untreated seeds in revalidated seeds. The higher pod yield resulted from significantly improved germination, speed of emergence, per cent field emergence, ultimately better crop establishment and in turn higher plant stand. The beneficial effects of hydration followed by Thiram dressing was more pronounced in the low vigour seed lot (revalidated) than in the high vigour lot (fresh). The study highlighted the efficacy of hydro priming followed by Thiram dressing.

References Abdul-Baki, A.A. and J.D. Anderson. 1973. Vigour determination of soybean seed by multiple criteria. Crop Science, 13: 630-633. Acosta-Motos, J.R., M.F. Ortuño, A. Bernal-Vicente, P. Diaz-Vivancos, M.J. Sanchez-Blanco. 2017. Plant responses to salt stress: Adaptive mechanisms. Agronomy, 7: 1-18. Adebisi, M.A., T.O. Kehinde, A.W. Salau, L.A. Okesola, J.B.O. Porbeni, A.O. Esuruoso and K.O. Oyekale. 2013. Influence of different seed size fractions on seed germination, seedling emergence and seed yield characters in tropical soybean (Glycine max L. Merrill). International Journal of Agricultural Research, 8: 26-33. Alagusundarum, K., D.S. Jayas, W.E. Muri, N.D.G. White and R.N. Sihna. 1995. Distribution of introduced carbon dioxide through wheat bulks contained in bolted metal bins. Transaction of the ASAE, 38: 895-901. Almedia, F. de A.C. and J. de. S. Morais. 1997. Effects of conditioning type of packaging and storage atmosphere on physiological quality of groundnut seeds. Revista Brasileira de Armazenament, 22: 27-33. Ambika, S., V. Manonmani and G. Somasundarum. 2014. Review on effect of seed size on seedling vigour and seed yield. Research Journal Science of Seed, 7: 31-38. Amen, R.D. 1964. A model of seed dormancy. Bot. Rev., 34: 1-31. Anon. 1966. Determination of moisture content. International rules for seed testing. Proc. Int. Seed Test. Ass., 31: 128-134. Asibuo, J.Y., R. Akromah, S. Kantanka, A. Dapaah, K. Hans et al. 2008. Inheritance of fresh seed dormancy in groundnut. African Journal of Biotechnology. 7: 421-424. Balmer, A., J. Pastor, V. Gamir, Flors and B. Mauch-Mani. 2015. The “prime-ome”: Towards a holistic approach to priming. Trends in Plant Science, 20: 443-452. Bandyopadhyay, A., P.C. Nautiyal, T. Radhakrishnan and H.K. Gor. 1999. Role of testa, cotyledons and embryonic axis in seed dormancy of ground nut (Arachis hypogaea L.). Journal of Agronomy and Crop Science, 182: 37-41. Baskin, C.C. and J.M. Baskin. 1998. Seeds – Ecology, Biogeography, and Evolution of Dormancy and Germination. San Diego, CA, USA: Academic Press. Benech-Arnold, R.L., R.A. Sanchez, F. Forcella, B.C. Kruk, C.M. Ghersa et al. 2000. Environmental control of dormancy in weed seed banks in soil. Field Crops Research, 67: 105-122. Benson, E.E. 1990. Free radical damage in stored plant germplasm. International Board of Plant Genetic Resources, Rome. Bewley, J.D. 1997. Seed germination and dormancy. The Plant Cell, 9: 1055-1066.

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Bewley, J.D. and M. Black. 1994. Seeds: Physiology of Development and Germination. 2nd edition. Plenum Press, New York, USA. Bewley, J.D., K.J. Bradford, H.W.M. Hilhorst and H. Nonogaki. 2013. Seeds: Physiology of Development, Germination and Dormancy. 3rd Edition, Springer, New York. Bhanuprakash, K., H.S. Yogeesha and M.N. Arun. 2010. Physiological and biochemical changes in relation to seed quality in ageing Bell pepper. Ind. J. Agric. Sci., 80: 9-11. Bindu, M. 1996. Evaluation of groundnut (Arachis hypogaea L.) genotypes for storability and vigour. M.Sc. (Ag.) Thesis, Tamil Nadu Agricultural University, Coimbatore. Boswell, V.R., E.H. Toole, V.K. Toole and D.F. Fisher. 1940. A study of rapid deterioration of vegetable seeds and methods for its prevention. Bull. U.S. Dep. Agric. No. 708, pp.40 Chen, K. and R. Arora. 2011. Dynamics of the antioxidant system during seed osmopriming, post-priming germination, and seedling establishment in spinach (Spinacia oleracea). Plant Science, 180: 212-220. Christensen, C.M. 1972. Microflora and seed deterioration. pp. 59-93. In: E.H. Roberts (ed.). Viability of Seeds. Chapman and Hall Ltd. London. Dhedhi, K.K., C.J. Dangaria, G.J. Parsana and A.K. Joshi. 2007. Effect of pre-sowing seed treatments for better crop establishment in summer groundnut. Seed Res., 35: 17-21. Dufourc, E.J. 2008. Sterols and membrane dynamics. J. Chem. Biol., 1: 63-77. Eastmond, P.J. and I.A. Graham. 2001. Re-examining the role of the glyoxylate cycle in oilseeds. Trends Plant Sci., 6: 72-78. Farooq, M., S.M.A. Basra, H. Rehman and M. Hussain. 2008. Seed priming with polyamines improves the germination and early seedling growth in fine rice. Journal of New Seeds, 9: 145-155. Gavrielit-Gelmond, Haya. 1971. Moisture content and storage of peanut seed (Arachis hypogaea L.). Proc. Int. Seed Test. Ass., 36: 159-171. Ghassemi-Golezani, K., A. Hosseinzadeh-Mahootchy, S. Zehtab-Salmasi and M. Tourchi. 2012. Improving field performance of aged chickpea seeds by hydro-priming under water stress. International Journal of Plant, Animal and Environmental Sciences, 2: 168-176. Girish, G.K., R.K. Goyal and Krishnamurthy. 1972. Ethylene dibromide as a grain fumigant. Grain Technol., 12: 120-130. Gorecki, R.J., K. Kulka and J. Puchalski. 1996. Biochemical aspects of seed deterioration during storage. pp. 50-60. In: T. Gass, W. Podyama, J. Puchalski and S.A. Eberhart (eds.). Proceedings of an International Conference on Crop Germplasm Conservation with Special Emphasis on Rye, July 1996, Warsaw, Poland. International Plant Genetic Resources Institute, Rome. Haferkamp, M.E., L. Smith and R.A. Nilan. 1953. Studies on aged seeds. I: Relation of age of seed to germination and longevity. Agron. J., 45: 424-437. Harris, D., A. Rashid, G. Miraj, M. Arif, H. Shah et al. 2007. ‘On-farm’ seed priming with zinc sulphate solution – A cost-effective way to increase the maize yields of resource poor farmers. Field Crops Res., 102: 119-127. Holaday, P.E. and J.L. Pearson. 1974. Effects of genotype and production area on the fatty acid composition, total oil and total protein in peanuts. J. Food Sci., 39: 1206-1209. Huang, A.H.C. 1992. Oil bodies and oleosins in seeds. Annu. Rev. Plant Physiol. Plant Mol. Biol., 43: 177-200. Hussein, M., M.L.K. Balbaa and M.S. Gaballah. 2007. Salicylic acid and salinity effects on growth of maize plants. Res. J. Agric. Biol. Sci., 3: 321-328. ICRISAT. 1992. Workshop recommendations. pp. 375–378. In: Nigam, S.N. (ed.). Groundnut – A global perspective. Proceedings of an International Workshop, 25–29 November. 1991. ICRISAT Asia Centre, Patancheru, India. Justice, O.L. and L.N. Bass. 1978. Practices of seed storage. Agriculture Handbook, 506: 57-77. Kamithi, K.D., F. Wachira and A.M. Kibe. 2016. Effects of different priming methods and priming durations on enzyme activities in germinating chickpea (Cicer arietinum L.). American Journal of Natural and Applied Sciences, 1: A1-A9.

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Kaydan, D. and M. Yagmur. 2008. Germination, seedling growth and relative water content of shoot in different seed sizes of triticale under osmotic stress of water and NaCl. African Journal of Biotechnology, 7: 2862-2868. Khalfaoui, J.B. 1991. Inheritance of seed dormancy in a cross between two Spanish peanut cultivars. Peanut Science, 18: 65-67. Khan, A.A. and E.C. Waters. 1969. On the hormonal control of post harvest dormancy and germination in barley seeds. Life Sciences, 8: 729-736. Khan, A.A., C.E. Heit, E.C. Waters, C.C. Anojuly, L. Anderson et al. 1971. Discovery of a new role for cytokinins in seed dormancy and germination. Search Agriculture, 1: 1-12. Kibinza, S., D. Vinel, D. Come, C. Bailly, F. Corbineau et al. 2006. Sunflower seed deterioration as related to moisture content during ageing, energy metabolism and active oxygen species scavenging. Physiologia Plantarum, 128: 496-506. Kumar, A.S.T., M.V.C. Gowda and H.L. Nadaf. 1991. Seed dormancy in erect bunch genotypes of groundnut (Arachis hypogaea L.) I. Variability for intensity and duration. J. Oilseeds Res., 8: 166-172. Lawles, K., W. Raun, K. Desta and K. Freeman. 2012. Effect of delayed emergence on corn grain yields. Journal of Plant Nutrition, 35: 480-496. Limede, A.C., C.E. da Silva Oliveira, A. Zoz, A.M. Zuffo, F. Steiner and T. Zoz. 2018. Effects of seed size and sowing depth in the emergence and morphophysiological development of soybean cultivated in sandy texture soil. Australian Journal of Crop Science, 12: 93-98. Misra, J.B., P.C. Nautiyal, S. Chauhan and P.V. Zala. 1992. Reserve mobilization and starch formation in cotyledons of germinating groundnut seeds. pp. 451. In: S.N. Nigam (ed.). Groundnut – A Global Perspective. Proc. of an International Workshop, 25-29 Nov. ICRISAT, Patancheru, India. Mishra, A., S.L. Swamy, S.S. Bargali and A.K. Singh. 2010. Tree growth, biomass and productivity of wheat under five promising clones of Populus deltoids in agri silviculture system. International Journal of Ecology and Environmental Sciences, 36: 167-174. Mohamed, H.A. 1984. Varietal differences in the temperature responses of germination and crop establishment. Ph.D. Thesis, University of Nottingham, Nottingham, U.K. Morgan, P.W., D.L. Ketring, E.M. Beyer and J.A. Lipe. 1970. Functions of naturally produced ethylene in abscissian, dehiscence and seed germination. pp. 502-509. In: D.J. Carr (ed.). Plant Growth Substances. Proc. 7th Symp. Plant Growth Substances, Canberra, Australia, 7 Dec. 1970. Springerverlog Berlin, Heidelberg, New York. Munns, R. and M. Tester. 2008. Mechanisms of salinity tolerance. Annual Review of Plant Biology, 59: 651-681. Murugan, V. 1981. Seed quality studies in groundnut (Arachis hypogaea L.) cv. TMV 7. M.Sc. (Ag.) Thesis, Tamil Nadu Agricultural University, Coimbatore. Musa, A.M., C. Johansen, J. Kumar and D. Haris. 1999. Response of chickpea to seed priming in the High Barind Tract of Bangladesh. International Chickpea and Pigeon Pea Newsletter, 6: 20-22. Mut, Z. and H. Akay. 2010. Effect of seed size and drought stress on germination and seedling growth of naked oat (Avena sativa L.). Bulgarian Journal of Agricultural Science, 16: 459-467. Narayana Murthy, U.M. and W.Q. Sun. 2000. Protein modification by Amadori and maillard reactions during seed storage roles of sugar hydrolysis and lipid peroxidation. J. Exp. Bot. 51: 1221-1228. Nascimento, W.M., D.J. Cantliffe and D.J. Huber. 2004. Ethylene evolution and endo-βmannanase activity during lettuce seed germination at high temperature. Scientia Agricola, 61: 156-163. Nautiyal, P.C., Y.C. Joshi and P.V. Zala. 1994. Screening of Spanish groundnut cultivars for germination under simulated drought stress. Int. Arachis Newsl, ICRISAT, India Centre, Patencheru, Andhra Pradesh.

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Nautial, P.C. 2004. Groundnut Research in India, pp. 321-338. Pub. by NRCG, Junagadh. Ndoye, O. 2001. Screening techniques and mode of inheritance of fresh seed dormancy among crosses of Spanish-type peanut (Arachis hypogaea L.). Ph.D, Texas A&M University, p. 162. Paparella, S., S.S. Araújo, G. Rossi, M. Wijayasinghe, D. Carbonera et al. 2015. Seed priming: State of the art and new prospectives. Plant Cell Reports, 34: 1281-1293. Patee, H.E., E.B. Johns, J.A. Singleton and T.H. Sanders. 1974. Composition Changes of Peanut Fruit Parts during Maturation. Peanut Sci., 1: 57-62. Patee, H.E., D.K. Salunkhe, S.K. Sathe, N.R. Reddy, R.L. Ory et al. 1983. Legume lipids. CRC Crit. Rev. Food Sci. Nutr., 17: 97-139. Pereira, W.A., S.M.A. Pereira and D.D.C.F. Santos. 2013. Influence of seed size and water restriction on germination of soybean seeds and on early development of seedlings. Journal of Seed Science, 35: 316-322. Piskurewicz, U., V. Turečková, E. Lacombe and L. Lopez-Molina. 2009. Far-red light inhibits germination through DELLA-dependent stimulation of ABA synthesis and ABI3 activity. The EMBO Journal, 28: 2259-2271. Prestley, D.A.1986. Seed ageing: Implication for seed storage and preservation in the soil. Cornell University Press, Ithaca, New York. Probert, R.J. 2000. The role of temperature in the regulation of seed dormancy and germination. pp. 261-292. In: Fenner, M. (ed.). Seeds: The Ecology of Regeneration of Plant Communities. Wallinford, UK: CAB International. Rajjou, L., M. Belghazi, R. Huguet, C. Robin, A. Moreau et al. 2006. Proteomic investigation of the effect of salicylic acid on Arabidopsis seed germination and establishment of early defense mechanisms. Plant Physiol., 141: 910-923. Rao, M.R.K. and I.M. Rao. 1972. Influence of seed coat and leaching on mobilization of carbohydrates and on germination of dormant seeds of groundnut var. TMV-3 (Arachis hypogea L.). Indian J. Plant Physiol., 15: 89-94. Rao, R.G.S., P.M. Singh and R. Mathura. 2006. Storability of onion seeds and effects of packaging and storage conditions on viability and vigour. Scientia Hort., 100: 1-6. Rehman, H.U., S.M.A. Basra and M. Farooq. 2011. Field appraisal of seed priming to improve the growth, yield, and quality of direct seeded rice. Turkish Journal of Agriculture and Forestry, 35: 357-365. Rehman, S., P.J.C. Harris, W.F. Bourne and J. Wilkin. 1996. The effect of sodium chloride on germination and the potassium and calcium contents of Acacia seeds. Seed Science and Technology, 25: 45-57. Roberts, E.H. 1972. Storage environment and the control of viability. pp. 14-58. In: E.H. Roberts (ed.). Viability of Seeds. Chapman and Hall Ltd., London. Santini, B.A. and C. Martorell. 2013. Does retained-seed priming drive the evolution of serotiny in dry lands? An assessment using the cactus Mammilaria hernandezii. American Journal of Botany, 100: 365-373. Shahi, C., K. Vibhuti and S.S. Bargali. 2015. Effect of zinc and boron on growth. How seed size and water stress effect the seed germination and seedling growth in wheat varieties? Curr. Agri. Res., 3: 60-68. Shakirova, F.M., A.R. Sakhabutdinova, M.V. Biryukova, R.A. Fatkhutdinova, D.R. Fatkhutdinova et al. 2003. Changes in the hormonal status of wheat seedlings induced by salicylic acid and salinity. Plant Sci., 164: 317–322. Shelar, V.R., R.S. Shaikh and A.S. Nikam. 2008. Soybean seed quality during storage: A review. Agric. Rev., 29: 125-131. Sikder, S., M.A. Hasan and M.S. Hossian. 2009. Germination characteristics and mobilization of seed reserves in maize varieties as influenced by temperature regimes. J. Agric. Rural Dev., 7: 51-56. Singh, P.K., V.K. Chaturvedi and B. Bose. 2010. Effects of salicylic acid on seedling growth

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and nitrogen metabolism in cucumber (Cucumis sativus L.). J. Stress Physiology and Biochemistry, 6: 102-113. Smith, J.S. Jr. and J.I. Davidson Jr. 1982. Psychrometrics and kernel moisture content as related to peanut storage. Trans. Am. Soc. Agric. Engrs., 25: 231-236. Smith, M.T. and P. Berjak. 1995. Deteriorative changes associated with the loss of viability of stored desiccation-tolerant and desiccation-sensitive seeds. pp. 701-746. In: J. Kigel and G. Galli (eds.). Seed Development and Germination. Marcel Dekker Inc. New York. Smykal, P., J. Masın, I. Hrdy, I. Konopasek, V. Zarsky et al. 2000. Chaperone activity of tobacco HSP18, a small heat-shock protein, is inhibited by ATP. The Plant Journal, 23: 703-713. Soares, M.M., H.C. dos Santos Jr., M.G. Simões, D. Pazzin and L.J. da Silva. 2015. Estresse hídrico e salino em sementes de soja classificadas em diferentes tamanhos. Pesquisa Agropecuária Tropical, 45: 370-378. Sreeramalu, N. and I.M. Rao. 1969. Growth and endogenous gibberellins content of dormant groundnut embryonic axes as influenced by leaching and GA. Physiol. Plant, 22: 11341138. Sreeramalu, N. and I.M. Rao. 1971. Physiological studies on dormancy in seeds of groundnut (Arachis hypogea L.) III. Changes in auxin and growth inhibitors content during development of seeds of a dormant and a non-dormant cultivar. Aust. J. Bot., 19: 273-280. Sreeramulu, N. 1974. Changes in endogenous growth regulating compounds during the after ripening of the dormant seeds of groundnut. Int. J. Plant Physiol., 71: 101-110. Sulochanamma, B.N. and T.Y. Reddy. 2007. Effect of seed size on growth and yield of rainfed groundnut. Legume Res., 30: 33-36. Tavili, A., S. Zare, S.A. Moosavi and A. Enayati. 2011. Effects of seed priming on germination characteristics of Bromus Species under salt and drought conditions. American-Eurasian Journal of Agricultural and Environmental Sciences, 10: 163- 168. Tejovathi, G., M.A. Khadeer and S.Y. Anwer. 1988. Studies on certain enzymes in salt tolerant and sensitive varieties of sunflower. Indian J. Bot., 11: 113–117. Tobe, K., L.P. Zhang, G.Y.Y. Qiu, H. Shimizu, K. Omasa et al. 2001. Characteristics of seed germination in five non-halophytic Chinese desert shrub species. Journal of Arid Environments, 47: 191–201. Toh, S., A. Imamura, A. Watanabe, K. Nakabayashi, M. Okamoto et al. 2008. High temperatureinduced abscisic acid biosynthesis and its role in the inhibition of gibberellin action in Arabidopsis seeds. Plant Physiology, 146: 1368-1385. Toole, V.K., W.K. Bailey and E.H. Toole. 1964. Factor influencing seed dormancy of peanut seeds. Plant Physiol., 39: 822-855. Upadhyay, H.D. and S.N. Nigam. 1999. Inheritance of fresh seed dormancy in peanut. Crop Science, 39: 52-56. Urosevic, B. 1964. More important seed borne disease of Czechoslovakia Forest trees. Proceedings of FAO/IUFRO Sym. Int. Dangerous Forest Dis. and Insects, Oxford 20-30 July. Vaish, C.P., C.B. Singh, R.P. Katiyar and P.K. Katiyar. 1994. Dormancy studies in groundnut (Arachis hypogaea L.). Farm Sci. J., 69: 88-90. Ventura, L., M. Doná, A. Macovei, D. Carbonera, A. Buttafava et al. 2012. Understanding the molecular pathways associated with seed vigor. Plant Physiol. Biochem., 60: 196-206. Wahid, A., S. Gelani, M. Ashraf and M. Foolad. 2007. Heat tolerance in plants: An overview. Environ. Exp. Bot., 61: 199-223. Walters, C., D. Ballesteros and V.A. Vertucci. 2010. Structural mechanics of seed deterioration: Standing the test of time. Plant Science, 179: 565–573. Williams, E.J. and J.S. Drexler. 1981. A non-destructive method for determining peanut pod maturity. Peanut Sci., 8: 134-141. Wilson, W.O. and H.B. McDonald. 1986. The lipid peroxidation model of seed ageing. Seed Sci. Technol., 14: 269-300.

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Yaw, A.J., A. Richard, S. Osei, H.O.D. Seth, A. Adelaide et al. 2008. Inheritance of fresh seed dormancy. African Journal of Biotechnology, 7: 412-424. Young, C.T. and W.E. Schadel. 1990. Microstructure of peanut seed: A review. Food Struct., 9: 317-328. Young, C.T., H.E. Pattee, W.E. Schadel and T.H. Sanders. 2004. Microstructure of peanut (Arachis hypogaea L. cv. ‘NC 7’) cotyledons during development. LWT–Food Sci. Technol., 37: 439-445. Zhang, F., J. Yu, C.R. Johnston, Y. Wang, K. Zhu et al. 2015. Seed priming with polyethylene glycol induces physiological changes in sorghum (Sorghum bicolor L. Moench) seedlings under suboptimal soil moisture environments. PLoS One, 10: e0140620.

CHAPTER

3

Vegetative Growth During the first few days the developing seedlings are dependent on assimilates stored in the cotyledons. After 5-10 days, depending upon the cultivar and environmental conditions, the seedling grows autotrophically and is capable of absorbing minerals via the roots whilst the epicotyl is exposed to light and capable of photosynthesis. Stems are angular, green or pigmented and are initially solid, but as the plants grow, they tend to become somewhat hollow. The main stem develops from a terminal bud of the epicotyl and two opposite cotyledonary laterals grow at the soil level. The main stem can be upright or prostrate and from 12 to 35 cm long or may exceed 1 m in runner types. The early vegetative growth stage is mainly concerned with mainstem elongation and leaf production, whereas the formation of lateral branches dominates later growth. Mainstem leaves account for >50% of the leaf area of plants for the first 35 days, but at 90 days they account only for 10%. After flowering, dry matter accumulation is mainly in the reproductive structures. The growth and branching patterns differ between subspecies and botanical types. Subspecies hypogaea has alternating pairs of vegetative and reproductive nodes, while subspecies fastigiata has a sequential pattern of reproductive nodes. After 20 days, there may be 8–10 fully expanded leaves. Unlike most legumes, peanuts have four leaflets per leaf, which partially fold up at night. Peanut foliage can grow at a daily rate of 150–200 kg/ha once full canopy cover is reached. Peanuts are indeterminate in vegetative and reproductive development. This means the plant does not stop growing in order to flower and produce a crop. Plants continue to grow leaves and stems while flowering and setting pods. The pods must, therefore, compete with the shoots for carbohydrate and nutrients. Indeterminate crops are more likely to be able to compensate for low levels of insect damage. There are differences in determinacy between varieties. The Virginia types are more determinate than Spanish types. Newer varieties achieve higher pod yields than older varieties, because a larger portion of the newer varieties’ growth goes into pods than to vegetation.

3.1.

Vegetative Growth Stages

Determination of the vegetative growth stage (Table 3.l) is based on the number of developed nodes on the main axis of the peanut plant, beginning with the first cotyledonary node as zero. Nodes, rather than leaves, are used for stage determination because they are permanent, whereas early leaves may be lost. When a peanut leaf drops, the node can easily be identified by either the stipules or by the petiole scar or by the presence of a branch in the former axil of the leaf. Commonly, two cotyledonary

47

Vegetative Growth

branches develop at the cotyledonary node, one in each axil of the cotyledonary (seed) leaves. In spite of two seed leaves and two cotyledonary branches, this node is considered as one node and is designated node “zero” because it is the site of seed leaves. The first true leaf forms at the next node up, which is designated as node “one”. A node is counted as developed when its tetrafoliolate has developed sufficiently so its leaflets are unfolded and flat. Stage V1 is defined as one developed node with one tetrafoliolate leaf unfolded and its leaflets flat. Subsequent V stages up to VN are based on N developed nodes on the main axis of the plant, counting the uppermost last node having a tetrafoliolate unfolded with its leaflets flat. Rate of node development (V stage progression) is dependent on air and soil temperature, availability of soil water, and plant maturity. Figure 3.1 shows that the rate of node development of Starr and Florunner was initially rapid, but progressively slowed as the plants matured and set fruit. Further studies on many cultivars are needed before one can suggest any particular relationship of the V to the R stage, i.e., that R1 for a given cultivar occurs on a specific V stage. Nevertheless, this may be a worthy research objective, since both vegetative and reproductive development of peanut respond mainly to temperature with little effect of photoperiod (Cox and Martin, 1974; Emery et al., 1969; Mills, 1964; Wynne and Emery, 1974). Table 3.1. Vegetative growth stage descriptions for peanut Stage VE

Abbreviated stage title Emergence

V0

Description Cotyledons near the soil surface with the seedling visibly showing some part of the plant Cotyledons are flat and open at or below the soil surface

V1

First tetrafoliate

One to N developed nodes on the main axis

V(N)

To Nth tetrafoliate

A node is counted when its tetrafoliate is unfolded and its leaflets are flat.

Table 3.2. Growth stages of a Virginia peanut variety in southern and northern Queensland Growth stage

Days after planting southern Qld

Days after planting northern Qld

Planting to emergence

6-14

6-12

Emergence to first flower

20-40

28-38

Flowering

35-65

28-65

Pegging

45-75

36-75

Pod filling

60-130

55-130

Harvest maturity

140-150

125-150

Similarly, SVI decreased with increasing salinity, ranging between 1157 in ICGV 86590 in control and 0.0 in several cultivars in T4 (Table 3.2). In T4, SVI reduced drastically and the highest SVI was found in Kopergaon 3 (173) followed by

48

Physiology of the Peanut Plant

MH 2 (145). Average SVI values decreased from 632 in control to 27 in T4 and the sensitivity of cultivars to salinity varied maximum in this level. Among the cultivars, the reduction in SVI due to salinity ranged between 26% in TG 37-A and 86% in ICG (FDRS) 10. Various parameters of seed and seedling vigour calculated under salinity stress also decreased, but the magnitude of difference varied in each parameter; for example, GR decreased from 39 in control to 13 in T4 and CVG decreased from 23 in control to 14 in T4. Apart from germination, average root length also decreased with increasing salinity levels and ranged between 6.3 cm in control and 0.4 cm in T4 (Fig. 3.2). The magnitude of variations in root length in T4 was between 0.0 in cultivars Co 3, Jawan, DH 3-30, VG 9521 and ICG (FDRS) 10 and 1.97 cm in GG 4. The average hypocotyl length under salinity stress decreased in the following order: 1.64 cm (control) > 0.83 cm (T1) > 0.40 cm (T2) > 0.13 cm (T3) and > 0.03 cm (T4). However, the hypocotyl length in T4 was higher in cultivars TG 37-A, Kopergaon 3, ICGS 44, ICGV 86590 and GG 4 and lesser in GG 3, ICG (FDRS) 10, OG 521, VG 9521 and DH 3-30. Hypocotyl length reduced to half at each increase in the salinity level and was affected more than root length; in addition, about 18 cultivars did not show any hypocotyl growth in T4. Growth and initiation of secondary roots was affected adversely due to salinity and only a few cultivars were able to develop secondary roots (Fig. 3.2). For example, among the cultivars, number of secondary roots ranged between 39 and 1 in control and 6.3 and 0.0 in T2 and were completely inhibited in T3 and T4. Thus, higher salinity levels were found detrimental to the growth of secondary roots more than any of the parameters studied in this experiment. Based on number of secondary roots, cultivars Kopergaon 3, MH 2, Gangapuri, VRI 4 and MH 4 were found relatively tolerant, whereas Co 3, ICG (FDRS) 10, Tirupati 4, GG 3 and VG 9521 were susceptible. All the vigour parameters such as GR, GS, GC,

Fig. 3.1. Ontogenic changes of nodes on main axis in peanut

Vegetative Growth

49

Fig. 3.2. Root length of groundnut cultivars under different levels of salinity (T1 = 20% salinity; T2 = 40% salinity; T3 = 60% salinity; T4 = 80% salinity)

StG and CVG were affected adversely with increasing salinity levels. The ranking of cultivars based on STI, ranged between 90 and 0.0; however, among the top five, i.e., Kopergaon 3, GG 4, MH 2, ICGV 86590 and Gangapuri, three belong to the Valencia botanical group. These results showed that the effect of salinity was more on seedling vigour and not on the initial germination process per se. Groundnut is essentially a tropical plant and requires a long and warm growing season. The favourable climate for groundnut is a well-distributed rainfall of at least 500 mm during the crop-growing season with abundance of sunshine and relatively warm temperatures. A temperature in the range of 25 to 30°C is optimum for plant development (Weiss, 2000). Once established, groundnut is drought tolerant, and to some extent it also tolerates flooding. A rainfall of 500 to 1000 mm will allow commercial production, although the crop can be produced on as little as 300 to 400 mm of rainfall. Groundnut thrives best in well-drained sandy loam soils, as light soils help in easy penetration of pegs and their development and harvesting. The productivity of groundnut is higher in soils with pH between 6.0 and 6.5. A summary of the reported water use of groundnuts (reproduced from Sivakumar and Sharma, 1986) in Table 3.3 shows that it varies from 250 mm in the rainfed conditions to 830 mm under irrigated conditions (with irrigation at weekly intervals). Naveen et al. (1992) reported that spraying of 3% Kaolinite during dry periods at 35 and 55 days after sowing showed significant yield increases over control. From the lysimetric studies in groundnut (ICGS-76) at Rakh Dhiansar, Jammu region of India, the water requirement of the crop was estimated at 494 mm and 500 mm in two individual years and crop water use was observed to be maximum (crop coefficient 1.9) during pod formation stage (AICRPAM, 1997; AICRPAM, 1998). Doorenbos and Kassam (1979) worked out stagewise water requirement as well as total water requirement of the crop. The water requirement of the crop ranged from 500 to 700 mm for the total growing period. The growing period of the crop is divided into five stages. The

50

Physiology of the Peanut Plant

data show that the midseason stage (pod formation and filling) requires higher water as indicated by the high crop coefficient value. In a field experiment conducted with JL-24, a bunch variety in two summer seasons in eastern India, water use recorded for three treatments with applied irrigation of 0.9, 0.7 and 0.5 of cumulative pan evaporation were 434, 391 and 356 mm, respectively (Bandopadhyay et al., 2005). The maximum average Kc value of 1.19 occurred around nine weeks after sowing in the same experiment (Table 3.3). Table 3.3. Summary of reported values of total water use (mm) of groundnut Reference

Total water use (mm)

Remarks

Ali et al., 1974

530

Irrigated at 60% water depletion

Angus et al., 1983

250

Rainfed

Charoy, 1974

510

Rainfed

Cheema et al., 1974

337

Rainfed

Kadam et al., 1978

597

Irrigated at 40% water depletion

Kassam et al., 1975

342

Rainfed

Reddy et al., 1980

438

Rainfed

Reddy et al., 1978

560

Irrigated, winter months

Reddy and Reddy, 1977

417

Rainfed

Panabokke, 1959

505

Irrigated at 25% water depletion October to January

Keese et al., 1975

404

Irrigated at 50% water depletion

Samples, 1981

500-700

Irrigated at 50% water depletion

Nageswara Rao et al., 1985

450-600

Irrigated at 7-10 day interval during winter months

Source: Sivakumar and Sharma, 1986

3.2.

Water

Groundnut is grown during the rainy, winter and summer seasons in India. The average productivity is relatively low in the rainy season. Groundnut has specific moisture needs due to its peculiar feature of producing pods underground. Some workers are of the opinion that early moisture stress restricts the vegetative growth which in turn reduces the yield, while others say that the peak flowering and pegging period is most sensitive as the peg cannot penetrate through dry and hard surface. The rabi crop avails the residual moisture and the scanty rainfall during winter and produces a substantial yield as compared to the kharif crop and few supplementary irrigations would improve the yield. Through high productivity under assured irrigation, groundnut cultivation in the summer season is gaining popularity. In irrigation scheduling, a climatologically approach based on IW/CPE ratio (IW - irrigation water, CPE - Cumulative pan evaporation) has been found most appropriate. This approach integrates all the weather parameters that determine water use by the crop and is likely to increase production at least 15-20%. Optimum scheduling of irrigation led to increase in pod yield and water use efficiency (WUE) (Taha and Gulati, 2001). Kadam and Patil (1989) were of the opinion that groundnut gave the highest pod yield with

Vegetative Growth

51

irrigation at 1.0 IW/CPE ratio rather than 0.7 or 0.5. However, Satapathy and Patro (1992) found no significant effect of levels of irrigation on pod yield of groundnut. Baliar Singh and Mahapatra (2015a, b) found that irrigation at all the critical stages of groundnut growth produced maximum pod yield in the summer groundnut, but the information on irrigation scheduling and suitable variety for the summer season is meagre. A larger share of water absorbed by the plants is used for their vegetative growth. Growth characters like shoot length, number of branches, number of leaves, leaf area index and dry matter production are affected by quantity and frequency of irrigation water. Khatri and Patel (1983) stated that vegetative development is one of the critical stages of irrigation for groundnuts. Lenka and Mishra (1973) reported that crops irrigated at different levels of soil moisture depletion had no differential effect on plant height, while Mathew et al. (1983) observed that the plant height increased with an increase in the irrigation level at IW/CPE ratios of 0.3, 0.6 and 0.9. Jana et al. (1989) observed that when groundnut was irrigated at different growth stages, maximum plant length was recorded at two irrigations given one at the flowering stage and the other at the pod development stage. It was noted that irrigation at IW/ CPE of 0.75 made the plants longer than those irrigated at IW/CPE of 0.5 or at three important growth stages, though not significant. Mathew et al. (1983) found that increased irrigation frequencies from 0.3 to 0.9 IW/CPE ratio, each with 50 mm of water exerted significantly influenced the number of branches per plant. Water stress at IW/CPE ratio of 0.4 at seedling stage, flowering, pegging and maturity reduced the number of branches per plant (Shinde and Pawar, 1984). Desai et al. (1985) made a comparison between the lowest number of six irrigations applied at 0.5 IW/CPE ratio and the highest number of 11 irrigations applied at 1.1 IW/CPE ratio and reported that the reduction in the number of primary branches was due to moisture stress, but researchers did not find any significant difference in the number of branches per plant by applying irrigation of different schedules such as 0.6 to 0.95 IW/CPE ratios or at different growth stages (sowing to pegging, pegging to pod formation and pod formation to maturity). The number of branches increased with increasing moisture around the root zone. The crops irrigated at all the critical stages have the maximum number of branches (6.27/plant) due to maintenance of the optimum soil moisture condition enhancing the growth processes resulting in a greater number of branches and leaves. Leaf area index is a better determinant of crop growth, which determines the photosynthetic capacity of the crop (Watson, 1952). LAI was very low at 30 DAS because of low initial growth. It increased gradually and reached its maximum at 90 DAS and decreased thereafter probably due to senescence. Leaf area index (LAI) is an important physiological attribute as it is directly associated with photosynthetic tissues of the plant (Hunt, 1990). Crop growth during the early stages is often restricted because leaf area is too small to intercept all the incident radiation. Rapid leaf area development may be attractive under a number of cropping conditions to enhance the vigour of the crop establishment and allows rapid canopy closure for maximizing light interception and shading of weed competitors. This study was undertaken to determine (1) if parameters describing leaf area development varied among the ten peanut (Arachis hypogeae L.) genotypes grown in field and pot experiments, (2) if these parameters were affected by the planting density, and (3) if these parameters varied between Spanish and Virginia genotypes. Leaf area

52

Physiology of the Peanut Plant

development was described by two steps: prediction of the main stem number of nodes based on phyllochron development and plant leaf area dependent based on the main stem node number. There was no genetic variation in the phyllochron measured in the field. However, the phyllochron was much longer for plants grown in pots as compared to the field-grown plants. These results indicated a negative aspect of growing peanut plants in the pots used in this experiment. In contrast to phyllochron, there was no difference in the relationship between plant leaf area and main stem node number between the pot and field experiments. However, there was genetic variation in both the pot and field experiments in the exponential coefficient (PLAPOW) of the power function used to describe leaf area development from node number. This genetic variation was confirmed in another experiment with a larger number of genotypes, although possible G × E interaction for the PLAPOW was found. Sowing density did not affect the power function relating leaf area to main stem node number. There was also no difference in the power function coefficient between Spanish and Virginia genotypes. SSM (Simple Simulation Model) reliably predicted leaf canopy development in groundnut. Indeed, the leaf area showed a close agreement between predicted and observed values up to 60000 cm2 m−2. The slightly higher prediction in India and slightly lower prediction in Niger reflected G×E interactions. Until more understanding is obtained on the possible G×E interaction effects on the canopy

Fig. 3.3. Illustration of plant leaf area considered as a function of main stem node number described by the power function for two peanut genotypes: (a) ICG 1834 and (b) Fleur 11

Vegetative Growth

53

development, a generic PLAPOW value of 2.71, no correction for sowing density, and a phyllochron at 53°C could be used to model canopy development in peanuts (Fig. 3.3). Dry matter accumulation is the cumulative growth of various plant parts and acts as an important index of efficient photosynthetic activity. Nageswar Rao et al. (1985) reported that soil moisture stress at seedling, flowering, pegging and pod maturity stages resulted in a drastic reduction in dry matter accumulation due to a lesser number of leaves. The vegetative growth rate of a crop under water stress may be severely restricted, resulting in reduced total dry matter and smaller leaf area than where water is unlimited. In an investigation the crop growth rate (CGR) was slow at the beginning and then increased rapidly upto 90 DAS. The CGR was maximum at 60-90 DAS in all the treatments and decreased thereafter which was due to senescence of older leaves. In a clay-loam soil Gulati et al. (2001) observed peak values of CGR and NAR between 56 and 90 days in summer groundnut at Chipilima. Application of irrigation water ensures steady availability of soil moisture to the crop, which consequently improves uptake of nutrients, fertilizer use efficiency, growth and development. It ultimately reflects on the accumulation of higher dry matter in aerial parts (Dutta and Mandal, 2006) along with the high CGR, RGR and NAR values. To ascertain the effect of different methods of irrigation (drip, micro sprinkler and surface) and transient water stress (at early vegetative stages up to 20, 30 and 40 DAS) on groundnut (Arachis hypogaea L.), a field experiment was conducted during the summer seasons of 2015 to 2017 on calcareous clayey soil at Junagadh (Gujarat, India). The results indicated that drip irrigation method significantly enhanced growth, yield and quality attributes viz., plant height, branches/plant, LAI, dry matter/plant, mature pods/plant, pod weight/plant, test weight and shelling, which in turn resulted in higher pod (18.30 q ha-1) and haulm yield (42.58 q ha-1) as well as water use efficiency (2.253 kg ha-1 mm-1) over the surface irrigation method. Water stress up to 40 DAS decreased plant height and dry matter/plant, while it increased branches/plant and LAI over the water stress up to 20 DAS. However, yield and quality parameters as well as pod and haulm yields remained statistically equivalent under various stress periods. The water use efficiency was increased with each increment in the stress periods. It is suggested that drip irrigation method and water stress in early vegetative stage up to 30 DAS could be adopted to enhance water use efficiency and to increase the yield of groundnut in the summer season. Prabawo et al. (1990) reported that irrigation applied before and/or after early pod filling stages increased pod yields of Spanish type groundnuts (100 days) to 2.4 t ha-1 compared with 0.53 t ha-1 in dryland groundnut crop. Nageswara Rao et al. (1985) confirmed that irrigation could be withheld during much of the vegetative period without any apparent effect on pod yield, implying that water stress during the vegetative stage has no effect on yield. Nautiyal et al. (1999) proved that soil moisture deficit for 25 days during the vegetative phase was beneficial for growth and pod yield of groundnut while Stirling et al. (1989) observed the insensitivity of pod yield to early moisture deficit. Sivakumar and Sharma (1986) imposed drought stress or soil moisture deficit at all the growth phases of groundnut during three growing seasons and observed that stress from emergence to pegging gave increased yields over control in all the three years while stress in other stages decreased the yield. Moisture stress also affects physiological characters like photosynthesis, stomatal conductance, leaf water potential, radiation and water use efficiencies, partitioning

54

Physiology of the Peanut Plant

of dry matter (Williams and Boote, 1995). Bhagsari et al. (1976) observed large reductions in photosynthesis and stomatal conductance as the relative water content of groundnut leaves decreased from 80 to 75% (due to moisture stress). Subramanian and Maheswari (1990) reported that leaf water potential, transpiration rate and photosynthesis rate decreased progressively with increasing duration of water stress. Black et al. (1985) recorded lower water potential and stomatal conductance when moisture stress was imposed. Clavel et al. (2004) reported that water deficit decreased leaf area index, relative water content and transpiration at about three weeks after the occurrence of water deficit at the soil levels.

3.3.

Temperature

The early vegetative growth stage is mainly concerned with mainstem elongation and leaf production, whereas the formation of lateral branches dominates later growth. Mainstem leaves account for >50% of the leaf area of plants for the first 35 days, but at 90 days they account only for 10%. After flowering, dry matter accumulation is mainly in the reproductive structures. The groundnut leaves are tetrafoliate, peripinnate with two pairs of opposite subsessile, obovate leaflets with entire ciliate margins, are born spirally in 2/5 phyllotaxy and the arrangement is distichous. Stipules are prominent, linear and adenate to some length and become free at the pulvinus. In general, the sub species hypogaea has dark green foliage with small leaflets and sub sp. fastigiata has light green and larger leaflets. The leaflets have hairs mainly on the abaxial surface and on the margins which is related to resistance to leaf hoppers. The leaf size ranges from 4 cm2 in the first seedling stage up to 80 cm2 in upper leaves of a fully developed stand. The specific leaf weight ranges from 4.1 to 6.7 mg cm-2 in young fully expanded leaves. The leaf is the photosynthetic unit and in groundnut it exhibits nyctiotropic movements daily where the adaxial surfaces of leaflets come together and the petiole bends downwards. Stomata appear on both sides of the leaf. The groundnut leaves show exponential increase in their number from 20 to 90 days after sowing, but the leaf production during this period differs with botanical types and is higher in the Virginia-runner followed by semi-Virginia bunch and erect Spanish types (Table 3.4). Table 3.4. The average number of leaves produced by three cultivars at three constant temperatures Weeks after sowing

Schwarz 21 33°C

28°C

Mallorca

24°C

33°C

28°C

Ukraine 24°C

33°C

28°C 24°C

1

4

-

-

4

-

-

4

-

-

2

13

6

4

7

6

4

7

5

4

4

21

19

16

14

20

12

18

14

9

6

43

30

28

33

32

22

32

22

22

7

58

43

36

52

47

34

42

33

30

10 12

72 80

50 55

36 36

67 74

60 65

36 36

55 64

45 63

32 32

The common relationship between leaf appearance rate and leaf number at 30/24 and 40/28°C for all but three genotypes in Fig. 3.4 suggests that a temperature of

Vegetative Growth

55

40/28°C (mean 34°C) was not supra optimal for the leaf appearance in most genotypes, i.e. the thermal time per leaf was the same at 30/24 and 40/28°C. A comparison of leaf appearance rates at 30/24 and 40/28°C against thermal time or growing degree days (GDD, calculated using the mean daily temperature above a base temperature of 10°C; Ong (1986) confirmed that there was no significant effect (P 0.50) of temperature on thermal time for the appearance of successive leaves (h) in four genotypes, values of h ranging from 66 to 78 GDD/leaf. However, in ICGV 86021, 796, 55-437 and 47-16 either increased or decreased with high temperatures. In this experiment most of the groundnut genotypes were clearly able to develop vegetatively and grow as well at 40/28°C as at 30/24°C. Several other studies have shown similar results. For example, Talwar et al. (1999) reported significantly greater leaf numbers and vegetative biomass in three genotypes of groundnut grown at mean temperatures of 32±5 compared to 25°C, while Leong and Ong (1983) reported a constant value of 56 GDD/leaf for cv. Robut 33-1 (now known as Kadiri-3) grown at mean temperatures from 19 to 31°C. Net photosynthesis rates have also been reported to be 26% greater at 32±5 compared to 25°C (Talwar et al., 1999), and only reduced by 25% at 40 °C relative to 30°C (Ketring et al., 1982). However, Nigam et al. (1994) found plant growth rates in three Spanish genotypes to be lower at 30/28 than at 26/22°C. Nonetheless, these results support the view that groundnut is well adapted to warm climates and that the optimum temperature for vegetative development and growth is probably between 30 and 33°C, as Williams and Boote (1995) suggest.

Fig. 3.4. Relation between mainstem leaf number and leaf appearance rate per day in eight genotypes of groundnut grown at 30/24°C (+) and 40/28°C (E). Bar shows s.e.d.

The groundnut plant has a distinct main stem and a varying number of lateral branches and carriage of laterals is an important character determining the growth habit. The bunch types have a thicker stem than runners. The internodes are short and highly condensed at the base but are longer at the higher nodes (Fig. 3.5). Spreading (runner, trailing, procumbent and prostrate) and erect (upright, erect bunch and bunch) are two distinct growth habits in groundnuts. However, six types of growth

56

Physiology of the Peanut Plant

habits (procumbent 1 and 2; decumbent 1, 2, and 3 and erect) have been described for groundnuts (IBPGR-ICRISAT, 1981). The groundnut stems develop anthocyanin pigments in their epidermal cells, the colour of which may be purple (violet), pink, dark red, light red or green. The stem contains long shoot and glandular hairs with a bulbous base. The leaves of the erect types are larger than of the spreading variety. Varietal differences in the pattern and coverage of the canopy in different plant types under non-competitive conditions are observed. The leaf area and dry matter regularly increased from the 3rd leaf stage up to peg formation. With plant age, the leaf number increased most for spreading varieties and leaf weight increased most for semispreading varieties (Velu and Gopalakrishnan, 1987).

Fig. 3.5. Main stem elongation of the Mallorca cultivar grown at constant temperatures of 24°, 28° and 33°C.

In a growth chamber study, Ketring (1984) showed that when groundnut plants were transferred from 30/25 ±1°C to experimental temperatures (30/22, 32/22, and 35/22°C) the leaf area of two cultivars (Tamnut 74 and Starr) progressively decreased with the increase in temperature when observed at 63 and 91 DAP. At harvest (91 DAP) the decrease in leaf area per plant was about 49% for Tamnut 74 and about 80% for Starr at 35/22 ºC as compared to leaf area of respective cultivars at 30/22°C. Stem elongation was significantly inhibited by both 32/22°C and 35/22ºC for Tamnut 74 and by 35/22ºC for Starr. Contrary to Ketring’s results, Talwar et al. (1999) in a glasshouse study observed that all vegetative growth parameters (such as leaf area,

Vegetative Growth

57

stem elongation etc.) of three genotypes (ICG 1236, ICGS 44 and Chico) increased at 35/25ºC as compared to those observed at 25/25ºC. These contradicting results between the two studies may be caused by lower light intensity in growth chamber studies.

3.4. 3.4.1.

Photosynthesis Light Intensity

Pallas and Samish (1974) determined the net photosynthetic rate (Pn) for several peanut genotypes as a function of the flux density of photosynthetically active radiation (PAR, 400-700 nm) with various light sources in a growth chamber. The genotypes were grown at 255 JLE m-2s-1, while photosynthetic measurements were made at given flux densities for 15 min. They found no genotype quite photosaturated at the highest intensity (1546 JLE m-2s-1), but the slope relating Pn to PAR was distinctly attenuated above 500-700 JLE m-2s-1. Similar results were found by Trachtenberg and McCloud (1976). The results of these studies, however, are based on rather short term observations and the plants have little time to adjust to the change in their environment. Therefore, the results are not likely applicable for estimating net assimilation for a longer period. Shading experiments have been conducted in the field for longer term evaluations. Ono and Ozaki (1971) found that increasing the degree or length of shading of peanuts resulted in less growth. The net assimilation rate (NAR) decreased linearly with relative light intensity between 100 and 26%. It was not directly proportional, however, as 55% of the NAR that existed with no shade was found when only 26% of the light remained. This indicates this would have to be marked as a non-linear effect at low light intensities. A non-linear effect could be related to stomatal diffusion resistance. This resistance has been found to increase logarithmically with a decrease in irradiance (PAR), especially at less than 500 JLEm-2s-1 (Allen et al., 1976). Two phytotron experiments were conducted in which light intensity and the duration of light were varied and Florigiant peanuts grown. Dry weight of leaflets, petioles and stems, leaf area, and number of flowers of young non-competitive plants were measured at four to five day intervals over a 39 or 46 day growth periods. Top dry weight increased curvilinearly with increasing photosynthetically active radiation becoming asymptotic above about 23 E m-2day-1. Leaf area differed due to light treatment much as did top dry weight but differences in light did affect the leaf area due to light treatment much as it did affect the leaf area per gram of leaflet and leaflet to top ratio. Peanuts (Arachis hypogaea L.) which are commonly grown in intercropping systems often suffer from shading caused by the associated crop. In a study an attempt has been made to estimate the effect of different levels of shade at different growth stages on crop yield. Field experiments were laid out during the monsoon and winter seasons of 1985 and 1986 by creating artificial shading up to 25 and 50 per cent of the day/natural light at flowering‐pegging, pod filling and maturity stages of a Spanish bunch type peanut. Dry matter production has shown a linear response to light intensity and due to 50 per cent shading it was reduced by 55 per cent. Vegetative growth rate during the pod filling stage was very poor as a result of an increase in shading at this stage. In shaded plants the nodulation was less and some reduction in chlorophyll content was also observed. However, oil content in the kernel was not affected by shading. Shading caused significant reduction in pod number and kernel weight and thus there was decrease in pod yield. Flowering to

58

Physiology of the Peanut Plant

pegging and pod filling stages seemed to be sensitive to shading while an increase in shading at the maturity stage did not cause any reduction in yield. It could be possible to obtain about 90 per cent pod yield by avoiding shading in the duration from the flowering to the pegging stage (45 DAS).

3.4.2.

Carbon Dioxide

Foliage and stem fresh and dry weights were greater at 800 than at 400 µmol·mol-1 CO2 but declined at 1200 µmol·mol-1 (Table 3.5). Fibrous root dry weight increased linearly as CO2 increased. Increasing CO2 had no effect on dry matter accumulation in individual leaves (Table 3.5), but area per leaf and branch length (main and cotyledonary) increased linearly with increasing CO2. In contrast, specific leaf area decreased linearly as CO2 concentration increased from ambient to 1200 µmol·mol-1. Table 3.5. Effects of CO2 enrichment on leaf and stem characteristics, net photosynthesis, stomatal conductance, and carboxylation efficiency of ‘Georgia Red’ peanut Observation Dry weight/leaf (mg) Area/leaf (cm2) SLA (m2.kg-1) Stem length (cm) Main Cotyledonary Net photosynthesis (µmol·mol-1) Stomatal conductance (mol.m-2.s-1) Carboxylation efficiency (µmol. m-2s-1)

CO2 (400 µmol·mol-1) 193.0 30.9 24.8

CO2 (800 µmol·mol-1) 188.0 36.4 21.2

CO2 (1200 µmol·mol-1) 183.0 38.9 22.1

Regressions

25.3 23.1 17.1

29.3 26.2 22.1

36.5 33.2 13.0

L* L* L*

0.468

0.261

0.234

L*

0.111

0.116

0.069

L*

NS L*** L*

L = linear, Q = quadratic; significant at P 80%), gs and Pn under stress appears to be imparting drought tolerance in groundnut. The translocation of photosynthates involves movement of metabolites from mesophyll cells to phloem tissue, phloem loading, translocation in phloem, unloading and metabolism of photosynthates in the site of utilization. The rate of translocation is positively correlated with net photosynthetic rate; however, environmental factors affect efflux and distribution of photosynthates. During the early stages the partitioning of dry matter is into the leaf and stem but in later stages it accumulates simultaneously in the vegetative parts and fruits as these two phases overlap in groundnut. Thus, the quicker cessation of the vegetative growth (dry matter accumulation in stems) makes more photosynthates available for pods and is a desirable character. Jun et al. (1999) using a gas-phase oxygen electrode measured the photosynthesis and respiration of 13 groundnut cultivars grown in pots in a natural light phytotron under 28/23°C day/night temperatures, 60% RH and 380 ppm CO2 and reported that though the cultivar differences in the rates of photosynthesis and respiration were within a range of mean values >17%, the cultivars Jenkins Jumbo, Posados 64 and Satonoka with high photosynthetic oxygen evolution (24% higher than the mean) had high respiratory oxygen absorption (17% higher than the mean), and there was a weak correlation between photosynthesis and respiration. Moisture stress also affects physiological characters like photosynthesis, stomatal conductance, leaf water potential, radiation water use efficiencies and partitioning of dry matter (Williams and Boote, 1995). Bhagsari et al. (1976) observed large reductions in photosynthesis and stomatal conductance as the relative water content of groundnut leaves decreased from 80 to 75% (due to moisture stress). Subramanian and Maheswari (1990) reported that leaf water potential, transpiration rate and photosynthesis rate decreased progressively with increasing duration of water stress. Black et al. (1985) recorded lower water potential and stomatal conductance when moisture stress was imposed. Clavel et al. (2004) reported that water deficit decreased leaf area index, relative water content and transpiration at about three weeks after the occurrence of water deficit at the soil levels.

3.5.

Characteristics of Peanut Plant Parts

Groundnut is a herbaceous annual with a fairly developed root system and a tap root. The tap root appears on the second day after seed germination and has a massive root cap. It elongates rapidly and grows almost vertically. It may vary from a few millimetres in diameter in annual species to 10 cm in perennial species. The welldeveloped tap root may penetrate to a depth of 130 cm but rarely goes beyond 90 cm. The root system is normally concentrated at a depth of 5 to 35 cm and root spread is confined to a radius of 12 to 14 cm. The root system of spreading types is usually more vigorous than the bunch types. The lateral roots appear on the third day after seed germination. They are basically similar to tap roots but they lack the central pith and they multiply very quickly (as many as 100-120) on the fifth day and grow to a length of 15-20 cm (Fig. 3.6).

62

Physiology of the Peanut Plant

Fig. 3.6. Root system in peanut

The young stems are angular, usually pubescent and solid with a large pith. As the plant grows, the stems become hollow and tend to be cylindrical and shed hairs. The thickness of the stem is highly variable. Generally, the bunch types have thicker stems. Internodes are short and highly condensed at the base but are longer at the higher nodes (Fig. 3.7).

Fig. 3.7. Stem in peanuts

The peanut Arachis hypogaea is an annual legume, unusual in its genus being polyploid (4x = 40). It can interbreed only with another species A. monticola, the probable wild progenitor of the crop. The cultivated peanut plant is an erect or prostrate, usually 15 to 60 cm tall. It is sparsely hairy and has a well-developed tap root system with many lateral roots. Roots are usually devoid of hairs, and a distinct epidermis. A unique characteristic of the peanut plant is the nyctinastic movements of the leaflets. The leaf blade consists of four oval to obovate leaflets attached to the midrib by small articulations which allow for movement. During dark periods and hot sunny days, the paired leaflets are close together in a vertical position, and on normal day leaflets are separated from each other in a horizontal position.

63

Vegetative Growth

The leaves in the genus are tetrafoliate. Occasionally small and abnormal leaflets may appear. The leaves of cultivated species are paripinnate with two pairs of opposite, sub-sessible, obovate (variable), shortly by mucronate leaflets with entire ciliate margin. The leaflets are borne on a slender, grooved and jointed rachis. Groundnut cultivars differ in leaf characteristics such as leaf colour (foliage colour), shape, hairiness and size. Stomata appear on both sides of the leaf (Fig. 3.8).

Fig. 3.8. Leaves of peanuts

The cultivated groundnut has a distinct main stem and a varying number of lateral branches. The carriage of laterals is an important character which determines the growth habit of the plant. Two distinct forms of growth habits have been reported in groundnut. They are, Spreading (runner, trailing, procumbent and prostrate) and Erect (upright, erect and bunch). The spreading form is usually characterized by an erect conspicuous main stem with procumbent or documbent lateral branches. In the erect types the main axis becomes indistinguishable from the laterals. An intermediate semi-spreading form (spreading bunch, bunch runner and runner bunch) is also reported. The two main botanical sections of Arachis hypogaea differ in the distribution of vegetative branches and inflorescence in the axils of the leaves on the main axis and the branches. The main branch (axis) of cultivated groundnuts is designated as ‘n’ and the first, second and third branches are called n+1, n+2 and n+3 respectively. In all forms of the species, primary vegetative branches (n+1) arise on the axis of the cotyledons and at a number of higher nodes on the main axis. In the sequential type (bunch types) inflorescence are borne at a second and several subsequent nodes of the primary branches. The first node on such a branch may bear secondary branches (n+2) but often it bears an inflorescence, so that the first flowers are initiated very soon after the development of n+1 branches of the alternate branching type (spreading type), the first two nodes of n+1 branches normally bear vegetative branches (n+2), the next two bear inflorescence and the next two again vegetative branches and so on. The same sequence is repeated for n+n+n n+2 branches. In the Spanish group of the sequential type, the n+1 branches grow upwards from the very beginning whereas in the Valencia

64

Physiology of the Peanut Plant

group, these branches grow outwards first and then upwards. These two groups are generally referred as bunch types. In the alternate branched section, the runner forms have prostrate n+1 branches, whereas spreading forms more upright branches, both constituting spreading types. The sequential and alternate branching types also differ in many other agronomic characters: the duration is 110-120 days in the sequential and 130-150 days in the alternate branched; seeds are dormant in the alternate branched and non-dormant in the sequential branched; and plants are light green in sequential and dark green in alternate branched. The production of leaves and increase in shoot weight are considered as a measure of vegetative growth. The period of maximum growth is between 56 and 97 days in bunch varieties and 70 and 125 days in spreading ones. A higher rate of growth during early stages and more dry matter accumulation was seen in bunch types in comparison with spreading types. The total number of leaves per plant showed an exponential increase from about 21 days to 90-100 days after sowing and it ranged from 93 to 112 in bunch varieties and from 206 to 346 in spreading types. Maximum Leaf Area Index reported was 4.0 and for maximum yield the leaf area index at the 14th leaf stage should be more than 4, the total plant dry matter more than 500 g.m-2 and the leaf dry matter more than 175 g.m-2. As the plant grows, the root develops very rapidly in comparison to the shoot. By 10 days after planting, root growth can reach 12 inches. By 60 days, roots can extend 35 to 40 inches deep. Late season measurements have found peanut roots down to 6 to 7 feet. Roots grow at a rate of about one inch per day as long as soil moisture is adequate. The hypocotyl pushes the plumule upward causing “ground cracking”. After emergence, the plumule is called a shoot and consists of a main stem and two cotyledonary lateral branches. At emergence the main stem has at least four immature leaves and the cotyledonary lateral branches have one or two leaves also. The seedling develops slowly showing as few as eight to 10 fully expanded leaves 3 to 4 weeks after planting. Leaves are attached to the main stem at nodes. There is a distinct pattern by which these leaves are attached. There are five leaves for every two rotations around the main stem, with the first and fifth leaves located one above the other. Leaves attached to the cotyledonary laterals and other lateral branches are two-ranked, so there is one leaf at each node, alternately occurring on opposite sides of the stem.

Fig. 3.9. Branching in peanuts

65

Vegetative Growth

The species Arachis hypogaea L. is highly variable morphologically. Branching pattern and growth habit are important characters in the classification of variety groups and cultivar clusters (Gibbons et al., 1972). Normally ‘Schwarz 21’ and ‘Matjan’ plants develop lateral branches in the axils of the cotyledons and the two lowermost foliar leaves (Fig. 3.9). These four primary axes are always formed virtually. Depending on external conditions more vegetative laterals may be formed. By preference these appear in the axils of the prophylls (cataphylls) of the cotyledonary branches and at the higher nodes of the main stem. However, Fortanier (1957) demonstrated that each axillary bud has the potential to develop into a vegetative branch. The complex structure of the groundnut plant becomes quite clear. Figure 3.10 is a photograph of an exceptionally well developed 21 days old ‘Matjan’ plant. Noteworthy are: 1. A difference between vegetative branches and generative branches. According to Purseglove (1968): Dimorphic branching with monopodial vegetative branches and reduced reproductive branches. 2. The presence of solitary flowers in the axils of bracts or prophylls only. 3. The presence of two partly fused but clearly distinct bracteoles at the base of the pedicel of each flower.

Fig. 3.10. ‘Matjan’ plant in 21 days

The main stem and cotyledonary laterals determine the basic branching pattern of the shoot. The main stem develops first and in runner type plants the cotyledonary laterals eventually become longer than the main stem. Additional branches arise from nodes on the main and lateral stems. The growth habit of peanut is described as bunch, decumbent or runner. Spanish and Valencia market types are classified as “bunch”, with their upright growth habit and flowering on the main stem and lateral branches. Most Virginia and runner market types are considered to have a prostrate (flat) growth habit and do not flower on the main stem. Decumbent varieties have an intermediate growth habit between a runner and bunch. Several Virginia varieties are classified as decumbent. Peanuts are indeterminate in both vegetative and reproductive development (similar to cotton).

66

3.6.

Physiology of the Peanut Plant

Nodulation

Groundnuts are modulated by a wide range of species of rhizobia. Proliferation of compatible rhizobia in the rhizosphere of legumes is the first step towards nodule formation. There is no clear specificity in groundnut nodulation and root colonization. In most legumes rhizobia enter through root hairs via an “infection thread”, but in groundnut roots the invasion process is rather different. Normal root hairs are absent; instead, tufted rosettes of hairs are found in the junctions of the root axils. It is at these junctions that nodulation occurs. Once the rhizobia have entered the root and occupied the space between the root hair wall and the adjoining epidermal and cortical cells, the cells adjacent to the point of Rhizobium penetration separate at their middle lamellas and the resultant spaces become filled with bacteria, forming intercellular zones of infection. The bacteria penetrate into progressively deeper cell layers and intracellular infection then occurs. Soon after intracellular infection, the bacteria multiply rapidly. Further development of the nodule occurs by repeated division of the infected host cells, and the bacteria become transformed, into different morphological forms known as bacteroids. As the size of the bacteroids is relatively large, the number of bacteroids per unit nodule weight in groundnut is small when compared to other tropical legumes such as cowpea. The initial spherical shape of legume nodules can develop into various morphological variants, but in groundnut they remain more or less spherical throughout. The bacteroids contain the nitrogenase enzyme which reduces gaseous nitrogen. The effectiveness of the nodule to fix nitrogen depends on the presence of leghaemoglobin, which gives a pink coloration to the nodule tissue. Quantitative determination of leghaemoglobin during certain stages of plant growth (e.g. flowering) could be used as an indicator of dinitrogen fixation in groundnuts. The host plant provides energy in the form of carbohydrates for dinitrogen fixation and the carbon skeleton for the assimilation of reduced nitrogen (NH4). The fixed nitrogen is transferred to shoots mainly in the form of o-methylene glutamin. Not all the rhizobia that produce nodules fix nitrogen. It is important therefore to select good inoculant strains for survival in the rhizosphere, competition with indigenous strains, infectivity and fixation potential. The effect of different salinity levels on the main root length and lateral root length of groundnut ranged between 1.33±0.66 cm in 250 mM and 4.63±0.63 cm in control treatment. Root length decreased with increasing salinity levels, as reported Table 3.6. Effects of salt solutions on the root length of groundnut Salt conc. (mM)

Root length (cm)

Lateral root length (cm)

0

5.70±0.10a

4.63±0.63a

50

4.40±0.05b

4.06±0.47a

100

bc

3.93±0.06

3.40±0.30ab

150

3.23±0.14cd

2.83±0.16ab

200

de

2.23±0.14

2.46±0.26ab

250

1.16±0.60e

1.33±0.66b

F-ratio P-value
80%), gs and Pn under stress appears to be imparting drought tolerance in groundnut (Table 3.9). However, in an experiment with groundnut cultivars and seed size in major and minor seasons no appreciable difference had been noted among the varieties but a significant difference was noted between seasons (Table 3.10).

71

Vegetative Growth

Table 3.9. Growth parameters of groundnut as influenced by varieties and plant population Treatment

Plant height (cm)

LAI

DMP (kg ha-1)

SCMR

Days to 50% flg.

Days to maturity

Variety Abhaya TAG 24 Dharani Kadiri-6 SEm± CD(0.05)

34.8 24.8 39.1 44.3 0.32 0.92

2.64 2.62 2.75 2.78 0.006 0.018

7574 7455 7675 7950 25.0 71.5

37.7 37.2 38.5 36.0 0.67 NS

34.09 32.04 33.57 33.30 0.10 0.29

110.40 103.80 105.06 105.01 0.10 0.33

Plant population (lakh.ha-1) 3.33 4.44 5.00 6.66 SEm± CD(0.05)

27.6 32.9 39.2 43.2 0.32 0.92

2.41 2.61 2.75 3.01 0.006 0.018

5902 7397 7810 9544 25.0 71.5

38.3 37.8 37.5 35.9 0.67 NS

36.17 34.16 32.16 30.52 0.10 0.29

106.5 106.2 106.0 105.7 0.10 0.33

Variety x Plant population SEm± CD(0.05)

0.64 2.04

0.013 0.041

49.0 158.3

1.35 NS

0.20 0.65

0.20 NS

Table 3.10. Effect of cultivar and seed size on number of branches of groundnut in the major and minor seasons of 2007 Treatment

Major 35DAP

Variety Adepa Azivivi Jenkaah Nkosour LSD (5%) Seed size Small Medium Large LSD (5%) CV (%)

3.8.

Minor

49DAP 63DAP

35DAP

49DAP 63DAP

17 17 16 16 1.60

17 17 18 16 1.48

19 19 20 18 1.32

13 13 12 12 1.59

13 13 14 13 2.04

15 15 14 14 2.35

16 17 17 0.68 11.60

17 17 17 1.28 10.50

19 19 19 1.14 8.40

12 13 13 1.45 15.90

13 13 13 1.17 12.10

15 14 15 0.73 6.90

Temperature × Carbon Dioxide

Optimum temperature for the growth of most peanut (Arachis hypogaea L.) genotypes is near 30°C. During the 1980 growing season, temperatures ranged from 35 to 40°C for many days in peanut producing regions in USA, and severe crop losses attributable to the high temperatures and drought occurred. Experiments were initiated to determine the effect of temperature separate from drought on peanut development. Plants were

72

Physiology of the Peanut Plant

grown in controlled environments at 30/25°C, 12/12 h light/dark temperatures to obtain a population of plants with uniform development. Measurements of individual leaf areas, leaf dry weights, and seedling height were made 21 days after planting to establish plant growth state. Then temperature treatments expected to occur during the summer peanut growing season in the southwest USA, were begun. Temperature during the dark period was held constant at 22°C for all treatments. Temperatures during the light period were 30, 32 and 35°C. The 35°C treatment decreased individual leaf areas and dry weights at both 63 and 91 days after planting. Plants harvested at 91 days after planting showed reduced total leaf area and stem elongation was decreased in two experiments at 35°C. A study was undertaken to investigate the growth and development responses of shoots and roots of peanut (Arachis hypogaea L.) grown under different combinations of atmospheric [CO2] and temperature. The study comprised a longterm experiment, in which plants were grown in growth chambers for 112 days, and a short-term experiment, in which growing plants in rhizotrons for 17 days. In the long-term experiment, peanut cultivar Tainan 9 was grown in 20-L containers fitted with minirhizotron observation tubes at 5 cm soil depth and placed in controlled environment chambers under three levels of [CO2] (400, 600, and 800 μmol·mol-1) and two levels of air temperature (25/15ºC and 35/25ºC day/night temperature). In the short-term experiment, two peanut seedlings were grown in each of the 18 acrylic rhizotrons with a 6-mm thick soil layer. Rhizotrons with plants were placed in the same growth chambers as above. At 3- to 4-day intervals, rhizotrons were placed on a flatbed scanner to collect digital images from which root length and number were measured using RMS software. At 25/15ºC, plants grown at 600 and 800 μmol·mol-1 CO2 had main stems that were 24 and 44% longer than those grown at 400 μmol·mol-1, while at 35/25ºC the main stem length was similar in all [CO2] levels. At 25/15ºC, plants showed greater area and dry weight per leaf than at 35/25ºC. At harvest, high temperatures significantly reduced total leaf area to 574 cm2 for 35/25ºC compared with 921.2 cm2 for 25/15ºC. Specific leaf area at low temperatures was 22% lesser than at higher temperatures. The above ground biomass was increased by elevated CO2 in both temperature treatments. At high temperatures, above ground biomass was 56%, 24%, and 16% higher than at low temperature at [CO2] of 400, 600 and 800 μmol· mol-1, respectively. Pod dry weight increased with increasing [CO2] at 25/15ºC, but was not different among [CO2] levels at 35/25ºC. At 25/15ºC, pod dry weight was 50% higher than at 35/25ºC. As the temperature increased from 25/15ºC to 35/25ºC, pod dry weight was reduced by 40% at 400, 53% at 600, and 54% at 800 μmol mol-1 CO2. High temperature produced more root length in the containers, whereas low temperature did in the rhizotrons. There were significant interactions between temperature and [CO2] for their effects on the main stem length and the above ground biomass. High temperature enhanced growth of shoots and roots, but decreased pod dry weight. There was no interaction of elevated [CO2] with higher temperature on the reproductive growth, despite a tendency for beneficial temperature by [CO2] interaction on vegetative growth and total shoot dry weight. The beneficial effects of increased [CO2] on photosynthesis and growth were overwhelmed by the negative effect of high temperature on reproductive growth (Table 3.11). At the early growing period, plants growing at 35/25ºC had main stems longer than those at 25/15ºC. This result can be explained by the reports of Cox and Ong. They demonstrated that the mean optimal temperature range for vegetative growth of peanut is between 25 and 30ºC, while

73

Vegetative Growth

the optimum range for reproductive growth is between 22 and 24ºC. Elevated CO2 resulted in increased stem length at 25/15ºC, but the length did not vary with [CO2] at 35/25ºC. Individual leaf area and leaf dry weight increased with increasing [CO2], while specific leaf area decreased as CO2 increased in both temperature treatments. Plants grown at 25/15ºC had larger leaf areas and dry weights, but smaller SLA than plants grown at 35/25ºC. These results indicated that plants grown at higher temperatures have thinner leaves due to fewer cell layers, which leads to higher SLA (Wolfe and Kelly, 1992). In contrast, a decrease in SLA has been reported at elevated CO2 levels, which leads to extra palisade layer development (Mousseau and Enoch, 1989), increased mesophyll cell size (Conroy et al., 1986), and increased internal surface area for CO2 absorption (Radoglou and Jarvis, 1990). There were significant effects of temperature and CO2 and their interaction on the aboveground biomass. Aboveground biomass increased with increasing CO2 level and it was highest in plants grown at 800 µmol·mol-1 CO2 in both temperature treatments. This increase may have resulted from increased photosynthesis. In peanut, it has been recently reported that a doubling of ambient CO2 concentration enhances leaf photosynthesis by 27% and seed yield by 30% across a range of day-time growth temperatures from 32 to 44ºC. Chen and Sung (1990) reported that field-grown peanut plants produced more biomass and higher pod yield at 1000 μmol mol-1 CO2 than at ambient CO2. Stanciel et al. Table 3.11. Vegetative growth and total biomass at harvest of peanut grown in three atmospheric [CO2] levels and two air temperature treatments CO2 (µmol·mol-1) 400 600 800 Mean LSD(T)* LSD(C)+ LSD(TxC)++ CO2 (µmol·mol-1)

Main stem length (cm) 25/15°C 30 37 43 37

35/25°C 38 37 39 38 n.s 2.9 4.1

SLA (cm2g-1)

Branch number

Leaf area (cm2pl-1)

25/15°C 35/25°C 12 11 13 11 13 13 13 12 n.s n.s n.s

25/15°C 35/25°C 805 574 1052 605 908 543 922 574 79.5 97.4 n.s

Root dry weight (g.pl-1)

Shootdryweight(g.pl-1)

25/15°C

25/15°C

25/15°C

35/25°C

35/25°C

35/25°C

400

170

203

57

76

203

317

600

155

193

63

72

239

296

800

175

212

65

69

289

336

Mean

167

203

62

72

244

316

LSD(T)*

9.4

n.s

12.9

LSD(C)+

11.5

n.s

15.8

LSD(TxC)++

n.s

n.s

22.3

* Least significant difference (LSD) at P ≤ 0.05 for comparing means among the two air temperature treatments at CO2 concentrations of 400, 600, and 800 μmol·mol-1. † Least significant difference (LSD) at P ≤ 0.05 for comparing means between the three atmospheric CO2 concentrations at day/night air temperatures of 25/15 and 35/25ºC. ‡ Least significant difference (LSD) at P≤0.05 for temperature and CO2 interaction.

74

Physiology of the Peanut Plant

(2000) also found that foliage and stem fresh and dry weights were increased as the [CO2] increased from 400 to 800 μmol· mol-1, but declined at 1200 μmol·mol-1. The increased aboveground biomass obtained in this study agrees with the findings of Chen and Sung (1990) and Sanciel et al. (2000).

3.9.

Temperature × Photoperiod

Effects of temperature × photoperiod interaction on vegetative and reproductive growth were examined in three selected groundnut genotypes by growing them in controlled-environment growth chambers with three temperature regimes (22/18, 26/22 and 30/26°C, day/night) under long (12 h, long day), and short (9 h, short day) photoperiods. The effect of photoperiod on the total dry-matter production (TDM) was significant with the genotypes producing 32–72% greater dry matter under LD than SD. Temperature × genotype interaction effects were significant, with the dry-matter production being greatest at 26/22°C and least at 30/26°C and 22/18°C in two of the three genotypes. Leaf area (LA) was greater under LD than SD at all temperature regimes. LA accounted for 76% of the variation in shoot + root dry weight (R2 = 0.76, P < 0.01). A lack of relationship between LA and pod weight or pod numbers suggested that the pod development is controlled by factors other than carbon assimilation. The temperature × photoperiod interaction was significant for root growth, with the root weight being maximal and photoperiod effects being minimal at 22/18°C, while at 26/22°C, root weight declined and photoperiod effects became prominent. Low temperature (22/18°C) affected the reproductive development by reducing the proportion of reproductive nodes in total (vegetative + reproductive) nodes. The conversion of pegs into pods, as indicated by pod to peg ratio (PPR), was lower in LD than in SD conditions. Results suggested that the PPR could be used as an indicator of genotypic sensitivity to the photoperiod in groundnut. The peanut plant is indeterminate in growth habits and day-neutral with respect to flower initiation. Temperature (Alegre, 1957; Bolhuis and Egroot, 1959; Wood, 1968; Wynne et al., 1973), and photoperiod (Fortainer, 1957; Gautreau, 1973) influence vegetative and reproductive development of this plant. Generally, peanut cultivars initiation, number of flowers per plant, and pod formation are retarded at temperatures below about 25 to 27°C. Above 30°C vegetative growth may increase, but at a higher temperature the number of flowers per plant and pod production usually decrease. Quantitative differences between cultivars occur due to the photoperiod effect. At 30°C, Alegre (1957) found that shortening the photoperiod to 9 h reduced the amount of flowering. Fortanier (1957) found that with photoperiods of 12, 16, 20, or 24 h at constant irradiance (about 450 µEm-2s-1) and temperature (32°C) differences in lengths of the main axes and number of leaves between 12 and 24 h photoperiods were small, but there was an increase in dry weight of the plant as the photoperiod was lengthened. The number of flowers increased and lengthened from 12 to 24 h while the opposite occurred for fruit formation. A photoperiod of 9 to 14 h was suggested to be optimum for rapid fruit formation while longer photoperiods delayed fruiting. With day/night temperature regimes of 30/26°C, Wynn et al. (1973) found that the three botanical types of peanuts (Spanish, Virginia, and Valancia) responded to different photoperiods by producing a higher fruit weight to plant weight ratio under short

75

Vegetative Growth

(9 h) photoperiods. Gautreau (1973) concluded from tests with a Spanish-type cultivar that total radiation received by the plants was a predominant factor in the regulation of growth and fruiting. At a constant photoperiod of 15 h the early vegetative growth of a Virginia-type peanut cultivar increased as irradiance was increased (Cox, 1978). Under favourable temperature flowering was initiated and fruiting occurred under both short and long day conditions, i.e. there were no inductive effects of photoperiod (Alegre, 1957; Fortainer, 1957; Wynne et al., 1973). The peanut plant responded to both photoperiod and irradiance, but the extent to which these change the proportion of vegetative or reproductive growth has not been well defined, particularly for mature peanut plants. Two groups of 16 plants each were grown in separate chamber with 12 h light/12 h dark cycles and the same initial irradiance (530 µEm-2s-1) until 26 days from planting when flowering was initiated. At this time the irradiance was 500 µEm-2s-1 in both chambers. The irradiance was reduced to 300 µEm-2s-1 in one chamber. Plants were harvested at 91 and 95 days (Table 3.12). Table 3.12. Effect of irradiance on vegetative development of Spanish type peanut Irradiance (µEm-2s-1)

Leaves (No)

Total leaflet area (cm2)

Main axis length (cm)

Cotyledonary lateral lengths (cm)

500

66.9

1583.6**

25.3**

32.2*

31.5**

300

69.1

1875.7

47.2

50.6

51.0

** Means significant at 1% level.

3.10.

Defoliation

Wilkerson et al. (1984) stated that defoliation altered the normal partitioning of photosynthates between plants parts in “florunner” peanuts. The loss of leaf area at any stage of the crop growth at various degrees of defoliation results in the reduction of net photosynthetic area, the higher the degree of defoliation the more the loss in seed yield and production. Wilkerson et al. (1984) observed that defoliation imposed on florunner peanut plants uniformly and non-uniformly at 9, 12 and 16 weeks after planting and harvested at 2,4 or 6 weeks following treatment, all resulted in lower stem weight to length ratio, lower pods numbers and weights and equal or higher leaf numbers and weights. Studies on defoliation of peanuts have suggested that severe defoliation after the early vegetative stage reduced yields (Campbell, 1978; Turner, 1982; Santos and Sutton, 1983). Enyi (1975) reported that in groundnut the greatest reduction in seed yield occurred when the plants were defoliated during early pod stage. Defoliation reduced nodules formation and nitrogen fixation in a “Virginia” and a “Spanish”-type peanut (Arachis hypogaea) (Osman et al., 1983). Williams et al. (1976) noticed that in groundnut, the plant growth rate was reduced to 82 and 59% due to 50 and 75% defoliation respectively. Defoliation decreased both stem and pod growth rates. David and Timothy (1991) observed that foliar-feeding insects or foliar-fungal pathogens significantly reduced vegetative characters and yield such as height, number of leaves, leaf area, leaf dry weight, number of pods, pods dry weight, stem dry weight and stand density in “florunner”, “sunrunner” and “southernrunner” peanuts (Arachis hypogaea L.). The reduction in plant dry weight, nodule numbers,

76

Physiology of the Peanut Plant

nodules dry weight and nitrogenase activity was most severe for the 100% defoliation treatment in groundnut (Osman et al., 1983). Two growth chamber experiments were conducted to evaluate the effect of defoliation on the growth and development of peanuts. In Experiment one, peanuts (Arachis hypogaea L., cv. Flogiant) were uniformly defoliated by hand at 25, 50, 75 and 100% at the pod or the pod-filling stage. In Experiment two, 25 and 75% of the young leaves were removed by hand at the vegetative or the pod-performing stage. Results of these studies showed that defoliation lowered the leaf, stem, root, peg and pod masses. The growth stage of the plants determines the variables that would be most affected. The reduction in mass due to defoliation lasted for four to six weeks when defoliation occurred at the pod-forming or the pod-filling stage and about two to three weeks when 25% of the young leaves were removed at the vegetative stage. Plants defoliated at the 75% level gave greater priority for peg and pod growth than plants defoliated at the 25% level. Defoliated plants have the lighter pods at harvest. This experiment was designed to study the response of peanut (Arachis hypogaea L.) canopy carbon dioxide exchange rate (CER) to degrees of foliage loss at different dates throughout the season. Peanut plots were manually defoliated to 25, 50 and 75% on different dates during the season for comparison with control plots (0% defoliation). Weekly, CER was measured on control plots and on plots which had been defoliated at different dates. Canopy CER was initially reduced from 45 to 70%; then 75% defoliation reduced leaf area index (LAI) to about 1.0, but subsequent measurements revealed considerable CER recovery. Recovery of CER was related to two mechanisms, leaf area production and re‐adaptation of previously shaded leaves to the full sun. The re‐adaptation of leaves to full sun was most apparent from the inability of recently‐defoliated canopies to use all the light they intercepted for CER, but after 1 to 2 weeks, efficiency of utilizing photosynthetically active radiation improved without an increase in LAI. The re‐adaptation process appeared to be related to increasing specific leaf weight. Recovery mechanisms, especially leaf production, diminished as the peanut plants matured and progressed into pod set and pod fill. Leaf area index increased after early defoliations; however, the rate of LAI increase paralleled that of control plots. For defoliations late in the season, leaf growth ceased and was not stimulated as a result of defoliation. Four groundnut (Arachis hypogaea L.) varieties (SAMNUT 21, SAMNUT 22, SAMNUT 23 and SAMNUT 24) were investigated for the response of their vegetative parameters to defoliation treatments. The plants were subjected to five levels of defoliation: 0 (no defoliation), 25, 50, 75 and 100% at five weeks after planting (WAP). The treatments were laid out in a completely randomized design with three replications. The plants were sampled at 4, 7 and 10 WAP for assessment of growth parameters (plant height, root and shoot dry matter, root nodule count). The results of this study revealed that 75% defoliation increased plant height in most varieties at 7 and 10 WAP. The control and 25% defoliation were found to increase root nodules, shoot and root dry matter, and shoot and root relative growth rate (RGR) in most varieties. The 75 and 100% defoliation levels were found to significantly reduce vegetative growth parameters except plant height (at 75% defoliation) in groundnut varieties. The results also showed that varieties SAMNUT 22 and SAMNUT 21 exhibited higher values in vegetative growth parameters than the other varieties, while SAMNUT 23 which showed the lowest values gave a good indication of tolerance to

77

Vegetative Growth

defoliation especially at 25%. In conclusion, the impact of defoliation on vegetative growth parameters varies among the groundnut varieties and with defoliation levels (Table 3.13). Table 3.13. Effect of defoliation on root dry matter and root nodule count of four groundnut varieties Treatment

Root dry mass (g) 4 WAP

SAMNUT 21 0 25 50 75 100 Mean SEm± SAMNUT 22 0 25 50 75 100 Mean SEm± SAMNUT 23 0 25 50 75 100 Mean SEm± SAMNUT 24 0 25 50 75 100 Mean SEm± Interaction (VxD)

3.11.

Root dry mass (g) 7 WAP

Root dry mass (g) 10 WAP

Nodules/plant 10 WAP

0.23ab 0.13b 0.13b 0.33a 0.10b 0.19 0.05

1.00a 0.80a 0.70a 0.60a 0.63a 0.75 0.15

2.03a 1.70a 1.37a 1.03a 1.43a 1.51 0.34

137.33a 103.67ab 91.67ab 74.33b 78.00b 97.00 15.48

0.10a 0.13a 0.10a 0.10a 0.10a 0.11 0.02

0.97a 0.87a 0.90a 0.80a 0.60a 0.83 0.17

1.47ab 1.87a 1.40ab 1.17bc 0.80c 1.34 0.17

108.00b 160.67a 136.00ab 103.33b 100.67b 121.73 12.28

0.10a 0.17a 0.13a 0.10a 0.10a 0.12 0.02

0.57a 0.63a 0.60a 0.37a 0.47a 0.53 0.10

0.67a 1.07a 0.87a 0.50a 0.53a 0.73 0.22

116.33ab 128.33a 111.67abc 83.33bc 75.67c 103.07 11.74

0.10a 0.10a 0.13a 0.10a 0.10a 0.11 0.02 S

0.87a 0.33b 0.90a 0.40b 0.30b 0.56 0.15 NS

1.10a 0.83ab 0.70abc 0.60bc 0.33b 0.71 0.14 NS

145.00a 135.00ab 88.33bc 87.67bc 62.33c 103.67 15.90 NS

Salinity

The effects of different salinity levels on root fresh and dry weight are presented in Table 3.14. Root fresh and dry weight showed significant differences (P = 0.003 and 0.001) increased the rate of flower production and the total number of flowers produced per plant in several genotypes by more than 300%. However, thermal rates of flower production (flowers/GDD) were significantly (P > 0.05 to 0.001) less at 40/28°C than at 30/24°C in all genotypes except ICGV 86021 and ICGV 86015, where the rates were the same (P > 0.10), and 47-16, where the rate was significantly (P > 0.001) higher. Across both temperature regimes there was a good relation (r = 0.77) between total flower number and mainstem leaf number (Fig. 4.7), and greater flower numbers at 40/28°C can be partly explained by the increase in node (leaf) and axillary bud number per plant at this temperature (Bagnall and King, 1991). Increased flower production at a high temperature is also associated with reproductive sterility; groundnut flowers are borne on an inflorescence, but usually the first fertilized ovary exerts dominance over later formed flowers on the inflorescence, preventing their development. However, a high temperature prevents fruit set (see below) and so flowers continue to develop on each inflorescence resulting in an exponential increase in flower production, i.e. both rate and duration of flowering are increased by a high temperature. Similar observations have been made by Bolhuis (1958) from studies in which flowers were removed every day.

Fig. 4.7. Cumulative flower number/plant against days from sowing (DAS) in the Spanish genotype ICGV-SM 87003 (*, +) and the Virginia genotype 47-16 (D, E) grown at 30/24°C (open symbols) and 40/28°C (closed symbols).

Reproductive Development

93

The flowering in groundnut commences 20-30 days after emergence (DAE) depending upon genotypes and environment and most of the flowers appear in between 35-70 DAE. However, ‘Makalu Red’ flowers at 55 DAE in Rhodesia (Williams et al., 1975a). There are usually four stages of flowering, whose pattern depends upon the cultivar and the environment. At the first stage only a few flowers are produced followed by a stage of rapid flowering, peak flowering is reached at the third stage and decline in the fourth stage. The groundnut produces much more flowers than pods developed. Nearly 40% of the flowers fail to develop, while another 40% produce only pegs. The ratio of pods produced to flowers is generally 1:7 and removal of flowers results in prolonged flowering. The flower bud is only 6-10 mm long a day before anthesis, during the day hypanthium elongates to 10-20 mm, but at night elongation is faster and at the time of anthesis the buds are 50-70 mm long. The buds generally open at the beginning of the light period, but may be delayed due to cold or wet weather. The anther may dehisce 7-8 h before the opening of flowers. On warm and sunny days, the flowers wither within 5-6 h after flowering leaving the ovary and style which remain turgid after the day of the anthesis. As the daily mean temperature rises from 20 to 30°C the number of days required for the first flowering is reduced from 38 to 25 days in sub species hypogaea (Virginia runner) and from 35 to 24 days in sub species vulgaris (Valencia and Spanish) (Ono, 1979). The number of flowers produced per plant varies among genotypes and between botanical groups. Runners produced more flowers per plant and also had a longer duration of flowering than the erect ones. The number of flowers produced per plant ranged from 40-250 in runners and 100-150 in bunch types.

4.1.2.

Pollination

Flower buds were seen one day prior to anthesis. The buds developed rapidly and most buds reached the maximum size by the early afternoon (1430 h Malaysian Standard Time). Flowering occurred on the morning of the following day. Flowering was observed to begin from 0645 h on a fine morning with maximum blooming around 0730 h. On dull and wet mornings, it was delayed by half an hour (Fig. 4.8). In the improvement of groundnuts (Arachis hypogaea L.) information pertaining to the flowering, pollination, and seed set of the crop relevant to the local environment is necessary. For example, while it is generally known that groundnut flowers bloom early in the day, the time of flowering differs from place to place. Flowering has been reported to occur as early as 0300 h in the Philippines (Jose and Guevara, 1951) and between 0600 and 0800 h in India (Dainiel and Thulasiada, 1976). Dehiscence of the anthers has been reported to occur early, prior to the flower opening which enables self-pollination to take place within the closed petals (Culp et al., 1968). Groundnut, being a self-pollinated crop would have little need for insect assistance. However, it has been reported that only less than 10 per cent of the numerous flowers produced, develop into mature pods (Othman, 1979; Lim et al., 1980). The low pod set may be due to inefficient self-pollination. In some legume crops (e.g. Medicago sativa, Vicia faba) the flowers need to be tripped to effect pollination (Armstrong and White, 1935; Lawes, 1972; Lim and Knight, 1980). Pod set can also be influenced by the efficiency of fertilization. The distance between the stigma and the ovary ranges from 2.0 to 6.0 cm (Othmanhamdan, 1979) and slow pollen tube growth particularly where if the style is long it can influence the success of fertilization.

94

Physiology of the Peanut Plant

Fig. 4.8. Time of opening of peanut flowers

The dehiscence of the anthers was indicated by the presence of pollen within the keel petals. The anthers were found to dehisce beginning from 0545h until 0715h MST. Most anthers dehisced at 0615 h. Flowering followed about one hour after the pollen was released (Table 4.2). Table 4.2. Time of anther dehiscent and anthesis in peanuts Time (M.S.T)

Number of flowers with anthers dehisced

Number of flowers opened

0545 0600 0615 0630 0645 0700 0715 0730 0745 0800 0815

3 23 64 87 95 98 100 100 100 100 100

0 0 0 0 0 5 20 57 89 97 100

Two principal types of stigma were found in species of Arachis belonging to section Arachis and section Rhizomatosae. The different types did not correlate with sectional classification, genome groups or ploidy level, but did correlate with life forms. All the perennial species studied had very small stigmas, which were guarded by a ring of hairs and were difficult to pollinate effectively. Most of the annuals, including the cultivated peanut (A. hypogaea), had larger stigmas, which were not surrounded by hairs and which were easily pollinated. One annual species, A. spinaclava, had a stigma intermediate in many of its characteristics between the

Reproductive Development

95

other two types. Proteins, including esterase and acid phosphatase, were present in all stigmas before the flowers opened. Bud pollination produced apparently normal pollen tube growth in pistils of the wild annual A. spegazzinii and may have overcome certain interspecific incompatibilities. Various methods of pollinating stigmas of the perennial species were tried, but even the most successful produced pollen tubes in less than half of the flowers pollinated. Difficulties in pollination, due to stigma morphology, probably explain the limited seed set and poor performance as female parents in interspecific crosses which have been reported for perennial species of Arachis. The stigma becomes receptive about 24 h before the anthesis and its receptivity persists for about 12 h after anthesis (Hassan and Srivastava, 1966). However, Sastri and Moss (1982) reported that the stigma was not receptive before or after the day on which anthesis occurred. Pollen grains are smooth, oval, and sticky. Fertilization occurs about 6 h after pollination. In general, self-pollination is the rule. It has also pointed out that certain ovaries can remain dormant while others may develop in the normal way. This demonstrates that following syngamy certain ovaries remained dormant in their inflorescences for several weeks without losing their ability to develop pegs and eventually to produce mature pods and seeds. All these results demonstrate that 33°C is either directly or indirectly responsible for creating an unfavourable condition for ovary development. The viability of pollen originating from plants grown at constant temperatures of 24°, 28°, 33°C as well as in a greenhouse, was studied. Pollen germination and growth studies were performed

Fig. 4.9. Germination percentages obtained at six different temperatures, with and without H3BO3 (50 ppm) in the germination medium (Pollen collection took place during the third week of flowering; germination time 90 minutes)

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in media containing sucrose, agar and water, to which in some instances boric acid was added. All these studies were conducted in cabinets at temperatures ranging from 21° to 36°C. Plant growth temperature was found to exert great influence on pollen viability. At a growth temperature as high as 33°C, the viability of pollen was much lower than at the other growth temperatures. Addition of boron to the germination medium was usually found to stimulate both germination and pollen tube elongation. However when pollen viability dropped below a certain level, the stimulating effect of boron was found to be negligible. While an addition of boron to the germination medium had definite germinating and growth promoting properties, the same effect was not obtainable by boric acid plant sprays. The sampling date as well as the time of day when pollen was collected had a distinct effect on pollen viability. In all cases it was found that the viability of pollen was more or less impaired after an illumination period of 10 hours. The higher the temperature during illumination, the quicker the pollen viability decreased. The growth temperature during the vegetative phase had no influence on the behaviour of pollen, nor was the character of pollen seriously influenced by the temperature during the day of flowering. Further results indicated, however, that the behaviour of pollen is mainly dependent on the temperature 36 to 96 hours preceding the actual opening of the flowers. By studying the effect of growth temperature on pollen shed, the results brought to light that almost four times as many pollen grains were set free at 24°C at the control than at 33°C.

4.2.

Reproductive Stages

Determination of reproductive stages is based upon visually observable events related to flowering, pegging, fruit growth seed growth, and maturity (Stages are defined in Table 4.3). Reproductive stages are similar to those developed for soybean by Fehr and Caviness (1977), except that R2 was re-defined as “beginning peg” and the R9 stage was added to denote “over-mature pods”. Days from planting to specific R stages are given in Table 4.3 for Starr and Florunner. Table 4.3. Reproductive stages of growth R1

Beginning bloom

One open flower at any node on the plant

R2

Beginning peg

One elongated peg (gynophores)

R3

Beginning pod

The peg in the soil with ovary turned swollen at least twice the width of the peg

R4

Full pod

One fully expanded pod, to dimensions characteristic of the cultivar

R5

Beginning seed

One fully expanded pod in which seed cotyledon growth is visible when the fruit is cut in cross-section with a razor blade (Past the liquid endosperm phase) One pod with cavity apparently filled by the seeds when fresh

R6

Full seed

R7

Beginning maturity One pod sharing visible natural coloration or blotching of inner pericarp or testa

R8

Harvest maturity

One third to three fourths of all developed pods have testa or pericarp coloration. Fraction is cultivar dependent, lower for Virginia types of the testa

R9

Over-mature pod

One undamaged pod showing orange-tan coloration

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For populations, V stages can be averaged if desired. Reproductive stages should not be averaged. An R stage should remain unchanged until the date when 50% of the plants in the sample demonstrate the desired trait of the next R stage. The timing of a reproductive stage for a given plant is set by the first occurrence of the specific trait on the plant without regard to position on the plant (Table 4.4). Table 4.4. Days from planting to specific reproductive growth stages for Starr and Florunner peanut cultivars at Gainesville, FL in 1979 Reproductive elapsed time (days) from planting Stage Starr Florunner R1 31 31 R2 39 42 R3 46 51 R4 52 60 R5 57 62 R6 67 74 R7 80 93 R8 119 129 R9 NA 123

The number of flowers, pegs, and pods are the most important yield components that affect the yield potential of groundnut (Awal and Ikeda, 2003). After pollination and fertilization, the corolla closes, the calyx tube bends, the flower withers, and then the peg is formed. When the peg reaches its maximum depth in the soil it stops growing and the pod begins to develop. The pod continues to grow, reaching its maximum size after the penetration of the peg into the soil. It is well known that groundnut produces more flowers than the plant can sustain and develop into pods (Rao and Murty, 1994) and less than 15-20% of flowers produce mature pods (Lim and Hamdan, 1984). Caliskan et al. (2008) reported that dry matter accumulation in each part of the plant continues until maturity, although the accumulation rate differs according to plant age and genotype (Fig. 4.10 and Table 4.5). Table 4.5. Some reproductive growth parameters of eight peanut genotypes grown in Hatay,

Turkey, in 2001 and 2002 Genotypes

PI 269084 PI 355276 75/1073 Edirne NC9 Osmaniye2005 Com NC7 Mean LSD (5%)

Total flower (no. plant-1)

Total peg (no. plant-1)

Total pod (no. plant-1)

Flower to peg ratio (%)

2001

2002

2001

2002

2001

2002

2001

2002

597 733 864 676 756 342 1292 1516 847 18.1

804 872 791 866 695 471 828 906 779 14.8

146 182 203 209 208 145 214 265 197 5.2

186 187 169 173 178 144 186 207 179 5.9

54 45 60 58 54 43 54 64 55 4.8

50 48 63 51 47 46 57 65 53 4.3

24.3 24.7 23.7 31.3 27.7 42.3 17.0 17.3 26.0 1.2

23.0 21.3 21.3 20.0 25.7 30.7 22.7 23.0 23.5 1.0

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Fig. 4.10. Ontogenic changes of number of flowers per plant in peanut

4.3.

Drought

Drought is one of the major environmental factors that affect the groundnut yield and food safety worldwide. The severity of drought depends on the stage of crop development, the duration of the stress period and the magnitude of drought. Droughts affect membrane lipids and photosynthetic responses (Lauriano et al., 2000). The effects of soil moisture deficit on groundnut have been extensively studied and it has been concluded that water stress at the vegetative or early flowering stage is not detrimental and actually increases the yield. The dry spells during critical pheno phases like flowering and post flowering stages severely affect the morphological and physiological parameters and the yield substantially (Nautiyal et al., 1999). A pot culture study was conducted in Tamil Nadu Agricultural University, Coimbatore, to assess the morphophysiological characters of different groundnut genotypes viz., CO 7, COGn 4, TMV 7 and TMVGn 13 to water stress at different flowering phases viz., Pre Flowering Drought (PFD) between 15-30 DAS, Flowering Drought (FD) between 35-50 DAS and Post Flowering Drought (PoFD) between 75-90 DAS by withholding irrigation and a control was also maintained with irrigation to field capacity for comparison. Observations on various morphological (Plant height and Leaf area) and physiological aspects (Relative water content, SPAD chlorophyll Index and Photosynthetic rate) were studied during stress periods and after stress recovery. Among the treatments higher values of morphological and physiological parameters

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were observed under PFD after recovery and CO 7 performed better followed by TMV 7, TMVGn 13 and COGn 4. Peanut (Arachis hypogaea L.) displays a unique growth process called geocarpy during its life cycle. The unique process is that the peanut produces flowers aerially, and subsequently buries the fertilised ovules into the soil for the fruit and seeds to develop and mature underground (Smith, 1950; Shushu, 1990). The specialised downward growth organ that carries and sows the young seeds into the soil is known as the gynophore. The morphology and anatomy of peanut gynophores have been well established (Ziv, 1975; Periasamy, 1984; Pattee, 1987). Following fertilisation, the gynophore begins to form an intercalary meristem into a pointed stalk-like structure. Embryo development tentatively stops at this time and the gynophore elongates and bends downward to the ground. When its tip goes into the soil, the gynophore stops elongation and its tip containing the developing embryo begins to swell and elongates on the dorsal side. Finally, the tip forms mature peanut seeds in the horizontal orientation. Peanut (Arachis hypogaea L.) produces flowers aerially, but the fruit develops underground. This process is mediated by the gynophore, which always grows vertically downwards. The genetic basis underlying gravitropic bending of gynophores is not well understood. To identify genes related to gynophore gravitropism, gene expression profiles of gynophores cultured in vitro with tip pointing upward (gravitropic stimulation sample) and downward (control) at both 6 and 12 h were compared through a high-density peanut microarray. After gravitropic stimulation, there were 174 differentially expressed genes, including 91 upregulated and 83 downregulated genes at 6 h, and 491 differentially expressed genes including 129 upregulated and 362 downregulated genes at 12 h. The differentially expressed genes identified were assigned to 24 functional categories. Twenty pathways including carbon fixation, aminoacyl-tRNA biosynthesis, pentose phosphate pathways, starch and sucrose metabolism were identified. The quantitative real-time PCR analysis was performed for validation of microarray results. This study paves the way to better understand the molecular mechanisms underlying the peanut gynophore gravitropism. The growth of peg is geotropic until it penetrates the soil upto 5-7 cm. The tip then becomes diageotropic and the ovary starts developing into fruit. The peg begins rapid geotropic elongation and starts to penetrate the soil for about 7 to 14 days after fertilization of the flower. Following soil penetration, the ovary at the peg tip reinitiates growth and a groundnut fruit is formed. If the pegs fail to contact and enter the soil it usually withers; however, in humid conditions some cultivars belonging to fastigiata occasionally form underdeveloped small and green aerial pods. The portion of the peg in the soil is white, while the aerial portion of the peg normally develops a pink to purple colour due to anthocyanin pigments which is cultivar dependent and very much influenced by the environment. The thickness of the peg is 1-2 mm, the cultivars of subspecies fastigiata have thicker pegs than hypogaea. The peg growth is affected by relative humidity and on an average, the daily peg growth is 0.62 cm at 100% RH, and only 0.02 cm at 57% RH. But generally, the RH of the air is quite low in many groundnut-growing regions at the start of flowering and at the time of pegs penetrating the soil. In China, Jun and Ke (1988) controlled peg growth by adopting a crop technique `A n M’ by transforming the ridge into an arrow bank, which gave the base of the plant better ventilation and the distance between the pegs and the surface of the soil increased resulting in delayed peg penetration. This technique increased the accumulation of photosynthetic products in the ovaries, and increased the diameter of the vascular bundles in pegs and the pod yield by more than 20% in China.

100

4.4.

Physiology of the Peanut Plant

Temperature

Research in controlled environments has shown that peanuts are particularly sensitive to short episodes (≤6 d) of high air temperature (day/night; 38/22°C) starting from 6 d before until 15 d after flowering, and that the magnitude of sensitivity is related to the number of floral buds which are exposed to high temperature in the period before anthesis (Vara Prasad et al., 1999a). Warm days (>34°C) and nights (28°C) reduce fruit-set due to reductions in pollen number and pollen viability (Vara Prasad et al., 1999b). This reduction in pollen number and viability may be a consequence of high temperature effects on micro-sporogenesis (Warrag and Hall, 1984). In addition, peanuts are sensitive to high temperatures during the first 6 h of the daylight period (Vara Prasad et al., 2000a), which strongly suggests that pollination and/or fertilisation are also adversely affected by high temperature. Therefore, both pre- and post-anthesis stages of floral development may be sensitive to high temperatures. The time taken from microsporogenesis to pollination and fertilisation varies from 3 to 6 d in peanut (Martin et al., 1974; Xi, 1991). Peanut flowers typically open early in the morning, pollination occurs just before or during anthesis, and fertilisation is completed 5–6 h later, after which the flower withers (Lim and Gumpil, 1984; Bolhuis et al., 1965). Embryo formation occurs 2–3 d after fertilisation and a visible peg is typically formed about 6–9 d after fertilisation (Smith, 1956). The effects of high temperatures during the pre-anthesis (micro-sporogenesis, pollen shed and pollen viability) and post-anthesis (pollination, pollen tube growth, fertilisation and embryo development) stages of floral development may be different. In previous research (Vara Prasad et al., 1999b, 2000a), the effects of temperature during pre- and post-anthesis stages were confounded, as individual flower buds were not tagged and the duration of stress was 6 d. Therefore, this research was conducted to identify whether certain stages of pre- and post-anthesis floral development were more sensitive to high temperature than others, and to understand the mechanisms affected by high temperature stress. Peanut (Arachis hypogaea L.) crops are often exposed to day temperatures >35°C for short periods during flowering, resulting in lower yields. Research was conducted to study and quantify the effects of short episodes (1–6 d) of high temperatures during the pre- and post-anthesis stages of floral development on fruit-set, pollen viability, germination and tube growth. Plants of peanut cv. ICGV-86015 were grown in controlled environments at 28/22°C (day/night). High daytime air temperature treatments ranging from 28 (control) to 48°C were imposed at different times between 6 d before anthesis (DBA) and 6 d after anthesis (DAA) for 1, 3 or 6 d. Floral buds or flowers were tagged at different stages to determine fruit-set. Exposure to bud (tissue) temperatures ≥39°C for 1 d significantly reduced fruit-set compared to the control at 28°C, and the magnitude of the reduction varied with the stage of floral development. Floral buds were most sensitive to high temperatures at 4 DBA and at anthesis, coinciding with micro-sporogenesis and pollination or fertilisation, respectively. The critical bud temperature at these stages was 33°C, above which fruit-set was reduced by 6%°C-1. Lower fruit-set due to high temperatures at pre-anthesis and anthesis stages were due to pollen sterility and retarded pollen tube growth, respectively (Figs. 4.11 and 4.12). The principal effect of high temperature at microsporogenesis is on pollen number (Vara Prasad et al., 1999b) and pollen viability. The viability of pollen is determined

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101

Fig. 4.11. Effect of optimum (28°C) and high (39°C) floral bud temperatures imposed for 1 d at different stages of flower bud development relative to anthesis on the percentage of flowers setting fruits (fruit-set). Vertical bars denote a 95% confidence interval for the proportion

Fig. 4.12. Effect of optimum (28°C) and high (39°C) floral bud temperatures imposed for 1 d at different stages of flower bud development relative to anthesis on (a) pollen viability (%); (b) pollen germination (%); and (c) pollen tube growth (10–4 m). Vertical bars denote standard error for the trait and are shown where they exceed the size of the symbol

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during the early stages of floral bud development (De Beer, 1963). The result that viability was lower at 4 DBA than at 1 DBA is in agreement with the observations of Gross and Kigel (1994), who reported that pollen becomes less sensitive to high temperature as it matures. The mechanism has not been studied in peanuts; however, it is known from a number of other crop species that high temperatures at microsporogenesis reduces pollen production and viability due to impaired meiosis (Warrag and Hall, 1984), incomplete anther dehiscence (Rudich et al., 1977) and the inhibition of proline accumulation in pollen grains due to degeneration of the tapetal layer (Mutters et al., 1989). It was concluded that micro-sporogenesis and pollination or fertilisation are the most sensitive stages of flower development to high temperature stress in peanut. No pollen germination occurred below 65° or above 95°F. Growth was slow at temperatures below 75° and above 90°F. Pollen was kept successfully for several days when stored in a household refrigerator. The exception is peanut, a member of Fabaceae, which develops aerial cleistogamous flowers but subterranean geocarpic fruit (pods). These modifications have evolved to provide adaptations for a particular environment and are key to the strategy for reproduction for each plant type. The key adaptive trait in peanut is the formation of a structure known as the peg. The peg develops after double fertilization due to elongation of intercalary meristematic cells present at the basal region of the ovary. The peg has the capacity for positive gravitropism to move towards and penetrate the soil to form subterranean pods. Interestingly, failure of peg penetration into the soil leads to abortion of the developing embryo and thus incurs yield loss. Therefore, production of peanut is critically dependent on generation of pegs and their penetration into the soil (Luz et al., 2011). These unique features of peanut have attracted physiologists and researchers to explore the genetic control driving the positive gravitropic growth of the peg (Moctezuma, 2003; Chen et al., 2016a). Peanut has a unique mechanism to embed fertilized ovaries of flowers into the ground through specialized organs known as the peg or gynophore. Peg is a tube like structure which is formed after successful fertilization in flowers. After fertilization, the growth of the embryo remains arrested, and the intercalary meristematic cells beneath the ovary (part of the short stalk or pedicel) start to divide rapidly which leads to the formation of pegs (Moctezuma, 1998). Therefore, some botanists and researchers also prefer to use term “gynophore”. The peg elongation stops when it buries the embryo into the soil, and afterwards the growth of the embryo resumes. Peanut subterranean pegs are crucial for the growth of the developing pod as well as the plant. Webb and Hanse (1989) demonstrated that the subterranean pegs mediated plant survival through the excision of the main stem from the root system. The subterranean peg develops root hair-like structures that facilitate absorption of adequate amounts of moisture and nutrients from the soil required for the overall plant growth. Further, the peg absorbs nutrients and moisture from soil and develops fruits in a soil but not in the water (Harris, 1949; Van der Volk, 1914). Thus, the peg is a unique evolutionary adaptive structure that facilitates peanut reproduction and dispersion within close proximity of the parent plant through less inter-specific competition. This ecological adaptation significantly increases fitness by extending the duration of pod development and protecting pods from unfavourable abiotic conditions above the ground. However, this restricts the genomic fluidity of peanut germplasm causing dilution of genetic variation; arisen mainly because of selffertilization, prolonged cultivation, and local adaptation. Positive gravitropism is a

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peculiar feature of the peg. In peanut, successful fertilization results in initiation of the peg which carries mitotically arrested cells of the embryo (Moctezuma, 2003). The emerged peg senses gravity and bends downward. Initially the emerged aerial peg does not show any changes in the developing embryo as mitotic division is arrested with the embryo remaining at the proembryo stage. Later, the intercalary meristem residing at the base of the ovary divides rapidly resulting in peg elongation and implantation of the arrested embryo into the soil. Notably, after the peg tip penetrates the soil vertically, it reorients horizontally and perceives signals that prompt the resumption of embryo cell division, facilitating geocarpic pod development (Fig. 4.13). The perception of mechanical stimulus and darkness is essential for the transformation of the peg into a pod. Without these signals the embryo aborts (Moctezuma, 2003), leading to the formation of hard lignified green-aerial pods, which can also be observed under water deficit conditions.

Fig. 4.13. Flower and peg of peanut

4.5.

Peg Development

The anatomy of the peg typically resembles the shoot (Moctezuma, 1998). However, the anatomy of the unfertilized peg, compared to the fertilized peg, varies greatly, with the former lacking starch granules (Moctezuma and Feldman, 1998; Moctezuma and Feldman, 1999b). The aerial peg consists of multicellular trichomes of five to six cells in length, of which the terminal cell elongates compared to the first four to five proximal cells (Webb and Hanse, 1989). After the peg penetrates the soil, unicellular hairs similar to root hairs develop abundantly on the subterranean peg surface. Interestingly, while the anatomy of the peg is related to the stem, its behaviour and functionality changes after soil penetration, and resembles a root, especially just after soil penetration (Moctezuma, 2003). The exclusive features of aerial pegs are smooth epidermis and the presence of numerous stomata and lenticels, gradually disappearing and becoming obscured by tufts of hairs present in subterranean peg (Webb and Hanse, 1989). The aerial peg is self-sufficient for energy production because it possesses active photosynthetic physiological structures and machinery (Zhao C. et al., 2015). Additionally, the considerable photosynthetic activity of the sub-epidermal parenchyma tissue, the presence of stomata, and high starch content are consistent

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Physiology of the Peanut Plant

with the photosynthetic properties of aerial peg (Webb and Hanse, 1989; Zhao C. et al., 2015). For instance, proteome mapping at peanut reproduction and pegging stages identified the expression of approximately 34 photosynthesis-related proteins in the aerial peg such as photosystem II type I chlorophyll a/b-binding proteins, oxygen evolving enhancer protein 1/2, rubisco activase, plastocyanin, and more, representing a sub-set of core proteins involved in photosynthesis (Zhu et al., 2013; Zhu et al., 2014). The number of these photosynthetic proteins was drastically reduced in subterranean pegs/pods. In contrast to these photosynthetic proteins, other multiple energy metabolism related proteins such as glycolytic pathway proteins – fructose bisphosphate aldolase and triosephosphate isomerase were also identified in the subterranean peg. Similarly, proteomes of developing pegs revealed large-scale expression profile changes in the proteins related to fatty acid pathways: biosynthesis, elongation, and formation of unsaturated fatty acids (Zhu et al., 2014). This study also highlighted the differential expression of proteins involved in the biosynthesis of auxin, ethylene, and gibberellin and hormone signalling during pegging. Additionally, another recent proteome study emphasizes involvement of brassinosteroid during peg development (Zhao C. et al., 2015). The developing peanut peg also produces a significant amount of GA to promote cell elongation that ultimately facilitates peg elongation. Afterwards, GAs concentration declines once the peg penetrates the soil and buries (Shushu and Cutter, 1990). It was demonstrated that a combination of GA3 and auxin could restore peg

Fig. 4.14. Probable molecular regulatory mechanism involved during peg development, gravitropic bending, elongation and pod development. ABA - abscisic acid; ABRE, ABA responsive element; ABF, ABRE - binding factors; ACO - ACC; oxidase; ACS - ACC synthase; APX - Cytosolic ascorbate peroxidase; COP1, E3 ubiquitin – protein ligase constitutive photomorphogenic 1; CIP7, COP1 - interacting protein 7; CO - constans; EIN3 - ethyleneinsensitive3; GA - Gibberellic acid; GI - gigantea; GPx - glutathione peroxidase; IM,; LEA - late embryogenesis abundant; PAs - polyamines; PHY-B - phytochrome; PKS1 - phytochrome kinase substrate 1; SAM - S-adenosylmethionine; V- H(+) - ATPase subunit A - Vacuolar - H(+) - ATPase subunit A; V- H(+) - ATPase B-subunit - Vacuolar - H(+) - ATPase B-subunit

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growth in excised pegs as compared to the intact ones (Shushu and Cutter, 1990). However, unlike auxin, which affects only young peg cells, GA3 can promote cell elongation across the entire length of the peg (Shushu and Cutter, 1990). In contrast, cytokinin regulates cell division of the juvenile peg structure. As described earlier, during peg development cytokinin accumulates at the early stages to facilitate cell division (Moctezuma, 2003). Interestingly, kinetin (a type of cytokinin)-eg elongation in the dark, does not occur in decapitated pegs, suggesting that auxin and cytokinin interact during peg elongation (Shushu and Cutter, 1990). However, application of GA3 restores elongation of kinetin treated decapitated peg, suggesting that GA3 may act by elongating the newly dividing cell, facilitated by kinetin application. Ethylene levels tend to become elevated during peg burying or post-burying, and the white peg developed after soil penetration demonstrates the “triple response” phenotype shown by etiolated pea seedlings exposed to higher ethylene: thick and short hypocotyl, radial swelling, and horizontal growth habits (Shaharoona et al., 2007). Ethylene also regulates cell division by regulating a gene encoding microtubulestabilizing protein WAVE-DAMPENED2-LIKE5 (WDL5), a member of the WAVEDAMPENED2 (WVD2) protein family, which reorganizes cortical microtubules during cell elongation (Sun et al., 2015). Interestingly, over expression of WVD2 in Arabidopsis also results in “triple response” phenotype in seedlings (Yuen et al., 2003). In peanut, the role of the microtubule-associated protein has been reported through a combined transcriptome and proteome approach (Zhao C. et al., 2015). Additionally, Shlamovitz et al. (1995) demonstrated that application of the ethylene inhibitors aminooxyacetic acid and silver thiosulphate significantly affect the pod growth without altering the percentage of total pod formation (Shlamovitz et al., 1995). It is, thus, plausible that a peanut peg maintains a lower ethylene level in both the aerial peg and the subterranean pod to facilitate cell division and elongation suggesting that a basal level of ethylene might be required to maintain normal cell division, elongation and swelling of the developing peg . Abscisic acid (ABA) controls cell division and elongation of a developing embryo (Da Silva et al., 2008). Therefore ABA-deficient mutants have increased seed size and weight due to increased cell numbers in the embryo (Cheng et al., 2014). In peanut, the aerial peg exclusively produces high levels of ABA, which progressively decline after the peg penetrates the soil and during pod development (Shlamovitz et al., 1995). Ziv and Kahana (1988) have evaluated the response of the excised embryo in the dark and found that embryo development was arrested by the exogenous application of ABA. Therefore, it cannot be ruled out that high levels of ABA found in the aerial peg arrests the cell division of the developing embryo. Therefore, it is possible that in light conditions, the aerial peg maintains a high level of ABA to arrest embryo growth. However, in the dark, a signal is perceived for resuming embryo growth via supressing ABA levels (Fig. 4.14). In plants, photoreceptors perceive light and translate it into signals controlling various functions including phototropic response and reproduction (Gupta et al., 2014). In fact, an extended exposure to light can reduce the flowering and peg numbers as well as pod formation by limiting reproductive development of peanut (Quamruzzaman et al., 2018). Photoreceptor phytochromes play a central role in photomorphogenesis and are also likely to be involved in gravitropism. It was found that far-red and darkness can induce pod development by supressing peg elongation, which suggests that phytochromes may control peg and pod development (Moctezuma, 2003). Further, darkness induces loss of flavonoids via reduced gene expression and Chalcone synthase (CHS), a key enzyme in the flavonoid biosynthesis pathway (Xia et al., 2013),

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Physiology of the Peanut Plant

facilitates lignin development in the developing pod via diverting substrate to lignin biosynthesis. Genotypes had significantly different reproductive growth parameters. Total number of flowers per plant was negatively correlated with the percentage of flowers turned to pegs and pods, whereas the percentage of flowers turned to pegs and pods was positively correlated with pod yield. The highest pod yield was obtained from cv. Osmaniye2005, which had the lowest number of flowers per plant and the highest percentage of flowers turned to pods. The results of the current study showed that percentage of flowers turned to pegs and the percentage of pegs turned to pods were the most promising generative plant characteristics that could contribute to seed yield increase in groundnut production in a typical eastern Mediterranean climate. Depending on the genotype, peg formation (R2) started approximately 45 to 55 days after emergence in both years. The number of pegs increased steadily over the course of the two weekly sampling periods, and slowed down at the final harvest in 2001 and 2002 (Fig. 4.15). NC 7 produced the greatest number of pegs per plant, while the lowest number of pegs was obtained from Osmaniye2005, in both years (Fig. 4.15).n Correlation coefficients between some growth parameters and yield components are given in Table 4.6. The flower to peg ratio was negatively correlated with total number of flowers, pegs, and pods, and was positively correlated with pod yield (r = 0.347), but the correlations were not significant. According to our results, the flower to

Fig. 4.15. Changes with time in the total number of pegs of eight peanut genotypes grown in

Hatay, Turkey, in 2001 and 2002y

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pod ratio was negatively correlated with the number of flowers, pegs, and pods at final harvest, and was significantly correlated with the number of flowers and pegs. Total number of pods was positively correlated with the peg to pod ratio, but the correlation was not significant. In addition, pod yield had stronger correlations with the flower to peg ratio and the flower to pod ratio than to the peg to pod ratio (Table 4.6).t Table 4.6. The correlation coefficients of some growth parameters and yields for eight peanut genotypes grown in Hatay, Turkey, in 2001 and 2002 Component

Total flower number Total peg number Total pod number Pod yield

Flower to peg ratio

–0.814**

–0.520*

–0.478

0.347

Flower to pod ratio

–0.857**

–0.693**

–0.302

0.338

–0.433

–0.577*

0.265

0.038

Peg to pod ratio *P ≤ 0.05; **P ≤ 0.01

The groundnut plant has an indeterminate growth habit. This means that the flowering and fruiting of the crop occurs over a long period of time. Flowering of the groundnut genotypes occurred over a period of about 17 weeks, with peak production at around 13 weeks after sowing. At the end of the growing season, therefore, some of the pods were ready to harvest while some were not. Those immature and economically unacceptable pods waste a great amount of carbohydrates; therefore, cultivars with fewer flowers, and higher flower to peg ratio and peg to pod ratio are most suited to the Eastern Mediterranean region, and these traits could be used as selection criteria to improve pod yield in breeding programs. Higher number of flowers, pegs, and podsplant-1 may not always reflect higher yield, as the highest yielding cultivar, Osmaniye2005, had the lowest total number of flowers, pegs, and pods, while it had the highest flower to peg ratio and peg to pod ratio among the genotypes. The results of a study show that the flower to peg ratio, flower to pod ratio, and peg to pod ratio could be considered promising traits to enhance yield, and they may be used in breeding programs as selection criteria since they were significantly and positively correlated with pod yield. In groundnut, the flowering and fruiting are of an indeterminate type, which generally has an effect on pod yield and quality. The ideal type of groundnut would be one that sets all its young fruits within 4-5 days and spends the rest of the growing season filling them. At the same time there are two peaks of flowering in normal sown crops in many parts of India causing higher reproductive efficiency than the late sown crop which has no peak. More numbers of flowers produced upto 45 DAS cause a greater reproductive efficiency and higher pod yield. The continuous removal of flowers increased the daily flower production from 3 to 20. Talwar et al. (1992) found that vegetative growth was increased by flower removal in groundnuts cv. M 13, pod number increased by removing early phase (0-4 weeks) flowers and decreased by removing middle (5-9 weeks) phase flowers, or middle + late flowers (9 weeks to maturity). Gynophore removal in the late phase increased numbers of mature pods. Removal of pegs for one week from the onset of flowering increased root biomass and aerial pegs but reduced pod yield (Narayanan et al., 1984). The virginia cultivars compensated better for initial lost pegs than bunch types. Several workers have studied the temperature effects, on reproductive development,

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and data on the temperature dependence of peg and pod formation were obtained. The optimum temperature for peg formation ranged from 20⁰C to 32/23°C. In general, the spanish cultivars tend to develop a reproductive yield component (pegs and pods) sooner than valencia and virginia but considerable variation within botanical types especially the virginia also exist due to different phenology. The number of pegs and pods and harvest index at 35 and 65 days after flowering correlated with day length during emergence to flowering (Bell et al., 1991b). However, in these studies the day length effects could not be separated into the effect of photoperiod, irradiance or heat unit accumulation. Bell et al. (1991c) in their further studies contradicted the results and reported that the number of pods and pegs and total pod weight were reduced in long (16 or 17 h) photoperiods. Groundnut (Arachis hypogaea L.) is an autogamous and indeterminate legume crop. There is generally a big gap between the number of flowers produced and the number of mature pods formed from them (reproductive efficiency). The reproductive efficiency, defined as the percentage of viable reproductive tissues (Pattee and Young, 1992), in groundnut is assessed by flowers, pegs, mature pods, immature pods, sound kernels and unsound kernels (Coffelt et al., 1989). Less than 10 per cent of the flowers produced develop into mature pods (Othmanhamdan, 1979; Lim et al., 1980). There are four varietal forms of groundnut cultivated in India viz., Valencia (ssp. fastigiata var. fastigiata), Spanish bunch (ssp. fastigiata var. vulgaris), and Virginia bunch and Virginia runner (ssp. hypogaea var. hypogaea). Maturity differences between groundnut genotypes belonging to different varietal forms have been observed (Chunilal et al., 1997; Ghosh et al., 1997). The study was undertaken to understand the reproductive efficiency and flowering behaviour of different forms of cultivated groundnut, and to know the extent and scope of conversion of flowers into pegs, pods and kernels. The genotypes included in the experiment were taken two from each habit group. VL genotypes possessed high reproductive efficiency followed by SP in converting maximum flowers into pods. The correlation study indicated that flowers produced up to 50 DAS had a good association with a mature pod number and should be given due attention to select high yielding genotypes with a higher proportion of mature pods. Relative humidity recorded at 2 PM had an impact on flower production. The maximum temperature was negatively correlated with flowers for all the genotypes of VL and SP but it was positive with Virginia types except Somnath. The association of rainfall with flowers was mostly negative. The stepwise regression analysis revealed the importance of low temperature for flower production in both the rainy and summer seasons. The virginia runner genotypes were sensitive to rainfall. In the summer season no significant effect of relative humidity on flower production was observed. Gangapuri was found sensitive to the duration of sunshine hours. Impact of weather parameters on flower production was not found in both valencia genotypes and Somnath, but minimum temperature had significant effect in other genotypes (Table 4.7). Groundnut is affected by day length and light intensity. The crop prefers clear days with lots of sunlight for optimum production. It is a day-neutral plant with the flowering time controlled by temperature. However, photoperiod plays an important role in reproductive efficiency (flowers producing pegs and pods), and assimilate distribution during the post-flowering period. Long days promote vegetative growth at the expense of reproductive growth. During the post-flowering period, reproductive development is restricted when the photoperiod increases from 13 to 16 hours of

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Table 4.7. Reproductive efficiency indices (%) of eight genotypes belonging to four varietal forms of peanut Habit group (Genotypes)

RE1 R

RE2 S

R

RE3 S

R

RE4 S

R

RE5 S

R

RE5 S

R

S

Valencia Gangapuri MH2 Mean

45.9 31.0 65.3 38.0 55.6 34.5

77.3 42.1 34.1 25.7 64.0 57.4 36.3 37.6 70.7 49.8 35.2 31.7

44.5 61.5 56.3 66.2 50.4 63.9

82.0 58.6 27.8 15.0 91.0 56.2 33.0 21.1 86.5 57.4 30.4 18.1

Spanish JL24 GG2 Mean

54.8 34.0 65.7 36.5 60.3 35.8

48.6 47.5 26.4 31.0 40.3 39.1 21.5 27.7 44.5 43.3 24.0 29.9

54.9 63.8 25.6 59.4 40.3 61.6

76.1 65.1 19.2 20.1 79.6 84.9 16.6 21.8 77.9 75.0 17.9 21.0

47.8 37.0 34.5 28.0 41.2 32.5

39.4 32.9 15.5 16.2 39.5 56.1 62.4 44.7 25.8 21.6 41.4 50.5 50.9 38.8 20.7 18.9 40.0 53.3

67.4 50.4 10.5 9.2 68.9 45.8 17.9 10.3 68.2 48.1 14.2 9.8

46.9 39.1 36.1 24.3 41.5 31.7 4.2 1.9

77.4 46.9 19.4 24.2 66.3 79.7 20.2 35.2 71.9 63.3 19.6 29.7 5.4 5.1 2.6 2.5

52.6 61.2 10.2 14.8 69.8 41.5 13.9 14.7 61.2 51.4 12.1 14.8 4.1 4.8 2.9 1.7

Virginia Bunch BG1 Kadiri3 Mean Virginia Runner Somnath GAUG10 Mean S.Em.

25.6 53.3 30.1 44.6 27.9 49.0 4.3 2.6

day length. In addition, long days and high temperatures further reduce reproductive efficiency. Certain cultivars are also sensitive to photoperiods as the time of flowering is influenced by them.

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Hammons, R.O. 1964. Krinkle, a dominant leaf marker in the peanut (Arachis hypogaea L.). Crop Science, 4: 22-24. Harris, H.C. 1949. The effect on the growth of peanuts of nutrient deficiencies in the root and the pegging zone. Plant Physiol., 24: 150-161. Hassan, M.A. and D.P. Srivastava. 1966. Floral biology and pod development of peanut studied in India. Journal of the Indian Botanical Society, 45: 92-102. Ishag, H.M. 2000. Phenotypic and yield response of irrigated groundnut cultivars in a hot environment. Exp. Agric., 36: 303-312. Jose, M.E. and V.F. Guevara. 1951. The floral biology and fruitification of peanut (Arachis hypogaea L.). Philippine Agriculturist, 35: 137-142. Jun, S.Y. and Anke. 1988. Effect of controlling peg growth at the start of groundnut (Arachis hypogaea L.) flowering. Oleagineux, 43: 127-134. Knauft, D.A., A.J. Norden and D.W. Gorbet. 1987. Peanut. pp. 346-384. In: Principles of Cultivar Development. Vol. 2. New York, USA: Macmillan Publishing Company. Lauriano, J.A., F.C. Lidon, C.A. Carvalho, P.S. Campos, M.D.C. Matos et al. 2000. Drought effects on membrane lipids and photosynthetic activity in different peanut cultivars. Photosyn. (Prague), 38: 7-12. Lawes, D.A. 1972. The development of self-fertile field beans. Welsh Plant Breeding Station, Abery-stwyth Rpt. pp.739-751. Leong, S.K. and C.K. Ong. 1983. The influence of temperature and soil water deficit on the development and morphology of groundnut (Arachis hypogaea L.). J. Exp. Bot. 34: 15511561. Lim, E.S. and J.S. Gumpil. 1984. The flowering, pollination and hybridisation of groundnuts (Arachis hypogaea L.). Pertanika, 7: 61-66. Lim, E.S. and O. Hamdan. 1984. The reproductive characters of four varieties of groundnuts (Arachis hypogaea L.). Pertanika, 7: 25-31. Lim, E.S., S. Surjit and A. Amartalingam. 1980. Reproductive efficiency of groundnuts. Proc. Legumes in the Tropics, pp. 87-96. Lim, E.S. and R. Knight. 1980. The effect of inbreeding and hybridization on the seedset ability of Vicia faba L. SABRAO J., 12: 99-108. Luz, L.N., R.C. Santos and P.D. Melo Filho. 2011. Correlations and path analysis of peanut traits associated with the peg. Crop Breed. Appl. Biot., 11: 88-95. Martin, J.P., S. Cas and H. Rabechault. 1974. Cultures in vitro d etamines arachide (Arachis hypogea L.). 1. Stades du development des boutons floraux et microsporogenesis. Oleagineux, 29: 145-149. Moctezuma, E. 1998. Gravitropic mechanisms of the peanut gynophore. Berkeley, California: Ph.D. thesis, Department of Plant and Microbial Biology, University of California. Moctezuma, E. 2003. The peanut gynophore: A developmental and physiological perspective. Can. J. Bot., 81: 183-190. Moctezuma, E. and L.J. Feldman. 1998. Growth rates and auxin effects in gravi responding gynophores of the peanut, Arachis hypogaea (Fabaceae). Am. J. Bot., 85: 1369-1376. Moctezuma, E. and L.J. Feldman. 1999. The role of amyloplasts during gravity perception in gynophores of the peanut plant (Arachis hypogaea). Ann. Bot. (London), 84: 709–714. Mutters, R.G., L.G.R. Ferreira and A.E. Hall. 1989. Proline content of the anthers and pollen of heat-tolerant and heat-sensitive cow pea subjected to different temperatures. Crop Science, 29: 1497-1500. Narayanan, A., B. Reddy and S.R.K. Murthy. 1984. The effects of peg removal on the vegetative and reproductive parts of groundnut. J. Oilseeds Res., 1: 63-69. Nautiyal, P.C., V. Ravindra, P.V. Zala and Y.C. Joshi. 1999. Enhancement of yield in groundnut following the imposition of transient soil-moisture stress during the vegetative phase. Experi. Agric., 35: 371-385. Nigam, S.N., M.J.V. Rao and R.W. Gibbons. 1990. Artificial hybridization in groundnut. Information Bulletin No. 29. Patancheru, Andhra Pradesh, India: International Crops Research Institute for the Semi-Arid Tropics, 27 pp.

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Nigam, S.N., V. Ramanatha Rao and R.W. Gibbons. 1983. Utilization of natural hybrids in the improvement of groundnuts (Arachis hypogaea L.). Experimental Agriculture, 19: 355-359. Norden, A.J. 1980. Peanut. pp. 443-456. In: Fehr, W.R. and Hadley, H.H. (eds.). Hybridization of Crop Plants. Madison, Wisconsin, USA: American Society of Agronomy and Crop Science. Ono, Y. 1979. Flowering and fruiting of peanut plants. Japan Agricultural Research Quarterly, 13: 226-229. Othmanhamdan. 1979. Reproductive efficiency of groundnuts (Arachis hypogaea L.). Project Paper, Uniy. Pertanian Malaysia. 1978/79. Pattee, H.E. and H.T. Stalker. 1992. Reproductive efficiency in reciprocal crosses of Arachis duranensis and A. stenosperma with A. hypogaea cv. NC 6. Peanut Science, 19: 45-51. Pattee, H.E.M.S. 1987. Anatomical changes during ontogeny of the peanut (Arachis hypogaea L.) fruit: Mature megagametophyte through heart-shaped embryo. Botanical Gazette, 148: 156-164. Pattee, H.E. and C.T. Young. 1982. Peanut Science and Technology (ed.). Peanut Society of America, pp. 132. Periasamy, K.S.C. 1984. The morphology and anatomy of ovule and fruit development in Arachis hypogaea L. Annals of Botany, 53: 3. Pilumwong, J., C. Senthonga, S. Srichuwongb and K.T. Ingram. 2007. Effects of temperature and elevated CO2 on shoot and root growth of peanut (Arachis hypogaea L.) grown in controlled environment chambers. Science Asia, 33: 79-87. Prasad, P.V.V., P.Q. Craufurd and R.J. Summerfield. 1999b. Fruit number in relation to pollen production and viability in groundnut exposed to short episodes of heat stress. Ann. Bot., 84: 381-386. Prasad, P.V.V., P.Q. Craufurd, R.J. Summerfield and T.R. Wheeler. 2000a. Effects of short episodes of heat stress on flower production and fruit-set of groundnut (Arachis hypogaea L.). J. Exp. Bot., 51: 777-784. Quamruzzaman, M., M.J. Ullah, M.F. Karim, N. Islam, M.J. Rahman et al. 2018. Reproductive development of two groundnut cultivars as influenced by boron and light. Inf. Process. Agric., 5: 289-293. Ramanatha Rao, V. and U.R. Murty. 1994. Botany – Morphology and anatomy. pp. 43-95. In: J. Smartt (ed.). The Groundnut Crop. A Scientific Basis for Improvement. Chapman & Hall, London. Ramanatha Rao, V. 1988. Botany. pp. 24-64. In: Reddy, P.S. (ed.). Groundnut. New Delhi, India: Indian Council of Agricultural Research. Reddy, P.R. 1988. Physiology. pp. 77-118. In: Reddy, P.S. (ed.). Groundnut. New Delhi: ICAR. Rudich, J., E. Zamaski and Y. Regev. 1977. Genotypic variation for sensitivity to high temperature in the tomato: Pollination and fruit-set. Botanical Gazette, Chicago, 138: 448-452. Sastri, D.C. and J.P. Moss. 1982. Effects of growth regulators on incompatible crosses in the genus Arachis L. Journal of Experimental Botany, 33: 1293-1301. Savage, G.P. and J.I. Keenan. 1994. The composition and nutritive value of groundnut kernel. pp. 173- 213. In: Smartt, J. (ed). The Groundnut Crop: A Scientific Basis for Improvement. Chapman and Hall, London. Shaharoona, B., M. Arshad and A. Khalid. 2007. Differential response of etiolated pea seedlings to inoculation with rhizobacteria capable of utilizing 1-aminocyclopropane-1-carboxylate or L-methionine. J. Microbiol., 45: 15-20. Shlamovitz, N., M. Ziv and E. Zamski. 1995. Light, dark and growth regulator involvement in

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CHAPTER

5

Pod Growth and Yield Gynophore elongation, pod formation and pod orientation in the peanut plant (Arachis hypogaea L.) were studied in relation to the effects of light and dark conditions, mechanical stimulus, and growth substances. It was found that the proembryos control gynophore elongation, probably by secretion of growth regulators which stimulate cell division in the intercalary meristem located proximal to the ovules. The stimulus of pod production causes the development of the proembryo into a mature embryo simultaneously with the growth of pod tissues and the cessation of gynophore elongation. Darkness was found to be an essential factor for the induction of pod formation. Pod formation did not occur in any of the treatments performed in the light, including the application of different growth substances on the ovary. A mechanical stimulus is needed, in addition to darkness, for the normal thickening and diageotropic orientation of the pod, caused by a higher growth rate of the basal proximal side of the pod. The two ovules are always located on the upper wall of the diageotropically oriented pod (ventral suture). Normal pods (containing seed) of groundnut (Arachis hypogaea L.) (cv. TMV-2) were successfully raised in darkened, aerated, nutrient solutions, but not in the light. The onset of podding was evident 7 to 8 d after gynophores were submerged in the darkened nutrient solution. An examination of pods and submerged portions of gynophore surfaces by scanning electron microscopy showed the presence of two distinctly different protuberances: unicellular root-hair-like structures that first developed from epidermal cells of the gynophores and developing pods; and branched septate hairs that developed later from cells below the epidermal layer. The septate hairs became visible only after the epidermal and associated unicellular structures had been shed by the expanding gynophore and pods. The embryo which is dormant, during peg elongation begins to grow 3 to 4 days after the pod begins to develop. The basal ovule develops first and when the peg reaches its maximum depth. It turns diageotropic and almost horizontal. The pod starts developing only after the peg has ceased to elongate; the pods begin to enlarge from the base at the apex. The pod expands rapidly in the soil by the development of a large parenchymatous tissue (endocarp) lying between the ovules and shell layers. As the ovules begin to grow the endocarp recedes and totally disappears when the seeds mature fully. The inner side of the shell turns increasingly dark brown owing to increased tannin content, finally becoming very dark brown on maturation, which takes about 60 days after fertilization. The size and shape of mature pods and seeds are governed by several intrinsic and external factors including cultivar, soil and cultural practices. Once the peg enters the soil, the intercalary meristem ceases its activity

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(Periasamy and Sampoornam, 1984). Thompson et al. (1992) showed that seven days after entry into the soil phytochrome is present in the developing ovules and embryos, but not in the peg tissues. The cells derived from the innermost layers of the peg increase around the proximal ovule first followed by the distal ovule, forming an inner zone. This marks fruit initiation. Then cells surrounding the inner zone divide rapidly on the dorsal side near the proximal ovule. Due to this unequal growth of the inner zone tissue, the fruit assumes a horizontal position and runs parallel to the soil. The inner zone has spongy parenchymatous tissue contributing to the major portion of the fruit wall (Periasamy and Sampoornam, 1984). The epidermis of the outer zone is replaced by the periderm due to an increase in cell number. As the fruit enlarges in the soil, the vascular bundles become interlinked by lateral connections through fibrous plates. Based on physiological growth and biochemical changes, fruit development has been classified into various stages (Young et al., 2004; Boote, 1982; Pattee et al., 1974). The current classification system of peanut fruit development (Fig. 5.1) described below has been adapted from Young et al. (2004) and Paik-Ro et al. (2002).

Fig. 5.1. Stages of pod growth

Fruit Development Classification Stage 1: Very Immature Pod. The pod is very watery, soft and spongy. This appearance is due to the presence of parenchymatous tissue. The wall consists of an inner zone, outer zone, and 10-13 vascular bundles joined by lateral connections. At this stage, the pod acts as a storage organ for the developing seeds and has the highest levels of sugar and starch compared to later stages. Stage 1: Seed The seed is very small and flat with a white seed coat. Anatomically, the seed has an outer and inner surface epidermis having rectangular (20-40 μm) and irregular cells, respectively. The epidermal cells consist of a dense cytoplasmic network surrounding the organelles including starch grains and protein bodies. The provascular bundles range from 3-8 μm in diameter and are placed equidistantly. The mid region parenchyma cells consist of numerous vacuoles, protein bodies and starch granules. Lipid bodies can also be seen at this stage. Stomata cannot be distinguished with electron microscopy at this stage.

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Stage 2: Immature Pod The pod is watery and soft. However, it begins to show signs of dehydration. Seed – the seed is round. Its seed coat is pink at the tip of the embryo axis, but the remainder is white. Sugar levels are high in the seed coat. Cells of the outer epidermis are now 30-50 μm and the cells of the outer epidermis are angular. At this stage, stomata can be easily seen on the inner surface of the epidermis. At this stage of development, starch and sugar levels are approximately equal (Pattee et al., 1974); however the lipid content is low. The seed weight is approximately 300 mg. Stage 3: Mature Pod The parenchymatous tissue begins to disintegrate; the inner pericarp dries and cracks giving a white papery appearance. Sugar and starch contents decrease, however the content of hemicelluloses increases. Seed – the seed coat begins to dry out and turn a light pink. The epidermal cells of the outer surface increase in size to 60-80 μm. The inner surface epidermal cells possess angular shaped cells with distinct stomata. Lipid bodies start accumulating in the seed. Starch and sugar content gradually decreases in the seed coat and increases in the seed. The seed weight is approximately 600 mg. Stage 4: Very Mature Pod The parenchymatous tissue is gone. The outer and inner cell layers contain hemicellulose deposits. The 10-13 vascular bundles are interconnected through the sclerenchymatous layers. In cross section, the vascular bundles are in a Y-shaped groove of the sclerid layer. This Y-shaped grove is responsible for the reticulation of the outer surface of the pod (Halliburton et al., 1975). Brown black splotches are on the inner pericarp as a result of a complete loss of the parenchymatous tissue. Seed – the seed coat is completely dry and dark pink. The rounder outer surface epidermal cells are 70-100 μm in size and the inner surface epidermis possesses angular shaped cells. Vascular bundles can be seen in both cotyledons (Young and Schadel, 1990). A major portion of the cotyledons consist of parenchyma cells. Lipid bodies reach their maximal levels. Starch and sugar levels in the seed coat are the lowest since development initiated and are the highest in the cotyledons. The seed weight is approximately 600 mg (Fig. 5.2).

Fig. 5.2. Stages of peanut fruit development: (A) Pod stages; (B) Seed stages. 1: Very

Immature; 2: Immature; 3: Mature; 4: Very Mature (Pattee et al., 1974)

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Physiology of the Peanut Plant

The effect of gypsum supplementation on peanut seed and pod development was studied for two field varieties. Pod length was not affected by gypsum treatment. However, fewer two-segmented pods (P = 0.006), fewer pods with two seeds (P = 0.006), more immature or aborted distal seeds (P = 0.002), and more asynchronized fruit (P = 0.01) were observed in plots without gypsum applications. Non-treated gypsum plots at 100 and 130 DAP had the highest amount of aborted seeds (8%) and asynchronized fruits. The effect of gypsum on seed and fruit development was greater for C-99R than for Georgia Green. Gypsum application increased Ca and S concentrations of both pods and seeds, decreased Mg concentrations only in seeds, and decreased Na and P concentrations of both pods and seeds. Calcium concentrations were two times higher in pods compared to seeds. Georgia Green accumulated more Ca in pods and seeds than did C-99R. A linear increase in Ca concentration was observed in pods and seeds at the same physiological stages when sampled over time. Peanut seeds and pods were also analyzed for CDPK expression during their development in gypsum-treated and non-treated soils. In seeds, CDPK transcript and protein expression profiles were biphasic. High CDPK levels were observed in very immature seed stages and these levels dropped in immature stages, rose again to high levels in mature stages and then dropped significantly in very mature stages. In pods, CDPK transcripts and protein levels were consistently high until the very mature stage when levels were significantly diminished. Seeds at all developmental stages showed 2- to 3-fold lower CDPK transcripts and protein levels under gypsum treatment. Histology localization data showed decoration of immune reactive CDPK primarily in the outer most cell layers of the pericarp and around vascular bundles, as well as in the single vascular trace that supplies nutrients to the developing ovule. A recently-developed solution culture technique was used to study the effects of aeration and calcium (Ca) on groundnut (Arachis hypogaea L.) pod development. Two experiments were conducted with seven groundnut lines, TMV-2, Chico and A116L4 (Spanish), CBRR4 (Valencia), A125L25 (Valencia×Spanish), and Shulamit Strain 1 (SH-1) and Virginia Brunch Strain 1 (VB-1) (Virginia). Plants were grown in a potting mix, and the attached gynophores cultured in darkened polycarbonate jars containing nutrient solution. Non-aeration of solution prevented pod development, but pods and seeds of all lines developed in aerated, darkened nutrient solutions (ionic strength approx. 9 mm). Normal pods and seeds were produced by TMV-2, Chico and CBRR4, but constricted pods were developed in SH-1 and VB-1. A secondary gynophore developed between the basal and apical seed compartments in A116L4 and A125L25, and in VB-1 at high Ca (500–2500 μm) in solution. The secondary gynophores were similar to those produced in other Arachis spp. but not usually found in cultivated forms of A. hypogaea. Septate and non-septate hairs developed on submerged gynophores and pods, but were sparse on those of SH-1 and VB-1. The magnitude of the effects of aeration and Ca concentration on pod initiation and morphogenesis differed in experiments conducted in summer and winter and among the lines test. The podding zone of the peanut is located at the upper soil layer, about 4 cm below the soil surface. Therefore, environmental factors such as soil temperature and soil moisture are more variable in the podding zone than in the rooting zone which occupies the deeper layer. To examine the effect of environmental factors in the podding zone on the pod development of peanut plants, three experiments were

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119

carried out, by separating the podding zone from the rooting zone artificially. In an experiment performed in 1969 to investigate the effect of soil temperature on the pod development, the podding zones were kept at various temperatures, 15°C, 23°C, 32°C and 39°C, during a period of 7 weeks after the peg penetration into the soil. Results showed that when the podding zone was kept at 32°C, the swelling of ovaries began two days after and the pod size reached its maximum three weeks after the peg penetration into the soil. Keeping the podding zone at the temperature of 23°C or 39°C, commencement of swelling of ovaries was observed five days and seven days after the peg penetration, respectively. But, at the temperature of 15°C, days from peg penetration to ovary swelling were more than two weeks. The relation between the soil temperature in the podding zone and the pod development 7 weeks after the peg penetration is shown in Fig. 5.3. From those results, it was concluded that optimum soil temperature in the podding zone for fruiting of the peanut was in the range of 31-33°C. In the second experiment, the effect of soil moisture in the podding zone upon the pod development was investigated, changing the soil moisture content in the rooting zone. The experimental results indicated that regardless of the soil moisture content in the rooting zone, maximum pod development occurred at the soil moisture content of 4.0% to the total soil volume in the podding zone. The F2 population of a cross of MH2 BC28 with ICG(C) 8, a Valencia form of A. hypogaea × A. cardenasii, revealed a segregant with aerial pods and good seed development. The variant exhibited sequential branching with 6 thick primary branches and 4 secondary, and 3-4 stout, elongated pegs at each node. The aerial pods possessed 2-3 well developed seeds (2.48 g/10 seeds), as did the 30 subterranean pods (2.45 g/10 seeds). None of the parents or F1s exhibited the aerial podding attribute.

Fig. 5.3. Effect of soil temperature in the podding zone on the pod development Note: • The percentage or the number of developed pods to the number of pegs examined. o : Result in 1968, Result in 1969, I: 95% confidence interval of the mean

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Physiology of the Peanut Plant

Generally, a large number of early formed flowers develop into fruits and flowers that appear 70 days after flowering do not form pods and fail to increase the yield due to a low yield of mature pods (Knauft and Gorbet, 1989; Putnam et al., 1991; FAO, 1990). According to Bell et al. (1991) and Awal and Ikeda (2003) the production pattern of the groundnut flowers favours pod setting and pods take about 8 weeks to mature from the time of flowering and therefore, only the first three weeks of flowering may be considered to be useful but this varies among varieties (Fig. 5.4). Fruiting efficiency depends on the pattern of flowering (number of flowers at different periods of flowering), which is more important than the total number of flowers per plant (Lim and Hamdan, 1984; Lee et al., 1972). Plants whose flowering was stimulated set the largest percentage of pegs and highest growth rate of intact pegs but plants with the least number of flowers formed the lowest percentage of pegs, and the peg growth rate declined by 50% (Lim and Hamdan, 1984; Lee et al., 1972).

Fig. 5.4. Changes of flower production per plant with time

5.1.

Pod Growth

Corlett et al. (1992) and Önemli (2005) also indicated that the percentage of flowers turned to pods, but not the number of flowers, is important for determining the yield of groundnut. The flower to peg and peg to pod ratios were good indicators of pod yield and if the relationship between the beginning of the first flowering and maturity is known, the time of harvest can be estimated for healthy crops. Bailey and Bear (1973) demonstrated that the early onset of flowering and early accumulation of a given number of flowers (10 to 30) are important components of early maturity in groundnut and a high proportion of the first 25 flowers developed into mature pods. Similarly, the conversion of flowers to mature pods was the most important factor contributing to high pod yield (Songsri et al., 2009). Pod number and eventual crop yield is determined by the number of floral buds that form flowers, the number of flowers that are fertilized and produce pegs, and the number of pegs that successfully penetrate the soil surface and produce pods (Prasad et al., 1999b). Short or prolonged periods of high temperature during reproductive development of peanut are known to cause significant yield losses (Ketring, 1984; Ong, 1986; Wheeler et al., 1997). The ability of flowers to set fruits is an important factor determining fruit number and thus seed yield. Research in controlled environments

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Pod Growth and Yield

has shown that peanuts are particularly sensitive to short episodes (≤6 d) of high air temperature (day/night; 38/22°C) starting from 6 d before until 15 d after flowering, and that the magnitude of sensitivity is related to the number of floral buds which are exposed to high temperature in the period before anthesis (Vara Prasad et al., 1999a). Warm days (>34°C) and nights (28°C) reduce fruit-set due to reductions in pollen number and pollen viability (Vara Prasad et al., 1999b). Logistic regression statistics for Experiment 1 showed that there were significant differences in fruit-set due to temperature (P < 0.001), stage of floral bud development (P < 0.01), duration of exposure to high temperature (P < 0.05), stage X temperature interaction (P < 0.0001) and temperature X duration interaction (P < 0.0004). However, there was no stage X duration interaction or temperature X duration X stage interaction. In general, only bud temperatures >33°C significantly reduced fruit-set (Table 5.1). The values of fruit-set were reduced more following long rather than short exposure to high temperature. Furthermore, high temperature had a greater effect on fruit-set at 3 DBA and at anthesis than at other stages (Table 5.1). There was no significant effect on fruitset from exposure to 33°C for 1, 3 or 6 d at any stage of floral development. Floral bud temperatures >33°C significantly reduced fruit-set, from >55% at 28°C to 0.10), and a common line with a slope of 6%°C-1 could be fitted above a critical temperature of 33°C (Table 5.2). However, the response to temperature at 6 DBA, and 3 DBA/anthesis were far more sensitive to temperature than 3 DAA, with the critical temperature being 3–5°C higher at 3 DAA. Although the critical temperature was significantly greater, the slope was steeper at 3 DAA, where fruit-set was reduced by 10.3%°C-1 (Fig. 5.5). Table 5.2. Effect of 2, 4 or 6d exposure to floral bud temperatures of 33, 40 or 43°C during the day on the proportion of flowers producing fruits (fruit-set, angular transformed ) during the 6d treatment period; control plants were maintained at 28°C throughout Daytime floral bud temperature (°C) 28 (control) 33 40 43 Mean

Duration of exposure (d) 2 52.5 50.5 51.8 51.6

4 49.3 49.3 15.8 38.1

6 51.2 46.9 25.9 0.0 24.3

Mean 51.2 49.5 41.9 22.5

SED for duration of exposure (df 2, 18) = 5.43*** and for temperature × duration interaction (df 4, 18) = 5.16* *Significant at P < 0.05 probability level ***Significant at P < 0.001 probability level

Fig. 5.5. Per cent fruit set in peanut at varying days before and after anthesis under varying floral bud temperature

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123

Plants of cv. ICGV 86015 were grown in controlled environments at a day/night temperature of 28/22°C from sowing until 9d after flowering (DAF). Then, cohorts of plants were (a) exposed to day temperature of 28, 34, 42 or 48°C for 2, 4 or 6d; or were (b) exposed to 34, 42 or 48°C for 6d either throughout a 12h day (08.00 to 20.00 h, WD), or only during the first 6 h (AM) or second 6 h (AM) or second 6 h (PM) of the day. Values of RNt were significantly reduced by high temperature, by duration of exposure, and by timing of exposure. Variation in FN was quantitatively related to floral bud temperatures during the day over the range 28-43°C. In contrast only floral bud temperatures >36°C during AM and WD significantly reduced fruit-set and hence RNt, whereas high PM temperature had no effect on fruit-set (Fig. 5.6).

Fig. 5.6. Per cent fruit-set under mean floral bud temperature during AM

In the Prasad et al. (2003) study both pegging and podding were delayed above the 32/22°C to 36/26°C temperature range. As the temperatures increased from 32/22°C to 44/34°C pod number decreased from 353 to 74 m-2 under ambient CO₂ (350 μmol mol-1) and from 407 to 116 m-2 under elevated CO₂ (700 μmol mol-1). Similarly, with the same temperature increase, seed number decreased from 587 m-2 to 43 m-2 at ambient CO₂ and 709 m-2 to 132 m-2 at elevated CO₂. Across all temperatures, elevated CO₂ compared with ambient CO₂ increased pod number by 40% and seed number by 31%. The interaction between temperature and CO2 for pod and seed number was not significant. Bell et al. (1991) studied the effect of temperature and photoperiod on Spanish, Virginia and Valencia types of groundnut and reported strong photoperiod × temperature interaction for the number of pegs and pods produced. Photoperiod did not affect time to first flower, but the number of pegs and pods and total pod weight per plant decreased in long (16 or 17 h) photoperiods. For example, pod numbers of two cultivars, i.e. White Spanish and NC 17090, decreased with increasing photoperiod (17 h vs. 11.9-13.5 h) at two temperatures (33/17°C and 33/23°C). Similarly, Bagnall and King (1991b) studied the response of groundnut to temperature, photoperiod and irradiance on flowering and development of pegs and pods. Flower and peg number at 60 to 70 days from emergence were approximately doubled by 12 h days (SD) compared with plants with 16 h days (LD). Peg numbers were highly correlated to flower numbers and their ratio was independent of differing photoperiod treatments, suggesting that there was no major effect of day length on flower abortion. However,

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Physiology of the Peanut Plant

the pod number and, therefore, yield was more influenced by photoperiod than with flower or peg formation. In an experiment, it was noted that the number of pods formed increased linearly till 30 days after initiation of podding reaching a maximum and thereafter only pod development was maintained till maturity. However, pod weight increased gradually till maturity. Total weight of the plant also showed a trend similar to that of pod formation when analysed during the pod development stage. From this study it appears that the partitioning of photosynthates to the pod appears to be fractional i.e. 0.86, following the equation PART=PGR/0.606CGR (PART – Partitioning of assimilates, PGR – Pod growth rate and CGR – Crop growth rate during reproductive phase) (Fig. 5.7) (Basuchaudhuri, 1986).

Fig. 5.7. Number of pods per plant, pod dry weight (g.plant-1) and total dry weight (g.plant-1) changes after podding

Optimum air temperature for pod growth as suggested by various researchers appears to lie between 20-24°C (Williams et al., 1975; Cox, 1979). Cox (1979) observed that the individual and total pod weights and the rate of increase in pod weight were greatest at the mean temperature of 23.5°C. So, partitioning of dry matter to pods would, therefore, be expected to decrease as the temperature increases above 24°C (Ong, 1984). Pilumwong et al. (2007) found that as temperature increases from 25/15°C to 35/25°C, pod dry weight reduced by 50%. Pod weight reduction by high temperature (35/30°C vs. 25/25°C) was also reported by Talwar et al. (1999) for three genotypes. Nigam et al. (1994) reported that temperature had a significant effect (P < 0.01) on pod growth rate but there was no overall effect of the photoperiod. In the tested genotypes, highest pod growth rate was observed at 26/22°C compared to 22/18°C and 30/26°C. Photoperiod effects on pod growth rate for cvs. TMV 2 and Nc Ac 17090 were not significant in any temperature regimes. On the other hand, significantly greater pod growth rate for VA 81B occurred on a long day than on a

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125

short day at 26/22°C. The study may provide evidence of genotypic variability for photoperiod × temperature interaction which could influence adaptation for groundnut genotypes to new environments. Most of the sandy soils that are suitable for production of groundnut (Arachis hypogaea L.) in the tropic and subtropics are acidic. Whilst effects of pH in the root zone have been studied, the effects of pod-zone pH on groundnut productivity remain relatively unknown. To develop appropriate soil management practices for groundnut production on acid soils, it is essential to understand how low pH affects the reproductive growth of groundnut. Consequently, a glasshouse experiment was conducted in which attached groundnut gynophores were cultured in solution at a pH ranging from 3.0 to 7.0. A low pH delayed pod initiation, and resulted in almost no pod expansion at pH 3.0. Only 12% and 55% of the cultured gynophores developed into pods at pH 3.0 and 4.0, respectively, compared with 91–95% at pH ≥ 5.0. Pods produced at pH 3.0 contained no seeds and those produced at pH 4.0 had a hollow, dark coloured area in the cotyledon. Normal seeds and embryos were formed at pH ≥ 5.0, and plumule development was faster at a solution pH ≥ 5.0 than at pH 4.0. Pod and kernel dry mass were optimised (90% of maximum) at pH 5.62–6.69 and 5.65–6.78, respectively. Septate and non-septate pod hairs were formed at all solution pH regimes, but were denser and more persistent at the higher pH. Kernel calcium (Ca) concentration decreased with decreasing pH, and was highly correlated with the solution pH. Thus, pod-zone pH has important effects on the reproductive growth of groundnut, emphasizing the importance of managing pod-zone pH. Like other legumes, peanut plants are susceptible to soil salinity (Greenway and Munns, 1980) and genotypic difference for salinity tolerance exists within the species (Sun et al., 2013). Peanut plants are glycophytes indicating their vulnerability to highly saline soils (Banjara et al., 2012). In this context, marked salinity‐mediated reduction in the peanut pod production has been reported (Meena et al., 2016). Salinity can reduce peanut seed germination, seedling establishment, and the dry weight of plants (Meena et al., 2016; Parida and Jha, 2013). Moreover, salinity induced disruption in the photosynthetic apparatus and disturbance in nutrient uptake are believed to contribute to peanut yield losses (Qin et al., 2011a). Salinity induced downregulation of genes associated with photosynthetic light harvesting complex proteins and phenylalanine metabolism, and subsequent production of terpenoids, phenylalanine, tyrosine, and plant hormones has been reported (Chen et al., 2016). At the molecular level, salinity induced downregulation of 36 peanut genes along with upregulation of seven genes associated with the ROS network after 48 hr of salinity treatment has been reported (Chen et al., 2016). Efforts are required to develop salt tolerant peanut genotypes using transgenic approaches (Chen et al., 2010) so as to increase the yield of cultivated peanut. The use of good quality seeds and salinity management practices could contribute to improvement in peanut yield under salinity stress (Meena et al., 2016). The genotypes under study proved more salt tolerant during germination than during the vegetative stage of growth and the result identified Esan-Local, Ex-Dakar and RRB 12 as being more salt tolerant than the other genotypes under study. Treated plants maintained high heritability and genetic advance values in characters such as 100 seed weight, pods/plant and seeds/pod, indicating that the characters under study were controlled by additive genes and could be improved by selection. Thus salt tolerant traits from the tolerant genotypes (Esan-Local, RRB 12 and Ex-Dakar) could be a source for developing salt tolerant variants in groundnut (Table 5.3).

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Physiology of the Peanut Plant Table 5.3. Salinity effects on pods per plant in groundnut genotypes

Parameter Pods/plant

Cultivar Esan local RRB12 RMP91 Ex-Dakar RMP12 Mean

Salinity (mS/cm) 0.015

1.50

2.60

4.68

8.90

17.0

23.0 16.3 16.1 16.0 16.0 17.48

21.8 15.9 15.2 15.7 16.0 16.92

20.9 14.6 13.8 15.5 13.5 15.66

14.7 11.2 10.0 10.9 11.1 11.58

8.8 6.3 6.0 8.4 7.0 7.3

8.4 6.1 5.7 6.3 6.5 6.6

Drought mediated increase in top soil hardness can restrict or delay peg penetration into the soil and therefore restrict the pegging stage resulting in limited pod set and subsequent seed numbers (Haro et al., 2010, 2011). However, in contrast, if the drought affected pegs are re‐watered, viable pegs start penetrating the soil again (Haro et al., 2008). The resumption of pegging upon re‐watering is an adaptive trait in peanut plants that ensures long‐term survival of the fertilized embryo compared with other grain‐crops like maize and soybean where embryo viability is rapidly lost (Westgate and Boyer, 1986). Overall, a drought is the major abiotic stress‐limiting peanut crop yield (Boote et al., 1976; Haro et al., 2008). Furthermore, owing to a lack of genotypic variability for better water use efficiency among peanut plants (Gautami et al., 2011), the development of drought‐tolerant peanut genotypes is a major focus of plant breeders (Sarkar et al., 2016). A glasshouse experiment was conducted for evaluation of 4 peanut cultivars, Tainan 9, Khon Kaen 60-3, ICGV 98308 and ICGV 98324, under three regimes of water (field capacity, 1/2 available water and 1/4 available water) in earthen pots. A 4×3 factorial experiment in randomized complete block design with 4 replications was conducted. Total dry weight, pod yield, seed yield, 100 seed weight and shelling percentage were determined at the final harvest. At field capacity, all peanut cultivars performed well. Yield and agronomic characters of all cultivars were decreased under water stress; and significant response of genotypes was observed. Khon Kaen 60-3 and ICGV 98308 were more sensitive to water stress, comparing with Tainan 9 and ICGV 98324. Under water-limited conditions, pod yield of all peanut cultivars was decreased. Interaction between genotypes and water levels was found (Table 5.4). At FC, a significant difference was not found among genotypes (Table 5.4) but it was found to be significant at 1/2 AW and 1/4 AW. At 1/2 AW, Tainan Table 5.4. Effect of soil moisture regimes on pod yield of 4 peanut cultivars Genotype

Pod dry weight (kg/rai) FC

1/2 AW

1/4 AW

Tainan 9

443.94a

190.70b (57.0%)2/

66.25de (85.1%)

Khon Kaen 60-3

449.27a

18.88ef (95.8%)

4.14f (99.1%)

ICGV 98308

393.25a

89.42cd (77.3%)

35.49def (91.0%)

ICGV 98324

438.71a

145.89bc (66.8%)

140.92bc (67.9%)

1. Means followed by the same letter are not significantly different by DMRT at P < 0.05 2. Number in parenthesis is a reduction percentage.

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9 and ICGV 98324 exhibited high yield, while ICGV 98324 performed best at 1/4 AW. Seed yield reduction of Tainan 9 ranged from 57-85% while yield of ICGV 98324 was reduced by 67-68% (Table 5.5). Khon Kaen 60-3 and ICGV 98308 were highly sensitive to water stress. Pod yield of Khon Kaen 60-3 was decreased by 96-99%. Table 5.5. Effect of soil moisture regimes on seed yield of 4 peanut cultivars Variety

Seed yield (kg/rai) FC

1/2AW

1/4AW

137.61 (59.8%)

29.85ef (91.3%)

Tainan9

342.78

Khon Kaen60-3

306.25ab

6.31ef (97.9%)

0.0f (100%)

ICGV 98308

275.28

51.78 (81.2%)

15.33ef (94.4%)

ICGV 98324

315.08ab

91.32cd (71.0%)

57.22de (81.8%)

a

b

c

def

1. Means followed by the same letter are not significantly different by DMRT at P < 0.05 2. Number in parenthesis is a reduction percentage.

Drought at pod filling reduces growth, yield, and seed quality of peanut (Arachis hypogaea L.). A great root system can reduce yield loss under water stress. The pot experiments were conducted at Khon Kaen University, Thailand, in 2004-2005 and 2005-2006. A randomized complete block design was used with two factorials setup with four replicates. Factor A consisted of two water regimes (field capacity and 1/3 available water at 80 d after planting to harvest), and factor B comprised of 11 peanut genotypes. Data was recorded for root traits (root dry weight, root length, root surface, root diameter, and root volume), and peanut yield (pod dry weight, biomass, and harvest index) were measured at final harvest. Terminal droughts significantly decrease root characteristics (0.83-1.03 g plant-1 of root dry weight) and peanut yield (7.98-8.89 g plant-1 of pod dry weight). Yield responses to terminal drought were not correlated with root traits except root length and root volume (r = 0.71** and 0.83**, respectively). In some genotype, root traits seem to be correlated with peanut yield under terminal drought. ‘KK60-3’ showed high root traits, maintained pod dry weight under terminal drought, whereas Tifton 8 maintained biomass production. ICGV98348 had high root traits, maintained pod dry weight and harvest index under drought conditions. The results suggested that peanut contained high root characters which maintained yield under terminal drought (Table 5.6). Early peanut pod development is an important process of reproductive peanut development. Modes of DNA methylation during early peanut pod development are still unclear, possibly because its allotetraploid genome may cause difficulty for the methylome analysis. To investigate the functions of the dynamic DNA methylation during the early development of the peanut pod, global methylome and gene expression analyses were carried out by Illumina high throughput sequencing. A novel mapping strategy of reads was developed and used for methylome and gene expression analysis. Differentially methylated genes, such as nodulin, cell number regulator-like protein, and senescence-associated genes, were identified during the early developmental stages of the peanut pod. The expression levels of gibberellin-related genes changed during this period of pod development. From the stage one (S1) gynophore to the stage two (S2) gynophore, the expression levels of two key methyltransferase genes,

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Physiology of the Peanut Plant

Table 5.6. Total pod dry weight (PDW) of groundnut under moisture stress in 2005-2006 Genotype ICGV98300 ICGV98303 ICGV98305 ICGV98308 ICGV98324 ICGV98330 ICGV98345 ICGV98353 Tainan9 KK60-3 Tifton8 F-test Mean CV%

PDW (g plant-1)

PDW (g plant-1)

FC

1/3AW

10.64b 13.11ab 11.57ab 12.93ab 10.76b 10.93b 11.87ab 13.81a 11.07b 13.13ab 11.44ab * 11.93 14.95

8.11bcd 9.02ab 8.33bcd 7.00d 7.05cd 7.19cd 10.17a 7.67bcd 7.54bcd 8.81abc 6.87d ** 7.98 15.70

T-test Ns * ** ** * * Ns ** * ** *

T-test is for the difference among water regimes in 11 peanut genotypes. Means in the same column with the same letters are not significantly different. *Significant at P ≤ 0.05, **Significant at P ≤ 0.01

DRM2 and MET1, were up-regulated, which may lead to global DNA methylation changes between these two stages. The differentially methylated and expressed genes identified in the S1, S2, and stage 3 (S3) gynophore are involved in different biological processes such as stem cell fate determination, response to red, blue, and UV light, post-embryonic morphogenesis, and auxin biosynthesis. The expression levels of many genes were co-related by their DNA methylation levels. In addition, the results showed that the abundance of some levels of two key methyltransferase genes, DRM2 and MET1, were up-regulated, which may have led to global DNA methylation changes between these two stages. The differentially methylated and expressed genes identified in the S1, S2, and stage 3 (S3) gynophore are involved in different biological processes such as stem cell fate determination, response to red, blue, and UV light, post-embryonic morphogenesis, and auxin biosynthesis. The expression levels of many genes were co-related by their DNA methylation levels. In addition, our results showed that the abundance of some 24-nucleotide siRNAs and miRNAs were positively associated with DNA methylation levels of their target loci in peanut pods. To better understand the molecular mechanism of peanut pod development, attempts were made to perform the basic gene expression atlases regarding pod development. The transcriptome landscape of the peanut fruit was capable of building a theoretical foundation for the future illustrating the molecular details of hidden biological questions regarding the appearance of pod growth. Therefore, 20 separated seed and shell samples representing 11 distinct stages (P0–P10) of pod development were sequenced with RNA-seq. Consequently, a total of approximately 2.87 × 108 bp (approximately 62.45% of sequenced data) of uniquely mapped PEs (paired-end reads) were obtained for the subsequent abundance estimation (Fig. 5.8). Of the uniquely mapped PEs, the number of reads mapped to different transcripts ranged from 1 to 1,705,381 with a median of 1,266 for the transcriptome. For individual libraries, such numbers ranged from 1 to 810,833, and the average numbers ranged

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129

Fig. 5.8. RNA-seq analysis of peanut pod transcriptome. Shared and unique transcripts among aerial pod (P0), early subterranean pod (P1), seed (SD) and shell (SH) parts of subterranean pods

from 82 for P0 to 208 for P4SH (Fig. 5.8). The sequencing depth ranged from 0.24- to 175,758-folds with an average of 257-folds. The size of each transcript was plotted against the number of mapped reads on a logarithmic scale. The seed and shell tissues shared 51,264 transcripts with aerial pods (P0) and initial subterranean pods (P1) that are contained in the shell and the seed. The coverage analysis suggested that approximately 100 million short PEs should be sufficient to identify and measure all relevant transcripts during peanut pod development. A total of 226,587 transcripts were hit by more than 280 million PEs. Among expressed transcripts, 127,757 and 133,387 were expressed in seed and shell, respectively. In four scenarios, the number of transcripts that could be detected reached a plateau at approximately 100 million fragments. Moreover, it was observed that photosynthetic genes also played roles in seed development. A recent study has shown that fruit photosynthesis is not necessary for energy metabolism or development, but it plays a role in timed seed development. It was found that photosynthetic genes showed enrichment of up-regulated genes not only in the aerial pod, but also in P7 through P9. The expressions of genes encoding the biochemical reactions of the photorespiratory cycle and calvin cycle were mostly up-regulated. Our results indicated that photosynthesis had an effect on the aerial pod and the late pod development in the seed and shell (Fig. 5.9), while the biological reason was still unclear due to lack of sufficient study reference. Therefore, a further interpretation is able to portend the intellectual extension of plant fruit development under dark condition. Light represses peanut pod swelling and embryonic development in aerial pod. Pegs from underground flowers lack the light-induced inhibition to fruit enlargement. When the peg already underground starts off, there is no pause in embryo growth. The ubiquitin ligase COP1 (constitutively photomorphogenic 1) negatively regulates plant photomorphogenesis. It was found that COP1 gene was co-expressed across pod development, and the COP1 interacting protein 7 (CIP7) has been reported to be up-regulated by light. A previous study on peanut gynophore development in darkness has reported that the expression of CIP7 is drastically decreased in dark-grown gynophores. The study only covers three developmental stages, not including the

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Physiology of the Peanut Plant

Fig. 5.9. Photosynthesis involved in pod development. MapMan photosynthesis overview maps showing differences at the transcript level between aerial pods (P0) and subterranean developing seeds. Log2 ratios for average transcript abundance in seed across stages P2SD to P10SD were calculated

pod development. However, it was found that the expression of CIP7 was decreased when gynophores just penetrated the soil, and then it was monotonically increased during pod expansion underground. Moreover, other genes involved in light signaling transduction, such as PHYA (phytochrome A), PHABULOSA B, PHABULOSA C, CRY2 (cryptochromes 2) and CRY3 (cryptochromes 3), were also expressed during pod development. However, it is necessary to further investigate the roles of these genes in peanut fruit development in the absence of light.

5.2. Yield Peanut (Arachis hypogaea L.) production in Argentina is affected by frequent and unpredictable periods of water deficit that usually overlap the critical period for pod set of early sown crops. An indirect effect of water deficit is reduced pegging due to increased soil strength promoted by surface soil desiccation. There is no knowledge on the associated effects determined by peg production dynamics and variable plant water status. The responses of these traits were evaluated by means of field experiments (Exp1: 2002–2003; Exp2: 2005–2006) that included two peanut cultivars (ASEM 485 INTA and Florman INTA) cropped at different sowing dates and water regimes (IRR: irrigated; WS: water stress). Treatments allowed exploring a range of: (i) evaporative demands, (ii) surface soil strength levels, and (iii) soil water contents (θ). Computation of leaf area index (LAI), intercepted photosynthetically active radiation (IPAR), surface soil strength, degree of leaf folding, degree days of stress (SDD), crop (CGR) and pod growth rates (PGR) at critical periods, and radiation use efficiency (RUE). Seed yield and seed yield components (pod number per m2, seed number per m2 and individual seed weight) were determined at final harvest. WS

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131

promoted a significant decline (average of 73%) in seed yield (P ≤ 0.022), which was better explained (r2 = 0.98) by the decline in seed and pod numbers than by the decline in individual seed weight (r2 = 0.67). Seed number responded chiefly to CGR between R3 and R6.5, but WS plots of Exp1 departed from the general model fitted to IRR plots (40–53% decrease respect to predicted values). Biomass partitioning to reproductive sinks was also affected in WS plots. Enhanced soil strength promoted by soil drying reduced normal pegging patterns, and a generic bilinear model indicated a soil strength threshold of ca. 2.23 ± 0.10 MPa (θ = 0.119 cm3 cm−3) above which peg penetration decreased dramatically (r2 = 0.57, P < 0.001). WS reduced IPAR accumulation (10– 30% reduction) and biomass production (34–67% reduction). The former was affected only by direct WS effects (i.e., tissue expansion, leaf movements). The latter was affected additionally by indirect effects (i.e., those determined by reproductive sink activity). The larger response of biomass production than of cumulative IPAR to WS determined a significant (P < 0.05) decline in RUE with increased water deficit. Five experiments were conducted during 2001 and 2002 in North Carolina to evaluate peanut injury and pod yield when glyphosate was applied to 10 to 15 cm diameter peanut plants at rates ranging from 9 to 1,120 g ai/ha. Shikimic acid accumulation was determined in three of the five experiments. Visual foliar injury (necrosis and chlorosis) was noted 7 d after treatment (DAT) when glyphosate was applied at 18 g/ha or higher. Glyphosate at 280 g/ha or higher significantly injured the peanut plant and reduced pod yield. Shikimic acid accumulation was negatively correlated with visual injury and pod yield. The potential widespread adoption of cotton and soybean varieties with 2,4-D and dicamba resistance traits in the south eastern US will increase the risk of accidental exposure of peanut to these herbicides because of drift or application errors. When such accidents occur, growers must decide between continuing the crop and terminating it. In order to make this decision, growers need to estimate the potential yield reduction caused by 2,4-D or dicamba. Dose-response studies were conducted under field conditions in Citra and Jay, FL in 2012 and 2013 to determine peanut injury and yield reduction after exposure to 70, 140, 280, 560, and 1120 g ae ha−1 of 2,4-D or to 35, 70, 140, 280, and 560 g ae ha−1 of dicamba at 21 and 42 d after planting (DAP). Only herbicides by rate interactions were significant (P < 0.04). Dicamba caused 2 to 5 times higher peanut injury and 0.5 to 2 times higher yield reductions than 2,4-D. Injury ranged from 0 to 35% when peanut plants were treated with 2,4-D and from 20 to 78% with dicamba. The maximum yield reduction was 41% with 1120 g ha−1 of 2,4-D and 65% with 560 g ha−1 of dicamba. Linear regression indicated that the intercept for yield reduction was 12% for 2,4-D and 23% for dicamba, and there was a 2.5% and 7.7% increase in yield reduction per additional 100 g ha−1, respectively. Although high variability was observed for the different variables, there was a positive correlation between injury and peanut yield reduction (P < 0.0001) with Pearson’s Rho values ranging from 0.45 to 0.59 for 2,4-D and from 0.27 to 0.55 for dicamba. To study the effect of different preemergence herbicides on physiological growth parameters of groundnut (Arachis hypogaea L.) a field trial was conducted at Central farm, OUA&T, Bhubaneswar during Rabi 2014-15. This experiment was operated with five treatments such as weedy check (control), butachlor, alachlor, oxyfluorfen and weed free check (hand weeding) with three replications under Randomized Block Design (RBD). Studies revealed that among all pre-emergence herbicides application of butachlor (50% EC) @ 1000 ml ha-1 effectively controls weed population and recorded the highest values

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Physiology of the Peanut Plant

Fig. 5.10. Pod yield per hectare due to effect of pre-emergence herbicides on groundnut var. Devi

of growth. In contrast to this, application of alachlor adversely affected and noted the lowest growth value. The physiological growth parameters such as LAI, SLA, SLW, LAR, RGR, NAR and CGR significantly responded to herbicides application over the weedy check (control). All the above mentioned physiological parameters are highly correlated with pod yield of groundnut (Fig. 5.10). In Argentina, delayed sowing causes a decrease in seed yield and in radiation use efficiency (RUE) of peanut crops (Arachis hypogaea L.), but it is not known if RUE reduction is mainly due to reduced temperature during late reproductive stages or to a sink limitation promoted by a decreased seed number in these conditions. Seed yield determination and RUE dynamics of two cultivars (Florman and ASEM) in four irrigated field experiments (Expn) grown at three sites and five contrasting sowing dates (between 17 October and 21 December) in three growing seasons were analyzed. An additional field experiment was performed with widely spaced plants (i.e. with no interference among them) to evaluate the effect of peg removal on RUE and leaf carbon exchange rate (CER). Seasonal dynamics of mean air temperature and irradiance, biomass production (total and pods), and intercepted photosynthetically active radiation (IPAR) were followed. Seed yield and seed yield components (pod number, seeds per pod, seed number and seed weight) were determined at final harvest. Crop growth rate (CGR) and pod growth rate (PGR) were computed for growth phases of interest. RUE values for crops sown until 14 November were 1.89–1.98 g MJ−1 IPAR, within the usual range. RUE decreased significantly for cv. Florman in the late sowing of Exp1 (29 November) and for both cultivars in Exp3 (21 December sowing). Across experiments, seed yield (4.5-fold variation relative to minimum) was strongly associated (r2 = 0.87, P < 0.0001) with variations in seed number (3.5fold variation relative to minimum), and to a lesser extent (r2 ≤ 0.54, P ≤ 0.001) to variations in seed weight (1.9-fold variation relative to minimum). Seed number was positively related (P < 0.01) to CGR (r2 = 0.66) and to PGR (r2 = 0.72) during the R3–R6.5 phase (seed number determination window), while crop growth during the grain-filling phase (i.e. between R6.5 and final harvest) was positively associated with grain number (r2 = 0.80, P < 0.001). No association was found between RUE and mean air temperature, neither for the whole cycle nor for the phase between R6.5 and

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133

final harvest, which showed the largest temperature variation (16.4–22.4°C) across experiments. Use of mean minimum temperature records (range between 13.8 and 18.5°C) did not improve the relationship. However, grain-filling phase RUE showed a positive (r2 = 0.69, P = 0.003) linear response to seed number across experiments. This apparent sink limitation of source activity was consistent with the reduced RUE (from 2.73 to 1.42 g MJ−1 IPAR) and reduced leaf CER at high irradiance (from ca. 30 to 15 μmol m−2 s−1) for plants subjected to 75% peg removal. Peanut (Arachis hypogaea L.) production is frequently affected by unpredictable events of water deficit during pod set, which modulate water use, water use efficiency for biomass production (WUEB), and biomass partitioning to seeds. Studies on the effects of drought-induced impaired pegging on WUEB and the link between WUEB and photosynthetically active radiation use efficiency (PAR-UE). Field experiments were conducted that combined: two cultivars of contrasting pegging capacity (ASEM > Florman), two water regimes (irrigated and water stress) and different sowing dates. WUEB ranged between 6.1 and 6.7 g kPa/mm for irrigated plots, and between 2.9 and 7.1 g kPa/mm for water-stressed plots. WUE for pod production showed similar trends, but was larger for ASEM than for Florman because of higher biomass allocation to pods and pegging capacity of the former. The relationship between standardised values of WUEB and PAR-UE varied linearly for the post-R6 period, but fitted models differed between water regimes. This difference was attributed to the relative importance of stomata control on gas exchange (direct effects of water deficit) with respect to feedback effects on photosynthesis caused by reproductive sink size (indirect effects of water deficit). Variation in post-R6 PAR-UE could be linked exclusively to the latter, but variation registered in WUEB acknowledged both controls. More than 80% of the world’s peanut production comes from rainfed agriculture (Wright and Nageswara Rao, 1994). Erratic and insufficient rainfall is a major constraint of peanut production in rain-fed environments and water is increasingly becoming a scarce resource even in irrigated agriculture because of increasing demands for urban and industrial consumptions. Drought stress during the stages of pod and seed formation has shown to reduce pod yield (Nageswara Rao et al., 1989; Reddy et al., 2003; Vorasoot et al., 2003) and increase the likelihood of aflatoxin contamination (Rachaputi et al., 2002; Holbrook and Stalker, 2003). The relationships between pod yield and reproductive characters may be altered by drought. So far, the response of a genotype to drought in relation to reproductive characters and pod yield are not well understood. Previous studies indicated that the occurrence of drought during the vegetative phase has only a small effect on growth and yield of peanut (Nageswara Rao et al., 1988; Nautiyal et al., 1999). However, drought during the flowering and pod formation phases is severely detrimental to the yield of peanut (Nautiyal et al., 1999; Wright and Nageswara Rao, 1994) as it is lowered (Awal and Ikeda, 2002). Under water deficit conditions, pod yield was affected by decreasing pod growth and development (Reddy et al., 2003; Chapman et al., 1993) and drought also decreased the number of mature pods and pod yield (Nautiyal et al., 1999). Time to flowering is not modified by drought stress. Only severe water deficits of 35% of field capacity would delay flowering by about 1-2 days. However, the total flower number decreases when drought stress is experienced during the preor post-flowering stage. Reproductive development from meiosis to seed set is highly vulnerable to drought stress, which can cause pollen sterility, spikelet death or embryo abortion of newly formed seeds in groundnut. Drought stress either reduces or stops

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Physiology of the Peanut Plant

peg and pod development based on the degree of stress. The time of peg initiation, rate of peg elongation and pod initiation is delayed or inhibited by drought stress. Pod development is sensitive to drought stress, where it decreases the pod and seed growth. Adequate pod zone moisture is critical for developing pegs to pods; adequate water in the root zone cannot compensate for the lack of pod zone water for the 30 days of peg developing. However, after 30 days of adequate pod zone moisture, the pods can continue normal growth even if the pod zone is dry as long as roots can access adequate moisture. A decrease in dry matter, flower, peg and pod number, and a delay in peg and pod initiation and pod growth under drought stress conditions individually and in combination contribute to reductions in pod and seed yields. Reductions in pod yield are more pronounced when stress is imposed at the pod development and flowering phases than during the vegetative phase. Pod yield decreases in a linear fashion as the intensity of drought increases. The partitioning coefficient decreases from 0.52 (100% irrigation) to 0.24 (33% irrigation) as environments become less favourable. Under drought, already established pods have priority for partitioning of assimilates. Partitioning differences between genotypes are also attributed to ability of genotypes to initiate pods under drought conditions. The ability to produce pods under drought and the ability to recover from drought with greater pod growth are two different mechanisms for higher yields under drought conditions. Pod yield can be considered as a combination of the sequential processes of flower production, peg initiation, conversion of peg to pods and pod filling. The reproductive efficiency reflects the conversion of flowers, pegs and mature pods to pod yield. Multiple-linear regression was used to determine the relative contribution of yield component or reproductive efficiency to pod yield under FC, 2/3 AW and 1/3 AW. The analysis was based on the following statistical model (Hoshmand, 2006): Yi = α + β1X1i + β2X2i +…+ βnXni + δi where, Yi is pod yield of genotype i, α is the Y intercept, X1i, X2i and Xni are yield components or reproductive efficiency of genotype i, respectively, β1, β2 and β3 are regression coefficients for the independent variables X1, X2 and X3 and δi is the associated deviation from regression. The analysis was carried out by fitting the full model first and then determining the relative importance of the individual independent variables. A sequential fit was then performed by fitting the more important variables first. The relative contributions of the individual independent variables to pod yield under FC, 2/3 AW and 1/3 AW were determined from the percentages of regression sum of squares due to the respective independent variables to total sum of squares in the sequential fitted analysis. Drought significantly reduced pod yield by 51% at 2/3 AW and 82% at 1/3 AW conditions. ICGV 98348 and ICGV 98353 had high pod yield under FC. ICGV 98300 had high pod yield under both well-watered and drought conditions. The yield stabilizing strategy for this genotype should be largely due to their high starting yield at FC and in minor part due to their relatively low reductions (high DTI). ICGV 98324 exhibited high pod yield under drought conditions because of its high DTI (low yield reduction). Low reduction in pod yield was more important for stabilizing the yield of these genotypes under drought. The genotypes with poor performance for pod yield under both non-stressed and stressed conditions were Tifton-8 and KK 60-3. Tiffton-8 had the lowest starting point for yield (yield potential). The reductions in pod yield of these genotypes were also relatively high, indicating that they were most sensitive to drought. Two possible strategies of drought resistance

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Pod Growth and Yield

may be useful for explaining drought resistance in these peanut genotypes. Genotypes with high pod yield under drought conditions should be of either (1) high pod yield under well-watered conditions (e.g., ICGV 98300) or (2) ability to maintain a low rate of yield reduction under increasing stressed (high DTI) (e.g., ICGV 98324 and ICGV 98300) (Tables 5.7, 5.8 and 5.9). Table 5.7. Pod yield (g.plant-1) and drought tolerance index (DTI) for 11 peanut genotypes grown under different water regimes at harvest Pod yield (g.plant-1) Genotype ICGV98300 ICGV98303 ICGV98305 ICGV98308 ICGV98324 ICGV98330 ICGV98348 Tainan 9 KK60-3 Tifton-8 Mean

FC 7.66 6.79cd 7.36bc 5.97de 7.17c 7.01c 8.36a 7.23bc 7.34bc 5.48e 7.14

abc

2/3AW

1/3AW

DTI (2/3AW)

DTI (1/3AW)

4.90 3.71cd 3.44cd 3.48cd 4.51ab 3.33cd 3.76cd 3.27d 2.46e 1.10f 3.45

1.95 1.49bcd 1.07de 1.45cd 2.17a 1.18de 1.65bc 0.89ef 0.50fg 0.05g 1.28

0.65 0.55abc 0.46cd 0.59ab 0.62a 0.49bc 0.46cd 0.46cd 0.37d 0.22e 0.49

0.26ab 0.23bc 0.15de 0.25ab 0.31a 0.17cd 0.20bcd 0.13de 0.09e 0.01f 0.18

a

ab

a

Means in the same column with the same letters are not significantly different by Duncan’s multiple range test (DMRT) (at P ≤ 0.05). DTI for a genotype was calculated by the ratio of stressed. (2/3 available water (AW) or 1/3 AW)/non-stressed (field capacity; FC) conditions

Pegging and pod set responses of various peanut genotypes under drought varied substantially, leading to large differences in pod yield and the reductions in pod yield also varied among peanut genotypes (Nageswara Rao et al., 1989; Songsri et al., 2008). However, the genotypes producing the lowest number of flowers under normal conditions rarely produced high pod yield under water stress (Songsri et al., 2008). The peanut genotypes with extremely low flower production are therefore inferior to those with abundant flowers under drought conditions. However, in a parallel study, Songsri et al. (2008) did not provide the information in terms of reproductive efficiency contributing to pod yield. The results in this study indicated that the line ICGV 98324 with high reproductive efficiency had high HI and pod yield under severe drought conditions, and, under moderate drought conditions (2/3 AW), ICGV 98300 had high RET 4. In previous study, ICGV 98324 and ICGV 98300 were identified as drought resistant lines based on low specific leaf area (leaf thickness) and large root system, respectively (Songsri et al., 2009). The drought resistant trait in each genotype might contribute to reproductive efficiency and ultimately contribute to pod yield. Contribution of Reproductive Efficiency Traits (RET) to Pod Yield: Multiple regression analysis showed the contributions of RET to pod yield under FC, 2/3 AW and 1/3 AW conditions. The contribution of the conversion of reproductive sink to pods (RET 2) was highest under mild drought stress (59.03%). The contribution of the conversion of flower to mature pod (RET 4) was highest under severe drought stress conditions (71.08%). The result indicated that under mild drought stress (2/3 AW) the reproductive phase between after flower pollination to podding stage may

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Physiology of the Peanut Plant

Table 5.8. Harvest index (HI), number of mature pods per plant, 100 seed weight (g) and number of seeds per pod of 11 peanut genotypes grown under different water regimes at harvest Harvest index Genotype ICGV98300 ICGV98303 ICGV98305 ICGV98308 ICGV98324 ICGV98330 ICGV98348 ICGV98353 Tainan 9 KK60-3 Tifton-8 Mean

Mature pods plant-1

FC

2/3AW

1/3AW

FC

0.43 0.40abc 0.41abc 0.33d 0.41abc 0.37c 0.38bc 0.42ab 0.41abc 0.39abc 0.24e 0.38

0.36 0.32a-d 0.30cd 0.29d 0.37a 0.33a-d 0.31bcd 0.35abc 0.30d 0.21e 0.06f 0.29

0.14 0.13cd 0.15bcd 0.15bcd 0.24a 0.14bcd 0.19ab 0.18bc 0.10de 0.06e 0.00f 0.13

11.77 8.67cd 8.92cd 8.52cd 7.94cd 8.44cd 10.52b 9.10c 7.73d 5.75e 5.04e 8.40

a

ab

bcd

100 seed weight (g) Genotype

FC

2/3AW

ICGV98300 ICGV98303 ICGV98305 ICGV98308 ICGV98324 ICGV98330 ICGV98348 ICGV98353 Tainan 9 KK60-3 Tifton-8 Mean

31.34f 33.05ef 38.87bc 34.1def 40.40b 37.83bcd 32.30f 36.41cde 40.03bc 64.23a 60.54a 40.88

32.41cde 38.04abc 39.10abc 35.40bcd 42.60a 36.62a-d 30.84df 30.97de 36.56a-d 40.63ab 28.04b 35.56

2/3AW

1/3AW

5.58 4.21cd 4.25cd 4.75bc 4.96bc 4.48cd 6.00a 5.02bc 3.67de 2.85e 1.15f 4.27

2.13bcd 1.52de 1.79cde 2.33abc 3.04a 2.15bcd 2.81ab 2.85ab 1.25ef 0.63fg 0.00g 1.86

a

ab

No. seed per pod 1/3AW

15.67b 20.92b 16.20b 22.19b 29.93b 19.78b 19.52d 20.37b 19.21b 16.21b 0.00c 18.18

FC

2/3AW

1.69ab 1.77a 1.66abc 1.54c 1.74ab 1.67abc 1.70ab 1.71bc 1.77d 1.60b 1.34d 1.65

1.64abc 1.62abc 1.59bc 1.63abc 1.54bc 1.55bc 1.36cd 1.78bc 1.91a 1.59b 1.14d 1.58

1/3AW 0.92bc 0.98bc 1.16abc 1.19abc 1.22abc 1.23abc 1.31ab 1.45a 1.04abc 0.83c 0.00d 1.03

Means in the same column with the same letters are not significantly different by Duncan’s multiple range test (DMRT) (at P ≤ 0.05). Table 5.9. Contributions of yield components on pod yield Explained by regression (%) FC

2/3 AW

1/3 AW

Regression

96.44**

94.13**

97.47**

Mature pods

27.94

82.70**

89.22**

Seed size

41.83

8.88

5.96**

Seed pod

26.66

2.55

2 29**

* and ** are significant at 5% and 1% respectively.

be the most critical period to maintain high pod yield. The critical period was wider under more severe drought (1/3 AW). Under severe conditions, the fulfilment of all reproductive stages from flowering to mature pod were critical in maintaining high yield. Increasing pod yield while decreasing input costs is of major importance to

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137

peanut growers. Peanut seed is more expensive than most row crops, making it important to reduce seed requirements and costs, and increase yield. The single seeded precision sowing method (one seed in one planting hole) is a promising way to reduce cost and improve production (Wang et al., 1999; Li et al., 2004; Guo et al., 2008). The single seeded precision has long been used for grain crops, and may improve the propagation coefficient (Zhang and Wang, 1999; Zhao et al., 2011; Chen et al., 2012). Shen and Chen (1993) first studied the improving yield effect of uniformly singular seeding on summer peanuts covered with film. Wang et al. (1996) and Wan et al. (2006) reported through the studies of relations among peanut period, fertilization, density and production, that single-seed precision sowing with two lines in each ridge is better than single-seed precision sowing with three lines in each ridge for cultivation. Zhao et al. (2013) studied the relationship between planting density and the ecological characteristics of the population under the single-seed precision sowing mode and discovered that under the mode of single seeded precision, proper planting density increased the sowing-hole number, constructed a better growth environment for peanut population, and reduced the mutual influence among each plant. Feng et al. (2013) reported that single seeded precision sowing increases root growth, raises the root-shoot ratio, guaranteeing relatively stronger root growth predominance, promotes both individual and population coordination growth, and exerts the production potential of individual plants. However, there are fewer studies concerning single seeded precision sowing technology on peanut pod development. In the present study, Arachis hypogaea L. cv. Luhua 11 was used to determine the effects of planting density on pod development and yield of peanuts under the mode of single seeded precision sowing through field experiments. The results showed that an appropriate precision sowing density of 195,000-225,000 per hectare leads to an optimal pod number to produce pods, with dry matter accumulation resulting in significantly increased pod yield and harvest index (HI). In the same area sowing seed number of 255,000 per hectare, the kernel yield, pod yield, and HI per plant of the single seeded precision sowing method were higher than those of the double seeded precision sowing method (Fig. 5.11). There were a total of six treatments with different planting densities: 135,000 (M1), 165,000 (M2), 195,000 (M3), 225,000 (M4), 255,000 (M5) seeds ha-1, with a

Fig. 5.11. Effects of single-seeded precision sowing pattern on pod number in population

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single seed in each hole. Because peanuts were traditionally planted with double seeds in a hole, the planting density as the control (CK) is 127,500 holes ha-1 with double seeds in each hole, which had the same seed-planting amount with 255,000 (M5) seeds ha-1 under the single-seed sowing method (Table 5.10). Table 5.10. The effects of single-seed sowing pattern on peanut yield Treatment Biological yield

Pod yield

Kernel yield

Shelling percentage (%)

Economic coefficient

kg ha–1

%

Ml

9310.5

0.49b

4605.3c 100.7

3186.9c

92.2

90.9

69.2

M2

9344.0

0.54ab

5010.5b 101.0

3497.3b

100.3

99.7

69.8

M3

9613.5

0.58

a

5550.0

103.9

3973.8

a

111.1

113.3

71.6

M4

9652.5

0.56

a

5430.1

a

104.4

3877.1

a

108.7

110.6

71.4

M5

9307.5

0.55

a

5095.2

b

100.6 3612.5

ab

102.0

103.0

70.9

CK

9250.1

0.54ab

3506.6b

100.0

100.0

70.2

kg ha–1

%

a

4995.1b 100.0

kg ha–1

%

Note: Different small letters in the table meant significant difference among treatment at 0.05 level.

Canopy structure affects light efficiency and yield in peanut (Arachis hypogaea L.). The purpose of this study was to assess the influence of plant growth habit, planting pattern (inter and intra-row spacings), and seeding date on light interception and vegetative and reproductive growth, development, and yield of peanut. The two cultivars ‘Pronto’ (bunch-type) and ‘Florunner’ (spreading-type) were planted at a plant population density (PPD) of 95,000 ha−1 in 0.35 × 0.30, 0.70 × 0.15, and 1.05 × 0.10-m patterns in May, June, and August of 1982 and 1983 seasons at Gainesville, FL (lat. 29°38′N). The soil was an Arredondo fine sand (a loamy siliceous hyperthermic Grossarenic Paleudult). Standard commercial seedbed preparation, pest control, and irrigation practices were used to optimize growth. Beginning at 35 days after planting (DAP), plots were sampled at 2-week intervals for complete growth analysis in 1982 and for pod yield and market-grade analysis in 1982 and 1983. Leaf area index (L), canopy light interception, total dry matter and plant component yields (leaf, stem, branch, pod, and kernel) and their respective growth rates, as well as flower, peg and pod numbers were significantly affected (P < 0.05) by treatments. Leaf area index, light interception, pod and kernel yields were increased by treatments in the order of: May>June>August seeding date, 0.35 × 0.30 (square arrangement) > 0.70 × 0.15 > 1.05 × 0.10-m (hedge row) planting patterns, and Florunner > Pronto cv. Critical L’s (canopy light interception) were 4.3, 4.4 and 6.0 for the 0.35 × 0.30, 0.70 × 0.15, and 1.05 × 0.15-m planting patterns, respectively, indicating greater light interception in the square arrangement than in hedge rows. It was noted that percentage of sound mature kernels, as well as kernel size, were also greater for Florunner and the square planting pattern. It was concluded that early seeding of spreading-type cultivars (e.g., Florunner), planted in approximately square patterns, should maximize both vegetative and reproductive growth and development, capture more light, and produce more pod yield without sacrificing market quality. The response of ‘Georgia Red’ peanut (Arachis hypogaea L.) to elevated CO2 of 400 (ambient), 800, 1200, and

Pod Growth and Yield

139

1600 µmol mol-1, and photosynthetic photon flux (PPF) of 350 and 700 µmol m-2s-1 was evaluated under controlled conditions using the nutrient film technique (NFT). Growth chamber conditions included a 12 h photoperiod, 28/24°C thermoperiod and 70±5% relative humidity (RH). Plants were grown for 110 days using a modified halfHoagland nutrient solution with a pH range of 6.4-6.7 and an electrical conductivity of 1200 µS cm-1. Foliage fresh and dry weights increased with both CO2 and PPF, while harvest index (HI) increased with CO2 but was not significantly affected by PPF. Pod fresh weight declined with increased CO2 while pod dry weight increased with PPF. Seed yield was significantly enhanced by both increased carbon dioxide and high PPF. Elevated CO2 did not affect light or CO2 response curves. These results indicate that the level of PPF exerted a greater effect in enhancing peanut growth and seed yield in NFT than did the CO2 concentration. Peanut (Arachis hypogaea L.) may have one or more periods during development when low solar radiation intensity is particularly detrimental to high yield. The present studies were conducted in the field to determine the effect of shade on vegetative growth, partitioning of assimilates and yield components of peanut. In a 2-year experiment, 75% shade was applied for 7, 10, 14, or 21 day periods during flowering, pegging, podding, and maturing phases. The objective was to determine which reproductive phase was most sensitive to low solar radiation intensity. Flower number, peg development, pod formation, and dry matter accumulation and partitioning were measured at regular sampling intervals. Shade during the peak flowering period reduced the number of flowers per plant and inhibited peg formation. Shade during the pegging and podding phases reduced total peg and pod number and reduced pod dry weight. Shade during the maturing phase reduced seed fill as shown by reduced shelling percentage and a lower number of fruits achieving mature pod status. On an average, over all stages, 75% reduction of light intensity decreased the growth rate of vegetative parts by 85%, the reproductive growth rate by 67%, and the total biomass growth rate by 67%. It is noted that shade prior to podding increased partitioning to vegetative growth, by 20%, whereas shade during the podding phase (83 to 104 days) increased dry matter partitioning to pods by 127%. Seventy-five per cent reduction in solar radiation intensity reduced the yield of Florunner peanuts significantly only when the duration was for 14 or 21 day periods. Podding was the phase in which yield was most sensitive to shade with a 30% reduction in fruit yield from shade during 83 to 104 days of age. The maturing phase was next in sensitivity to shade, which decreased yield primarily by decreasing seed fill in existing fruits. Twenty-one days of shade at flowering did not reduce final fruit yield, since the plants had time to recover from the loss of active flowers and subsequently bear flowers and produce a normal pod load. In order to develop appropriate production practices, the effects of irrigation, inoculation and N fertilization on yield were investigated. Irrigation and inoculation each increased the yield by about 27%, and the effect of each of these factors was greater in the presence of the other. No significant yield difference (2992 kg/ha on average) was observed between the use of powdered peat or granular inoculants containing the same strains of rhizobia. However, a yield difference was observed between inoculants containing different strains of rhizobia. Nitrogen application at planting time did not increase the yield of uninoculated peanuts, but a split application, applied at planting 60 days later, increased the yield by 28% over the uninoculated control. Increasing the N application at planting decreased the yield and 100-kernel weight of inoculated peanuts (Table 5.11).

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Physiology of the Peanut Plant

Table 5.11. Effect of irrigation, inoculants, and nitrogen fertilizer on pod yield, kernel weight, SMK, and shelling of peanuts (Arachis hypogaea L.) in 1976 Pod yield (kg.ha-1)

Kernel yield (kg.ha-1)

SMK1 (kg.ha-1)

Shelling percentage

Unirrigated Irrigated No inoc. Peat inoc. Gran. inoc.

905 994 912 970 967

376 407 405 371 398

273 204 244 209 262

41.2 40.6 43.7 38.5 40.4

0 kg N.ha-1 25 kg N.ha-1 50 kg N.ha-1 100 kgN.ha-1

1096 953 918 810

496 409 370 290

310 252 232 161

44.8 43.4 40.1 35.3

Treatment

An experiment was conducted during the kharif season of 2010 to study the effect of different levels of added gypsum (0, 100, 200, 300 and 400 kgha-1) with RDF (NPK @ 25:50:20 kg ha-1) on growth and yield of groundnut (Arachis hypogaea L.). The results indicated that the effect of different levels of gypsum was significant on growth and yield of groundnut. Biological growth and yield attributes viz., plant height, number of branches plant-1, number of pegs, number of nodules, fresh weight of pod, dry weight (pod yield), straw yield were also significantly affected by the application of different levels of gypsum. The highest plant height and number of branches plant-1 at 90 DAS (23.16 and 9.80 cm respectively), number of pegs and number of nodules at 90 DAS (22 and 122 respectively), fresh weight of pod and dry weight (pod yield) (37.88 and 26.10 q ha-1 respectively) and straw yield (37.97 q ha-1) were found by the application of NPK – 25:50:20 kg ha-1 + gypsum @ 200 kg ha-1. However, it was evident from the present study that different growth parameters and yield of groundnut were increased significantly with application of gypsum @ 200 kg ha-1 (Table 5.12). Table 5.12. Effect of gypsum on yield attributes (per plant/g per plant) Treatments

Number of peg 45 DAS 90 DAS

Number of nodules 45 DAS

Fresh Dry weight weight (pod yield) 90 DAS of pod

Straw yield

T0

5

10

76

48

9.35

7.55

10.19

T1

10

20

128

110

17.50

13.40

18.09

T2

11

22

146

122

37.88

26.10

37.97

T3

9

18

96

74

23.95

17.05

23.02

T4

8

16

129

112

23.30

18.15

24.50

CD ( p = 0.05)

NS

4.10

19.81

35.39

12.78

7.78

12.40

The amount of dry matter accumulated in groundnut per unit input (light, water and nutrients) determines the efficiency of the production system. Crop growth rate, which indicates the rate of conversion of inputs into dry matter, is influenced by temperature. Temperatures above or below an optimum value reduce dry weight or biomass accumulation. An optimum temperature of 28°C has been identified for

Pod Growth and Yield

141

dry matter accumulation. Temperatures 4°C above or below this optimum reduce dry weight. Soil temperature is also important in determining groundnut yield as the pod growth occurs in the ground. Plant dry weight decreases with an increase in air and/or soil temperature. Pod yield is usually correlated positively with total dry matter accumulation and, therefore, any effect of temperature on total dry matter accumulation will affect pod yield. Once the peg penetrates the soil, the growth of the peg and that of the pod is influenced more by soil temperature than by air temperature. Maximum yield and quality will be produced when the geocarposphere temperature is between 21 and 29°C during pod addition and pod maturation periods. A 6 to 9°C increase in canopy temperature above 28°C and a 3 to 4°C increase in podding zone temperature above 23°C during the reproductive period resulted in adverse effects on pegging and pod formation. Pod production may also be reduced in circumstances where vigorous vegetative growth competes with reproductive organs for assimilates. Increased vegetative growth also involves greater stem elongation, which prevents the pegs from reaching the soil surface. Yield per unit area is reduced due to a decrease in the duration of the pod filling period under high soil temperature conditions. Combinations of high air and soil temperatures are especially detrimental to pod yields. Both high day and night temperatures result in reduced partitioning of biomass to yield in groundnut. Hot soil temperatures above 33°C significantly reduce pod dry weight and yield. Pod development is most seriously affected by the soil environment in the podding zone. When, in an experiment, the podding zone was submitted to hot soil temperatures of 37 to 39°C at successive 10-day intervals after peg penetration, it was shown that pod development was suppressed by the treatments given during the first 30 days, with greatest reductions occurring between 20 and 30 days after peg penetration. The optimum soil temperature for pod yields was in the range of 30 to 33°C. Soil temperature has a marked effect on reproductive growth and development of groundnuts, and day/night soil temperatures of 38/32°C compared with 26/20°C or 32/26°C, significantly reduce pod yields. This reduction was primarily due to soil temperature effects on the processes of pod initiation rate, pod growth rate, and 100% mature seed weight. Air and soil temperature both are important factors to determine the yield of groundnut as groundnut flowers develop aerially and as pods in the soil. The optimum soil temperature range for pod formation and development is between 31 and 33°C and soil temperatures above 33°C significantly reduce the number of mature pods and seed yields (Dreyer et al., 1981; Ono, 1979; Ono et al., 1974). However, Golombek and Johansen (1997) found that the greatest number of pods were produced at a slightly low range of mean soil temperatures i.e. between 23 and 29°C, while temperatures of 17°C and 35°C were sub- and supra-optimal, respectively. Prasad et al. (2000b) studied the individual as well as combined response of air and soil temperature on yield and yield components of groundnut. The effects of high air (38/22°C vs. 28/22°C) and high soil temperatures (38/30°C vs. 26/24°C) were imposed from flowering to podding. High air temperature had no significant effect on total flower production but significantly reduced the proportion of flowers setting pegs (fruit-set) and hence the fruit numbers. In contrast, high soil temperature significantly reduced flower production, the proportion of pegs forming pods, and 100-seed weight. The combined treatment of high soil and air temperatures reduced fruit-set and pod weight by 58 and 57% at podding and 49 and 52% at flowering, respectively, indicating high sensitivity to temperatures at the podding stage. The effects of high air and soil

142

Physiology of the Peanut Plant

temperature were mostly additive and without any interaction. Cox (1979) observed that accumulation of top dry weight in early growth was optimum at a weighted mean temperature of 27.5°C and no shoot growth was observed at 15.5°C indicating a positive linear function of growth above 15.5°C. However, a further increase in temperature above the optimum range may decrease dry matter production. Craufurd et al. (2002) observed that high temperature (38/22°C) significantly (P ≤ 0.001) reduced total dry weight of four groundnut cultivars (ICGV 86015, 796, ICGV 87282 and 47-16) by 20% to 35% as compared to the 28/22°C treatment. Similar results were obtained by Prasad et al. (2000b) in a poly tunnel study where the groundnut plants exposed to high air (38/22°C) and/or high soil temperature (38/30°C) significantly reduced total dry matter production, and its partitioning to pods and pod yields of groundnut. Cox (1979) reported that temperatures above 26/22°C (24°C mean temperature) reduced the pod weight per plant. Ong (1984) observed significant reduction in number of subterranean pegs and pods, seed size and seed yield by 30-50% at temperatures above 25°C. Using semi-closed chambers, Chen and Sung (1990) exposed peanut plants (cv. Li-Chih-Taze) to enriched CO2 atmospheres (1000 μL CO2 L-1) during two different growth periods, i.e., from pod formation (R3 stage) to final harvest (R8 stage) or seed filling (R5 stage) to final harvest. Groundnut plants produced more dry matter accumulation and higher pod yield in the enriched treatment (1000 μmol mol-1 CO2) as compared to the ambient treatment (340 μmol mol-1 CO2). The enrichment-stage effect on these parameters was not significant. Pilumwong et al. (2007) reported that above ground biomass of groundnut was increased by elevated CO2 (800 vs. 400 μmol mol-1) in both the low (25/15°C) and high (35/25°C) temperature treatments. Pod dry weight increased with increasing CO2 at 25/15°C, but was not different among CO2 levels at 35/25°C. At 25/15°C, pod dry weight was 50% higher than at 35/25°C. Highest above ground biomass production at 35/25°C, under 800 μmol mol-1 CO2, indicates that the high temperature regime chosen in this study was still in the optimum temperature range for biomass production of groundnut. Rao (1999) reported increased dry weight of shoots in the elevated CO2 (660 vs. 300 ppm) even at 40°C. Prasad et al. (2003) reported an increase in total dry matter production of groundnut with an increase in CO2 between temperatures of 32/22°C and 40/30°C. A further increase in temperature to 44/34°C decreased total dry matter under both ambient (350 μmol mol-1) and elevated CO2 (700 μmol mol-1). As the temperature increased from 32/22°C to 44/34°C, pod yield decreased by 89% and 87% under ambient and elevated CO2, respectively. With the same increase in temperature, the seed yield decreased by 90% and 88% under ambient and elevated CO2, respectively. Temperature and CO2 effects on the total dry matter, pod and seed yields were statistically significant, however, the interaction between temperature and CO2 for all yields were not significant. On average, total dry matter yield increased by 36% and both pod and seed yields increased by 30% under elevated CO2 across all the temperature regimes. The study showed that when the groundnut crop is exposed to high temperatures throughout the full season, total dry matter production is reduced at temperatures above 40/30°C (35°C), whereas the pod and seed yields are adversely affected above temperatures of 32/22°C (27°C). A study was undertaken to investigate the effects of early or late harvest times on the changing yield and yield component and the seed quality of peanuts. It was determined that harvest time had statistically significant effects on plant height, primary branch

Pod Growth and Yield

143

number, primary branch length, per plant pod number, per plant yield, 100 seed weight, shelling percentages, pod yield, fresh and dry weight, harvest index, oil content, protein content, carbohydrate and ash ratio and oleic/linoleic acid ratio. The highest pod yield was determined by the third harvest times in both years, because of a high pod per plant yield, per plant pod number, 100 seed weight, shelling percentage and harvest index. The protein and oil ratios were increased by delaying the harvest time, whereas the carbohydrate ratio decreased. In particular, it was revealed that the oleic/linoleic acid ratio increased because of increasing seed size and the amount of oleic acid throughout the delay in the harvest. As a result of this study although fourth and fifth harvest times were observed in terms of the highest 100 seed weight, there were problems due to the harvest lost and the formation of substances that threaten human health such as aflatoxin in the delayed harvest times. Therefore, there is no point waiting to take a very high pod yield at late harvest times. It was suggested that earlier cultivars or earlier planting and harvest times should instead be used for pod yield and healthy crops. Studies undertaken to evaluate the effect of harvesting time for optimum yield of groundnut pods for three Spanish varieties and to estimate yield losses as a result of harvesting time at two locations, namely: Nampula Research Station (PAN) and Mapupulo Agricultural Research Center (CIAM) in Nampula and Cabo Delgado provinces respectively. The experiment was laid out in a randomized complete block design in a split-plot arrangement with four replicates. The varieties (ICGV-SM-99568, ICGV-SM-01514 and JL-24) were the main factors and three harvesting times (10 days before physiological maturity, at physiological maturity and 10 days after physiological maturity) were the sub-plots. Highest pod yields of 1276.9 and 1503.6 kg/ha were recorded at CIAM and PAN as a result of harvesting at physiological maturity compared to harvesting 10 days before (904.6 and 950 kg/ ha) and 10 days after (826.8 and 1047.4 kg/ha) physiological maturity. Furthermore, yield losses ranged from (16-25%) and (30-40%) as a result of harvesting groundnut 10 days before and 10 days after physiological maturity respectively. It is therefore advisable that farmers’ harvest their groundnut crop at physiological maturity in order to obtain maximum pod yields of the groundnut (Fig. 5.12). Effects of temperature × photoperiod interaction on vegetative and reproductive growth were examined in three selected groundnut genotypes by growing them in controlled-environment growth chambers with three temperature regimes (22/18°C, 26/22°C, and 30/26°C, day/night) under long (12 h, long day), and short (9 h, short day) photoperiods. The effect of photoperiod on the total dry matter production (TDM) was significant with the genotypes producing 32-72% greater dry matter under LD than SD. Temperature × genotype interaction effects were significant, with the dry-matter production being greatest at 26/22°C and least at 30/26°C and 22/18°C in two of the three genotypes. Leaf area (LA) was greater under LD than SD at all temperature regimes. LA accounted for 76% of the variation in shoot + root dry weight (R2 = 0.76. P < 0.01). A lack of relationship between LA and pod weight or pod numbers suggested that the pod development is controlled by factors other than carbon assimilation. The temperature × photoperiod interaction was significant for root growth, with the root weight being maximal and photoperiod effects being minimal at 22/18°C, while at 26/22°C, root weight declined and photoperiod effects became prominent. Low temperature (22/18°C) affected the reproductive development by reducing the proportion of reproductive nodes in total (vegetative + reproductive)

144

Physiology of the Peanut Plant

Fig. 5.12. Effect of harvesting time on groundnut kernel yield

Mean values followed by a common letter do not differ significantly according to Fisher’s

protected LSD test at P = 0.05.

nodes. The conversion of pegs into pods, as indicated by pod to peg ratio (PPR), was lower in LD than in SD conditions. Results suggested that the PPR could be used as an indicator of genotypic sensitivity to photoperiod in groundnut (Table 5.13). Table 5.13. Reproductive node ratio (RNR), peg to reproductive node ratio (PRNR), pod to peg ratio (PPR) of three groundnut genotypes grown under two photoperiods (LD and SD) and three temperature regimes Genotype

Day/Night(°C)

RNR

PRNR

PPR

LD

SD

LD

SD

LD

SD

22/18 26/22 30/26 Mean NC Ac 17090 22/18 26/22 30/26 Mean VA 81B 22/18 26/22 30/26 Mean Overall Mean

0.11 0.33 0.22 0.22 0.08 0.27 0.31 0.22 0.20 0.28 0.21 0.23 0.22

0.16 0.23 0.20 0.20 0.09 0.27 0.22 0.19 0.17 0.28 0.22 0.22 0.21

1.39 1.63 1.27 1.43 1.84 2.08 2.22 2.05 1.43 1.68 1.52 1.54 1.67

1.37 1.94 1.56 1.62 1.74 1.84 1.34 1.64 1.39 1.61 1.39 1.46 1.57

0.49 0.42 0.29 0.40 0.02 0.21 0.06 0.10 0.54 0.46 0.29 0.43 0.31

0.49 0.66 0.56 0.57 0.01 0.53 0.36 0.30 0.54 0.42 0.48 0.48 0.45

SEDs

0.017 0.018 0.022 0.048 0.048 20

TMV2

CV (%)

(a) G over T&P (b) P over G&T (c) T over T&P (d) T effects (e) P effects

0.025 0.057 0.069 0.111 0.111 11

0.048 0.018 0.023 0.010 0.010 31

Pod Growth and Yield

145

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Pattee, H.E., E.B. Johns, J.A. Singleton and T.H. Sanders. 1974. Composition changes of peanut fruit parts during maturation. Peanut Science, 1: 57-62. Periasamy, K. and C. Sampoornam. 1984. The morphology and anatomy of ovule and fruitdevelopment in Arachis hypogaea L. Annals of Botany, 53: 399-411. Pilumwong, J., C. Senthonga, S. Srichuwongb and K.T. Ingram. 2007. Effects of temperature and elevated CO2 on shoot and root growth of peanut (Arachis hypogaea L.) grown in controlled environment chambers. Science Asia, 33: 79-87. Prasad, P.V.V., P.Q. Craufurd and R.J. Summerfield. 1999b. Fruit number in relation to pollen production and viability in groundnut exposed to short episodes of heat stress. Annals of Botany, 84: 381-386. Prasad, P.V.V., P.Q. Craufurd and R.J. Summerfield. 2000b. Effect of high air and soil temperature on dry matter production, pod yield and yield components of groundnut. Plant Soil, 222: 231-239. Prasad, P.V.V., K.J. Boote, L.H. Allen Jr. and J.M.G. Thomas. 2003. Super-optimal temperatures are detrimental to peanut (Arachis hypogaea L.) reproductive processes and yield at both ambient and elevated carbon dioxide. Global Change Biology, 9: 1775-1787. Putnam, D.H. , E.S. Oplinger, T.M. Teynor, E.A. Oelke, K.A. Kelling and J.D. Doll. 1991. Peanut. In: Alternative Field Crop Manual. University of Wisconsin-Extension, Cooperative Extension. Qin, L.Q., L. Li, C. Bi, Y.L. Zhang, S.B. Wan et al. 2011a. Damaging mechanisms of chilling and salt stress to Arachis hypogaea L. leaves. Photosynthetica, 49: 37-42. Rachaputi, N.R., G.C. Wright and S. Krosch. 2002. Management practices to minimise preharvest aflatoxin contamination in Australian peanuts. Aust. J. Exp. Agric., 42: 595-605. Rao, K.V. 1999. The combined effect of elevated CO2 levels and temperature on growth characteristics of groundnut (Arachis hypogaea L.). Indian J. Plant Physiol., 4: 297-301. Reddy, T.Y., V.R. Reddy and V. Anbumozhi. 2003. Physiological responses of peanut (Arachis hypogaea L.) to drought stress and its amelioration: A critical review. Plant Growth Regu., 41: 75-88. Sarkar, T., R. Thankappan, A. Kumar, G.P. Mishra, J.R. Dobaria et al. 2016. Stress inducible expression of AtDREB1A transcription factor in transgenic peanut (Arachis hypogaea L.) conferred tolerance to soil‐moisture deficit stress. Frontiers in Plant Science, 7: 935. Shen, Y.J. and W.M. Chen. 1993. Studies on seed reduction and hole increase of plastic-mulching summer-planting peanut. Laiyang Agri. Colle., 10: 1-4. Songsri, P., S. Jogloy, N. Vorasoot, C. Akkasaeng, A. Patanothai and C.C. Holbrook. 2008. Root distribution of drought resistance peanut genotypes in response to drought. Journal of Agronomy and Crop Science, 194: 92-103. Songsri, P., S. Jogloy, C.C. Holbrook, T. Kesmala, N. Vorasoot et al. 2009. Association of root, specific leaf area and SPAD chlorophyll meter reading to water use efficiency of peanut under different available soil water. Agric. Water Manage., 96: 790-798. Sun, L., R. Hu, G. Shen and H. Zhang. 2013. Genetic engineering peanut for higher drought‐ and salt‐tolerance. Food and Nutrition Science, 4: 1-7. Talwar, H.S., H. Takeda, S. Yashima and T. Senboku. 1999. Growth and photosynthetic responses of groundnut genotypes to high temperature. Crop Sci., 39: 460-466. Thompson, L.K., C.L. Burgess and E.N. Skinner. 1992. Localization of phytochrome during peanut (Arachis hypogaea) gynophore and ovule development. American Journal of Botany, 79: 828-832. Vara Prasad, P.V., P.Q. Craufurd and R.J. Summerfield. 1999a. Sensitivity of peanut to timing of heat stress during reproductive development. Crop Science, 39: 1352-1357. Vara Prasad, P.V., P.Q. Craufurd and R.J. Summerfield. 1999b. Fruit number in relation to pollen production and viability in groundnut exposed to short episodes of heat stress. Annals of Botany, 84: 381-386. Vorasoot, N., P. Songsri, C. Akkasaeng, S. Jogloy, A. Patanothai et al. 2003. Effect of water stress on yield and agronomic characters of peanut (Arachis hypogaea L.). Songklanakarin J. Sci. Technol., 25: 283-288.

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Wan, S.B., Y.P. Zheng, D.Z. Liu, B. Cheng, F. Wu et al. 2006. Optimization of peanut-wheat intercropping system on date, fertilizer and density. Chin. J. Oil Crop Sci., 28: 319-323. Wang, C.B., B. Cheng, Y.C. Chi, X.S. Sun, J.M. Zhang et al. 1996. Study on population density of high yield peanut under single-seed planting. Peanut Sci. Tech., 3: 17-19. Wang, C.B., B. Cheng, Y.P. Zheng, Y.C. Chi, X.S. Sun et al. 1999. Effects of different planting pattern and density on yield, yield characters and canopy characters under high-yield conditions. J. Peanut Sci., 1: 12-14. Westgate, M.E. and J.S. Boyer. 1986. Reproduction at low silk and pollen water potentials in maize. Crop Science, 26: 951-956. Wheeler, T.R., A. Chatzialioglou, P.Q. Craufurd, R.H. Ellis, R.J. Summerfield et al. 1997. Dry matter partitioning in peanut exposed to high temperature stress. Crop Science, 37: 15071513. Williams, J.H., J.H. Wilson and G.C. Bate. 1975. The growth of groundnuts (Arachis hypogaea L. cv. Makulu Red) at three altitudes. Rhodosian Journal of Agricultural Research, 13: 33-43. Wright, G.C. and R.C. Nageswara Rao. 1994. Peanut water relations. pp. 281-325. In: Smartt, J. (ed.). The Peanut Crop. Chapman and Hall, London. Young, C.T., H.E. Pattee, W.E. Schadel and T.H. Sanders. 2004. Microstructure of peanut (Arachis hypogaea L. cv. ‘NC 7’) cotyledons during development. LebensmittelWissenschaft Und-Technologie, 37: 439-445. Zhang, Z.X. and Y.L. Wang. 1999. Single-grain planting high breeding technology for new peanut variety Huaxuan 1. J. Peanut Sci., 1: 33-34. Zhao, C.X., C.L. Shao, Y.F. Wang, C.X. Song, M.L. Wang et al. 2013. Effects of different planting densities on population ecological characteristics and yield of peanut under the mode of single-seed precision sowing. J. Agri., 3: 1-5. Zhao, X., B.J. Tang, B.R. Feng, M.T. Liu, L. Huang et al. 2011. Primary exploration on harvest date and seed dehydration rate in later developmental period of single-grain-sown maize varieties. Acta Agri. Jiangxi., 23: 44-45.

CHAPTER

6

Plant Nutrition Unlike other plants, groundnut nutrition is unique as the pod develops under the soil and most of the seed nutrition is directly through the pod rather than from that transported from the root, shoot and back to the seed. Groundnut is an exhaustive crop and depending upon the yield it removes large amounts of macro and micronutrients. To produce 2.0 to 2.5 t/ha of economic yield, the groundnut requires, 160-180 kg of N, 20-25 kg of P, 80-100 kg of K, 60-80 kg of Ca, 15-20 kg of S, 30-45 kg of Mg, 3-4 kg of Fe, 300-400 g of Mn, 150-200 g of Zn, 140-180 g of B, 30-40 g of Cu and 8-10 g of Mo (NRCG, 1993, 1994; Singh and Chaudhari, 1995; Singh et al., 1990, 1995; Singh and Joshi, 1993). The dry matter accumulation in the groundnut crop follows the growth pattern characterized by a lag phase in early growth, exponential increases in weight from the vegetative to the flowering stage, a linear and maximum growth rate during late vegetative growth to early pod filling, and decline in weight during the late pod filling stage. There is a similar trend in the nutrient absorption and uptake in groundnut and the maximum amount is needed at the peak stage of growth. Being comparatively a drought tolerant crop with low transpiration, the groundnut is susceptible to nutritional disorders due to an insufficient supply of minerals (Beringer and Taha, 1976). The groundnut crop removes 4.6-11.9, 42.3-88.1 and 6.4-53% of the total nutrient during vegetative (0-25 days), reproductive (25-75 DAE) and pod development (75-105 DAE) stages, respectively (Polara et al., 1991). Leaf analysis is not well accepted as a good diagnostic tool for groundnuts; the final yield and quality of nuts do not generally relate well to leaf composition during growth. One factor that is probably involved is the restricted downward phloem movement of nutrients from the plant parts above the ground to the developing pods. However, leaf analysis is still important as a measure of the nutritional health of the plant through much of its development. Careful attention to plant age is necessary when interpreting foliar nutrient concentrations. Cox et al. (1970) found that N and P concentrations decreased steadily from plant age 2 to 21 weeks; K increased nearly a full per cent from week 2 to week 6 before decreasing slightly with further age; Ca and S were erratic but did not decrease significantly over the whole time period; Mg tended to increase slowly up to weeks 10-12 and then decreased slightly; Mn decreased slightly throughout; Zn increased up to week 6, then decreased markedly till week 21; Cu and B changed only slightly (Tables 6.1 and 6.2). Sufficiency ranges are:

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Physiology of the Peanut Plant Table 6.1. Plant analysis data (sufficiency levels for leaves) – Macronutrients

Plant part

Time

7 leaf

40 DAP

th

Upper mature Bloom leaves

% of dry matter N

P

K

Mg

Ca

3.3-3.9

0.15-0.25

1.0-1.5

0.30

3.0-4.5

0.20-0.50

1.7-3.0

0.30-0.80 1.25-2.0 0.20-0.35

2.0

S 0.19-0.25

Source: Gillier and Silvestre, 1969; Plank, 1989

Data from China for various plant parts at different stages of development are shown below. Table 6.2. Plant analysis data (China) – Macronutrients Plant part

Time

Leaf Leaf

% of dry matter N

P

K

Mg

Ca

Bloom

3.97

0.09

1.06

1.87

2.07

Maturity

2.95

0.26

0.65

1.79

2.00

Stem

Flowering

1.58

0.22

2.17

1.03

1.11

Stem

Maturity

1.15

0.12

0.79

1.63

1.12

Pod

Maturity

1.20

0.25

1.20

0.85

1.20

Seed

Maturity

4.76

0.46

0.24

0.58

0.17

Plant

Seedling

3.94

0.19

1.52

0.63 (root)

1.20 (leaf)

Plant

Flowering

3.86

0.23

1.31

0.76 (root)

1.64 (leaf)

Plant

Flowering

3.48

0.24

1.64

2.11 (leaf)

0.49 (young pod)

Plant

Maturity

3.70

0.28

1.59

2.70 (leaf)

0.89 (pod)

Plant

Maturity unfertilized

2.52

0.17

0.86

0.43

0.53

Plant

Maturity fertilized

2.61

0.22

0.79

0.41

0.48

Source: Academia Sinica, 1977; Wang Zaixu et al., 1982

Only 10-20% of the total uptake of nutrients occurs during the vegetative stage, the remainder being divided almost equally between the reproductive and ripening stages (Table 6.3). Table 6.3. Partitioning of total uptake of macronutrients by growth stage Growth stage

Percentages of total uptake N

P

K

Mg

Ca

Vegetative

10

10

19

11

10

Reproductive

42

39

28

48

53

Ripening

48

51

53

41

37

Source: Adapted from Longanathan and Krishnamoorthy, 1977

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151

Some workers believe that nutrient ratios are more important than individual concentrations. Roche et al. (1959) recommended ratios in the leaves of N: (N+P+K) = 0.5 to 0.65, P: (N+P+K) = 0.03 to 0.05, and K: (N+P+K) = 0.32 to 0.40. Since Ca is especially important for nut development, it has received considerable attention. Gaines et al. (1991) found the maximum yield and grade for the smallseeded runner type with 0.12% Ca in the hulls and 0.04% Ca in the nuts, and for the large-seeded Virginia type with 0.19% and 0.058% respectively. The accumulation of macro- and micronutrients at 150 DAE in descending order was: N>K>Ca>Mg>S>P and Fe>Zn>Mn>Cu>B, respectively (Fig. 6.1). According to Malavolta et al. (1997), the majority of crops, in general, obey the N>K>Ca>Mg>P ≈ S order of macro- and Fe>Mn>Zn>Cu ≈ B order of micronutrients. However, in peanuts, an inversion of Mn with respect to Zn occurs.

Fig. 6.1. Accumulation of macro- and micronutrients at 150 DAE in peanut

6.1.

Nitrogen

Groundnut produces approximately 36 kg of biomass per/kg of nitrogen assimilated (Nambiar, 1990). This large amount of nitrogen is supplied to the groundnut plant mainly by its root nodules. The groundnut absorbs mainly nitrogen from nitrates. The Rhizobia fix atmospheric nitrogen and supply it to the host plant in the form of an amide to be incorporated into proteins through the glutamyl phosphate pathway. The Virginia cultivars show high acetylene reduction activity, accumulate N at a faster rate, more in the vegetative parts and maintain a higher percentage in leaves than the Spanish groundnut, but this accumulation in leaves and stems ceases after the onset of pod development (70 DAS) (Tonn and Weaver, 1981). Williams (1979) reported that the crop accumulated 2.39 kg N/ha/day during vegetative growth and 3.77 during the first half of the reproductive growth after which N accumulation ceases with the cessation of vegetative growth. It is further reported that a total of 30 g N/m2 was

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Physiology of the Peanut Plant

accumulated by the crop and 66% of this was in the kernel. Nitrogen is an important constituent of proteins, chlorophyll, amino and nucleic acids, and is required for the vegetative and reproductive growth, nutrient absorption, photosynthesis and production of assimilates for developing pods. It also plays an active role in the enzyme reactions and energy metabolism. The nitrogen requirement of groundnut is much higher than cereals because of its high protein content; however, most of the soils where groundnut is grown in the world are deficient in nitrogen. The rationalization of fertilization programs requires the definition of stages of development during which a crop has higher nutritional requirements. Otherwise, in general, better management of fertilization would consist of application of the nutrients required at the exact moment before specific developmental stages of the plant (Nascimento et al., 2012). The establishment of this data and values are being conducted by studies termed “absorption march”, which consist of research intended to establish the absorbed amounts of nutrients according to the age and/or physiological stage of a plant (Echer et al., 2009). Thus, the determination of nutrient uptake and of accumulation during the different phases of plant development is important because it allows for identifying the times at which elements are required most during development of the crop and the distribution of the elements in the different structures of the plant, allowing adequate fertilization management. The efficiency of utilization of applied fertilizers and the fraction of nutrients supplied by the soil also should be taken into account (Laviola and Dias, 2008; Rosolem et al., 2012). The chemical composition as well as the accumulation of nutrients in leaves and fruits are essential information to determine the nutritional requirements of a plant. Subsequently, this information can aid estimation of the amount of nutrients to be supplied to plants through fertilization. To represent the total accumulation of the main nutrients, graphs of accumulation of macro- and micronutrients were constructed considering 120 days after the emergence (DAE) of the plants as a basis. All data obtained regarding the absorption rate of macro- and micronutrients and the growth rate of the plants were subjected to analysis of variance (using Sisvar® software 5.6) and regression analysis (using Microsoft Office Excel 2010 software), using the accumulation of nutrients, the dependent variable (Y) collection times of the plants, and the phenological stages of the crop, the independent variable (X). When evaluating the rate of N absorption in peanut plants, from 30 to 80 DAE an increase in the values of this variable were observed. These values remained in the range of 5.5 g day-1 until 120 DAE, which corresponded to the end of the vegetative phase and the beginning of the reproductive phase, from flowering to the beginning of grain filling. From 120 DAE, there was a sharp decrease in the rate of N uptake, reaching approximately 1 g day-1 until the end of the crop evaluation cycle (Fig. 6.2). Lobo et al. (2014), working with omission of nutrients in the culture of peanuts, concluded that, omission of N, P and K was most limiting in the treatments applied, demonstrating the importance of the application of these nutrients to peanut plants. Because it is a legume, the peanut plant fixes, under most conditions, sufficient amounts of N through the symbiotic association with bacteria of the genus Bradyrhizobium. According to Freire (1992), peanut plants fix up to 297 kg ha-1 year-1 of N through biological N fixation. In an area with an expected production in the range of 3 t ha-1 of pods, about 190 kg of N are removed (Bolonhezi et al., 2005). According to Stancheva and Dinev (1995), the maximum content of the photosynthetic pigments in corn and wheat leaves was observed in plants that exhibited the greatest vegetative growth. Correia et al. (2012) reported that the increase in peanut production is related

Plant Nutrition

153

to the increase in chlorophyll concentration in the leaves due to higher N absorption. Studies show that adequate B nutrition improves root uptake of phosphorus (P) and potassium (K) by maintaining proper functioning (through ATPase activity) and structure of root cell membranes. Boron has an important role in colonization of roots with mycorrhizal fungi, which contributes to root uptake of P.

Fig. 6.2. Nitrogen absorption rate in peanut (Silva et al., 2017)

The nitrogen deficient crop shows slow and stunted growth with a weak and prolonged stem and pale to yellowish green coloration of the older leaves. The chlorosis starts at the leaf tips, and dries up with pale brown necrosis. Nitrogen is very mobile and as the older leaves dies, it is mobilized from older to younger leaves in the form of amines and amides. Since there is no early senescence of leaves in groundnut, the characteristic deficiency symptom of nitrogen is the appearance of uniform yellowing of leaves including the veins due to decomposition of chloroplasts, being more pronounced on older leaves (Fig. 6.3). The Spanish, and Valencia bunch groundnut respond more to nitrogen than the Virginia cultivars because of lesser N2-fixation and short crop duration. The Native Bradyrhizobium are abundant and apparently able to fix adequate N at most of the places in India (except the rice fallows), China, USA and other groundnut growing countries leading to a lesser response to its inoculation. In Cyprus, the use of inoculation is advantageous to groundnut since it results in a 42% higher N yield over uninoculated one (Papastylianou, 1993). Thus, N is not needed unless the site is excessively low in N or effective Bradyrhizobium, otherwise it may lead to excessive growth and a decreased harvest index and pod yield. Increasing the levels of N, increased the concentrations of P, K, Ca and Mg in leaves, but did not affect S and Mg (Singh, 1999a). N is the most important nutrient involved in many processes of crop plants. Various strategies have been employed to improve its utilisation efficiency. Nitrogenous NFs have been reported by various scientists around the world (Millan et al., 2008; Kottegoda et al., 2011; Malekian et al., 2011; Perrin et al., 1998). For example, slow release of N was observed when urea (ammonium) was coated on zeolite chips (Millan et al., 2008). Similarly, urea-modified hydroxyapatite NPs were

154

Physiology of the Peanut Plant

Fig. 6.3. Nitrogen deficiency in peanut

encapsulated under pressure into the cavities of soft wood of Gliricidia sepium, and were tested for slow and sustainable release of N into the soil. Interestingly, N supply through this strategy was found optimum up to 60 days compared to conventional nitrogenous fertilizers, which gave more N supply to the plants in the beginning and very low at the later stage up to 30 days (Kottegoda et al., 2011).

6.2.

Phosphorus

The element is an essential macro-nutrient needed for crop growth and high yield. It is mainly supplied through chemical fertilizer application (George and Richardson, 2008). P fertilizer is also needed in many vital plant processes such as photosynthesis, root formation and nitrogen fixation (Grant et al., 2000), energy transformation and seed formation (Asibuo et al., 2008). The phosphorus appears in relatively small amounts in peanut plants, but they have the ability to absorb it from soils with a poor phosphorus content. Phosphorus has the function of transporting, accumulating and using energy (Tasso Jr., 2004). This element is considered the main productivity factor of the peanut crop, although it is extracted in smaller quantities compared to other macronutrients (Bolonhezi et al., 2005). According to Feitosa et al. (1993), more than 70% of the P absorbed by the peanut plant accumulates in the fruits, which shows the importance of this element in the formation and development of fruits. Phosphorus limits N2-fixation either directly by affecting nodule initiation, nodule development and N2-fixation or indirectly by affecting plant growth. The phosphorus increases the shelling percentage, oil yield and nodulation in groundnut. On a global level P is the most deficient element, and hence P deficiency is restricted to the areas which are not fertilized. A higher rate of P absorption was observed in peanut plants between 90 and 110 DAE, with a peak absorption at 110 DAE and a P absorption rate equivalent to approximately 0.16 g day-1. During that evaluation time, there was a decrease in the rate of P absorption to values below 0.03 g day-1 until the end of the crop cycle (Fig. 6.4).

155

Plant Nutrition

Fig. 6.4. Rate of P absorption in peanut (Silva et al., 2017)

In groundnut, P deficiency causes purpling of the leaf margin and stunted growth but a darker green colour. The deficiency first occurs on older leaves and later spreads to other leaves from the bottom, but it takes a minimum of 4 weeks for deficiency symptoms to appear on the plants. Some older leaves also show yellow symptoms of P deficiency. The older leaves become orange yellow, then brittle and finally fall. The low P availability decreases nodulation and the nitrogen fixation rate. The critical levels of P in Australian soil (required to attain a maximum yield of 90% is 7.3 ppm and 7.9 ppm for pods and kernels respectively at a depth of 10 cm (Bell, 1985b). However, using 0.5 M NaHCO3 the critical levels of P in Indian soil have been reported at 8.3 (Singh and Rana, 1979) and at 10 ppm (Cox et al., 1982) in soils in the USA. In laterite soils of India, the critical levels of available P are reported to be 5-10 ppm for groundnut depending upon the soil types, climate and groundnut genotypes (Dwivedi, 1988). The leaf at flowering is the most standard plant part and age to determine the critical level and it is 0.29% of P (Foster, 1980). However, Dwivedi (1986) reported 0.18% of P as the critical concentration in leaves. Bell (1985b) reported that the critical concentration of the uppermost fully expanded leaves was 0.3% and did not change with time during vegetative growth, but declined in a linear fashion over time during the reproductive phase and it was 0.27% at 60 DAE and 0.12% at 100 DAE. The leaf P content, at flowering is the most standard plant part and age to determine the critical levels and it is reported to be between 0.20-0.3% P by various workers, however for Indian groundnut the critical leaf P content is 0.22% for Spanish and 0.25% for Virginia cultivars. The sufficiency level of P concentration in leaves, at flowering, is reported to be 0.26-0.35% for Spanish and 0.29-0.50% for Virginia cultivars, which did not change during the vegetative phase but declined over the time and was lowest (0.12-0.18%) at harvest (Singh, 1999a). However, the sufficiency level of P in the seed at harvest is 0.4-0.62% (Singh, 1999a). Increasing the levels of P, increased the concentration and uptake of P in groundnut leaves, stems, seeds and shells. Otani (1997) reported that cell wall activity and P solubilizing activity of groundnut was superior indicating that it has a superior ability to take up P from soils with a low P content. Using “a double pot system”, it was demonstrated that Ca-P and Al-P fractions of the accumulated P in the acid soil around the mycorrhizal

156

Physiology of the Peanut Plant

root are mobile and available to the groundnut plant. Using 15N technique, the effect of Ca level on yield and nitrogen fixation of groundnut was assessed in Vietnam by Dang et al. (1997), where the highest values for all parameters was obtained at 213 kg Ca ha-1. Groundnut forms a symbiotic association with certain zygomycetous fungi known as vesicular buscular mycorrhiza (VAM). The VAM fungi are known to augment the plant phosphorus (P) uptake ability from soils deficient in this element. Groundnut roots show extensive VAM colonization. Inoculation with Gigaspora calospora resulted in highest growth stimulation while Glomus mosseae resulted in highest P uptake (ICRISAT, 1986). In a study conducted at ICRISAT, the mycorrhizal inoculated plant showed higher shoot dry matter and total P available P levels between 2.45 and 12.25 ppm with a rapid increase from 2.45 to 4 ppm (ICRISAT, 1986). However, Singh and Chaudhari (1996b) in a pot study reported that the inoculation of VAM fungus Glomus fasciculatus with various P doses showed a marginal response to groundnut in calcareous soil. Bell et al. (1988) in a glass house study found that the contribution of VAM to peanut plants, as an equivalent of applied P, was more than150 mg P/kg soil (240 kg P/ha) and the tissue P concentration in non-VAM plants were less. P uptake rate and dry matter accumulation has always been less than VAM colonised plants. The groundnut plants can respond to phosphorus nutrition to increase shelling percentage, oil yield and nodulation. P and S, though did not show any such interaction with other macronutrients. P and Zn are both essential nutrients needed for plant growth, but their combined effect at a certain level could be antagonistic especially when the application of soil-P is higher. Higher application of P may cause slower uptake of Zn by the plants or inadequacy of Zn in the soil (Mengel and Kirkby, 1987); this process results in yield decline in many crops. Studies have shown that a majority of the world soils are deficient in zinc. Yield loss and reduction in the nutritional quality of crops have been accounted for by micronutrient (Zn) deficiency in soils and crops worldwide (Sillanpää, 1982). Aboyeji et al. (2019) found that the effect of zinc was not significant on the vegetative parameters, but application of 8 kg Zn·ha-1 significantly increased the number of seeds, weight of seeds, seed yield per hectare, and seed quality. Results indicated that application of 8 kg Zn ha-1 and 120 kg P ha-1 had a synergistic effect on the growth parameters and antagonistic effect on the yield, yield parameters, some nutrient elements and beneficial heavy metals. Application of 8 kg Zn and 80 kg P ha-1 is therefore recommended on an Alfisol without necessarily increasing the concentration of non-beneficial heavy metals in groundnut seed. There are a number of reports about the non-responsiveness of groundnut to P application even in apparently low available P. This has been attributed mainly to low yield levels, a P-efficient crop as it can absorb P from its low concentration, and inadequate moisture as a limiting factor (Dwivedi et al., 1987; Patel and Kanzaria, 1985) and the beneficial effect of native mycorrhizal fungi. The P deficiency enhances root exudation of organic acids to mobilize sparingly soluble P on acid and calcareous soils. In acid soil, tartaric acid is the main component of exudates while malic and citric acids are in calcareous soil. The root “contact reactions” have shown that groundnut has superior ability to solubilize Fe- and Al-bound P, than many other crops and take up P from soils with low P. Looking to this advantage, research started on the selection for P-efficient genotypes and some of them are listed below:

Plant Nutrition

157

• P-efficient: GG 5, NRCG Acc 7085-1, 6919, 1308, 3498, and SP 250A • P-inefficient: VRI 3, B 95, PBS 16003, 20012 and 18057

6.3.

Potassium

Potassium is required for translocation of assimilates and is involved in maintenance of the water status of the plant especially the turgor pressure of cells and opening and closing of stomata, increase the solar energy harvesting efficiency and the availability of metabolic energy for the synthesis of starch and proteins (Beringer, 1978; Mengel and Kirkby, 1987; Dwivedi, 1989). It increases peg formation, synthesis of sugar and starch and helps in pod growth and filling. The nitrate reductase, EDTA-osmoticum, and productivity of groundnut under water deficit conditions were increased due to K application (Dwivedi et al., 1997). Application of potassium decreased transpiration and increased the stomatal resistance, solar energy harvesting efficiency and energy partitioning in the kernel in rainfed conditions in GAUG 1 groundnut (Dwivedi, 1989). Through the nutrient solution culture experiments has shown that increasing the levels of K, in the nutrient solutions, increased the concentrations of K, and N in leaves and only K in seeds, stems and shells, but decreased Ca and Mg concentrations of leaves, stems and shells. At low levels of K, the cations like Ca and Mg were absorbed more by the plant to balance the anions, but these were proportionally taken up at 50 ppm levels of K. The K is luxuriantly absorbed by groundnut at higher levels and accumulates in leaves, stems and shells, but not in seeds as they do not require much K unlike N (Singh, 1996b). The adequate K contents in groundnut leaves have been reported to be in between 1.6-3.0% (Dwivedi, 1988; Jones et al., 1991). However, Fageria (1974) reported 3.4-3.8% of K as adequate and 2.8% as the critical concentration in leaves of a 39 days old plant. Singh (1996b) reported that a 50 ppm level of K in the nutrient solution supports the sufficient K contents in leaves and stems at 60 DAE and seeds at harvest; in fact seed K content was the correct diagnostic measure for the sufficiency of K content. In relation to the rate of absorption of K, an absorption peak of 0.60 g day-1 was recorded at 20 DAE, followed by a decrease at 30 DAE, similar to that observed for the rate of N absorption. After this observed period, there was an increase in the rate of absorption of K, with its highest value observed at 110 DAE, equivalent to approximately 1.6 g day-1. The K absorption rate values were reduced to 0.2 g day-1 at 140 DAE (Fig. 6.5). The amount of K can vary. K is very important to plants and is the second-most absorbed element, overcome only by N. K has the physiological function of an enzymatic activator and, once absorbed, can be transferred from the older parts of the aerial portions to the newer parts (Tasso Jr. et al., 2004). K plays an important role in the formation of fruits, and in the transport of photo assimilates in the phloem (Taiz and Zeiger, 2013). The deposition of biomass in fruit is necessarily accompanied by the accumulation of K. In addition, K is a required nutrient in the activation of several enzymes essential to the synthesis of organic compounds including starch (Laviola and Dias, 2008). The responses with K and peanut are, in most cases, lower than expected, even in soils with low levels of this element (Freire et al., 2007). According to Bolonhezi et al. (2005), the K levels applied should be considered relative to the levels of other cations, especially Ca, as they compete for absorption for the development of the

158

Physiology of the Peanut Plant

Fig. 6.5. Rate of K absorption in peanut (Silva et al., 2017)

pods. Uchôa et al. (2011) and Salvador et al. (2011) cautioned that excessive KCl applications may inhibit Ca2+, Mg2+ and P uptake. The concentrations of Ca and Mg decreased with increasing levels of K but were high in all the plant tissues except seeds at very low levels of K. Similarly, the concentrations of K and Mg in plant tissues decreased with increasing Ca levels. As the K requirement of groundnut is very high, the K deficiency occurs when the K levels of the soil and plant go down. The potassium deficiency in groundnut is more common in the older leaves. Drying up of the leaf margin with yellowed hallow margins and necrotic symptoms and reddish coloration of the tip of branches is common. The stem becomes red accompanied by an excess storage of starch and the leaves become light green. Sometimes, there is interveinal chlorosis. K deficiency reduced the flowers, peg and kernels. The peanut roots are highly efficient in obtaining K from the soil.

Fig. 6.6. K deficiency symptoms in peanut leaves

Understanding the characteristics of the balanced nutrient requirements for peanut to achieve target yields is paramount when formulating fertilizer management

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strategies to increase yields and avoid fertilizer loss. Nutritional requirement estimation models can provide effective alternatives for the estimation of the optimum crop balanced nutrient requirements under varied agricultural conditions which are less time consuming and expensive. In the present study, the quantitative estimation of the optimum crop balanced nutrient requirements of peanut in China were obtained using quantitative evaluation of fertility of tropical soils (QUEFTS) model. The database covered the main agro-ecological region for peanut crops in China between 1993 and 2018. The predicted results of the QUEFTS model indicated that nutrient uptake requirements increased linearly with increasing pod yields until the yields had reached approximately 60% to 70% of the potential pod yields. It was found that with the increasing pod yields during the nutrient linear absorption stage, the plants had required 38.4 kg N, 4.3 kg P, and 14.0 kg K in total to produce 1000 kg of pods, and the corresponding internal efficiencies were 26.0 kg N/kg, 235.0 kg P/kg, and 71.6 kg K/kg, respectively. In addition, the balance rates of removal of the nutrient in the pods were determined to be 29.4 kg N, 2.9 kg P, and 4.9 kg K per 1000 kg of pod yield, or approximately 76.5%, 67.4%, and 34.7% of N, P, and K in the total plants, respectively (Fig. 6.7).

Fig. 6.7. Relationships between pod yields and the N, P, and K accumulations in the total dry plant matter at maturity (a–c), and the N, P, and K removal in the pods (d–f) under different potential yields predicted by the quantitative evaluation of fertility of tropical soils (QUEFTS) model. In the figure, YD, YA, and YU represent the maximum dilution, maximum accumulation, and balanced uptake of the N, P, and K in the total dry plant matter or in the pod dry matter, respectively; These parameters were calculated by the QUEFTS model after excluding the upper and lower 2.5th percentiles of all the internal efficiency data (HI range: 0.4 to 0.82; and potential pod yield range: 6 to 10 t/ha

6.4.

Calcium

Calcium maintains the cell integrity and membrane permeability, enhances pollen germination, activates the number of enzymes for cell division and takes part in protein synthesis and carbohydrate transfer. Calcium is more important for groundnut; often

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a lack of Ca reduces the yield and quality more than any other element. The calcium requirement is very high especially for gynophore development and pod filling. Field work showed that calcium and potassium levels in the fruiting zone affected seed quality (Cox et al., 1982; NRCG, 1985, 1991, 1996; Zharare et al., 1997). Early ovule abortion was prevented by adequate calcium supply. Acid soils due to their low base saturation are deficient in Ca for which liming is a must. Nicholaides and Cox (1970) observed the Ca deficiency if the concentration was less than 1.2% in 9-weekold plant. The adequate concentration of Ca in leaves is in between 1.2 and 2.0% (Nicholaides and Cox, 1970; Jones et al., 1991; Singh, 1996b). Since Ca is especially important for nut development, it has received considerable attention. Gaines et al. (1991) found maximum yield and grade for the small-seeded runner type with 0.12% Ca in the hulls and 0.04% Ca in the nuts, and for the largeseeded Virginia type with 0.19% and 0.058% respectively. Since Ca is especially important for nut development, it has received considerable attention. The Ca absorption rate increased from 30 to 100 DAE, with values varying from 0.001 g to about 0.4 g day-1, respectively. However, the highest Ca uptake peak was observed at 110 DAE, with an absorption rate of 1.20 g of Ca day-1. During this evaluation period, there was a marked decrease in the rate of Ca absorption to values below 0.20 g day-1 at 120 DAE until the last evaluation (Fig. 6.8).

Fig. 6.8. Rate of Ca uptake in peanut (Silva et al., 2017)

Calcium is taken up directly from the soil by pods, and an inadequate supply results in pods without seeds called “Pops” or blackened plumules inside the seed known as “Black heart” (Cox and Reid, 1964). The plumule damage in the seed is due to calcium deficiency which is eliminated by application of gypsum (Cox and Reid, 1964). Since the calcium is immobilized in the older leaves, the deficiency occurs on the fresh and emerging leaves. The calcium deficiency in the leaf is characterized by development of a localized pitted area on the lower surface of the leaves which later on converts into large necrotic spots (Fig. 6.9). Cracking of the basal stem and dieback

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of the shoot at a later stage of growth are also reported. The calcium ion (Ca++) is transported exclusively in the xylem tissue upward with the transpiration stream but its downward movement from the leaves through phloem is practically nil (Mengel and Kirkby, 1978). As soon as the peg penetrates the soil it ceases to transfer the root absorbed water and hence loses access to root absorbed Ca, thus developing fruit absorbed Ca from the soil solution. Excess of Ca content will produce a deficiency of either Mg or K.

Fig. 6.9. Ca deficiency symptom in peanut

Ca participates in many metabolic processes of plants like cell elongation, strengthens the cell wall structure via the formation of calcium pectate, improves stomatal functions, induces heat shock proteins, and protects against various fungal and bacterial diseases. Ca-NPs have also been formulated and tested for their role in increasing the crop growth and productivity. CaCO3 NPs (20–80 nm, 160 mg l-1as Ca) in Hoagland solution were tested as a source of Ca for peanut, grown in sand for 80 days, and were compared with control (without Ca) and with a soluble source of Ca as Ca(NO3)2 (200 mg l-1). A significant improvement in fresh biomass compared to the control was observed; however, this enhancement was similar on a dry weight basis in comparison to the soluble source of Ca [Ca(NO3)2]. Ca uptake by seedling stems and roots was enhanced compared to the control, which makes it justifiable that CaNPs enhanced Ca uptake and its transport from the root to the shoot due to their high surface area for being scavenged by the root surface of the plant in the rhizosphere. Moreover, when there was combined application of Ca-NPs and humic acid (1 g l-1), maximum increase in seedling dry weight, i.e. 30% and 14% compared to the control and treated with Ca(NO3)2, respectively, was observed.

6.5.

Magnesium

Magnesium is a component of chlorophyll and it serves as a cofactor in most of the enzymes that activate phosphorylation processes, as a bridge between pyrophosphate

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Physiology of the Peanut Plant

structures of ATP or ADP and the enzyme molecules. Under high rain fall, the Mg can bleached out more easily in acid and sandy soils. The concentration of Mg in leaves depends upon the crop genotype age and position of the leaf. There is wide variation in the critical levels of Mg in leaf tissues which is reported at 0.6% for 9-week-old plant tops in field grown crop (Nicholaides and Cox, 1970) and 0.25-0.3% in solution culture experiments for 39-day-old plant tops (Fageria, 1976). Walker et al. (1989) suggest a critical concentration of 0.2% Mg in the recently mature leaf of runner-type groundnut at 100 days after planting. However, Schimdt and Cox (1992) did not find any Mg deficiency with leaf Mg concentration as low as 0.15%. Usually, Mg deficiency occurs when the tissue concentration drops below 0.3%. Mg absorption rates of less than about 0.1 g day-1 in 110 DAE were observed in the peanut plants, followed by a peak absorption of this nutrient at 120 DAE, with a value close to 0.90 g day-1. The Mg absorption rate returned to values close to 0.1 g day-1 only at 160 DAE, at the end of the cycle (Fig. 6.10).

Fig. 6.10. Rate of Mg absorption in peanut (Silva et al., 2017)

In Mg deficient plants the interveinal chlorosis of basal leaves starts from the leaf margin and advances towards the midrib. In acute deficiency, the young leaves are also affected. The Mg deficiency is conducive to the occurrence of tikka disease. The concentration of Mg in leaves depends upon the crop genotype age and position of leaf. There is wide variation in the critical levels of Mg in leaf tissues which is reported at 0.6% for 9-week-old plant tops in field grown crops (Nicholaides and Cox, 1970) and 0.25-0.3% in solution culture experiments for 39-day-old plant tops (Fageria, 1976). Walker et al. (1989) suggest a critical concentration of 0.2% Mg in the recently mature leaf of runner-type groundnut at 100 days after planting (Fig. 6.11). However, the authors observed an increase in the uptake of Mg in different plant tissues compared to the control and regular application of Mg, which suggests that Mg uptake increases with the application of Mg-NPs.

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Fig. 6.11. Mg deficiency in leaves of peanut

6.6.

Sulphur

Sulphur, in groundnut, constitutes methionine, cysteine and cystine amino acids and increase oil synthesis. It improves nodulation, and pod yield besides reducing the incidence of diseases and is as important as phosphorus for oilseed crop. Sulphur increases chlorophyll and decreases chlorosis in calcareous soil (Singh et al., 1990a). The Groundnut grown on course-textured sandy soils generally suffer from S deficiency due to leaching of SO4-S. Plant take up sulphur mainly as SO42ions. The elemental sulphur added to soil is subjected to microbial transformation by Thiobacillus sp. into SO42- before it is taken up by the plants. There was an increase of approximately 0.03 g day-1 from 30 DAE to 0.25 g -1 day at 40 DAE in the rate of absorption of sulphur in peanut plants. During this evaluation period, there was a tendency for increasing rates of sulphur absorption, with the highest absorption peak observed at 120 DAE, reaching a sulphur absorption rate of 0.45 g day-1. The sulphur absorption rate reduced to values below 0.05 g day-1 at 130 DAE until the end of the cycle (Fig. 6.12). Singh (1996b) in a nutrient solution culture reported that increasing S levels, increased the concentration of S in leaves, seeds, stems and shells up to 20 ppm of S only. The concentration of S was 0.2-0.32% in leaves at 60 DAE and 0.22-0.25% in seeds at 20 ppm of S levels of the nutrient solution (Singh, 1996b). An adequate concentration of S in the leaves is between 0.2-0.35% (Singh and Chaudhari, 1995; Singh et al., 1990; Jones et al., 1991) and the critical S concentrations in leaves are 0.18 and 0.2%, at pegging and pod formation growth stages respectively (Supakamnerd et al., 1990). Bockelee-Morvan and Martin (1966) as referred by Cox et al. (1982) suggested that the critical levels of S are related to the N content and is 0.2% at 2.5­ 3.0% N and 0.25% at 3.5-4.0% N. This N:S ratio of about 15:1 was confirmed by Lund and Murdock (1978). Sulphur requirement of groundnut can be met through a number of S-containing materials such as gypsum, elemental S, pyrite and phosphogypsum (Biswas et al., 1988; Sahu et al., 1991; Singh and Chaudhari, 1995; Singh et al., 1991b). Generally, 20 kg of S/ha is sufficient to meet the nutrient requirement of groundnut (Singh and

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Physiology of the Peanut Plant

Fig. 6.12. Rate of S absorption in peanut (Silva et al., 2017)

Chaudhari, 1995), but in a recent pot study Singh and Chaudhari (1996a) observed the yield response of groundnut along with P up to 50 kg S/ha (Table 6.4). Table 6.4. Influence of sulphur and phosphorus on the concentrations of P and S at pegging (45 DAE) and their uptake by groundnut at harvest (Singh and Chaudhari, 1996) Treatment

Sulphur S0 S50 LSD 0.05 Phosphorus P0 P50 P100 P150 P200 LSD 0.05

Nutrient concentration (%) in leaves at 45 DAE

Nutrient uptake at harvest (mg/pot)

S

P

S

P

0.189 0.258 0.012

0.252 0.278 0.010

21.9 30.6 2.01

29.7 36.5 2.31

0.190 0.220 0.230 0.237 0.240 0.019

0.198 0.230 0.265 0.280 0.352 0.016

18.3 22.4 25.9 29.8 35.0 3.18

21.4 27.1 31.9 38.5 46.5 3.65

Generally, application of 30-40 kg S/ha to groundnut was more beneficial (Singh et al., 1991b; Kale, 1993; Patra et al., 1995). Application of 1 kg of nutrient sulphur produced 12.9 kg more pod and 4.3 kg more oil/ha in the experimental plot (Singh et al., 1991b). The symptom of S deficiency is like nitrogen but it occurs in young leaves. The Fe and S deficiency symptoms in groundnut appear together in most parts and distinction is not possible. The S deficiency occurs when the leaf concentration falls below 0.17% (Brownfield, 1973). Though this element is mobile, in groundnut the symptoms appear mainly on young leaves first and extend to the middle showing a pale yellow colour with the vein showing white (Fig. 6.13).

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Plant Nutrition

Fig. 6.13. Sulphur deficiency in leaves of peanut

An attempt was made to study the response of nano-sulphur and conventional sulphur in groundnut in completely randomized block designs and replicated thrice during 2013-2014. The results indicated that nano-sulphur @ 30 kg ha-1 recorded of 0.76 mg, 40.5 mg, 14.9 mg, 3.09 mg root, shoot, kernel and shell sulphur uptake plant-1 respectively, whereas conventional sulphur @ 40 kg ha-1 registered root, shoot, kernel and shell sulphur uptake of 0.53, 35.8, 11.4 and 2.46 mg plant-1 , respectively. The highest pod yield was recorded of 12.4 g plant-1 with nano-sulphur application @ 30 kg ha-1 when comparison to conventional sulphur @ 40 kg ha-1 registered 10.7 g plant-1. The higher oil, crude protein, methionine, cysteine and total free amino acid content of 48.3%, 27.2%, 3.44 mg 100 g protein-1, 1.89 mg 100 g protein-1 and 46.3 mg plant-1 were recorded under nano-sulphur application respectively rather than other sulphur sources. Finally, the study concluded that nano-S @ 30 kg ha-1 is sufficient to attain higher sulphur use efficiency with reduction of sulphur fertilizer to the tune of 25% besides augmenting the soil sulphur reserve without harming the environment (Fig. 6.14).

Fig. 6.14. Response of SUE (%) and sulphur (kg ha-1) under conventional and nano sulphur.

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6.7.

Physiology of the Peanut Plant

Iron

Most iron in soils is unavailable for plant absorption (Meng et al., 2005). For example, iron deficiency is common in calcareous soils (which have a high pH), as iron availability to plants decreases with increasing pH. On the other hand, availability of iron for plants is generally high in acid tropical soils. Iron is a component of cytochrome oxidase, ferredoxin protein, chlorophyll and several enzyme systems. It is involved in nitrate and sulphate reductase nitrogen assimilation and energy (NADP) production. Among all micronutrients, iron deficiency is most commonly observed in groundnut. In plants, iron is involved in chlorophyll synthesis, and it is essential for the maintenance of chloroplast structure and function. It is essential because of its numerous functional roles in growth and development of plants. • • • • • •

Photosynthesis Respiration Nitrogen fixation Uptake mechanisms (Kim and Rees,1992) DNA synthesis through the action of the ribonucleotide reductase (Richard, 1993) Cofactors of many enzymes necessary for phytohormone synthesis, like ethylene, lipoxygenase, 1-aminocyclopropane acid-1-carboxylic oxydase (Siedow, 1991), or abscisic acid. • Iron-sulphur proteins (e.g., Ferredoxin, super oxide dismutase) • Redox systems such as cytochromes, catalases and peroxidises (Mengel and Kirby, 1987; Marschner, 1995b) An increase in the Fe uptake rate values by the plants from 10 to 110 DAE was observed, with an absorption peak of 25 mg day-1 at 110 DAE. After this evaluation period, there was a decrease in uptake rate of this micronutrient until the end of the crop cycle (Fig. 6.15).

Fig. 6.15. Rate of Fe absorption in peanut (Silva et al., 2017)

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The high free CaCO3, HCO3-, moisture, heavy metals, pH and available P, poor aeration, heavy manuring and low organic matter content in acid soils and root damage enhance iron deficiency of groundnut (Chaney, 1984; Chen and Barak, 1982; Hartzook, 1975; Mengel and Geurtzen, 1986; Singh, 1994a; Singh et al., 1991a, b; Wallace et al., 1976). In groundnut the Fe-deficiency appear 10-15 days after emergence in the field and remains throughout the cropping season, but the maximum intensity was in between 30-70 DAE (Singh and Chaudhari, 1991, 1993). Crops suffering from iron deficiency will grow slower than normal and are more susceptible to disease (Cakmak, 2002; Kulandaivel et al., 2004; Rashid and Ryan, 2004; Wiersma, 2005; Chatterjee et al., 2006). All three components of Strategy I (proton pumping, ferric chelate reductase gene expression and enzyme activity, and IRT1 expression) increase markedly when plants are grown in iron-deficient conditions. Iron is translocated from roots to shoots as a ferric-citrate chelate form, and this is transported to actively growing shoots. Iron chlorosis in groundnut (appearance of papery whitish yellow bud leaves) is another problem of growing concern in many alkaline calcareous soils where bicarbonate ions hinder the uptake and translocation of Fe in the plant (Patel et al., 1993) (Fig. 6.16). The lime induced iron chlorosis can be managed by soil application or foliar spray of ferrous sulphate. They also reported that splitting the application rate of 2 kg Fe ha-1 into 4 sprays of 0.5 kg each at 30, 45, 60 and 70 days after emergence resulted in the highest recovery from chlorosis and also produced the highest pod and haulm yield.

Fig. 6.16. Fe chlorosis in peanut

Inoculation with Bradyrhizobium and Pseudomonas improve iron nutrition by synthesis of chelates (siderophores) that keep iron in soluble form (Jurkevitch et al., 1988; O Hara et al., 1988). Pseudomonas inoculation is reported to correct iron chlorosis of peanut in calcareous soil also (Jurkevitch et al., 1988). Nambiar and Sivaramakrishnan (1987) showed that NC 92 grown in culture media produced more siderophore bound iron than NC 43.3. The soil application up to 500 ppm of Fe did not show any toxic symptoms in groundnut leaves (Singh, 1994b). Iron may accumulate to several hundred ppm without any toxicity symptoms. Toxicity of Fe produces a bronzing of the leaves with tiny brown spots which sometimes occur in the acid soils.

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Physiology of the Peanut Plant

The total Fe content in the shoots and roots of peanut plants significantly increased in the EDTA-Fe and Fe2O3 NPs treatments. More Fe was taken up into roots than into shoots, probably because Fe is absorbed into plants via the roots. The Fe content in roots was higher in the EDTA-Fe treatment and Fe2O3 NPs treatments than in the control. The highest Fe content in shoots was in the 10 and 250 mg⋅kg-1 Fe2O3 NPs treatments (consistent with the SPAD results), followed by the 1000 mg⋅kg-1 Fe2O3 NPs and EDTA-Fe treatments. The exposure of peanut seeds to NZVI at all the tested concentrations altered the seed germination activity, especially the development of seedlings. In comparison with the de ionized water treated controls (CK), all of the NZVI treatments had significantly larger average lengths. Further investigations with transmission electron microscopy (TEM) and thermogravimetric analysis (TGA) suggested that NZVI particles may penetrate the peanut seed coats to increase the water uptake to stimulate seed germination. The growth experiments showed that although NZVI at a relatively high concentration (320 μmol/L) showed phytotoxicity to the peanut plants, the lower concentrations of NZVI particles stimulated the growth and root development of the plants. At certain concentrations (e.g. 40 and 80 μmol/L), the NZVI treated samples were even better than the ethylenediaminetetraacetate-iron (EDTA-Fe) solution, a commonly used iron nutrient solution, in stimulating the plant growth. This positive effect was probably due to the uptake of NZVI by the plants, as indicated in the TEM analyses. Since, low concentrations of NZVI particles stimulated both the seedling development and growth of peanut, they might be used to benefit the growth of peanuts in large-scale agricultural settings.

Fig. 6.17. Effects of nano iron on water uptake, germination of seeds and seedling development

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A pot experiment with factorial design involving normal and calcareous soil and five genotypes with differential response to iron deficiency chlorosis (IDC) viz., ICGV 86031 and A30b (resistant), TG 26 (moderately resistant), TAG 24 and TMV 2 (susceptible) were tested for various traits like VCR and SCMR, chlorophyll a, b and total chlorophyll, active iron content, specific activity of peroxidase at five different stages and the effect of IDC on yield and yield components. Iron deficiency chlorosis resistant genotypes recorded significantly lower VCR, higher SCMR, higher active iron content, chlorophyll a, b and total chlorophyll and peroxidase activity in leaves across all stages compared to susceptible genotypes. A strong and positive correlation was observed between peroxidase activity and leaf iron content. Yield and yield components were significantly reduced in susceptible genotypes compared to resistant genotypes. Iron deficiency chlorosis is an important abiotic stress affecting groundnut production worldwide in calcareous and alkaline soils with a pH of 7.5– 8.5. To identify genomic regions controlling iron deficiency chlorosis resistance in groundnut, the recombinant inbred line population from the cross TAG 24 × ICGV 86031 was evaluated for associated traits like visual chlorosis rating and SPAD chlorophyll meter reading across three crop growth stages for two consecutive years. Thirty-two QTLs were identified for visual chlorosis rating (3.9%–31.8% phenotypic variance explained [PVE]) and SPAD chlorophyll meter reading (3.8%–11% PVE) across three stages over 2 years. This is the first report of identification of QTLs for iron deficiency chlorosis resistance-associated traits in groundnut. Three major QTLs (>10% PVE) were identified at a severe stage, while majority of other QTLs were having small effects. Interestingly, two major QTLs for visual chlorosis rating at 60 days (2013) and 90 days (2014) were located at the same position on LG AhXIII. The identified QTLs/markers after validation across diverse genetic material could be used in genomics-assisted breeding.

6.8.

Zinc

Zinc (Zn) deficiency in soils has been reported worldwide, particularly in calcareous soils of arid and semiarid regions. In a global soil survey study, SillanpaÈaÈ (1990) found that about 50% of the soil samples collected in 25 countries were Zn deficient. Zinc deficiency is a particularly widespread micronutrient deficiency in wheat, leading to severe depressions in wheat production and nutritional quality of grains (Graham et al., 1992; Cakmak et al., 1996d; Graham and Welch, 1996) . Availability of zinc for plants is particularly low in calcareous and alkaline soils, while absolute zinc contents tend to be low in highly weathered acid tropical soils. • Zn is required for the synthesis of tryptophan which is a precursor of IAA, and it also has an active role in the production of an essential growth hormone auxin. • Zn is required for the integrity of cellular membranes to preserve the structural orientation of macromolecules and ion transport systems. Its interaction with phospholipids and sulfhydryl groups of membrane proteins contributes to the maintenance of the membrane. • The regulation and maintenance of gene expression required for the tolerance of environmental stresses in plants are Zn dependent. • Stabilization of ribosomal fractions. • Synthesis of cytochrome.

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Physiology of the Peanut Plant

• Plant enzymes activated by Zn are involved in carbohydrate metabolism, protein synthesis and pollen formation. • Influence the activities of enzymes hydrogenase and carbonic anhydrase. A large increase in Zn absorption rate was observed from 80 to 110 DAE. During that time of evaluation, the highest-peak Zn absorption by peanut plants was observed. After 110 DAE, a decrease was observed in the values of this variable up to 150 DAE (Fig. 6.18).

Fig. 6.18. Rate of Zn absorption in peanut (Silva et al., 2017)

Zinc (Zn) distribution and transport in plants is affected by the level of Zn supply and plant species. When plants have low to adequate Zn supply, Zn concentrations are usually higher in growing tissues than in mature tissues; this is true for roots, vegetative shoots and reproductive tissues. In plants tolerance of toxic levels of Zn accumulation has been observed in the root cortex and in leaves. In these tissues, Zn accumulates in cell walls or is sequestered in vacuoles. The possible involvement of Zinc-Regulated Transporter, Iron-Regulated Transporter (ZRT-IRT)-like proteins (ZIPs) in cellular Zn2+ uptake was established by expressing cDNAs from Zn-deficient plants in a yeast zrt1zrt2 mutant (Grotz et al., 1998). Some plant members of the so-called Cation Diffusion Facilitator (CDF) family of metal cation/proton antiporters, members of which have also been named ZAT (Zinc Transporter of Arabidopsis thaliana) and MTP (Metal Tolerance Protein or Metal Transport Protein), act in the removal of Zn from the cytoplasm. Visible deficiency symptoms include: • Chlorosis – yellowing of leaves; often interveinal; in some species, young leaves are the most affected, but in others both old and new leaves are chlorotic; • Necrotic spots – death of leaf tissue on areas of chlorosis; • Bronzing of leaves – chlorotic areas may turn bronze coloured;

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Plant Nutrition

• Rosetting of leaves – zinc-deficient dicotyledons often have shortened internodes, so leaves are clustered on the stem; • Stunting of plants – small plants may occur as a result of reduced growth or because of reduced internode elongation; • Dwarf leaves (little leaf) – small leaves that often show chlorosis, necrotic spots or bronzing (Fig. 6.19); • Malformed leaves – leaves are often narrower or have wavy margins.

Fig. 6.19. Zn deficiency in peanut leaves

Leaf tissue less than 20 ppm of Zn showed deficiency of Zinc (Dwivedi, 1986; Singh, 1994b). At the available levels more than 12 ppm in the soil and 220 ppm in the plant tissue, Zn showed toxic effects (Keisling et al., 1977). The zinc toxicity causes early senescence of leaves and stunted growth. Leaf zinc was affected more by soil pH than by soil Zn and an increase in soil Zn from 1.0 to 10 mg/kg increased leaf Zn 202 mg/kg at soil pH 4.6 and only 9 mg/kg at pH 6.6 (Parker et al., 1990).Peanut plants are more susceptible to zinc toxicity than other crops and minimization of Zn uptake by the hulls would evidently be beneficial in aiding peanut plants in tolerating high soil Zn levels while producing economic yields (Davis et al., 1995) The data from growers’ fields indicated the Ca : Zn ratio of 50 or less was required for Zn toxicity of peanuts rather than high concentration of leaf Zn per se in Georgia, USA (Parker et al., 1990).Treatments consisted of four levels of phosphorus (0, 40, 80 and 120 kg P ha-1) and three levels of zinc (0, 4 and 8 kg Zn ha-1). An experiment was laid out in a Randomized Complete Block Design (RCBD), replicated four times. The following parameters were taken: plant height, plant spread, total biomass, number of pods/ plots, weight of pods/plot, number of seeds/plot, weight of seeds/plot. Laboratory analysis of some chemical constituents of groundnut seed was carried out to determine the nutrient and heavy metals composition. Results indicated that application of 8 kg Zn ha-1 and 120 kg P ha-1 had a synergistic effect on the growth parameters and antagonistic effect on the yield, yield parameters, some nutrient elements and beneficial heavy metals. Application of 8 kg Zn and 80 kg P ha-1 is therefore recommended on

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Physiology of the Peanut Plant

an Alfisol without necessarily increasing the concentration of non-beneficial heavy metals in groundnut seed. Peanut seeds were separately treated with different concentrations of nanoscale zinc oxide (ZnO) and chelated bulk zinc sulphate (ZnSO4) suspensions (a common zinc supplement), respectively and the effects this treatment had on seed germination, seedling vigour, plant growth, flowering, chlorophyll content, pod yield and root growth were studied. Treatment of nanoscale ZnO (25 nm mean particle size) at 1000 ppm concentration promoted both seed germination and seedling vigour and in turn showed early establishment in soil manifested by early flowering and higher leaf chlorophyll content. These particles proved effective in increasing stem and root growth. Pod yield per plant was 34% higher compared to chelated bulk ZnSO4. Consequently, a field experiment was conducted during summer seasons of 2008– 2009 and 2009–2010 with the foliar application of nanoscale ZnO particles at a15 times lower dose compared to the chelated ZnSO4 recommended and we recorded 29.5% and 26.3% higher pod yield, respectively, compared to chelated ZnSO4. The inhibitory effect with higher nanoparticle concentration (2000 ppm) reveals the need for judicious usage of these particles in such applications. Biofortification (delivery of micronutrients via micronutrient-dense crops) can be achieved through plant breeding and offers a cost-effective and sustainable approach to fighting micronutrient malnutrition. The present study was conducted to facilitate the initiation of a breeding programme to improve the concentration of iron (Fe) and zinc (Zn) in peanut (Arachis hypogaea L.) seeds. The experiment was conducted with 64 diverse peanut genotypes for 2 years in eight different environments at the International Crops Research Institute for the Semi-Arid Tropics, Patancheru, India to assess the genetic variation for Fe and Zn concentrations in peanut seeds and their heritability and correlations with other traits. Significant differences were observed among the genotypes and environments for Fe (33–68 mg/kg), Zn (44–95 mg/kg), protein (150– 310 mg/g) and oil (410–610 mg/g) concentration in seeds and their heritability was high, thus indicating the possibility of improving them through breeding. As seen in other plants, a significant positive association between concentrations of Fe and Zn was observed. Trade-offs between pod yield and Fe and Zn concentrations were not observed and the same was also true for oil content. Besides being high yielding, genotypes ICGV 06099 (57 mg/kg Fe and 81 mg/kg Zn) and ICGV 06040 (56 mg/ kg Fe and 80 mg/kg Zn) had stable performances for Fe and Zn concentrations across environments. These are the ideal choices for use as parents in a breeding programme and in developing mapping populations.

6.9.

Manganese

Both pH and redox conditions influence Mn bioavailability in soils (Marschner, 1995; Porter et al., 2004). In most acid soils at low pH (S1>S2. The 100-pod mass, 100-kernel mass, kernel rate to pod, and pod mass per plant were reduced under salt stress, and the trend was CK>S1>S2. The distribution proportion of dry matter in different organs of the peanut plant was changed to adapt to such stress. Roots under salt stress were intensively distributed in a 0–40 cm soil layer for salt resistance. Dry mass proportion in stems and pods increased during the vegetative stage and early period of the reproductive stage, respectively. The maximum growth rates of the pod volume, pod dry weight, and seed kernel dry weight all declined, and the pod and kernel volume at harvest were reduced, improving the seed plumpness under salt stress. This finding could be useful in growing peanut in saline soil (Fig. 7.3).

Fig. 7.3. Changes in photosynthetic characteristics under salt stress. CK, non-salt stress; S1, 0.15% salt stress; S2, 0.3% salt stress. Ci, intercellular CO2 concentration; gs, stomatal conductance; Pn, net photosynthetic rate; Evap, transpiration rate. D20, D35, D50, D65, D85, and D120, the 20th, 35th, 50th, 65th, 85th, and 120th days after planting, respectively. The small letters indicate significant differences at the 0.05 level. Bars mean SD (n=5)

Elevated CO2 also ameliorates the adverse effect of salinity (Bazzaz, 1989; Pérez-López et al., 2012, 2013). These observations suggest that the CO2 enriched plants of the future will tolerate salinity better. Few investigations showed a gradual decrease in net photosynthetic rate (Pn) under increased salinity and were sometimes even stimulated by low salt concentration (Heuer and Plaut, 1981). Four peanut (Arachis hypogaea L.) cultivars (cvs. TPT-1, TPT-4, JL-24 and TMV-2) were grown in open-top chambers at 350 and 600 μmol CO2/mol in soil amended with 0 (control), 50, 100 and 200 mmol solutions of NaCl. The net photosynthetic rate (Pn), stomatal conductance (gs), transpiration (E) and dry biomass of the leaf, stem and root were measured 60 days after sowing. The plant growth and photosynthesis increased in both NaCl treated and control plants with elevated CO2. The gs and E

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Physiology of the Peanut Plant

decreased under elevated CO2 and the CO2 effect was highly significant under salt stress mitigating the adverse effect on these components in all the four cultivars tested. A positive correlation was observed between Pn and dry biomass under elevated CO2 and salt stress. Enhanced CO2 helps to increase growth and photosynthesis in peanut cultivars and it ameliorates the adverse effects induced by salt stress (Fig. 7.4).

Fig. 7.4. Genotypic variations of net photosynthesis under salinity levels

The effect of NaCl and Na₂SO₄ treatments on chlorophyll content, rate of ¹⁴C assimilation and products of photosynthesis in peanut (Arachis hypogaea L.) variety TMV-10 has been investigated. It was observed that chlorophyll content was affected mainly by NaCl. Na₂SO₄ treatment lowered the rate of photosynthetic ¹⁴CO₂ fixation. The analysis of labelled products revealed that the salts affect the carbon metabolism differently. The radioactivity was found to be accumulated in fractions of sugars and sugar phosphates in the leaves of NaCl treated plants. Na₂SO₄ treatment brought about a considerable decline in the labelling of sugars and an increase in the labelling of amino acids and sugar phosphates. A field experiment was conducted using two differentially salt-responsive cultivars and three levels of salinity treatment (control, 2.0 dS m-1, 4.0 dS m-1) along with two levels (with and without) of potassium fertilizer (0 and 30 kg K2O ha-1). Salinity treatment incurred significant changes in the overall physiology in the two peanut cultivars, though the responses varied between the tolerant and the susceptible one. External K application resulted in improved salinity tolerance in terms of plant water status, biomass produced under stress, osmotic adjustment and better ionic balance. Tolerant cv. GG 2 showed better salt tolerance by excluding Na from the uptake and lesser accumulation in the leaf tissue and relied more on organic osmolyte for osmotic adjustment. On the contrary, susceptible cv. TG 37A allowed more Na, K to accumulate in the leaf tissue and relied more on inorganic solute for osmotic

Photosynthesis

199

adjustment under saline condition, hence showed more susceptibility to salinity stress. Application of K resulted in nullifying the negative effect of salinity stress with a slightly better response in the susceptible cultivar (TG 37A). The study identified Na-exclusion as a key strategy for salt-tolerance in tolerant cv. GG 2 and also showed the ameliorating role of K in salt-tolerance with a varying degree of response amongst tolerant and susceptible cultivars (Fig. 7.5).

Fig. 7.5. Differences in (A) photosynthetic efficiency (Pn, mmol m-2s-1) and (C) SPAD value of two peanut cultivars (TG 37A, GG 2) over three salinity gradients (I0: control, I1: 2 dSm-1, I2: 4 dSm-1) under potassium supplemented (K 30:30 kg K2O ha-1) and without potassium (K 0:0 kg K2O ha-1) conditions

In this study, photosynthetic performance, pigment content, chlorophyll a fluorescence, and leaf anatomy in peanut (Arachis hypogaea) subjected to zinc (Zn) stress were investigated. Zn stress resulted in reduction of photosynthetic and transpiration rates, pigment contents and root biomass. Zn-induced xerophyte structure in peanut leaves (i.e. thick lamina, upper epidermis, and palisade mesophyll, as well as abundant and small stomata) also contributed to a decreased transpiration rate and stomatal conductance. This in turn, partially contributed to the limitation of photosynthesis (Table 7.3a).

200

Physiology of the Peanut Plant

Table 7.3a. The effect of different Zn concentrations on growth, gas exchange, chlorophyll fluorescence, photosynthetic pigments, and anatomic traits of leaves in A. hypogaea 0.2 μM

200 μM

500 μM

1000 μM

Root Zn content [mg g-1 (d.m.)]

0.13±0.00

2.74±0.28

3.47±0.55

3.81±0.88a

Shoot Zn content [mg g-1 (d.m.)]

0.07±0.00c

1.05±0.12b

1.25±0.23a

1.29±0.16a

Root biomass [g plant-1]

0.51±0.01a

0.43±0.01b

0.27±0.01c

0.26±0.03c

Shoot biomass [g plant-1]

1.55±0.06a

1.50±0.07a

1.45±0.05a

1.38±0.03a

Pn [µmol (CO2) m-2 s-1]

8.4±0.4a

5.7±0.3b

5.0±0.2c

4.7±0.2c

gs [mol (H2O) m-1 s-1]

0.03±0.00a

0.02±0.00b

0.01±0.00c

0.01±0.00c

E [mmol (H2O) m s ]

4.8±0.3

2.7±0.4

1.6±0.1

1.7±0.1c

c1 [µmol (CO2) mol-1]

187±4c

WUE

1.8±0.1c

2.6±0.2b

3.3±0.2a

3.0±0.2ab

Fv/Fm

0.82±0.00a

0.81±0.01ab

0.81±0.00ab

0.80±0.00b

F0/Fm

4.6±0.1a

4.3±0.2ab

4.2±0.1ab

4.0±0.1b

fPS2

0.76±0.00a

0.74±0.01a

0.69±0.025b

0.72±0.01ab

d

-2

-1

a

c

b

258±8b

b

c

291±12a

313±lla

Chl a [mg g–1 (d.m.)]

36.4±2.4a

25.2±2.6b

24.9±1.4b

22.8±1.6b

Chl b [mg g–1 (d.m.)]

8.9±1.0a

6.1±0.7b

6.0±0.6b

5.5±0.5b

Chl [mg g–1 (d.m.)]

45.3±3.5a

31.3±3.3b

30.9±1.9b

28.3±2.1b

Car [mg g–1 (d.m.)]

7.2±0.6a

4.5±0.6b

4.8±0.4b

4.4±0.4b

Chl a/b

4.12±0.21a

4.13±0.08a

4.18±0.20a

4.20±0.09a

Car/Chl

6.31±0.04b

7.02±0.16a

6.51±0.10b

6.45±0.08b

Stomatal density in upper epidermis [mm-2]

190±3a

190±3a

214±4b

194±3a

Stomatal density in lower epidermis [mm-2]

167±2d

199±3c

239±4a

215±3b

Stomatal length in upper epidermis [µm]

26.3±0.3a

24.5±0.1b

24.1±0.1bc

23.8±0.1c

Stomatal length in lower epidermis [µm]

27.5±0.2a

27.2±0.2a

26.6±0.2b

24.9±0.2c

The upper epidermis thickness [µm]

17.2±0.8b

21.7±0.7a

21.8±0.6a

20.7±0.5a

The lower epidermis thickness [µm]

17.4±0.5a

16.5±0.5a

16.3±0.5a

16.2±0.5a

The palisade tissue thickness [µm]

74.6±2.1c

98.9±2.8b

95.4±1.8bc

102.9±2.4a

201

Photosynthesis The spongy tissue thickness [µm]

43.7±1.9a

42.2±1.5a

44.8±1.4a

46.7±1.4a

The palisade to spongy thickness ratio

1.85±0.11b

2.5±0.12a

2.23±0.08a

2.34±0.11a

The lamina thickness [µm]

152.8±3.6b

179.3±3.6a

178.3±2.5a

186.4±2.8a

Means ± SE, n = 3 for Zn content, biomass and pigment content, 18 for gas exchange and Chl fluorescence parameters, 60 for stomatal characters and 36 for other anatomic traits. Means in the same row followed by the same letters are not significantly different at P < 0.05 based on Duncan’s multiple range test.

7.4.

Water

In a field experiment three irrigation treatments were given to twelve peanut genotypes through drip. At 80 days after sowing (DAS) the amount of irrigation applied was 20% higher than the evaporative demand (ET) in T1, 25% less than ET in T2 and 48% less than ET in T3 against the cumulative evaporative demand of 412 mm. The relative water content (RWC) of peanut leaves reduced by cutting irrigation from 93.5% in T1 to 91.1% in T2 and 77.2% in T3 but, net photosynthetic rate (Pn) was higher in T2 (29.6 μ mol m-2 s-1) than T1 (28.6 μ mol m-2 s-1) and T3 (24.3 μ mol m-2 s-1) at 75–80 DAS. Peanut genotype ICGV 91114 showed the highest Pn (30.9 μ mol m-2 s-1) which was statistically at par with GG 20, ICGV 86590, TAG 24, SB XI, TMV 2 and TPG 41. The non-photochemical quenching (NPQ) varied with different irrigation treatment with lowest in T2 and highest in T3. The de-epoxidation state (DeS) was 38% in T1 and T2 but, increased to 47% in T3 due to the sever water deficit stress. Applying 20% higher irrigation than the ET demand (T1) does not warrant any extra benefits in terms of higher photosynthesis in peanut at 75–80 DAS. Further, a reduction of 25% of the ET (T2) in peanut seems to be the ideal condition for photosynthesis and desirable chlorophyll fluorescence parameters at 80 DAS. Girnar 3 and ICGV 91114 showed a NPQ value above 2.2 and higher de-epoxidation state, and maintained the least deviation in Fv/Fm and Fv′/Fm′ under severe water deficit condition which are promising peanut genotypes (Tables 7.4 and 7.5). Water deficit stress significantly reduced the rate of photosynthesis in all the genotypes; however, there were enough variations observed in the genotypes of different habit groups (Fig. 7.6). In terms of percentage change in net photosynthetic rate Virginia genotypes showed greater reduction compared to the Spanish type. At individual genotype level, HNG 10 showed the highest reduction (32.7%) in photosynthesis rate followed by Somnath (29.7%) and Kadiri 3 (28.1%). This result suggested, for photosynthetic parameters relatively greater susceptibility of Virginia type peanut cultivars to water deficit stress than Spanish type. A drought is one of the main constraints in peanut production in West Texas and eastern New Mexico regions due to the depletion of groundwater. A multi-seasonal phenotypic analysis of 10 peanut genotypes revealed C76-16 (C-76) and Valencia-C (Val-C) as the best and worst performers under deficit irrigation (DI) in West Texas, respectively. Net photosynthesis at the leaf level was measured during the water deficit treatment to monitor the impact of deficit irrigation (DI) stress on the plant metabolism. In 2013, the water deficit treatment resulted in a moderate decrease in

202

Physiology of the Peanut Plant

Table 7.4. Total chlorophyll content (mg g−1dw), net photosynthetic rate (Pn - μ mol m−2 s−1) and stomatal conductance (gs m sec−1) in peanut genotypes at 75–80 DAS grown at various soil moisture regimes Genotypes/ Treatment

Total chlorophyll content (mg.g-1dw) T1

T2

Mean

T1

AK 265

5.68g

6.31e

9.37ab

7.12def

26.1bcd

27.9bc 26.1ab

26.7bc

GG 20

6.48ef

9.25c

8.91b

8.21a

31.2abc

32.6a

23.5bc

29.1ab

Girnar 3

7.79ab

6.52e

7.28de

7.19cdef

29abcd

25.9c

23bcd

26bc

Girnar 2

8.03

5.1

9.21

7.45

31.4

26.1

19.4

25.6bc

ICGV86590

7.26bcd

5.92e

9.69a

7.63abcd

29.6abcd 30.4ab

25.5ab

28.5abc

ICGV9114

6.5

7.94

8.99

7.81

34.5

34.1

24.1

bc

30.9a

JL 24

4.93h

10.23b

9.42ab

8.19a

24.3cd

27.3bc

22.0cd

24.5c

SB XI

6.2

11.56

6.84

8.2

28.8

26.0

29.0

27.9abc

SG 99

7.59abc

4.32g

7.86cd

6.59fg

23.2d

32.4a

23.4bcd

26.3bc

TAG 24

6.8

6.07

8.07

6.89

30.2

31.3

25

abc

28.8ab

TMV 2

7.9ab

8.8c

6.86e

7.85ab

29abcd

30.3ab

22.8bcd

27.3abc

TPG 41

6.97

4.47

7.65

6.36

26.5

30.7

28.2

28.5abc

Mean

6.84

7.21

8.35

7.47

28.6

29.6

24.3

a

ef

fg

f

d

a

def

cde

e

fg

T3

ab

b

e

c

cd

bcde

abc

a

Pn (µmol.m-2.s-1)

ab

a

abcd

efg

g

abcd

bcd

T2

c

a

c

ab

ab

T3

Mean

d

a

a

27.5

Table 7.5. Non-photochemical quenching (NPQ) and relative de-epoxidation state (DeS) in peanut genotypes at 75-80 DAS, grown at various soil moisture regimes Genotype/ Treatment

NPQ

DeS (%)

T1

T2

T1

T2

T3

Mean

AK 265

1.77a

1.3bcde

1.85bc

1.64ab

34.8e

30.8f

48.0de

37.9d

GG 20

1.61

a

1.17

2.19

1.66

ab

35.1

43.0

44.4

40.8c

Girnar 3

1.44

a

2.06

2.13

1.88

a

31.8

37.7

38.8

36.1de

Girnar 2

1.67a

2.03a

1.58abc

1.76ab

45.9a

40.4d

54.3b

46.8b

ICGV86590

1.14

0.78

1.78

1.23

40.9

44.4

a

60.7

48.7a

ICGV9114

1.72a

1.54abc

2.17a

1.81ab

29.9h

38.1e

44.4f

37.5d

JL 24

1.22

0.82

1.25

1.1

33.1

38.1

51.0

40.8c

SBXI

1.35a

1.79ab

1.73abc

1.63ab

30.1h

25.2g

49.2cd

34.9ef

SG 99

1.30

0.96

1.54

1.26

45.2

49.2

47.2

47.2ab

TAG 24

1.43a

0.9cde

1.06c

1.13b

45.9a

37.7e

54.5b

46.0b

TMV 2

1.75

1.53

1.88

1.72

32.3

32.6

36.5

h

33.8f

TPG 41

1.11a

0.97cde

1.28bc

1.12b

43.2b

41.5cd

55.5b

46.8b

Mean

1.46

1.32

1.7

1.5

37.4

38.2

48.7

41.4

a

a

a

a

bcde a

e

de

cde

abcd

T3

Mean a a

ab

bc

abc

ab

ab

b

d g

c

ef

ab

ab

a

fg

bc e

b

e

a

f

f g

c

e

net photosynthesis for most of the genotypes, but no significant differences were observed during stress, recovery, or complete irrigation among the genotypes. In 2014, treatment resulted in a significant decrease in net photosynthesis (ranging from 18 to 38 µmolesm−2s−1) compared to the fully irrigated treatment (ranging from 20 to

Photosynthesis

203

Fig. 7.6. Changes in net photosynthesis rate (Pn) in groundnut leaves under water deficit stress

40 µmolesm−2s−1) but again no statistically significant differences could be observed between the genotypes. General trends emerged in both the years for photosynthetic responses, but in this case, net photosynthesis was used to guide the collection of leaf material for transcriptome studies (Fig. 7.7a, b). Further, SLA and chlorophyll contents were also measured in 2014 but the values obtained were not statistically significant between the treatments of the same genotypes (Fig. 7.7c, d). The photosynthetic response of three Arachis hypogaea L. cultivars (57-422, 73-30, and GC 8-35) grown for two months was measured under water available conditions, severe water stress, and 24, 72, and 93 h following re-watering. At the end of the drying cycle, all the cultivars reached dehydration, relative water content (RWC) ranging between 40 and 50%. During dehydration, leaf stomatal conductance (gs), transpiration rate (E), and net photosynthetic rate (Pn) decreased more in cvs. 57-422 and GC 8-35 than in 73-30. Instantaneous water use efficiency (WUEi) and photosynthetic capacity (Pmax) decreased mostly in cv. GC 8-35. Except in cv. GC 8-35, the activity of photosystem I (PSI) was slightly affected. PS2 and ribulose-1,5bisphosphate carboxylase/oxygenase (RuBPCO) were the main targets of water stress. After rewatering, cvs. 73-30 and GC 8-35 rapidly regained gs, E, and Pn activities. Twenty-four hours after re-watering, the electron transport rates and RuBPCO activity strongly increased. Pn and Pmax fully recovered later (Table 7.6). Peanut (Arachis hypogaea L. TVM-2) was grown in fields under drought stress in combination with paclobutrazol (PBZ) and abscisic acid (ABA) to study their individual and combined effects on leaf anatomical characteristics. The thickness of the leaf, upper and lower epidermis and the number of cells per unit area in the palisade and spongy regions were significantly reduced under drought stress. The palisade and spongy layers of mesophylls were well-differentiated, and the cells were wider and longer as compared to shorter palisade and spongy parenchyma of control. The number of palisade and spongy cells increased per unit area with all treatments as compared with drought stressed and unstressed plants. The vascular bundles of the PBZ-treated plants were narrow and dense when compared to control. The xylem vessels of PBZ and ABA-treated leaves were much narrower when compared to

204

Physiology of the Peanut Plant

Fig. 7.7. Physiological observations from fully irrigated and deficit irrigation plots at different time intervals: (a, b) Mean gas exchange measurements in the years 2013 and 2014. (c) Mean specific leaf area (SLA) during irrigated and deficit irrigation conditions in selected genotypes. (d) Mean chlorophyll content during irrigated and deficit irrigation conditions in selected genotypes

control plants. Among the treatments, the present findings revealed that the growth regulator treatments to the drought stressed plants have a great impact on the anatomy of A. hypogaea plants (Table 7.6, Table 7.7 and Fig. 7.8). Water stress is one of the most important environmental factors inhibiting photosynthesis due to damage of chlorophyll pigments, thereby reducing their light harvesting capacity (Graan and Boyer, 1990). During photosynthesis, chlorophyll pigments in photosystem-II are excited by sunlight releasing electrons and energy in the form of an ATP molecule required for the breakdown of the water molecule (H2O) into ½O2 and 2H+ maintaining an electron gradient which is further excited in photosystem-I to produce NADP++ H+. This highly energetic NADPH molecule is then fed into the Calvin cycle to carry out carbon fixation. Hence, overexpression of the Photosystem-I reaction centre subunit-II of the chloroplast precursor protein in the 20 DS plants of ICGV 91114, ICGS 76 and J 11 varieties might help in the uninterrupted supply of NADPH during photosynthesis to ensure the supply of substrates to the carbon skeleton for various metabolic pathways conferring increased drought tolerance. However, the decline in the intracellular levels of CO2 might create an oxidative stress leading to the destruction of photosynthetic apparatus, changes in the conformation of chloroplast proteins, imbalances in the ions and other macromolecules making JL 24 the most susceptible (Lauer and Boyer, 1992; Bray et al., 2000). Ribulose-1,5-biphosphate carboxylase/oxygenase (RuBisCo) is the key photosynthetic enzyme that plays a significant role in the fixation of CO2 in C3 plants which can be quickly remobilized under stress (Jensen and Bahr, 1977). Many studies reported the reduced expression of RuBisCo under drought stress (Parry et al., 2002) while the studies in the drought-tolerant peanut cultivar, Vemana by Katam et al. (2016) reported high abundance of RuBisCo under stress conditions. Kottapalli et al. (2009)

205

Photosynthesis

Table 7.6. Relative water content, RWC [%], stomatal conductance, gs [mmol(H2O) m-2 s-1], net photosynthetic rate, Pn [μmol(CO2) m-2 s-1], transpiration rate, E [mmol(H2O) m-2 s-1], Ci [cm3 m-3], and water use efficiency, WUEi [Pn/E] under control and severe water stress conditions as well as at 24 and 72 h after re-watering Cv. 57-422

73-30

Control RWC

94 549±12

Pn

16.2±0.9a;r

E ci

a;r

36±4

24

72

95

94

91±9

c;s

105±21b;t

b;s

0.8±0.0c;s

7.4±0.2b;s

16.1±1.7a;r

6.2±0.2

0.7±0.0

5.5±0.06

4.7±0.7b;s

296±1a;r

272±14a;s

235±1b;t 1.5±0.01c;r

a;r

c;t

WUEi

2.7±0.0a;r

1.3±0.0c;r

RWC

93

45-55

255±32

b;c

100±0

c;r

b;s

214±8b;t 3.4±0.02a;r

95

94

337±52

375±37a;s

a;r

Pn

9.0±0.1b;r

1.2±0.0a;r

6.1±0.2c;t

14.0±0.3a;s

E

5.0±0.0b;s

2.6±0.0c;r

9.1±0.4a;r

9.2±0.0a;r

ci

289±2

a;s

WUEi

?

Time after re-watering

45-55

gs

gs

GC 8-35

Severe water stress

1.8±0.3a;s

RWC

93

287±2

a;r

285±9

282±7a;r

a;r

0.9±0.0b;s

0.7±0.0b;s

45-55

94

1.5±0.0a;s 93

gs

454±19

32±2

Pn

13.0±0.4b;s

0.5±0.0d;s

8.8±0.1c;r

18.0±3.0a;r

E

6.0±0.1b;r

1.5±0.0c;s

10.6±0.1a;r

11.0±2.0a;r

ci

a;s

299±1a;r

WUEi

2.2±0.1a;rs

c;s

266±14b;s

304±21

b;r

261±0b;s

0.3±0.0c;t

453±120a;r

258±2b;s

0.8±0.0d;s

1.7±0.0b;s

Means ± SE, n = 5. Different letters indicate significant differences: r, s, t among cultivars and a, b, c, d among treatments. Table 7.7. Effect of PBZ, ABA and drought and their combination induced changes in the leaf anatomy of Arachis hypogaea in 80 DAS Parameter

Control Drought

Leaf thickness Epidermal thickness Number of palisade Palisade mesophyll No. of spongy cell

215.21 10 39.54 98 35.47

192.62 69 25.47 84 23.62

Drought+ Drought+ PBZ ABA PBZ 10 mg l-1 ABA 10 µg l-1 10 mg l-1 10µg l-1 34.27 223.15 243.42 28.41 81 31 11 12 42.13 41.18 49.39 45.24 137 117 143 126 38.62 36.43 43.96 41.33

Spongy mesophyll Xylem vessel

124 19

971 23

581 21

461 15

651 17

524 16

Values are the mean of seven replicates and expressed in μ meters. CD: Critical difference, PBZ: Paclobutrazol, ABA: Abscisic acid, DAS: Days after sowing.

206

Physiology of the Peanut Plant

Fig. 7.8. Effect of paclobutrazol, abscisic acid and drought and their combination induced changes on the leaf anatomy of Arachis hypogaea in 80 days after sowing

reported the reduction in photosynthesis-related proteins in tolerant genotypes during water stress which includes proteins like Rubisco LSU and SSU, oxygen evolving enhancer protein of PS II and chlorophyll a/b binding proteins. However, both larger and smaller subunits of Rubisco exhibited down-regulation in all the stressed plants of 4 varieties compared to their respective controls while PS I reaction centre subunit II protein showed overexpression in severely stressed plants of ICGV 91114, ICGS 76 and J 11 cultivars. Overproduction of reactive oxygen species (ROS) during drought stress, cleaves the large subunit of Rubisco directly or modifies it to become more susceptible to proteolysis (Feller et al., 2008).

Photosynthesis

7.5.

207

Temperature

Relatively mild night temperature scan reduce leaf carbon dioxide exchange rates (CER) and dry matter (DM) accumulation in peanut (Arachis hypogaea L.). To investigate differences among cultivars in response to long-term exposure to a range of night temperatures, three peanut cultivars (OAC Ruby, Chico, and Early Bunch) with known differences in chilling sensitivity were grown in controlled environment cabinets at the University of Guelph, Ontario. Effects of long-term exposure to night temperatures from 9 to 20°C were assessed in terms of leaflet and whole plant CER, DM accumulation, and phenological development. Effects of night temperature on the rate of phenological development and DM accumulation were consistent with differences in the accumulation of degree-days. Cultivars did not differ in daytime leaf CER response to all night temperatures except 9°C, at which the CER for OAC Ruby was higher than for Early Bunch or Chico. The CER in the 9°C treatment was 92% of the CER at 20°C for OAC Ruby and 80% for Early Bunch and Chico. Continuous exposure to night temperatures of 10°C reduced CER sensitivity to low daytime temperatures in OAC Ruby, but Early Bunch was unaffected (Fig. 7.9).

Fig. 7.9. Effects of long-term exposure to various night temperatures on CER of young, fully expanded leaflets measured during the day at 29.5±1°C with ambient CO2 concentrations of 315±15 mL L-1 and 650±15 mmol m-2sec-1 PPFD. Data are shown for the cultivars OAC Ruby, Chico, and Early Bunch, with the vertical bar indicating a LSD (0.05).

Arachis hypogaea L. is a tropical crop that is slow-growing at temperatures below 25°C. The unadapted CO2-assimilation rate (A) showed insufficient variation between 15 and 30°C in the short term (hours) to explain this marked reduction in growth. However, at longer periods (12 d), A was depressed as were growth and leaf production rates. To examine the possible relationship between growth, A and sink demand, plants were transferred from 30°C, which is near the optimum for growth, to a suboptimal temperature (19°C). In the first 2 d of cooling, A decreased by 50–70%, the stomata stayed open, and the intercellular CO2 concentration (Ci) rose, i.e. the decrease in A of the cooled plants was the result of non-stomatal factors. Changes in dark respiration did not account for the decline in A.

208

Physiology of the Peanut Plant

Clear evidence was obtained of the sink control of A by independently manipulating the temperature of different leaves on the plant. Cooling (to 19°C) most of the plant (the sink) led to a 70% decline in A of the remaining leaves at 30°C after 3 d, whereas the converse treatments (30°C sink, 19°C source) resulted in small changes (17%). In plants at 19°C which were exposed to low CO2 concentration to prevent photosynthesis, A was not reduced when measured at normal CO2 concentrations, indicating that carbohydrate accumulation was responsible for the decline in A. Drymatter build-up at suboptimal temperatures was also consistent with end-product inhibition of photosynthesis. Breeders have been trying to develop new varieties to resolve the problem of peanut chilling damage and have made certain progress, and a few cold-tolerance early-maturing cultivars with ability to germinate in cooler soils have been released (Ntare et al., 2001; Gorbet and Shokes, 2002; Upadhyaya et al., 2003, 2006). However, cold tolerance in plants is an intricate quantitative trait that always occurs in combination or in succession and it is not controlled by a single regulatory pathway or gene, making conventional breeding approaches for challenging cold tolerance (Kumar et al., 2015; Wang et al., 2017). With the development of biotechnology in agriculture, extensive and in-depth studies on the mechanism of cold tolerance in plants in terms of morphological, anatomical, physiological, biochemical, and molecular biology have been conducted. Lyons (1973) proposed that chilling damage initially occurs at the cellular and organ levels. The bio membrane system, including cell, nuclear and organelle membranes, is the initial site of injury, particularly in terms of its structure, function, stability, and enzyme activity, thereby resulting in substantial metabolic imbalance, especially involving respiration and photosynthesis. These changes in turn affect the plant growth and development and eventually incur damages at the whole-plant level, leading to the occurrence of chilling damage. The bio membrane is also the main repository of lipids for peanut plants (Yu, 2008), and fatty acids and is the main component of the bio membrane, which has been used as the primary index to evaluate peanut quality. Recent studies have further shown that chilling tolerance in peanut is closely correlated with the composition and structure of the membrane lipids, particularly the saturation of membrane fatty acids (Tang, 2011). The complex physiological, biochemical, and molecular mechanisms between membrane lipid metabolism and cold tolerance is being continuously explored to improve cold tolerance by means of high-throughput gene identification, gene editing, and transgenic technology. Talwar et al. (1999) recorded higher net photosynthetic rate in three groundnut genotypes grown at 35/30°C as compared to those grown at 25/25ºC at 30 and 60 DAS. They also observed genotypic differences in net photosynthesis at both temperatures. In crops like groundnut (C3 crops), Rubisco is not saturated by the current concentration of CO2 in the atmosphere. So, an increase in CO2 concentration will improve the balance of CO2 and O2 at Rubisco site, thus improving the CO2Exchange Rate (CER) of the plant by providing more substrate for photosynthesis. Prasad et al. (2003) reported that doubling of ambient CO2 concentration (350 vs. 700 μmol mol-1) enhanced leaf photosynthesis of groundnut by 27% across a range of daytime temperatures (32 to 44ºC), but they found no CO2 by temperature interaction on leaf photosynthesis.

Photosynthesis

7.6.

209

Carbondioxide

Peanut (Arachis hypogaea L. cv. Florunner) was grown from seed sowing to plant maturity under two daytime CO2 concentrations ([CO2]) of 360 μmol mol-1 (ambient) and 720 μmol mol-1 (elevated) and at two temperatures of 1.5°C and 6.0°C above ambient temperature. The experiments were undertaken to characterize peanut leaf photosynthesis responses to long-term elevated growth [CO2] and temperature, and to assess whether elevated [CO2] regulated peanut leaf photosynthetic capacity, in terms of activity and protein content of ribulose bisphosphate carboxylase-oxygenase (Rubisco), Rubisco photosynthetic efficiency, and carbohydrate metabolism. At both growth temperatures, leaves of plants grown under elevated [CO2] had higher midday photosynthetic CO2 exchange rates (CER), lower transpiration and stomatal conductance and higher water-use efficiency, compared to those of plants grown at ambient [CO2]. Both activity and protein content of Rubisco, expressed on a leaf area basis, were reduced at elevated growth [CO2]. Declines in Rubisco under elevated growth [CO2] were 27–30% for initial activity, 5–12% for total activity, and 9–20%

Fig. 7.10. Leaf CER response curves for the 360 µmol CO2 mol-1 (ambient) and 720 µmol CO2 mol-1 (elevated) also 95 µmol CO2 mol-1 peanut plants of the TA +1.5°C treatment. Measurements were performed in situ on uppermost mature leaves, 68 days after seed planting, from 1000 to 1100 EDT (1400–1500 mol m-2s-1 solar PPFD). The single attached leaflet was enclosed in the 1 dm3 assimilation chamber of the LI-6200 Portable Photosynthesis System, and CER responses to declines in CO2 concentration of the air inside the closed chamber (Cca) were monitored through the “drawdown” procedure described by McDermitt et al. (1989). Extrapolation of the curves gave an interception at the abscissa of about 95 mol CO2 mol-1 for both CO2 treatments. The initial slopes of the CER response curves were 0.123 for the ambient-CO2 plants and 0.073 for the elevated-CO2 plants.

210

Physiology of the Peanut Plant

for protein content. Although Rubisco protein content and activity were downregulated by elevated [CO2], Rubisco photosynthetic efficiency, the ratio of midday light-saturated CER to Rubisco initial or total activity, of the elevated-[CO2] plants was 1.3- to 1.9-folds greater than that of the ambient-[CO2] plants at both growth temperatures. Leaf soluble sugars and starch of plants grown at elevated [CO2] were 1.3- and 2-folds higher, respectively, than those of plants grown at ambient [CO2]. Under elevated [CO2], leaf soluble sugars and starch, however, were not affected by high growth temperature. In contrast, high temperature reduced leaf soluble sugars and starch of the ambient-[CO2] plants. Activity of sucrose-P synthase, but not adenosine 5′-diphosphoglucose pyro phosphorylase, was up-regulated under elevated growth [CO2]. Thus, in the absence of other environmental stresses, peanut leaf photosynthesis would perform well under rising atmospheric [CO2] and temperature as predicted for this century. Growth at elevated [CO2] resulted in a down-regulation of both activity and protein content of Rubisco, expressed on a leaf area basis (Table 7.8). Elevated [CO2] reduced the initial activity of Rubisco by 27% at near-ambient temperature and 30% at high temperature. Reductions in total activity by elevated [CO2], however, were less: about 5% at near-ambient temperature and 12% at high temperature. Similarly, reductions in Rubisco protein content by long-term elevated growth [CO2] were about 15% at near-ambient temperature and 20% at high temperature. In addition, Rubisco activation, the ratio of the initial to the corresponding total activity, was also reduced under CO2 enrichment. Rubisco activation was 73 and 74% under ambient growth [CO2], compared to 57 and 58% under elevated growth [CO2], for the near-ambient and elevated temperature treatments, respectively (Table 7.8). Table 7.8. Activity, activation, protein content and photosynthetic efficiency of Rubisco in midday-sampled, fully-developed leaves of ‘Florunner’ peanut plants grown for a season under 360 and 720 µmol CO2 mol-1 and at average temperatures of 1.5°C and 6.0°C above outdoor ambient temperature (TA) CO2 (µmol. mol-1)

Temp. (°C)

360

TA+1.5 TA+6.0

38.0a 41.1a

51.3ab 56.3a

74.1 73.0

2.25a 2.23a

85.3 83.2

63.2 60.8

720

TA+1.5 TA+6.0

27.6b 28.7b

48.5b 49.4b

56.9 58.1

1.92b 1.78b

158.7 139.0

90.3 80.8

Activity (µmol m-2s-1) Initial

Activation (%)

Total

Protein (g.m-2 leaf area)

Photosynthetic efficiency (%) CER/Rin

CER/Rto

Leaf sampling was performed at midday (~1800 mol m-2s-1 PPFD), 76 days after seed planting. Values are the mean and S.E. (parentheses) of three determinations. Rubisco activation is computed as the ratio of initial to total activity. Rubisco photosynthetic efficiency is computed as the ratio of midday leaf CER (Table) to Rubisco initial (Rinitial) and total (Rtotal) activity. Values with different letters in the same column are significantly different at P < 0.05 using a Duncan Multiple Range Test. Continuing increases in atmospheric carbon dioxide concentration (CO2) will likely be accompanied by global warming. Researches were (a) to determine the effects of season-long exposure to daytime maximum/night time minimum

Photosynthesis

211

temperatures of 32°/22°, 36°/26°, 40°/30° and 44°/34°C at ambient (350 µmol mol-1) and elevated (700 µmol mol-1) CO2 on reproductive processes and yield of peanut, and (b) to evaluate whether the higher photosynthetic rates and vegetative growth at elevated CO2 will negate the detrimental effects of high temperature on reproductive processes and yield. Doubling of CO2 increased leaf photosynthesis and seed yield by 27% and 30%, respectively, averaged across all temperatures. There were no effects of elevated CO2 on pollen viability, seed-set, seed number per pod, seed size, harvest index or shelling percentage. At ambient CO2, seed yield decreased progressively by 14%, 59% and 90% as temperature increased from 32/22 to 36/26°, 40/30 and 44/34°C, respectively. Similar percentage decreases in seed yield occurred at temperatures above 32/22°C at elevated CO2 despite greater photosynthesis and vegetative growth. Decreased seed yields at high temperature were a result of lower seed-set due to poor pollen viability, and smaller seed size due to decreased seed growth rates and decreased shelling percentages. Seed harvest index decreased from 0.41 to 0.05 as the temperature increased from 32/22°C to 44/34°C under both ambient and elevated CO2. It was concluded that there are no beneficial interactions between

Fig. 7.11. Leaf photosynthesis, stomatal conductance and transpiration in peanut under day and night temperatures

212

Physiology of the Peanut Plant

elevated CO2 and temperature, and that seed yield of peanut will decrease under future warmer climates, particularly in regions where present temperatures are near or above optimum (Fig. 7.11). Peanut (Arachis hypogaea L.), a kind of tropical thermophilic crop originated in South America, is not only an important oil crop, but also an important source of proteins around the world (Yu, 2008; Mukesh et al., 2011). Low temperature, one of the main environmental factors, limits peanut geographical distribution and production. Currently, chilling injury is a serious problem worldwide, especially low night temperature stress which has an adverse impact on peanut seedling development in high latitude regions including northern China (Bagnall et al., 1988; Wan, 2007; Lin et al., 2011). Low temperature affects physiological metabolisms of cold-sensitive plants in multiple aspects, of which photosynthesis (including sensitive photosynthetic membrane system) is one of the processes to be affected mostly (Berry and Bjorkman, 1980; Damian and Donald, 2001; Yu et al., 2002; Liu et al., 2010, 2011).

7.7.

Nutrition

Both deficient and excessive exogenous P supplies significantly reduced leaf growth, relative chlorophyll concentration and dry matter production in two high-yielding peanut cultivars. The optimum P range was 0.8–1.1 mM for peanut seedlings. Through principal component analysis (PCA) and data fitting, it was found that the trade-off of the normalized actual quantum yield [Y(II)] and non-regulatory quantum yield [Y(NO)] in photosystem II (PSII) under light is one of the best proxies to determine the suboptimal, supra optimal, deficient and toxic P supplies, because they are the two key factors with major positive and negative effects of PC1, accounting for 75.5% of the variability. The suboptimal P range was 0.41–0.8 mM and the supra optimal P range was 1.1–1.72 mM. The suboptimal P supplies corresponded with a leaf P concentration range of 4.8–8.1 mg P g−1 DW, while the supra optimal P supplies corresponded with a leaf P concentration range of 9.9–12.2 mg P g−1 DW. Both deficient and toxic P levels severely inhibited leaf growth and photosynthesis of peanut, and these unfavourable conditions were associated with a significant reduction of biomass and photosynthesis, and photodamage extending beyond PSII. Ca2+, an essential element, is not only a structural matter for the plant cells, but also the second messenger for the extracellular signals and intracellular physiological reaction. Being capable of sustaining the stability for cell wall, cell membrane and membrane protein, it plays a crucial role in plant growth and response to the environment, intensively participating and regulating physiological and bio-chemical reactions in the plant (Sun, 1998; Anireddy et al., 2011; Li et al., 2012). The specific changes of Ca2+ intracellular concentration (known as Ca2+ fingerprint) induces specific physiological reactions (Fig. 7.12). Peanut is one of the calciphilous plants. Calcium (Ca) serves as a ubiquitous central hub in a large number of signalling pathways. The effect of exogenous calcium nitrate [Ca(NO3)2] (6 mM) on the dissipation of excess excitation energy in the photosystem II (PSII) antenna, especially on the level of D1 protein and the xanthophyll cycle in peanut plants under heat (40°C) and high irradiance (HI) (1200 µmol m−2 s−1) stress were investigated. Compared with the control plants [cultivated in 0 mM Ca(NO3)2 medium], the maximal photochemical efficiency of PSII (Fv/Fm) in Ca2+-treated plants showed a slighter decrease after 5 h of stress, accompanied

Photosynthesis

213

Fig. 7.12. Effects of different CaCl2 concentrations on net photosynthetic rate (A), stomatal conductance (B), intercellular CO2 concentration (C), and transpiration rate (D) in peanut under low night temperature stress. Pure water was sprayed onto those peanut seedlings under nonstress as control (CK)

Fig. 7.13. Analysis of D1 protein in peanut seedlings. qRT-PCR for psbA expression in peanut leaves of CK and CA before and after heat (40 uC)and high irradiance (1200 mmol m-2 s-1 PFD) treatment for 3 h and 6 h (A). *Significant difference compared with CK using Student’s t-test at P, 0.05. Thylakoid membrane proteins were separated by SDS-PAGE and then probed with D1 antibody. Relative optical density (OD) has been added to ensure the difference in intensity (B). The same thylakoid membrane proteins were separated by SDS-PAGE and then stained with Coomassie brilliant blue R250 (C).

by higher non-photochemical quenching (NPQ), higher expression of antioxidative genes and less reactive oxygen species (ROS) accumulation. Meanwhile, a higher content of D1 protein and a higher ratio of (A+Z)/(V+A+Z) were also detected in Ca2+treated plants under such stress. These results showed that Ca2+ could help protect the peanut photosynthetic system from severe photoinhibition under heat and HI stress

214

Physiology of the Peanut Plant

by accelerating the repair of D1 protein and improving the de-epoxidation ratio of the xanthophyll cycle. Furthermore, EGTA (a chelant of Ca ion), LaCl3 (a blocker of Ca2+ channel in cytoplasmic membrane), and CPZ [a calmodulin (CaM) antagonist] were used to analyze the effects of Ca2+/CaM on the variation of (A+Z)/(V+A+Z) (%) and the expression of violaxanthin de-epoxidase (VDE). The results indicated that CaM, an important component of the Ca2+ signal transduction pathway, mediated the expression of the VDE gene in the presence of Ca to improve the xanthophyll cycle (Fig. 7.13).

7.8.

Intercropping

Wheat (Triticum aestivum L.)–peanut (Arachis hypogaea L.) relay intercropping rotation systems are a mainstay of the measures to improve the economic and food security situation in China. Therefore, a 2-year field study (2015–2017) was conducted to evaluate the effect of different N fertilizer management regimes on the photosynthetic characteristics and uptake and translocation of N in peanut in the wheat–peanut rotation system. Using common compound fertilizer (CCF) and controlled-release compound fertilizer (CRF) at the same N–P2O5–K2O proportion. (The contents of N, P2O5, and K2O in the two kinds of fertilizer were 20, 15, and 10%, respectively.) The fertilizer was applied on the day before sowing, at the jointing stage or the flag leaf stage of winter wheat, and at the initial flowering stage of peanut in various proportions, with 0 kg N ha-1 as the control. The content of N, P2O5, and K2O in the two kinds of fertilizer was 20, 15, and 10%, respectively. The amount of applied fertilizer was 1500 kg ha-1 (converted into pure form, N: 300 kg ha-1, P2O5: 225 kg ha1, and K O: 225 kg ha-1), part of which was manually distributed over the soil surface 2 prior to sowing and then ploughed into the soil at a depth of 20 cm as a basal dressing. For top dressed N, manually performed ditching and fertilizing at the jointing stage and flag leaf stage of winter wheat, and the initial flowering stage of peanut. The fertilizer was applied on the day before sowing and at the mentioned growth stages in the following splits: 50%–50%–0–0 (JCF100), 35%–35%–0–30% (JCF 70 and JCRF 70), 50%–0–50%–0 (FCF 100), and 35%–0–35%–30% (FCF 70 and FCRF 70), with 0 kg N ha-1 as control (CK). Results showed that split applications of N significantly increased the leaf area index (LAI) and chlorophyll content and improved photosynthetic rate, thus increasing the pod yield of peanut. Topdressing N at the jointing stage (S1) or at the flag leaf stage of wheat (S2) and supplying part of the N at the initial flowering stage of peanut increased pod yield. Withholding N until the flag leaf stage (S2) did not negatively affect wheat grain yield; however, it increased N accumulation in each organ and N allocation proportions in the peanut pod, ultimately improving pod yield. With the same N–P2O5–K2O proportion and equivalent amounts of nutrient, CRF can decrease malondialdehyde (MDA) and maintain a relatively high LAI and chlorophyll content at the late growth stage of peanut, prolong the functional period of peanut leaves and delay leaf senescence, resulting in an increase of pod yield over that with CCF. At S1, CRF resulted in a better pod yield than CCF by 9.4%, and at S2 it was 12.6% higher. In summary, applying N fertilizer in three splits and delaying the topdressing fertilization until the flag leaf stage of winter wheat increases total grain yields of wheat and peanut. However, gradual application with the growth process (Table 7.9), indicated that the degradation rate of Chl a was greater than that of Chl b. Compared

215

Photosynthesis

with CK, the application of N fertilizer significantly increased Chl a/b, and the efficacy of CRF was higher than that of CCF. Table 7.9. Effect of different N fertilizer management regimes on the chlorophyll content and Chl a/b of peanut Stage

Treatment

Pegging

Pod setting

Pod filling

Maturity

Chl Chl (a+b) a/b mg.g-1 FW

Chl Chl (a+b) a/b mg.g-1 FW

Chl Chl (a+b) a/b mg.g-1 FW

Chl Chl (a+b) a/b

Jointing CK

1.29

1.86

2.16

1.72

1.80

1.60

1.02

1.41

(S1)

JCF 100

1.61

1.89

2.63

1.78

2.24

1.62

1.20

1.43

JCF 70

1.98

1.96

2.86

1.80

2.40

1.63

1.22

1.47

JCRF 70

2.09

1.97

3.68

1.86

2.73

1.65

1.44

1.58

Flag leaf

FCF 100

1.74

1.87

2.73

1.81

2.69

1.68

1.18

1.61

(S2)

FCF 70

2.05

1.96

3.38

1.90

2.70

1.71

1.40

1.65

FCRF 70

2.21

1.97

3.53

1.91

2.86

1.75

1.77

1.68

As shown in Fig. 7.14, compared with CK the application of N fertilizer significantly increased Pn. The overall trend in Pn was consistent among the treatments. Initially, Pn increased up to the pod-setting stage, and decreased again starting with the pod-filling stage. Further, the Pn values under treatments JCF 70 and JCRF 70 at different stages were higher than those under JCF 100 by 4.9–23.2 and 10.5–42.4%, respectively, while the Pn values under FCF 70 and FCRF 70 at different stages were higher than those under FCF 100 by 2.5–10.8 and 6.7–24.2%, respectively. Treatment with CRF at the pegging stage gave no significant difference in Pn compared to values under the CCF treatments but resulted in significant and substantial (in the range of 2.5–15.6%) increases in Pn at the pod-filling and mature stages despite the same application ratios of N–P2O5–K2O and equal nutrient doses. At all the corresponding growth stages,

Fig. 7.14. Effect of different N fertilizer management regimes on the net photosynthetic rate (Pn) of peanut

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Physiology of the Peanut Plant

the difference in average Pn under the two topdressing fertilizer regimes showed that S2 > S1. These results illustrate that by splitting N application and postponing N supply, a relatively high Pn can be maintained at the later growth stages of peanut and that the flag leaf stage is the optimum stage for topdressing the fertilizer. Application of nitrogen fertilizer at anthesis to groundnuts cv. Luhua 11 and Fu 8707 was investigated with reference to effects on senescence. Nitrogen application delayed the senescence process of the whole plant, improved the photosynthetic capacity of the canopy, increasing the content of chlorophyll and net photosynthesis rate in leaves, and enhanced the soluble protein content of leaves. Genes related to photosynthesis showed significant discrimination between two contrasting genotypes especially during FI conditions. The genes related to light reactions of photosynthesis of photosystem I and II were found to be downregulated in Val-C versus C-76 during FI conditions. The downregulated genes related to the subunits of photosystems I and II under drought stress conditions, reduced plant tolerance levels to drought stress in sorghum. Decreased expression of genes involved in light-harvesting chlorophyll a/b-binding proteins (LHCBs) and photosystems I and II results in decreased drought tolerance in plants. During irrigated conditions, the genes encoding P700 apo A1 of photosystem I, and D2 of photosystem II exhibited comparatively high expression in C-76 than Val-C. The D2 protein of photosystem II plays a vital role in stress tolerance. Among the different DEGs identified, genes related to photosynthesis showed a distinct expression in both genotypes. Therefore, it is essential to comprehensively study the photosynthetic genes associated with drought stress in peanuts, which gives a better idea in the development of droughttolerant genotypes.

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Leplat, F., P.R. Pedas, S.K. Rasmussen and S. Husted. 2016. Identification of manganese efficiency candidate genes in winter barley (Hordeum vulgare) using genome wide association mapping. BMC Genomics, 17: 775. Li, X.G., Q.W. Meng, G.Q. Jiang and Q. Zou. 2003. The susceptibility of cucumber and sweet pepper to chilling under low irradiance is related to energy dissipation and water-water cycle. Photosynthetica, 41: 259-265. Li, X.G., Y.P. Bi, S.J. Zhao, Q.W. Meng, Q. Zou et al. 2005. Cooperation of xanthophyll cycle with water-water cycle in the protection of photosystems 1 and 2 against inactivation during chilling stress under low irradiance. Photosynthetica, 43: 261-266. Li, T.G., K.C. Wang and Q.Y. Luo. 2012. Effects of exogenous Ca2+ on physiological and photosynthesis of Fritillaria anhuiensis under high temperature stress. Plant Nutrition and Fertilizer Science, 18: 765-770 (in Chinese). Lin, L.Q., L. Li, C. Bi, Y.L. Zhang, S.B. Wan et al. 2011. Damaging mechanisms of chilling and salt stress to Arachis hypogaea L. leaves. Photosynthetica, 49: 37-42. Liu, T., M. Sheng, C.Y. Wang, H. Chen, Z. Li and M. Tang. 2015. Impact of arbuscular mycorrhizal fungi on the growth, water status, and photosynthesis of hybrid poplar under drought stress and recovery. Photosynthetica, 53(2): 250-258. Liu, Y.F., T.L. Li, H.Y. Qi, C.Q. Xu, J.Y. Li et al. 2010. Effects of grafting on carbohydrate accumulation and sugar-metabolic enzyme activities in muskmelon. African Journal of Biotechnology, 9: 25-35. Liu, Y.F., H.Y. Qi, C.M. Bai, M.F. Qi, W.Z. Chen et al. 2011. Grafting helps improve photosynthesis and carbohydrate metabolism in leaves of muskmelon. International Journal of Biological Sciences, 7: 1161-1170. Lyons, J.M. 1973. Chilling injury in plants. Annu. Rev. Plant Physiol., 24: 445-466. Masuda, T. and K.I. Takamiya. 2004. Novel insights into the enzymology, regulation and physiological functions of light-dependent protochlorophyllide oxidoreductase in angiosperms. Photosynth. Res., 81: 1-29. McDermitt, D.K., J.M. Norman, J.T. Davis, T.M. Ball, T.J. Arkebauer et al. 1989. CO2 response curves can be measured with a field‐portable closed‐loop photosynthesis system. Annals of Forestry Science, 46: 416-420. Mukesh, J., P.P. Bhuvan, C.H. Alice, L.T. Barry and G. Maria et al. 2011. Calcium dependent protein kinase (CDPK) expression during fruit development in cultivated peanut (Arachis hypogaea) under Ca2+-sufficient and -deficient growth regimens. Journal of Plant Physiology, 168: 2272-2277. Muller̈ , P., X.P. Li and K.K. Niyogi. 2001. Non-photochemical quenching. A response to excess light energy. Plant Physiol., 125: 1558-1566. Murata, N., S. Takahashi, Y. Nishiyama and S.I. Allakhverdiev. 2007. Photoinhibition of photosystem II under environmental stress. Biochim. Biophys. Acta Bioenerg., 1767: 414-421. Nageswara Rao, R.C., H.S. Talwar and G.C. Wright. 2001. Rapid assessment of specific leaf area and leaf N in peanut (Arachis hypogaea L.) using chlorophyll meter. Journal of Agronomy and Crop Science, 189: 175-182. Nautiyal, P.C., V. Ravindra and Y.C. Joshi. 1995. Gas exchange and leaf water relations in two peanut cultivars of different drought tolerance. Biological Plantarum, 7: 371-374. Nautiyal, P.C., N.R. Rachaputi and Y.C. Joshi. 2002. Moisture-deficit induced changes in leafwater content, leaf carbon exchange rate and biomass production in groundnut cultivars differing in specific leaf area. Field Crops Research, 74: 67-79. Nishiyama, Y., S.I. Allakhverdiev, H. Yamamoto, H. Hayashi, N. Murata et al. 2004. Singlet oxygen inhibits the repair of photosystem II by suppressing the translation elongation of the D1 protein in Synechocystis sp. PCC 6803. Biochemistry, 43: 1321-1330. Niyogi, K.K., A.R. Grossman and O.Björkman. 1998. Arabidopsis mutants define a central role for the xanthophyll cycle in the regulation of photosynthetic energy conversion. Plant Cell, 10: 1121-1134.

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Niyogi, K.K., C. Shih, W.S. Chow, B.J. Pogson, D. DellaPenna et al. 2001. Photo protection in a zeaxanthin- and lutein-deficient double mutant of Arabidopsis. Photosynth Res., 67: 139-145. Niyogi, K.K. 1999. Photoprotection revisited: Genetic and molecular approaches. Annu. Rev. Plant Physiol. Plant Mol. Biol., 50: 333-359. Ntare, B.R., J.H. Williams and F. Dougbedji. 2001. Evaluation of groundnut genotypes for tolerance under field conditions in a sahelian environment using a simple physiological model for yield. J. Agr. Sci., 136: 81-88. Parry, M.A.J., P.J. Andralojc, V. Khan, P.J. Lea, A.J. Keys et al. 2002. RuBisCO activity: Effects of drought stress. Ann. Bot., 89: 833-839. Pérez-López, U., J. Miranda-Apodaca, A. Mena-Petite and A. Muñoz-Rueda. 2013. Barley growth and its underlying components are affected by elevated CO2 and salt concentration. Journal of Plant Growth Regulation, doi: 10.1007/s00344-013-9340-x Pérez-López, U., A. Robredo, M. Lacuesta, A. Mena-Petite, A. Muñoz-Rueda et al. 2012. Elevated CO2 reduces stomatal and metabolic limitations on photosynthesis caused by salinity in Hordeum vulgare. Photosynthesis Research, 111: 269-283. Prasad, P.V.V., K.J. Boote, L.H. Allen Jr. and J.M.G. Thomas. 2003. Super-optimal temperatures are detrimental to peanut (Arachis hypogaea L.) reproductive processes and yield at both ambient and elevated carbon dioxide. Global Change Biology, 9: 1775-1787. Schmidt, É.C., R.W. Santos, C. Faveri, P.A. Horta, R.P. Martins et al. 2012. Response of the agarophyte Gelidium floridanum after in vitro exposure to ultraviolet radiation B: Changes in ultrastructure, pigments, and antioxidant systems. Journal of Applied Phycology, 24: 1341-1352. Schmidt, S.B., P. Pedas, K.H. Laursen, J.K. Schjoerring, S. Husted et al. 2013. Latent manganese deficiency in barley can be diagnosed and remediated on the basis of chlorophyll a fluorescence measurements. Plant and Soil, 372: 417-429. Sheshshayee, M.S., H. Bindumadhava, N.R. Rachaputi, T.G. Prasad, M. Udayakumar et al. 2006. Leaf chlorophyll concentration relates to transpiration efficiency in peanut. Annals of Applied Biology, 148: 7-15. Singh, A.L. and Y.C. Joshi. 1993. Comparative studies on the chlorophyll content, growth, N uptake and yield of groundnut varieties of different habit groups. Oleagineux, 48: 27-34. Skotnica, J., M. Matouskova, J. Naus, D. Lazar, L. Dvorak et al. 2000. Thermoluminescence and fluorescence study of changes in Photosystem II. Photochemistry in desiccating barley leaves. Photosynth. Res., 65: 29-40. Su, Q., G. Frick, G. Armstrong and K. Apel. 2001. POR C of Arabidopsis thaliana: A third lightand NADPH-dependent protochlorophyllide oxidoreductase that is differentially regulated by light. Plant Molecular Biology, 47: 805-813. Sun, D.Y. 1998. Cellular Signal Transduction. Science Press, Beijing, China (in Chinese). Takebe, M., T. Yoneyama, K. Inada and T. Murakam. 1990. Spectral reflectance of rice canopy for estimating crop nitrogen status. Plant and Soil, 122: 295-297. Talwar, H.S., H. Takeda, S. Yashima and T. Senboku. 1999. Growth and photosynthetic responses of groundnut genotypes to high temperature. Crop Sci., 39: 460-466. Tang, Y.Y. 2011. Screening of Peanut Genotypes for Low Temperature Tolerance and Identification of Low Temperature Responsive Genes. Qingdao: Ocean University of China. Upadhyaya, H.D., R. Ortiz, P.J. Bramel and S. Singh. 2003. Development of a core collection using taxonomical, geographical and morphological descriptors. Genet. Res. Crop Evol., 50: 139-148. Upadhyaya, H.D., L.J.Reddy, C.L.L. Gowda and S. Singh. 2006. Identification of diverse groundnut germplasm: Sources of early maturity in a core collection. Field Crop Res., 97: 261-271. Wan, S.B. 2007. Peanut Quality Sciences. Chinese Agricultural Science and Technology Press, Beijing, China (in Chinese).

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CHAPTER

8

Respiration Aerobic respiration is the major energy producing process in higher plants, and the energy efficiency in the breakdown of glucose to CO2 and H2O by the EmbdenMeyerhof-Parnas (EMP) glycolytic sequences and the tricarboxylic acid (TCA) cycle is more than 90% (Beevers, 1970). Chemical processes of the bio-synthesis of polysaccharides, lipids, proteins, and other plant constituents have been reasonably well described. However, little information is available as to how respiration as a whole is linked with growth because of the greater complexity of these synthetic processes and the uncertainty of bioenergetics in biomass (Lehninger, 1965). A concept of the growth efficiency (GE) has been proposed by Tanaka and Yamaguchi (1968) to evaluate the significance of respiration in dry matter production. The GE is defined as W/(W + R), where W is the amount of dry matter produced and R is that of substances respired in a given period of plant growth. Thus, the GE is the proportion of the amount of growth in a given number of substrates. They demonstrated that the GE of rice plants in the tropics was about 60% during active vegetative growth stages and decreased at later growth stages. Attempts have also been made to classify the respiration of plants (R) into two components, i.e., growth respiration (Rg) and maintenance respiration (Rm) (Hansen and Jensen, 1977; Hesketh et al., 1971; Loomis et al., 1971; McCree, 1970, 1974; McKinion et al., 1974; Penning DeVries, 1972, 1973; Ryle et al., 1976; Tanaka, 1972; Thornley, 1970; Yamaguchi and Tanaka, 1970). McCree (1970, 1974) defined Rm as the efflux of CO2 from the plant after more than 48 hours in the dark. It was found that the rates of Rg and Rm per unit weight (Rg and Rm) were dependent on gross photosynthesis (Pg) and plant dry weight (DW), respectively: R = rg.Pg+rm.DW, and estimated that Rm were 15 and 5.4 mg glucose g-1 DW day-1 in white clover and sorghum, respectively. Ryle et al. (198l) described the total respiratory efflux of 14CO2 in terms of two main components: an intense efflux characterized by a half-life of 4-8 hours, which was identified with the biosynthesis of new tissue (Rg); and a much less intense efflux characterized by a half-life of 26-120 hours, which was identified with the maintenance of metabolic activity (Rm). Minor fractions of the products of photosynthesis remain at the site of production in fully expanded leaves. Most of them are translocated to other organs where they are either used as building blocks for various cell constituents or deposited as storage products (Sharma et al., 1981). Top leaves translocate more photosynthates to the developing pods than basal ones (Choudhary et al., 1987). A certain proportion is always lost through respiration during transport as well as at the final storage site. The unfilled pegs and pods in peanut indicate the potential for yield (Williams, 1979). The

222

Physiology of the Peanut Plant

information regarding the effect of the range of light intensities on translocation of assimilates at different stages of peanut is inadequate. Labelled 14CO2 was fed to the fully opened tetra foliate leaves of the main stem of M-13 peanut (Arachis hypogaea L.) which were exposed to 75%, 50% and 0% light intensities continuously at different stages of plant growth. Translocation of assimilated 14CO2 was measured from different parts of the plant. As light intensity decreased, the translocation of assimilates to the pods also decreased proportionately. Reduction of light intensity at the podding stage (70-105 DAS) showed that most of the assimilates were retained in fed leaves and only a small amount of assimilates were transported to pods. Reduction in light intensity at flowering and pegging or later pod filling stages had less effect on partitioning of assimilates than that at the initial podding stage. Translocation of assimilates at the podding stage is one of the factors that limited the yield under reduced light. The effect of temperature on respiration and photosynthesis must be regarded as considerably important. Went (1953) reported that at lower temperatures the ratio of photosynthesis to respiration is over 10, but that this ratio decreases at high temperatures. According to the latter author, these lower photosynthesis-respiration ratios may indicate why many plants have a more vigorous growth in the temperate regions than in the tropics.

8.1.

Dormancy and Seed Germination

Seed germination is regulated in a concerted manner that involves generating growth potential in the embryo to overcome the mechanical resistance of the endosperm. The wake-up call of a dry seed includes the reorganization of subcellular structures and the reactivation of metabolism in a dense, oxygen-poor environment. Pools of unbound metabolites and solutes produced by the degradation of storage reserves, including starch, proteins and oils, in the embryo can contribute to the generation of the embryo growth potential and radicle protrusion. Recent genomic studies have contributed a vast amount of data on protein, metabolite and gene transcript profiles during germination, which can be integrated to explore the seed metabolism from water imbibition to radicle protrusion. Figure 8.1 shows the decline in oxygen inside the mason jars at 7 and 8% moisture contents in hermetically sealed jars containing sound peanuts and 3% broken peanuts. The difference in the respiration rate of the two tests clearly indicates the enhancing effect of the presence of broken peanuts in the jars. Respiration of the 8% m.c. sound peanuts reached the lowest oxygen concentration after 28 days while that of the peanuts containing 3% broken ones reached it after 18 days. Respiration of the peanuts with 7% m.c. containing 3% broken ones reached the lowest oxygen concentration after 68 days while sound peanuts reduced their oxygen concentration to 1% which remained constant. Packaging bags composited with selective barrier films and moisture absorbent nonwoven fabrics were prepared to design a kind of functional bag, which can inhibit the growth of aflatoxin of peanuts. The influences of the super‐absorbent fibre (SAF) and jute fibre on the internal relative humidity (RH) were investigated. It was found that jute nonwoven/selective barrier film composite bags can prevent the growth of aflatoxin B1 of peanuts under the environment studied because peanuts with higher moisture content can reduce O2 content inside the bag by aerobic respiration, achieving

Respiration

223

Fig. 8.1. The decline in oxygen inside the mason jars at the 7 and 8% moisture content with

or without broken nuts (3%) both hermetically stored at 30°C. Results are an average of

3 replicates

the modified atmosphere packaging (MAP) effect. In addition, a low RH micro‐ environment can be achieved by using SAF as a moisture absorbent. It is promising to design a packaging bag with the effect of inhibition on the growth of aflatoxin of peanuts, by selecting proper moisture absorbents and selective barrier films of the composite bag. The respiration rate at any time of measurement was higher in GG 2 (a non-dormant cultivar) than the ICGS 11, a dormant seeded cultivar. For example, the highest respiration rate was 8.0 μg g-1 sec-1 CO2, at 55 h after imbibition in GG 2 and 6.8 μg g-1 sec-1 CO2 at 75 h after imbibition in ICGS 11 (Fig. 8.2). Seed and seedling vigour calculated following various methods showed a higher germination rate (GR) in GG 2 (53%/day) than the ICGS 11 (51%/day). Germination speed (GS) was also higher in GG 2 (30%/day) than ICGS 11 (28%/day). The co-efficient of velocity of germination was almost equal, i.e., 24.0 in GG 2 and 23.74 in ICGS 11, whereas the vigour index (VI) was much higher in ICGS 11 (46) than GG 2 (37). Thus, the higher vigour index in ICGS 11 is ascribed to higher hypocotyls plus root lengths. In addition, growth and development of secondary roots both in terms of length and number was also higher in ICGS 11. These results clearly indicated that seedling vigour index is a combination of various attributes that provide vigour to the developing seedling, and not the seed respiration rate alone. Wilson and Bonner (1971) observed that mitochondria isolated from peanut embryos were essentially deficient in Cyt C for up to 16h following imbibitions. However, even in the apparent absence of Cyt C, O2 uptake in these mitochondria was greater than 70% inhibited by 0.1 mM KCN suggesting that most of the respiratory flux was via the main Cyt pathway. Respiratory activity increased as seed size decreased. Oxygen consumption and RQ were higher in small seeds than in large ones (Table 8.1). Increased respiration (oxygen consumption) is a characteristic of its maturity (Kyalvie and Altschul, 1946; Olafson et al., 1954; Smirnov et al., 1943). The higher RQ of a small seed can also

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Physiology of the Peanut Plant

Fig. 8.2. Seed respiration rate during different hours of germination in non-dormant (GG 2) and dormant (ICGS 11) seeded Spanish market type groundnut cultivars (CD at P = 0.05 for variety × time = 0.11)

be related to immaturity. If RQ is considered as an indicator of the substrate being respired, then the RQ of 0.75 for the large seed suggests a lipid substrate which would be expected in mature peanuts. The RQ of 0.86 for medium peanuts also indicates a basic lipid substrate but suggests that oil synthesis has not quite reached the point of completion it has in large (mature) peanuts. The RQ of 0.91 for small seeds approached that of a carbohydrate substrate also suggesting that oil synthesis had not been completed in them. At least a portion of the primary respiratory substrate in small seeds could have been a carbohydrate, a protein not yet synthesized into structural material, or intermediates of lipid synthesis. Table 8.1. Respiratory characteristics of Spanish peanuts varying in size (weight) Lot Large Medium Small

2 HAI

4 HAI

6 HAI

O2*

CO2

RQ

O2

CO2

RQ

O2

CO2

RQ

58.2

37.7

0.65

80.0

54.4

0.68

96.9

72.8

0.75

82.1

54.3

0.66

113.2

89.5

0.79

119.7

102.4

0.86

121.7

65.3

0.54

149.7

117.1

0.78

148.3

135.7

0.91

HAI - Hours after imbibition * Respiration expressed on a per gram of seed per hour basis. Measured in microliters of gas exchange. RQ = CO2/O2.

Enzymatic activity paralleled respiration. As seed size increased, enzymatic activity decreased (Table 8.2). GADA was 103% higher, dehydrogenase activity 75% higher, and catalase activity 54% higher in smaller than in larger seeds. A high level of enzymatic activity is another characteristic of immature seeds (Kyalvie and Altschul, 1946; Smirnov et al., 1943). This higher level of enzymatic activity in small peanut seeds further suggests immaturity.

225

Respiration Table 8.2. Enzymic activity in seed size classes of Spanish peanuts Lot

GADA*

Dehydrogenase

Catalase

Large

17.50

0.21

112.0

Medium

19.50

0.25

124.5

Small

35.50

0.48

172.0

* GADA (Glutamic Acid Decarboxylase Activity) expressed in mm of CO2 evolved per 20 grams of ground seed per 30 minutes; dehydrogenase expressed as absorbance of light by formazan at 520 millimicrons as compared to pure acetone as zero; catalase expressed as mm of O2 evolved per 0.2 ml extract per 20 minutes.

The data suggest that exposure of high moisture peanut seeds to freezing injured the protein synthesizing system. This system is partially a membrane bound sequence of biochemical reactions and membranes have been found to be damaged by freezing (Benedict and Ketring, 1972; Mayland and Cary, 1970; Mazur, 1969). Carbon dioxide production from mitochondrial activity also could have been reduced by damage to these membranes. The results were reduced ethylene production, germination and seedling growth or a complete loss of most of these functions by a majority of the seeds (Fig. 8.3).

Fig. 8.3. Effect of exposure to subfreezing temperatures on ethylene production (left), carbon dioxide production (centre), and seedling growth (right). LSD: Least Significant Difference

For assessing seed and seedling vigour, especially for cultivation of groundnut in stressed environments a genotype with high vigour is required to cope with adverse situations, if any occurs during germination and early seedling establishment stages. For instance, the pre- and post-radicle emergence seed vigour in relation to O2 consumption, as measured by the rate of respiration during different hours of germination in dormant and non-dormant seeded cultivars, is not related with ultimate seedling vigour. The respiration rate, in fact was always higher in the non-dormant, while the length and number of secondary roots was higher in dormant seeded cultivars.

226

Physiology of the Peanut Plant

Further, the growth and development of secondary roots in combination with medium or higher seed weights seems to be beneficial in establishing the crop stand under water scarcity environments. It is also reported that during germination in fully viable embryos and axes the activity of mitochondria increases with time after imbibition (Bewley and Black, 1994). The synthesis of ATP commences almost immediately and during germination becomes more efficiently coupled to O2 consumption. However, the degree of correlation between O2 uptake and seedling vigour varies with the time after imbibition at which respiration is measured, and the number of days after which seedling vigour was determined (Halmer and Bewley, 1984). As is evident from the respiration rate and vigour parameters, it is clear that each cultivar is characterized with different combinations of seed and seedling vigour parameters. Cultivars such as GG 2 have higher GR and GS and respiration rate, but these values did not match with the ultimate seedling vigour required to establish a healthy seedling, especially under adverse conditions. Therefore, the overall development of a seedling, including the development of secondary roots as in the case of ICGS 11, seems to be an adaptation to a water scarcity environment. It is also suggested that while assessing groundnut seedling vigour, the growth and development of secondary roots must be taken into consideration. Seeds of the variety Luhua No. 14, which undergo nondeep dormancy, were selected, and their transcriptional changes at three different developmental stages, the freshly harvested seed (FS), the after-ripening seed (DS) and the newly germinated seed (GS) stages, were investigated by comparative transcriptomic analysis. The results showed that genes with increased transcription in the DS vs FS comparison were overrepresented for oxidative phosphorylation, the glycolysis pathway and the tricarboxylic acid (TCA) cycle, suggesting that after a period of dry storage, the intermediates stored in the dry seeds were rapidly mobilized by glycolysis, the TCA cycle and the glyoxylate cycle; the electron transport chain accompanied by respiration was reactivated to provide ATP for the mobilization of other reserves and for seed germination. In oxidative phosphorylation pathways, a series of protein complexes in the electron transport chain (consisting of complexes I to V) within the inner membrane of mitochondria carried out the sequential redox reactions to oxidize nutrients and to release energy. Results found that large numbers of the enzyme genes related to this pathway were significantly enriched, which included the genes encoding NADH DEHYDROGENASE (ND) subunits 1, 2, 4, 4L, 5 and 6 as well as NADH DEHYDROGENASE (UBIQUINONE) IRON-SULFUR (NDUFS) subunits 1, 2, 7, and 8 and FLAVOPROTEIN 2 (NDUFV2) from complex I (NADH-COENZYME Q OXIDOREDUCTASE); SUCCINATE DEHYDROGENASE (UBIQUINONE) IRON-SULFUR SUBUNIT 2 (SDHB2) from complex II (SUCCINATE-Q OXIDOREDUCTASE); UBIQUINOL-CYTOCHROME C REDUCTASE IRONSULFUR SUBUNIT (ISP), CYTOCHROME B SUBUNIT (CYTB), CYTOCHROME C1 SUBUNIT (CYT1) and UBIQUINOL-CYTOCHROME C REDUCTASE SUBUNIT 7 (QCR7) from complex III (CYTOCHROME BC1 COMPLEX); CYTOCHROME C OXIDASE (COX) subunits 1, 2 and 3 from complex IV; and different kinds of ATPASE subunits from complex V (F-TYPE H+-TRANSPORTING ATPASE subunit α; subunits a, b and g; and V-TYPE H+-TRANSPORTING ATPASE subunits B, D, E, G, and H as well as a 21 kDa PROTEOLIPID subunit) (Figs. 8.4A, B, C).

Respiration

227

Fig. 8.4. DEGs between the DS and FS stages in several metabolic pathways. A. The majority of upregulated genes represented in the KEGG pathways are involved in glycolysis, the tricarboxylic acid (TCA) cycle, the glyoxylate cycle, ASP and ALA metabolism, and oxidative phosphorylation. (1) Invertase; (2) Hexokinase (HK); (3) Glucose-6-phosphate dehydrogenase (G6PDH); (4) Pyrophosphate-dependent phosphofructokinase (PFP)/ diphosphate-fructose-6-phosphate1-phosphotransferase; (5) Phosphofructokinase (PFK); (6) Fructose-biphosphate aldolase (FBA); (7) Pyruvate kinase (PK); (8) Pyruvate dehydrogenase complex; (9) Citrate synthase (CSY); (10) Citrate hydrolase and citrate hydroxymutase; (11) Isocitrate dehydrogenase (IDH); (12) 2-Oxoglutarate dehydrogenase (OGDH); (13) SuccinylCoA:acetate CoA transferase/SSA-CoA synthetase; (14) Succinate dehydrogenase (SDH); (15) Fumarate hydratase; (16) Malate dehydrogenase (MDHm); (17) Isocitrate lyase (ICL); (18) Malate synthase (MSY); (19) Aspartate aminotransferase (AspAT); (20) Malate dehydrogenase (MDHc); (21) NAD-dependent malic enzyme 2 (NAD-ME2); (22) Alanine aminotransferase (AlaAT). The numbers in parentheses marked in red represent the upregulated genes encoding the key enzymes in the related pathway. B. Heatmaps of the 59 DEGs among the FS, DS and GS stages involved in the oxidative phosphorylation pathway; C. Heatmaps of the 59 DEGs among the FS, DS and GS stages involved in the carbon metabolic pathway.

228

Physiology of the Peanut Plant

In carbon metabolism pathways, many genes associated with glycolysis, the tricarboxylic acid (TCA) cycle (also named the citrate cycle), and the glyoxylate cycle were significantly upregulated during this stage (Fig. 8.4). Among them, twenty-one genes encoding different dehydrogenases, including the PYRUVATE DEHYDROGENASE COMPLEX, ISOCITRATE DEHYDROGENASE (IDH), 2-OXOGLUTARATE DEHYDROGENASE (OGDH), SUCCINATE DEHYDROGENASE (SDH), and MALATE DEHYDROGENASE (MDH), accounted for 1/3 of the upregulated genes, which, by a series of oxidation reactions of intermediates in the glycolysis pathway and TCA cycle, catalyse one pyruvate molecule to produce CO2, one molecule of ATP, four NADH molecules and one FADH2 molecule. The transcriptional level of four genes encoding aminotransferase (ASPARTATE AMINOTRANSFERASE and ALANINE AMINOTRANSFERASE 2 from mitochondria; SERINE-GLYOXYLATE AMINOTRANSFERASE, and PHOSPHOSERINE AMINOTRANSFERASE 1 from chloroplasts) was markedly elevated in results (Fig. 4A and 4C). The resulting NADH and FADH2 molecules enter the electron transport chain and are further oxidized to produce energy by oxidative phosphorylation. In addition, the expression of several genes encoding GLYCERALDEHYDE-3-PHOSPHATE DEHYDROGENASE also increased, which catalyses the oxidation and phosphorylation of glyceraldehyde-3-phosphate to produce 1,3-bisphospho-D-glycerate in glycolysis. The expression levels of several key genes involved in complexes I to V of the electron transport chain and crucial dehydrogenase genes in the glycolysis pathway and TCA cycle were verified by qRT-PCR. They all displayed the enhanced expression in dry seeds than those in freshly-harvested seeds, while some of them expressed in newly germinated seeds at significantly downregulated ways than in dry seeds (Fig. 8.5). Rate of respiration, glutamic acid decarboxylase, dehydrogenase and catalase activity were measured in small, medium and large seeds of Spanish peanuts. Rate of respiration, O2 consumption and enzymatic activity increased as seed size increased. The relatively high activity of the small seeds as compared to large ones suggests immaturity. The culture presents a more efficient stomatal closure mechanism, reducing respiration before the photosynthetic activity is irreversibly damaged, which is a characteristic of resistance to climatic elements, like water availability (Anjum et al., 2011). However, the level of damage caused to the peanut crop by water deficiency is determined by the intensity, duration of stress and phenological stage in which the crop is found (Duarte et al., 2013).

Fig. 8.5. Analysis of the mRNA transcript levels of several DEGs between the DS and FS stages by real-time fluorescent quantitative RT-PCR. A. DEGs related to the electron transport chain in the oxidative phosphorylation pathway; B. DEGs involved in the dehydrogenation reaction in glycolysis and the TCA

Respiration

8.2.

229

Respiration in Peanut Plant

The circadian like endogenous change in the dark respiration of peanut leaves varied between 0.5 and 0.2 mg CO2 dm-2hr-1 (Fig. 8.6). An endogenous rhythm in the rate of carbon dioxide output has been reported to exist in the leaves of several species (Chia-loot and Cumming, 1972; Hillman, 1972; Wilkins, 1959). Circadian like changes in the dark respiration of roots, tubers, and germinating and ungerminated seeds are also known to exist (Brown et al., 1970; Bryant, 1972; Huck et al., 1962). The CO2 compensation point of the cultivar Florigiant shows rhythmic changes throughout a 24-hour light period (Fig. 8.6). The range for plants amounted to 30 µl/l; however, the range between maxima and minima of CO2 compensation points of other plants has been greater and lesser than 30 µl/l. The rhythm is out of phase with photosynthesis, dark respiration, and transpiration.

Fig. 8.6. Photosynthesis, transpiration, dark respiration, and carbon dioxide compensation of peanut leaves in continuous light

All growth and phenology depends on respiration; they are the prices for changes in state and form. Unless differences in the efficiency of respiration exist, higher rates are usually advantageous and indicative of greater growth or developmental rates. It is generally accepted that many processes and phenomena are linked to common respiratory processes at the biochemical level and are therefore influenced by environmental factors, such as respiration, temperature and substrate supply. Little research has been conducted directly on the respiration of peanut. Generally, respiration in peanut is considered to be the same as in other plants. The existing simulation models use generic respiratory coefficients to compute the respiration associated with the production of carbohydrates, proteins and oils. There is no evidence for major differences in specific respiratory costs for any process within the species. Peanut respiration responses to temperature are similar to those of most plants. Watterott (1991) observed that root respiration was influenced by both recent photosynthesis and temperature. The Q10 for temperature was 2.07, which is similar to that observed for most plants (1.8 to 2.2), and the value Q10 = 2.0 is generally recommended for modelling of respiration (van Keulen et al., 1982). Limitations to respiration associated with substrate supply are more difficult to document at the biochemical level, but many processes have been successfully

230

Physiology of the Peanut Plant

modelled using a substrate limitation hypothesis providing circumstantial evidence that the underlying respiration has been limited by substrates. The direct evidence for substrate limitations to peanut respiration and respiration related processes was shown by Watterott (1991) for roots. The respiration was strongly influenced by the temperature and photosynthesis occurring over the few proceeding hours. There were strong diurnal trends in respiration (after the effects of temperature had been taken into account), which imply that substrate limitations to root respiration occur during dark periods. Varieties had varying respiration rates, but these were considered more a result of growth differences than those in maintenance respiration. Groundnut had a larger number and high frequency of primary and secondary roots as well as a larger total root length and leaf area. There was a significantly high correlation between the root respiration rate and N uptake activity among the crop groups. Since N uptake should be one of the major functional activities of the root, the low root respiration can be considered to be an adequate index to estimate the capacity of root function. The slope of the regression line indicates the N uptake efficiency in terms of requirement or respiratory energy, which is 1.8 times higher in legumes than in cereals (Table 8.3 and Fig. 8.7). Table 8.3. Rates of root respiration, transpiration, total nitrogen content in root and shoot, and nitrogen uptake activity for 3 legumes and 3 cereals Parameter Root respiration rate (µmole O2 g root FW-1) Transpiration rate (cm2m-2h-1) Root-N (% on DW) Shoot-N (% on DW) N uptake activity (mgN g root FW-1)

Pigeonpea Chickpea Groundnut Sorghum Pearl millet 17.1 223

10.3 340

16.3 155

14.1

16.3

99.4

97.1

Maize 9.10 84.2

1.7

1.5

1.8

0.57

0.87

0.70

1.8

2.7

1.9

0.87

0.97

0.67

4.83

2.48

4.14

2.03

2.39

1.05

FW = Fresh weight, DW = Dry weight

Studies deal with the changes in respiratory activity of peanut (Arachis hypogaea var. Big Japan) and linseed (Linum usitatissimum var. NP [RR] 5) plants as influenced by different levels of the employed calcium sand culture technique revealing that low levels of calcium cause stimulation of leaf respiration. Under complete deficiency of calcium, however, while a high respiration rate is observed in linseed leaves in the case of peanut plants the respiration rate is almost similar to that of control in most harvests. Furthermore, it has been observed that the magnitude of the increase in respiration varies with the age of the plant. In peanut plants, though, high levels of calcium initially result in the promotion of respiration but at later stages of growth it shows inhibitory effects. In case of linseed plants, initially there is an inhibition which seems to recover at subsequent growth periods. The effects of light on plant respiration have been studied for many years (Azcón‐ Bieto et al., 1983; Azcón‐Bieto and Osmond, 1983; Kromer et al., 1988; Azcón‐Bieto et al., 1989; Gardeström et al., 1992; Hill and Bryce , 1992; Kromer , 1995). Light

Respiration

231

Fig. 8.7. Correlations between root respiration rates and N uptake activity in legumes (pigeonpea, chickpea and groundnut) and cereals (sorghum, pearl millet and maize). Each symbol represents an individual measure with 4 replications for each crop

is known to regulate gene expression of several key respiratory enzymes through the action of phytochrome, including cytochrome c oxidase (Hilton and Owen, 1985) and phosphoenolpyruvate carboxylase (Sims and Hague, 1981). It has also been suggested that blue light can cause an increase in total respiration (Kowallik, 1982). Furthermore, there is a differential expression of alternative oxidase genes between light‐grown and dark‐grown tissues (Obenland et al., 1990; Finnegan et al., 1997). Other workers have reported an indirect effect of light and photosynthesis on respiration, with the cellular concentration of sugars suggested to play an important role in the regulation of respiration in leaves (Azcón‐Bieto et al., 1983 ) and roots (Bingham and Farrar, 1988). The presence of two terminal oxidases, branching from the ubiquinone pool in the mitochondrial electron transfer chain has been known for many years, along with the fact that the cyanide‐resistant, alternative respiratory pathway is not coupled to the synthesis of ATP (Moore and Siedow, 1991). Studies of the kinetics of ubiquinone oxidation by the alternative oxidase and complex III of the cytochrome pathway supported the notion of Bahr and Bonner (1973) that the redox level of the ubiquinone pool is a major factor controlling the electron partitioning between the alternative oxidase and the cytochrome pathways (Dry et al., 1989). However, this behaviour can be modified by effectors which presumably function in the metabolic regulation of the alternative oxidase. These effectors include a redox‐sensitive disulphide bond (Umbach and Siedow, 1993; Ribas‐Carbo et al., 1997) and α‐keto acids (Millar et al., 1993; Ribas‐Carbo et al., 1997). Several workers have suggested that soluble sugars could regulate electron flow to the alternative pathway (Azcón‐Bieto et al., 1983). Studies used inhibitors to show that the flux through the alternative pathway in wheat leaves was higher at the beginning of the night, when the soluble sugar concentration was higher, than at the end of the night (Azcón‐Bieto et al., 1983). In light of these results we were interested in observing the effects of an extended period of darkness,

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Physiology of the Peanut Plant

which would lead to a depletion of cellular sugar, on the electron partitioning between the two respiratory pathways. Carbon isotope discrimination during photosynthetic CO2 assimilation has been extensively studied and rigorous models have been developed, while the fractionations during photorespiratory and dark respiratory processes have been less well investigated. Whilst models of discrimination have included specific factors for fractionation during respiration (e) and photorespiration (f), these effects have been considered to be very small, i.e. not significantly modifying the net discrimination expressed in organic material. The fractionation effects associated with specific reactions set against the overall discrimination which occurs during source-product transformations are considered. Studies which have recently shown that discrimination occurs during respiration at night in intact C3 leaves, lead to the production of CO2 enriched in 13C (i.e., = –6), and modifying the signature of the remaining plant material. Under photorespiratory conditions (i.e., increased oxygen concentration and high temperature), the photorespiratory fractionation factor may be high (around, +10), and significantly alters the observed net photosynthetic discrimination measured during gas exchange. Fractionation factors for both respiration and photorespiration have been shown to be variable among species and with environmental conditions. Specific respiration rates of whole plants above-ground DM were significantly greater for Ruby than for Early Bunch. Although specific respiration rates of plants of Ruby acclimated to 10°C nights were higher than for plants at 20°C, the difference was not statistically significant. Similar data for leaf specific respiration rates (Table 8.4) showed no differences between Ruby and Early Bunch in plants acclimated to 20°C and measured near that temperature, but differences became increasingly apparent as the night temperature at which plants were acclimated declined. Measurement temperature effects on whole-plant specific respiration rates showed Q10 ranging from 1.77±0.23 to 2.07±0.27 and a mean Q10 across all treatments of 1.95, very similar to the expected value of 2.0 (Johnson and Thornley, 1985; Amthor, 1989) (Table 8.4). Table 8.4. Nitrogen concentration (N), total respiration rates, specific leaf weights, and calculated specific respiration rates for 10 fully expanded leaves of Ruby and Early Bunch grown at 20, 15 and 12°C night temperatures Cultivar

Night temp. °C

Temp °C

Total N (%)

Total res. rate mg CO2 m-2h-1

Specific leaf wt. (g.m-2)

Specific res. rate mg CO2 g-1DMh-1

Ruby Early Bunch

20 20

21.9

3.84 4.22

145.7±9.0 144.7±8.9

69.9±6.0 68.5±11.3

2.08±0.09 2.11±0.13

Ruby Early Bunch

15 15

16.7

3.62 4.17

89.3±6.5 76.1±4.6

74.1±6.0 74.1±3.1

1.21±0.12 1.03±0.10

Ruby Early Bunch

12 12

12.5

3.60 4.21

80.9±4.0 62.4±6.4

74.1±3.8 76.8±6.9

1.09±0.14 0.81±0.06

The PG and RD were higher in cv. M 13 in both the rainy seasons (Table 8.5).

233

Respiration

Table 8.5. Mean rates of gross photosynthesis (PG) and respiration (RD) during the growth period (20 to 90 d after sowing). Parameter

Rainy season I

Rainy season II

cv. J 11

cv. M 13

cv. J 11

PG (g m d )

7.31

9.90

8.88

9.91

RD (g g d )

-0.030

-0.046

-0.030

-0.036

-2 -1

-1 -1

cv. M 13

Carbon isotope composition in respired CO2 and organic matter of individual organs were measured on peanut seedlings during early ontogeny in order to compare fractionation during heterotrophic growth and transition to autotrophy in a species with lipid seed reserves with earlier results obtained on beans. Despite a high lipid content in peanut seeds (48%) compared with bean seeds (1.5%), the isotope composition of leaf- and root-respired CO2 as well as its changes during ontogeny were similar to already published data on bean seedlings: leaf-respired CO2 became 13C-enriched reaching –21.5‰, while root-respired CO became 13C-depleted reaching 2 around –31‰ at the four-leaf stage. The opposite respiratory fractionation in leaves vs. roots already reported for C3 herbs was thus confirmed for peanuts. However, contrarily to beans, the peanut cotyledon-respired CO2 was markedly 13C-enriched, and its 13C-depletion was noted from the two-leaf stage onwards only. Carbohydrate amounts being very low in peanut seeds, this cannot be attributed solely to their use as respiratory substrates. The potential role of isotope fractionation during glyoxylate cycle and/or gluconeogenesis on the 13C-enriched cotyledon-respired CO2 is noted. When the foliage of a branch had been fed 13CO2 at the vegetative stage, the loss of the assimilated 13C by respiration was about 40% of the total assimilated 13C within 23 d and about 65% within 93 d after the exposure, and a small amount of photo assimilates was detected in the fruit. On the other hand, at the seed-filling stage, about 35% of the photo assimilates were utilized for seed growth within 10 d after the end of exposure. These results suggest that in the peanut plant, the carbon source of nodules mainly depends on the branch, and the main stem plays an important role as a carbon source for the fruit, that a sink organ for carbon is connected with a specific source leaf by the vascular bundles, and that most of the carbon sources for the growth of peanut fruit depend on the photo assimilates at the reproductive stage.

8.3.

Temperature

Respiration can be highly affected by temperature (Atkin et al., 2005), and its rate is determined by the status of carbohydrate and supply of adenylate (enzyme catalysing the conversion processes) (Fig. 8.8). The sucrose content of the tissue can govern the capacity of mitochondrial respiration (Farrar and Williams, 1991), and mitochondrial respiration plays a great role in growth and survival of plants (Atkin et al., 2005). It is expected that at least a short period increases the respiration rate from parts of plants that show increased growth and assimilation due to elevated [CO2], that is source leaves, individual sink tissue (fruit, seed, steam, root) and total sink tissue. Nevertheless, a few reports concluded that long-term treatment with increased concentration of CO2 resulted in declined whole-plant respiration (Farrar and Williams, 1991). Whereas, results of a few other experiments show that a short-term increase in temperature on

234

Physiology of the Peanut Plant

plants growing in cold climate areas such as Arctic have resulted in a greater potential impact on plant respiration than in plants growing in warmer areas (tropics) (Atkin and Tjoelker, 2003). One of the reasons might be that tropical plants better acclimatise to higher temperatures than the Arctic cold area plants. Research on volatiles present in developing groundnut seeds by Pattee et al. (1970) indicates that the maximum metabolic activity of seeds occurs during stage II of seed development and is probably related to larger seed size while moisture contents are still adequate, rather than high levels of metabolic activity per se. Schenk’s (1961) data for pod respiration shows that this reaches a maximum around the beginning of stage II of seed development. During active lipid synthesis the respiratory quotient of developing kernels approaches two, falling to unity as this process stops. One of the most noteworthy aspects of the study by Pattee et al. (1970) was the high level of lipoxygenase activity detected from the ninth week of development onwards; the activity of this enzyme might be expected to reduce the levels of polyunsaturated fatty acids accumulated by the maturing seed. Levels of conserved lipoxygenase in dried kernels may be important in subsequent storage behaviour of seeds. Zinc was known to be a constituent of many enzymes which stimulated various metabolic activities such as nucleic acid metabolism, protein synthesis, photosynthesis, respiration and carbohydrate metabolism. Marschner (1986) indicated that cation and anion uptake by the cell could be actively regulated by electrogenic proton pumps (H+-ATPase), transmembrane redox pumps (NAD (P) oxidase), and ion channels. Davis et al. (1995) indicated that the plants exposed to higher concentrations of zinc disturbed the mitochondrial structure and reduce the energy. Respiration is necessary for many processes in living organisms; for instance, it is crucial for maintenance of photosynthesis activity, mainly because of the energy demands of sucrose synthesis. Moreover, it plays a role in determining the carbon budget of individual plants and the concentration of CO2 in the atmosphere; it contributes up to 65% of the total CO2 released to the atmosphere (Atkin et al., 2005). The influence of abiotic stress is complex, given that it is often confounded

Fig. 8.8. Changes in the rate of photosynthesis and respiration of (C3) crops as a function of temperature (Porter and Semenov, 2005)

Respiration

235

and associated with hot temperatures, water deficits and high light intensities. Heat and drought stress significantly affect physiological processes, e.g., inhibition of photosynthesis, disruption of respiration, changes in membrane permeability, and interference with nutrient mobility. The response of plant respiration to long-term change in temperature is dependent on the level of effects of temperature on plant development, and on other direct and interactive effects of temperature and abiotic factors (e.g. irradiance, nutrient availability and drought). Evidence shows that the response of respiration to temperature is dynamic, with plant respiration often acclimatizing to long-term changes in temperature. In addition, both degree of acclimation and value of Q10 (proportional change in respiration with a 10°C increase in temperature) vary in response to the surrounding environment and/or the metabolic condition of the plants. There is variability in Q10 as the day and night time temperatures vary (e.g. nights are increasing to a larger extent than daytime). The Q10 of leaf R is often not always reduced in the light compared with the Q10 of leaf R in dark, and Q10 values are often lower in water-stressed plants than in their fully-watered counterparts, root and leaves also differ in their Q10 values as upper and lower canopy leaves. Q10 of both root and leaf R dark generally decreased as temperature increased. Rise in [CO2] does not show a predictable, systematic effect on Q10 of dark R of stems root or leaves. Different studies show a variation in the effect of rise in atmospheric [CO2] on the Q10 value of R in the above ground plant parts in dark conditions, but the overall results indicate that elevated [CO2] has little impact on the average Q10 values (Atkin et al., 2005) (Fig. 8.9). Genotypes used in this study differed in their response to temperature treatments. Genotypes did not differ in their vegetative weight, indicating that the source was not limiting. Thus, processes like photosynthesis or respiration, responsible for the source, are not altered much. In contrast, pod yield was reduced in both the genotypes.

8.4.

Heat Stress

Heat stress affects respiration as it retards or increases mitochondrial activity depending on the crop (Paulsen, 1994; Stone, 2001). Initially, the rate of respiration increases exponentially with increasing temperature but beyond threshold levels, respiration decreases due to damage to respiratory mechanisms (Prasad et al., 2008). Decreased respiration under high temperature has been reported in chickpea (Kumar et al., 2013), and was most likely due to the impaired structure and function of mitochondria and proteins and the effect on the electron transport rate. The rate of both photorespiration and dark respiration of cotton leaf increased with increasing temperature (Salvucci and Crafts-Brandner, 2004). Prasad et al. (1999) found that increase in respiration rate especially during the night can increase ROS, resulting in cell damage and reduced pollen viability. Respiration, particularly the mitochondrial ETC, is responsible for the production of ROS in the dark (Kromer, 1995). A well-drained soil facilitates adequate exchange of air to meet nitrogen, carbon dioxide and oxygen requirements of the crop. The respiration of roots is affected when oxygen supply is low due to lack of proper drainage. This results in inhibition of root growth and retards metabolic functions. In the absence of adequate oxygen in the root zone, the nitrogen fixing bacteria are ineffective and the roots are unable to take up soil nitrogen.

236

Physiology of the Peanut Plant

Fig. 8.9. Relative photosynthesis (A), stomatal conductance (B), respiration (C) and nitrogen content (D) changes with night temperature changes in peanut

Respiration

8.5.

237

Salt Stress

Generally, the growth failure in saline environments stems from the fact that water intake into the seed is hindered (Coons et al., 1990; Mansour, 1994). In addition, yield reduction in saline conditions is due to the toxic effect caused by excessive concentration of Na and Cl ions, breakdown of crop ion balance, problems in nutrient uptake and transport, and decrease in physiological processes such as respiration and photosynthesis (Levitt, 1980; Yeo and Flowers, 1983; Leopold and Willing, 1984; Marschner, 1995). Nitric oxide (NO) is a redox, gaseous, highly reactive nitrogen species (RNS) produced in living cells under normal as well as under conditions of biotic and abiotic stress. When the concentration of ROS becomes toxic to a plant, NO may act as a detoxifier and minimize any detrimental effects (Lipton et al., 1993). NO also has a role in respiratory function, namely electron transport pathways in mitochondria, where it modulates ROS thereby activating defence mechanisms through enhanced antioxidant production in plants exposed to various abiotic stresses (Zottini et al., 2002). Additionally, exogenous supply of NO leads to activation of antioxidant enzymes, especially superoxide dismutase, and restricts superoxide anion and lipid O2 and organic radicals (R) (Shi et al., 2007). However, NO was also shown to be an endogenous modulator of several plant hormones, in addition to inhibiting the induced programmed cell death and aiding in stomatal functions in several plant species such as Arabidopsis, wheat, and pea (Bright et al., 2006, Leshem and Haramaty, 1996; Mata, 2001). NO provokes a wide range of physiological responses in plants, including germination, development, flowering, senescence, and abiotic stress. Moreover, NO possesses several additional properties favourable to activity as a signalling messenger during unfavourable or multiple stress conditions such as the presence of free radicals, small size redox molecules, neutral, and easily diffusible through a cell membrane, all these make it a very significant agent to act as a dynamic molecule (Domingos et al., 2015). NO is known to interact directly or indirectly with a wide range of targets, leading to altered gene expression and protein function, thereby affecting the phenotypic response. NO-mediated stress responses and the underlying mechanisms have been extensively studied by several scientists (Spoel and Loake, 2011; Yu et al., 2012), though with variable results due to the use of different approaches (Ahmad et al., 2016; Begara-Morales et al., 2014; Manai et al., 2014). Variations in endogenous NO levels and/or exogenous NO application have shown to regulate abiotic stress resistance suggesting that this approach may contribute to enhancing crop production under stress conditions (Ahmad et al., 2018; Farooq et al., 2009; Siddiqui et al., 2011). Different studies report molecular details of the adaptive responses of plants to salt stress in the presence of exogenous application of NO donors. Polyamines have a known role in plant defences against salt stress, and their induction is known to be closely associated with NO production (Guo et al., 2009). Therefore, NO-polyamine interaction may be one of the molecular mechanisms for stress tolerance in plants. NO, induces the expression of the plasma membrane (PM) H+-ATPase required for a balanced K+ :Na+ ion ratio providing protection against salt stress (Zhao et al., 2004). Evidence for NO mediated protection against salt stress in vivo has been shown in Arabidopsis Atnoa1 mutants with impaired endogenous NO levels as these plants show enhanced sensitivity to salt stress, as well as reduced survival rates compared to wild plant types.

238

Physiology of the Peanut Plant

By drying freshly harvested peanuts at various temperatures Picket (1957) found that 49°C was the least satisfactory as far as flavour and aroma were concerned, and observed that this temperature was in the critical range for life processes. The involvement of a life process in curing off-flavour was postulated in 1957 from studies that used oxygen atmospheres during curing (Dickens, 1957). Peanuts bulkcured in an oxygen atmosphere gave better flavoured products than those cured in nitrogen or carbon dioxide atmospheres (Emery and Gupton, 1968). These results suggested that anaerobic respiration was the cause of curing off-flavour. Under slowcuring conditions sufficient oxygen could diffuse into the kernels to supply metabolic demands, but under high curing temperatures the oxygen could not diffuse in at a rate sufficient to supply the increased metabolic demands and consequently an aerobic condition occurred with a resultant production of off-flavour. Studies with Whitkar and Dickens (1964) indicated that the level of off-flavour in peanuts was related to the amount of anaerobic respiration that occurred during curing. The results also showed that immature peanuts cured at 35°C and had more off-flavour than mature peanuts cured at 52°C.

8.6.

Temperature × CO2

Uprety et al. (1996) concluded that with the type of climate in the northern belt of the Indian subcontinent, viz, variation in temperatures and CO2 concentration, the production of Brassica crop (an oilseed crop) is likely to increase and is likely to be shifted in some more relatively drier region as than where it is grown presently. Hundal and Kaur (1996) examined the climate change impact on productivity of wheat, rice, maize and groundnut crop in Punjab using CERES-wheat (Godwin et al., 1989), CERES-rice (Singh et al., 1993), CERES-maize (Ritchie et al., 1989) and “PNUTGRO” (Kaur, 1993) crop simulation models. They concluded that, if all other climate variables were to remain constant, temperature increases of 1, 2 and 3°C from present day conditions, would reduce the grain yield of wheat by 8.1, 18.7 and 25.7%, rice by 5.4, 7.4 and 25.1%, maize by 10.4, 14.6 and 21.4% and seed yield in groundnut by 8.7, 23.2 and 36.2%, respectively. In general, the simulation results indicate that increasing temperature and decreasing radiation levels pose a serious threat in decreasing growth and yields of cereals and oilseeds crop. Increased CO2 levels are expected to favour growth and increase crop yields and, therefore, will be helpful in counteracting the adverse effects of temperature rise in future. Temperature has been shown to have a major influence on the growth and development of peanuts, with the optima for growth and dry matter production generally considered to lie within a mean temperature range of 27 to 32°C (Bell and Wright, 1998). Recent studies (Bell et al., 1992, 1993, 1994) have indicated that low night temperatures of less than 20°C significantly decrease crop productivity in many environments. Such temperature conditions are encountered in many regions of southern Australia.

8.7.

Water

Drought tolerance is a cost-intensive phenomenon, as a considerable quantity of energy is spent to cope with it. The fraction of carbohydrate that is lost through respiration determines the overall metabolic efficiency of the plant (Davidson et al., 2000). The

Respiration

239

root is a major consumer of carbon fixed in photosynthesis and uses it for growth and maintenance, as well as dry matter production (Lambers et al., 1996). Plant growth and developmental processes as well as environmental conditions affect the size of this fraction (i.e., utilized in respiration). However, the rate of photosynthesis often limits plant growth when soil water availability is reduced (Huang and Fu, 2000). A negative carbon balance can occur as a result of diminished photosynthetic capacity during drought, unless simultaneous and proportionate reductions in growth and carbon consumption take place. There are two mitochondrial electron transport pathways from ubiquinone to oxygen in plants. The alternative pathway branches from the cytochrome pathway and donates electrons to oxygen directly by alternative oxidase (Moore and Siedow, 1991). The alternative pathway is not coupled with adenosine triphosphate synthesis, but can be induced in response to stress or inhibition of the main electron transfer pathway (Wagner and Moore, 1997). When plants are exposed to drought stress, they produce reactive oxygen species, which damage membrane components (Blokhina et al., 2003). In this regard, alternative oxidase activity could be useful in maintaining normal levels of metabolites and reducing reactive oxygen species production during stress. Oxygen uptake by sugar beet was characterized by a high rate, distinct cytochrome oxidasedependent terminal oxidation and up to 80% inhibition of respiration in the presence of 0.5 mM potassium cyanide. At an early drought stage (10 days), a decrease in the activity of the cytochrome-mediated oxidation pathway was largely counter-balanced by the activation of mitochondrial alternative oxidase, whereas long-term dehydration of plants was accompanied by activation of additional oxidative systems in sensitive to both potassium cyanide and salicyl hydroxamate (Shugaeva et al., 2007). In summary, water deficit in the rhizosphere leads to an increased rate of root respiration, leading to an imbalance in the utilization of carbon resources, reduced production of adenosine triphosphate and enhanced generation of reactive oxygen species. Water deficit stress during the pod-development phase is detrimental to several physiological and biochemical processes (Nautiyal et al., 1991). Water stress conditions disturb photosynthetic activity of plants and thereby affects further vegetative growth and the mobilization of assimilates towards storage or sink tissues. Sugars in plants, derived from photosynthesis, act as substrates for energy metabolism and the biosynthesis of complex carbohydrates, providing sink tissues with the necessary resources for growth and development. Responses to a specific stress can vary with the genotype, but some general reactions occur in all. Under sugar depleted conditions, substantial physiological and bio-chemical changes occur to sustain respiration and other metabolic processes (Journet et al., 1986) Sucrose and glucose either act as the substrates for cellular respiration or as the osmolytes to maintain cellular osmotic potential (Gupta et al., 2005). However, the experiment using low CO2 and cool temperature shows that the reduction in A was not a result of photoinhibition. Further, during the first 2 d of sub-optimal temperatures and normal CO2 in the source leaves, subsequent rewarming led to complete recovery within 1 d. Assimilation rate also increased to pre-cooling rates within 1 h of rewarming the source leaf to 30°C. Overnight and fast recovery of the assimilation rate in the peanut indicates some non-damaging inhibition, possibly involving deactivation and reactivation of ribulose-bisphosphate carboxylase-oxygenase (Rubisco) or changes in the concentration of endogenous inhibitors of Rubisco (e.g. 2-carboxyarabinitol 1-phosphate (Berry et al., 1987). During respiration at night under low-temperature,

240

Physiology of the Peanut Plant

sink treatment increased by no more than 0.5 pmol.m-2.s-1 compared with plants at 30°C . This represents a 2% change compared with a change in A of 50% or more. The slightly higher rate of respiration at night in the cooler plants may be associated with increased dry matter accumulation (Azcön-Bieto and Osmond, 1983). Clearly, then, changes in dark respiration cannot account for the change in A. Similarly, increases in photorespiration are unlikely to account for the changes in A (Fig. 8.9). The oxygen sensitivity of photosynthesis remained unaltered after cooling although the degree of oxygen sensitivity depends on the species and on the pre-treatment temperatures (Cornic and Louason, 1980; Sage and Sharkey, 1987). According to Azevedo et al. (2014) in irrigated agriculture it is necessary to know the determining factors in irrigation management that directly interfere with higher or lower water consumption levels, and to determine the water needs of the crops according to the different phenological stages.

Fig. 8.10. Changes in assimilation with time in peanut

8.8.

Nutrition

Root respiration could be used as an index of the root activity which was positively correlated with the N uptake activity, indicating the importance of the role played by root respiration in the total nitrogen status of the plant (Bloom et al., 1992). The respiratory efficiency for N uptake can be calculated from the slope of the regression and does not seem to be significantly different among the groups of crops. Using fieldgrown crops, Ito et al. (1996) reported that pigeon pea exhibited an extremely low efficiency of N uptake in terms of respiration requirements. Sulphur is now widely accepted as fourth major plant nutrient along with N, P and K. It is involved in the synthesis of essential amino acids and oils in oilseeds, being a vital component of the co-enzyme involved in oil synthesis. It is also involved in various metabolic and enzymatic processes including photosynthesis, respiration and legume-rhizobium symbiotic nitrogen fixation. This role of sulphur in plant makes it of fundamental importance in increasing the productivity of crops especially legume oilseeds in India, where more than 50% of soils have been reported to be deficient in sulphur (Tewatia et al., 2006). In plants, Fe participates in photosynthesis, respiration, the biosynthesis of phytohormones and chlorophyll, and in electron transfer in redox reactions (Sánchez-Alcalá et al., 2014). Results indicate that adding Fe2O3 NPs to the

Respiration

241

soil increased the biomass, chlorophyll content, and total Fe content of peanut plants. Overall, both Fe2O3 NPs and EDTA-Fe could notably increase peanut growth in terms of dry biomass and total chlorophyll content. The evidence for the reduction of antioxidant enzyme activities suggested that the additions of both types of Fe sources did not result in oxidative stress in plants, but stimulated plant growth by producing certain amounts of ROS, which are known as signalling molecules to trigger root elongation and plant development. The rate of photosynthesis, growth and pod yield were maximum at 0.5 ppm of B in the nutrient solution, indicating that this is the optimum concentration of B. The respiration rate in groundnut roots was more at the toxic levels (5 ppm) of B. The toxic effect of Al on roots has a clear effect on plant metabolism by decreasing mineral nutrition and water absorption. Aluminium has been shown to interfere with cell division in plant roots; inhibits the respiratory activity of mitochondria; increases pectin, hemicellulose and cellulose contents of root cell walls; reduces DNA replication; decreases cell permeability by coagulating protein and inhibiting cell division; reduces root respiration; precipitates nucleic acid by forming strong complexes; inhibits cation transport across the plasma membrane; blocks K+ uptake in root hairs; interferes with the uptake, transport, and use of several nutrients (P, K, Ca, Mg, Zn and Fe) and water by plants (Kamprath and Foy, 1984; Keltjens, 1990; Keltjens and Djikstra, 1990; Baligar et al., 1993; Delhaize and Ryan, 1995; Kochian, 1995). Manganese toxicity alters the activities of enzymes and hormones in plants; causes the destruction of indole-3-acetic acid (IAA) by increasing the activity of IAA oxidase, amino acid imbalance, lower respiration rate, reversal of growth inhibition of roots caused by enhanced auxin production (Robson, 1988).

8.9.

Root Nodules

Peanut has a deep taproot, but most of the 1st-order lateral roots show a shallow distribution (Ketring and Reid, 1993), and nodules are formed dominantly on the shallow lateral roots. Such a distribution of nodules in the root system possibly occurs because root nodules need a large amount of oxygen for nitrogen fixation and the formation and activity of nodules are often inhibited by low levels of oxygen (Weisz and Sinclair, 1987). The C cost determined by CO2 released from nodulated roots is generally higher than that from nodules as the former includes root respiration. The C cost of N fixation also varies with growth stages (Ryle et al., 1979; Twary and Heichel, 1991), but it is a matter of debate that the C cost increases (Warembourg and Roumet, 1989; Voisin et al., 2003) or decreases (Adgo and Schulze, 2002) with the course of the legume life cycle. In addition, the strain of Rhizobium may affect the C cost significantly. Carbon provision and assimilation to bacteroids in legume nodules has been reviewed recently by Lodwig and Poole (2003). In summary, photosynthate is supplied to nodules as sucrose in the phloem. Sucrose synthase activity in the nodule is primarily responsible for the hydrolysis of the sucrose. If carbon supply to the nodule is more than sufficient, some of the released monosaccharides can be stored as starch within the nodule. Sugars destined for utilization by the bacteroids pass through glycolysis and enzymes of the tricarboxylic acid cycle, to form malates, fumarates and succinates, the three dicarboxylic organic acids that are the primary carbon sources for bacteroids in the nodules. The transport of these dicarboxylic acids across the peri

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bacteroid membrane and their utilization by bacteroids have been studied extensively (Day and Udvardi, 1993; Day et al., 2001; Lodwig and Poole, 2003). Nitrogenase is sensitive to inhibition by molecular oxygen. The exact mechanism of the inhibition is unknown (Gallon, 1992), but it may be that O2 itself is a substrate of nitrogenase, and its reduction leads to highly reactive oxygen species (e.g., O2) which result in the denaturation of the nitrogenase complex. Regardless of the mechanism, all diazotrophs containing the Mo–Fe nitrogenase complex must protect nitrogenase from O2. In aerobic bacteria such as rhizobia, Frankia, and cyanobacteria, this is a particular dilemma because the organisms require an adequate O2 flux for oxidative phosphorylation to provide the energy required for nitrogenase activity, but not so high a flux that it will result in inhibition of nitrogenase. In fact, despite existing in an aerobic milieu, it is necessary that the environment immediately surrounding the nitrogenase complex be microaerobic. In its symbiotic state, the legume nodule creates a microaerobic environment to protect nitrogenase from O2, but a relatively high flux of O2 is maintained to the bacteroids for respiration. These seemingly contradictory functions (low O2 concentration/high O2 flux) are accomplished via an O2-diffusion barrier in the cortex of the nodules, and facilitated diffusion of O2 bound to the transporting haemoprotein, leghaemoglobin. The subject of the regulation of O2 flux to bacteroids within legume nodules has been well reviewed (see Hunt and Layzell, 1993; Lodwig and Poole, 2003). In brief, an O2-diffusion barrier exists in a region of densely packed cells in the inner cortex of legume nodules. There is some controversy whether expression of Enod2 is related to the development of this diffusion barrier (Wycoff et al., 1998). The exact nature of this diffusion barrier is unknown, but there is evidence that the path length of intercellular water (Denison, 1992) or the abundance of intercellular glycoprotein (James et al., 2000) may play roles in establishing the diffusion resistance to O2. A very important feature of this diffusion barrier is that it can quickly change (i.e., in seconds to minutes) its resistance to O2 diffusion when either the external concentration of O2 or the internal demand for O2 changes. In fact, it appears that a number of stresses (drought, temperature, supplemental mineral N, carbohydrate limitations) decrease nitrogenase activity indirectly by decreasing O2 diffusion into the infected zone of the nodule (Kuzma and Layzell, 1994; Serraj et al., 1999; Vessey et al., 1988). In the infected zone of the nodule, leghaemoglobin acts as a shuttle, binding O2 from the intercellular spaces within the infected zone, diffusing down the oxyleghemoglobin concentration gradient, and delivering O to the sites of respiration (cytochrome oxidase) in the bacteroids (Becana and Klucas, 1992; Bergersen, 1996; Denison and Okano, 2003). Leghaemoglobin represents approximately 5% of the total protein of a mature nodule and is coded for by at least four lb genes, of which lbc 3 is the most intensively studied (e.g., Cvitanich et al., 2000). Respiration and nitrogen fixation in legume root nodules is considered to be limited by the rate at which O₂ from the atmosphere can enter the nodules. Athin diffusion barrier in the inner cortex, restricts access to the central tissue where there is a high demand for O₂ at a low concentration. Observed variations in rates of nodule activities in response to imposed stresses, are often attributed to variations in the diffusion resistance of the barrier. Aspects of nodule structure and metabolism underlying nodule activities are reviewed in terms of components of the symbiotic system, the nature of steady states and in relation to homeostasis of low O₂ concentrations within the bacteroidfilled host cells. It is suggested that variations in O₂-demand of both mitochondria and

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bacteroids, serve to preserve nitrogenase activity by poising O₂ concentration within ‘safe’ limits. Further, data from isolated soybean bacteroids suggest that nitrogenase is converted to a less active but more robust form, in the presence of O₂ in excess of about 70 nM, thus protecting nitrogenase from irreversible inactivation by excess O₂. This regulation is rapidly-reversible when O₂ concentration falls below about 0.1 µM. Respiration by large numbers of host mitochondria in the periphery of infected nodule cells, adjacent to gas-filled intercellular spaces, is considered to play an important part in maintaining a steep gradient of O₂ concentration in this zone. Also, it is possible that variations in nodule O₂ demand may be involved in the apparent variations in resistance of the diffusion barrier. It is concluded that there are many biochemical components which should be considered, along with possible changes to the diffusion barrier, when the effects of imposed stresses on nodule activities are being analyzed. The energy for splitting the nitrogen gas in the nodule comes from sugar that is translocated from the leaf (a product of photosynthesis). Malate as a breakdown product of sucrose is the direct carbon source for the bacteroid. Nitrogen fixation in the nodule is very oxygen sensitive. Legume nodules harbour an iron containing protein called leghaemoglobin, closely related to animal myoglobin, to facilitate the diffusion of oxygen gas used in respiration. It is only produced in fully functioning root nodules, and functions to bind and regulate the levels of oxygen in the nodule. Since the enzyme nitrogenase is sensitive to oxygen, free oxygen in nodule cell cytoplasm would inhibit the levels of nitrogen fixation in the nodule. Leghaemoglobin seems to transport enough oxygen to allow the rhizobia to carry out cellular respiration, but not too much to inhibit the action of nitrogenase. This heme protein is jointly produced by the legume and bacterium; the legume produces the apoprotein while the bacterium produces the heme (porphyrin ring bound to an iron atom). Differences in symbiotic qualities between swollen and nonswollen bacteroids have been previously explored in peanuts and cowpeas by Sen and Weaver (1980, 1981, 1984), who also hypothesized that swollen bacteroids are more beneficial to the host plant than nonswollen ones. They found 1.5 to 3 times greater acetylene reduction by nitrogenase (as well as plant nitrogen) per nodule mass in peanuts than in cowpeas at multiple nodule ages (Sen and Weaver, 1980). Acetylene reduction per bacteroid was also greater in peanuts than in cowpeas when measuring whole nodules, but this difference disappeared when isolated bacteroids were assayed (Sen and Weaver, 1984). They concluded that swelling of peanut bacteroids per se was not responsible for the higher rate of nitrogen fixation per bacteroid. They suggested that in cowpea nodules, with greater numbers of smaller bacteroids per nodule volume, availability of oxygen to each bacteroid might be restricted such that the rate of oxidative phosphorylation, necessary for nitrogen fixation, is reduced. Fixation rates per bacteroid may be different between hosts due to nodule gas permeability or bacteroid crowding within nodules. However, fixation efficiency (nitrogen fixed per carbon respired) would not necessarily be affected by these and may be more important for the host than the rate of fixation (Fig. 8.11). The measured fixation efficiency in 12 different legume genera found no consistent difference between those species with swollen bacteroids and those with nonswollen ones. For example, peas ranged from 2.25 to 4.52, CO2 : C2H4 moles whereas beans ranged from 2.65 to 3.29 CO2 : C2H4 moles, depending on the host cultivar and rhizobia strain. They also found no clear difference between cowpeas

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and peanuts (1.97 CO2 : C2H4 moles in cowpeas and 2.08 CO2 : C2H4 moles in peanuts) that were nodulated by the same strain of rhizobia RCR 3824. These comparisons by Witty et al. (1983), among others (Hunt et al., 1989), gave similar values H2 : CO2 ratios assuming 1 mol of H2 for 1 mol of C2H4 conversion. Further comparisons of single strains nodulating both hosts that do or do not impose bacteroid swelling would be informative. Unfortunately, such dual-host strains are rare. Even R. leguminosarum A34 did not effectively nodulate a wide array of bean cultivars, so the experiment could not be extended even within bean. Interestingly, A34 could effectively nodulate other pea cultivars, including Green Arrow, albeit with delayed nodulation with even higher efficiency (0.70 H2/CO2±0.18 SD, n = 3) than in our first tested cultivar, Maestro.

Fig. 8.11. Nitrogen fixation efficiency measured as a marginal increase in ratio of nitrogenase activity (μmol H2 g−1 h−1) with increasing respiration (CO2 μmol g−1 h−1) in pea and bean nodules nodulated by R. leguminosarum A34 (A) and in peanut and cowpea nodules nodulated by Bradyrhizobium sp. 32H1 (B). Error bars indicate one SD, with n = 3; all differences significant at *P < 0.05, **P < 0.001.

The effects of salt stress on nodulation and nitrogen fixation of legumes have been examined in several studies (Abdel-Wahab and Zahran, 1981; Abdel-Wahab et al., 1991; Delgado et al., 1994; El-Shinnawi et al., 1989; Ikeda et al. 1992; Nair et al., 1993; Velagaleti et al., 1990; Zahran, 1986). The reduction of N2-fixing activity by salt stress is usually attributed to a reduction in respiration of the nodules (Delgado et al., 1994; Ikeda et al., 1992; Walsh, 1995) and a reduction in cytosolic protein production, specifically leghaemoglobin, by nodules (Delgado et al., 1993, 1994). The depressive effect of salt stress on N2 fixation by legumes is directly related to the salt-induced decline in dry weight and N content in the shoot (Cordovilla et al., 1995). The saltinduced distortions in nodule structure could also be reasons for the decline in the N2 fixation rate by legumes subjected to salt stress (Sprent and Zahran, 1988; Zahran, 1986; Zahran and Abu Gharbia, 1995). Reduction in photosynthetic activity might also affect N2 fixation by legumes under salt stress (Georgiev and Atkinos, 1993). The disaccharide trehalose plays a role in osmoregulation when rhizobia are growing under salt or osmotic stress (El-Sheikh and Wood, 1990; Hoelzle and Streeter, 1990). Trehalose accumulates to higher levels in cells of R. leguminosarum (Breedveld et al., 1991) and peanut rhizobia (Ghittoni and Bueno, 1996) under the increasing osmotic pressure of hyper salinity. Fast-growing peanut rhizobia accumulate trehalose

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in the presence of many carbon sources (mannitol, sucrose, or lactose), but the slow growers accumulate trehalose only when cultured with mannitol as the carbon source. In a medium supplemented with 400 mM NaCl, the content of trehalose increased intracellularly throughout the logarithmic and stationary phases of growth of peanut rhizobia (Ghittoni and Bueno, 1995). The disaccharides sucrose and ectoine were used as osmo protectants for Sinorhizobium meliloti (Gouffi et al., 1999). However, these compounds, unlike other bacterial osmo protectants, do not accumulate as cytosolic osmolytes in salt-stressed S. meliloti cells.

8.10.

Other Aspects

Storing pods in jute bags provides conditions conducive to mold growth, especially with A. flavus. Jute bags are highly porous and can easily absorb moisture, and therefore foster the rapid growth and multiplication of these aflatoxigenic molds. Alternatively, hermetic storage offers a new alternative to traditional storage of grains and pods, and is a sustainable practice. Hermetic storage works on the principle of creating airtight conditions in which oxygen levels are lowered for insect, fungal and seed respiration. In a recent study conducted at ICRISAT, Purdue Improved Crop Storage (PICS) bags that rely on the principle of hermetic storage were used to safeguard groundnuts against A. flavus infestation, and subsequently lowered aflatoxin contamination levels in storage. Peanut and foxtail millet intercropping can produce a better production increase and synergy, but there is no research report on the photosynthetic response characteristics of the intercropping system. In this test, research was conducted on the variation characteristics of leaf photosynthetic response and chlorophyll fluorescence parameters under the intercropping conditions of peanut and foxtail millet. The increase in Fo (dark level fluorescence) of intercropping foxtail millet leaves can reduce the reversible inactivation of hard light on foxtail millet leaves; increase in Fm (maximum fluorescence) and Fv/Fm (maximum quantum yield of photosystem II) of intercropping peanut can improve conversion efficiency of the PS II (photosystem II) original light energy of peanut. Meanwhile, dark respiration rate and the light compensation point of crops in the intercropping system decreased and the apparent quantum efficiency and light saturation point increased, which can show that the peanut and foxtail millet intercropping can improve the light energy utilization efficiency of them. The causes of off-flavours in peanuts include lipid oxidation, induction of anaerobic respiration, and external contamination with compounds such as limonene, antioxidants, or insecticides (Ory et al., 1992). Lipid oxidation is one of the leading causes of off-flavours in raw and roasted peanuts, due to a high content of unsaturated fatty acids (Warner et al., 1996; Lee et al., 2002). Oxidation of the fatty acids in peanut oil can be caused by light, heat, air, metal contaminations, microorganisms, or enzymatic activity (Ory et al., 1992; Sanders et al., 1993). Hydroperoxides formed during lipid oxidation subsequently break down into alcohols, alkanes, ketones, and aldehydes, which can be the source of off flavours in the peanut. High concentrations of certain compounds such as ethanol, ethyl acetate, and acetaldehyde were found in high temperature cured peanuts (Pattee et al., 1965). In addition, a fruity fermented off-flavour has been shown to occur predominantly in immature peanuts undergoing high temperature curing (Sanders et al., 1989; Didzbalis et al., 2004). An enzyme called lipase is present in both animal and plant tissues. The presence of this enzyme

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enables the lipolysis of lipids to form FF A and glycerol, triggering hydrolytic rancidity (McWilliams, 1993). Crude oils are particularly sensitive to lipase, so manufacturers briefly heat treat oils to desensitize these enzymes and prevent extensive splitting of triglycerides (Patterson, 1989). Polyunsaturated FF A are susceptible to further degradation by oxidative enzymes. The interaction of products formed from the breakdown of FF A and the products from the breakdown of hydroperoxides, results in the formation of keto acids, methyl ketones and lactones, all having an individual, yet distinct pungent flavour (Patterson, 1989).

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CHAPTER

9

Nitrogen Metabolism and Biological Nitrogen Fixation A large proportion (50-80%) of nitrogen in peanut crops is derived from N2-fixation (Boddey et al., 1990; Khan and Yoshida, 1994). Application of nitrogen fertilizer increased the peanut yield only in few cases (Nambiar, 1990). Without any supply combined nitrogen from soil or fertilizer the initial growth of soybean plants sometimes becomes poor especially before the initiation of nitrogen fixation (Hatfield et al., 1974). Heavy application of nitrogenous fertilizers is known to depress the nodulation and nitrogen fixation activity, and in some cases results in over-luxuriant growth and a lower yield (Semu and Hume, 1979). Peanut is considered to be a poor utilizer of nitrogen fertilizer (Nambiar et al., 1986). A large proportion of fertilizer is lost through volatilization, denitrification, and leaching (Sisworo et al., 1990) and considered as an environmental hazard. The addition of 200 kg N ha-1 as urea decreased the N2-fixation of cowpea by 57%, while the N2-fixation of peanut decreased only by 31% (Yoneyama et al., 1990). Again, nitrogen fixation in soybean was reduced by 20-25% with the application of 160 kg N ha-1 as Ca(NO3)2 (Rennie et al., 1982). However, the application of plant materials (Broadbent et al., 1982) and ground plant materials or oxamide (Witty and Ritz, 1984) did not affect the symbiotic nitrogen fixation, presumably because of the slow release of N from these sources. The use of nitrogen fertilizer to maximize the benefits depends on the proper selection of fertilizers. Application of slow release nitrogen fertilizers provides not only an insight into the N-economy of the crop, but also results in a better utilization of atmospheric nitrogen and higher yields (Tables 9.1 and 9.2). Total N, fertilizer N-utilization, and percentage of N2 fixed in peanut estimated by the A value method shows that the total amount of N in peanut increased with the increase in the levels of N-fertilizer, irrespective of the source of fertilizer and time of sampling. At 75 DAS, the % Ndfa in 100, 200, 400 mg.pot-1 N was 61, 54 and 29 respectively when N was applied as 15N Urea (Table 9.2), and at 98 DAS at the same levels of 15N Urea application the % Ndfa was 69, 63 and 52 respectively (Table 9.1). The utilization of 15N Urea by the plants in the pots treated with 100, 200 and 400 mg.pot-1 was 55, 55 and 62%, respectively and at 75 DAS was 69, 71 and 68% respectively at 98 DAS. In the case of 15N Super IB, the % Ndfa in the 100, 200 and 400 mg.pot-1 was 63, 62 and 61 respectively at 75 DAS (Table 9.2) 69, 69 and 66 respectively at 98 DAS (Table 9.1). The utilization of 15N Super IB by the plants at 75 DAS was 46, 41 and 31% respectively in the pots treated with 100, 200 and 400

257

Nitrogen Metabolism and Biological Nitrogen Fixation Table 9.1. Effect of different concentrations of 15N Urea on symbiotic N2-fixation of peanut at 98 DAS

Total N in plant (mg.pot-1) Atom % 15N excess in plant % N utilization % Ndff A soil+fix (mg.pot-1) A soil (mg.pot-1) A fix (mg.pot-1) % Ndfa % Ndfs

Non-nodulating

Nodulating

Urea (mg.pot-1) 100

Urea (mg.pot-1)

337 2.27 64.8 21.0 376 79.0

100

200

1047 0.71 69.3 6.6 1413 1037 68.5 24.9

LSD0.05 400

1089 1108 1.40 2.66 70.3 68.3 12.9 24.7 1351 1222 976 846 62.8 52.2 24.3 23.1

118 0.11 11.6 1.0 113 113 2.7 1.3

The atom % 15N excess of fertilizer was 10.8 for both nodulating and non-nodulating peanuts and for all the fertilizer rates of fertilizer-N. Table 9.2. Effect of different concentrations of 15N Super IB on symbiotic

N2-fixation of peanut 75 DAS

Total N in plant (mg.pot-1) Atom % 15N excess in plant % N utilization % Ndff A soil+fix (mg.pot-1) A soil (mg.pot-1) A fix (mg.pot-1) % Ndfa % Ndfs

Non-nodulating

Nodulating

Super IB (mg.pot-1) 100

Super IB (mg.pot-1)

147 1.85 34.4 23.5 376 76.5

LSD0.05

100

200

400

530 0.69 46.1 8.7 1048 722 62.9 28.4

556 1.15 40.6 14.6 1171 845 61.6 23.8

576 1.72 31.4 21.8 1440 1114 60.5 17.7

63 0.11 5.5 1.4 121 121 2.7 1.2

The atom % 15N excess of fertilizer was 7.89 for both nodulating and non-nodulating peanuts and for all the rates of fertilizers.

mg.pot-1 and at 98 days at the same levels of 15N Super IB the utilization was 32, 48 and 42% respectively. Hence, it can be concluded that higher levels of 15N Urea markedly decreased the symbiotic nitrogen fixation and that in contrast the application of 15N-Super-IB did not decrease the nitrogen fixation, even at higher doses. The utilization of 15N Urea was very high presumably because water was applied very carefully and there was no hole at the bottom of the pots, hence 15N Urea was not lost through runoff or leaching. The utilization of 15 N-super-IB was less remarkable than that of 15N-Urea, because 15N-super-IB is a slow release N-fertilizer (Table 9.3). Nitrogen requirements and utilization of mineral nitrogen (N) by sorghum and groundnut were compared. At the maximum N use level, sorghum genotypes showed greater N use efficiency (120 kg biomass/kg N harvested) than groundnut genotypes

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Physiology of the Peanut Plant Table 9.3. Effect of different concentrations of 15N Super IB on symbiotic

N2-fixation of peanut at 98 DAS Non-nodulating fertilizer application (mg.pot-1) 100

Total-N in plant (mg.pot-1)

Atom% 15N excess in plant % Ndff % N-utilization

Asoil+Fix (mg.pot-1)

Asoil (mg.pot-1)

Afix (mg.pot-1)

% Ndfa

% Ndfs

279 124 15.8 43.8 543 ­ ­ 84.2

Nodulating fertilizer application (mg.pot-1) 100

200

300

1095a 0.38c 4.8c 52.2a 2024a 1481a 69.4a 25.8a

1139a 0.66b 84b 47.9a 2188a 1645a 68.8a 22.8ab

1175a 1.12a 14.2a 41.8a 2426a 1883a 66.4a 19.4b

LSD(0.05)

93 0.15 1.9 11.2 479 ­ 479 6.6 4.9

The atom % 15N in excess of fertilizer was 7.89 for both nodulating and non-nodulating peanuts and for all the rates of fertilizer-N. In a row, means followed by the same letters are not significantly different at 5% significance level.

(36 kg biomass/kg N harvested). Using a non-nodulating groundnut genotype (Nonnod) or sorghum as controls for soil N uptake, the amounts of N, fixed by the nodulated groundnut genotypes were estimated to be 183-190 kg N/ha. Nitrogen fertilization increased harvest index and percentage N translocated to seeds in sorghum genotypes, but decreased harvest index and had variable effects on the percentage of N translocated to seeds in groundnut genotypes (Fig. 9.1). Leaf nitrate reductase activity (NRA) and nitrate content in the leaves of two sorghum genotypes, one nodulating, and ‘Non-nod’ groundnut genotypes were also compared. The concentration of nitrate was lower in sorghum than in groundnut leaves, but NRA was higher in sorghum. It is suggested that either NRA in the groundnut leaves has relatively lower affinity for the substrate (higher Km, the Michaelis-Menten constant) or higher nitrate is required for the induction of nitrate reductase in groundnut than in sorghum. This implies that groundnut is a poor utilizer of nitrogen fertilizers (Table 9.4). Table 9.4. Effect of N fertilization on total nitrogen harvested (kg/ha) Nitrogen applied (kg/ha) 0 50 100 150 200 SE

Sorghum CSH 8R 30 58 64 82 104 ±5.01a

Groundnut

M35-1 25 40 56 66 87

Non-nod 29 43 60 97 90

Robut 33-1 218 208 231 222 246 ±11.8b

J11 212 203 188 195 206

The reduction of nitrate (NO3 to NO2) is believed to be the rate-limiting step in plants growing on mineral nitrogen, and the enzyme NR in the leaves is inducible by the substrate NO3, in many plant species (Beevers and Hageman, 1969; Srivastava,

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1980). Significant positive correlations between NRA and plant growth have been obtained in many plant species (Zieserl et al., 1963; Croy and Hageman, 1970; Dykstra, 1974; Singh et al., 1976). Hence, examination of the relationship between leaf nitrate content and leaf nitrate reductase activity of groundnut and sorghum was undertaken. Both groundnut genotypes showed low leaf NRA, even at very high levels of nitrate content in the leaves, whereas sorghum genotypes showed higher NRA at low levels of leaf nitrates (Fig. 9.1) relative to groundnut genotypes. This could be because of (a) higher nitrate concentration being required to induce nitrate reductase in groundnut than in sorghum, or (b) nitrate reductase in groundnut having a lower affinity for the substrate (higher Km, the Michaelis-Menten constant) compared to that of sorghum. This implies that groundnut is a poor utilizer of mineral nitrogen at all stages of growth. This may be one of the reasons for the poor response of groundnut to N application; differences in symbiotic N fixation in different soil types could also contribute to the erratic responses of groundnut to N application. Regression analyses of nitrate content in the leaves vs NRA of the three canopy portions at 71 DAS are shown in Fig. 9.2. Although slope differences between genotypes of sorghum and those of groundnut are significant, slope differences within

Fig. 9.1. Translocation of amino acids in groundnut

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Fig. 9.2. Relationship between the NRA/mg leaf per h and nitrated leaf in the top (A), middle (B), and lower (C) leaves of sorghum () and groundnut () genotypes

groundnut genotypes and sorghum genotypes were not significant. Hence only pooled data for groundnut (Robut 33-1 and Non-nod) and sorghum (M 35-1 and CSH 8R) are presented in Fig. 9.2. This data indicates high NRA activity at a low nitrate content in the leaves of sorghum, and low NRA even at a high nitrate content, in the leaves of groundnut. Gene action for nodulation associated with biological nitrogen fixation has been studied in groundnut. Both additive and non-additive gene interactions are prevalent for nodule number/plant, nodule dry weight/plant, average weight of nodules and nitrogenase activity of the nodules. The genotypes ‘Gunajato’ and ICGV-86055 were the best general combiners for nodule characters and nitrogenase activity in the nodules. The cross ICGV-86055xEC-21989 appeared to be a promising combination for high nodule numbers and nitrogenase activity. Fluctuations in pH, nutrient availability, temperature, and water status, among other factors, greatly influence the growth, survival, and nitrogen fixation metabolic activity of bacteria. The subsequent inhibition of nitrogenase would result in O2

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accumulation in the infected zones, inducing a decrease in nodule permeability. Poor nodulation of legumes in arid soils is likely due to decreases in population levels of rhizobia during the dry season. Fixation, therefore, also tends to decrease with legume age, mainly because of the concomitant increase in soil N. Calcium deficiency, with or without the confounding influence of low pH also affects attachment of rhizobia to root hairs. Rhizobia may have different tolerances to soil acidity factors than the host plant. A relatively, high-root temperature has also been shown to influence infection, N2-fixation ability, and legume growth. Also, root nodulation by the bacteria can be dependent on the formation of mycorrhiza. Phosphorus (P) is second only to nitrogen as an essential mineral fertilizer for crop production. At any given time, a substantial component of soil P is in the form of poorly soluble mineral phosphates. A high phosphorus supply is needed for nodulation. When legumes dependent on symbiotic nitrogen receive an inadequate supply of phosphorus, they may therefore also suffer from nitrogen deficiency. Apart from nitrogen, phosphorus is also an essential nutrient for an efficient growth and yield improvement of the groundnut (Hemalatha et al., 2013). Also, the

Fig. 9.3. Estimation of symbiotic and nodule parameters recorded after phosphorus

supply and strains inoculations.

Ctrl: Control, T1: Plants supplied with 50 kg P2O5.ha-1, T2: Plants inoculated with STM 5945,

T3: Plants inoculated with STM 5945 and supplied with 50 kg P2O5.ha-1, T4: Plants inoculated

with STM 5894, T5: Plants inoculated with STM 5894 and supplied with 50 kg P2O5.ha-1, T6:

Plants inoculated with WSM 4412, T7: Plants inoculated with WSM 4412 and supplied with

50 kg P2O5.ha-1

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phosphorus can not only result in an increase of the crop nitrogen fixation rate (Jones, 1982) but also can increase the nodulation of roots (Nwaga and Ngo Nkot, 1998). The competition observed among soil native bacteria strains is minimized with an appropriate dose of phosphorus (Basu and Bhadoria, 2008; Basu, 2011). On inclusion in the cropping system, groundnut is known to help phosphorus solubilization in soils which can improve the soil physical environment, and soil microbial activity to restore most of its organic matters (Ghosh et al., 2007). Potassium and sulphur are usually not limiting nutrients for nodulated legumes, although a K+ supplement for osmoadaptation has to be considered for growth in saline soils. Among mineral nutrients, B and Ca are undoubtedly the nutrients with a major effect on legume symbiosis. Both nodulation and nitrogen fixation depend on B and Ca2+, with calcium more necessary for early symbiotic events and B for nodule maturation. Copper plays a role in proteins that are required for N2 fixation in rhizobia. Cu deficiency decreases nitrogen fixation in subterranean clover. Iron is required for several key enzymes of the nitrogenase complex as well as for the electron carrier ferredox in and for some hydrogenases. A particularly high iron requirement exists in legumes for the heme component of haemoglobin. As molybdenum is a metal component of nitrogenase, all N2-fixing systems have a specifically high molybdenum requirement. Molybdenum deficiency-induced nitrogen deficiency in legumes relying on N2 fixation is widespread, particularly in acid mineral soils of the humid and sub humid tropics. A specific role for nickel in nitrogen-fixing bacteria is now well established with the determination that a nickel-dependent hydrogenase is active in many rhizobia bacteria. Cobalt is required for the synthesis of leghaemoglobin and, thus, for the growth of legumes relying on symbiotically fixed nitrogen, an essential mineral nutrient. It has been established that Rhizobium and other N2-fixing microorganisms have an absolute cobalt requirement whether or not they are growing within nodules and regardless of whether they are dependent on a nitrogen supply from N2 fixation or from mineral nitrogen. The nutritional status of plants, nodulation (number of nodules and nodule dry matter per plant), nitrogenase activity, and nitrogenase specific activity were evaluated at 45 and 64 days after emergence (DAE). The yield components and kernel yield were evaluated at the end of the growing season. Nitrogenase enzyme activity at 64 DAE approximately doubled, and the number of pods per plant was greater with inoculation than without, both of which led to greater yields of pods and kernels. In long-term pasture areas, inoculation and molybdenum fertilization greater than the currently recommended rate appear to be necessary to increase pod and kernel yield per hectare of peanut when managed under no-tillage. The principal forms of amino nitrogen transported in xylem were studied in nodulated and non-nodulated peanuts (Arachis hypogaea L.). In symbiotic plants, asparagine and the nonprotein amino acid, 4-methylene glutamine, were identified as the major components of xylem exudates collected from root systems decapitated below the lowest nodule or above the nodulated zone. Sap bleeding from detached nodules carried 80% of its nitrogen as asparagine and less than 1% as 4- methylene glutamine (Fig. 9.1).Pulsefeeding nodulated roots with 15N2 gas showed asparagine to be the principal nitrogen product exported from N2-fixing nodules. Maintaining root systems in an N2-deficient (argon: oxygen, 80:20, v/v) atmosphere for 3 days greatly depleted asparagine levels in nodules. 4-methylene glutamine represented 73% of the total amino nitrogen in the xylem sap of non-nodulated plants grown on nitrogen-free nutrients, but relative levels

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of this compound decreased and asparagine increased when nitrate was supplied. The presence of 4-methylene glutamine in xylem exudate did not appear to be associated with either N2 fixation or nitrate assimilation, and an origin from cotyledon nitrogen was suggested from a study of changes in the amount of the compound in tissue amino acid pools and in root bleeding xylem sap following germination. Changes in xylem sap composition were studied in nodulated plants receiving a range of levels of 15N-nitrate, and a 15N dilution technique was used to determine the proportions of accumulated plant nitrogen derived from N2 or fed nitrate. The abundance of asparagine in xylem sap and the ratio of asparagines: Nitrate fell, while the ratio of nitrate: total amino acid rose as plants derived less of their organic nitrogen from N2. Assays based on xylem sap composition are suggested as a means of determining the relative extents to which N2 and nitrate are being used in peanuts.

9.1. Ammonia Assimilation The ammonium available for plants can be derived from the atmospheric nitrogen fixed by the Rhizobiaceae bacteria in the legume nodules or from the nitrate absorbed from the soil. NH4+, organic nitrogen is also taken up from soil. Whatever its origin may be, the ammonium is then assimilated and incorporated into the nitrogenous compounds. The enzymes glutamine synthetase and glutamate synthase participate in this process by catalysing the reactions: • L-Glutamate + NH4+ + ATP → L-Glutamine + ADP + Pi • L-Glutaine + 2-oxoglutarate + NADPH (or Fdx.red) → 2L-Gutamate + NADP + (or Fdx.ox). The two major classes of GOGAT enzymes in higher plants are ferredoxindependent GOGAT (Fdx-GOGAT) and NADPH-dependent GOGAT (NADPHGOGA T), both plastid localized. Fdx-GOGAT is the predominant GOGAT isoenzyme in leaves and plays a major role in the reassimilation of photorespiratory ammonia. Whereas the NADPH-GOGAT isoenzyme is present in low amounts in leaves, but constitutes the main isoenzyme in non photosynthetic tissues, NADPH-GOGAT may function predominantly in primary nitrogen assimilation. A field experiment of two cultivars under four nitrogen levels (0, 45, 90 and 180 N kg.hm-2) investigated the soluble protein content and free amino acid content in leaf, stem, root and pod of peanut, as well as the activities of the nitrate reductase (NR), glutamine synthetase (GS) and glutamate dehydrogenase (GDH) in these organs. With the nitrogen application, the soluble protein content and free amino acid content were increased, and the activities of the nitrate reductase (NR), glutamine synthetase (GS) and glutamate dehydrogenase (GDH) also increased. When excessive nitrogen was used, the NR activity and kernel protein content were increased, while the activities of GS and GDH were decreased. Soluble protein content, free amino acid content, NR, GS and GDH along with the growth periods were not affected by nitrogen level, but with suitable nitrogen the activities of NR and GS in different organs could be increased. Also, nitrogen level affected GDH activities in the leaf and kernel, with lower effect on the GDH activities in the stalk and root. In brief, nitrogen level could affect the correlating enzyme activities of nitrogen metabolism in peanut, resulting in changes of soluble protein content and free amino acid content in organs. The best nitrogen level for peanut was 90 N kg.hm-2 (Fig. 9.4).

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Fig. 9.4. The soluble protein content in organs in different growing periods A1, A2, A3, A4: 0, 45, 90 and 180 N kg.hm-2 treatments

Although the GS activity in leaves is considerable, it is known that it largely depends on the chloroplastic enzyme involved in the secondary assimilation of NH4+, which mainly derives from photorespiration and is not related to the primary assimilation. The specific activities of GS and GOGAT were considerably higher in roots compared to other plant organs. The specific activity of GS determined in the roots of plants inoculated with Bradyrhirobium sp. showed a significant increase with respect to non-inoculated roots (Figs. 9.5 and 9.6). Although it is known that the GS activity increases by inoculation (Smith and Gallon, 1993), it reached its maximum in inoculated roots cultivated in acidic soil (pH 5.5). These results suggest that the soil acidity can stimulate an increase of the root GS activity only in inoculated plants. In the nodule extract, the GS activity did not show differences at both pHs (7.0 and 5.5). On the other hand, the comparison of the GS activity and symbiotic effectiveness in nodulated roots and nodules revealed a high correlation between them at both pHs. It is supposed that GS plays a leading role in both the primary assimilation of ammonium produced during symbiotic fixation of molecular nitrogen in nodules, and in its secondary assimilation in roots, in order to keep constant the shoot nitrogen content in peanut plants growing under acidic conditions. The results obtained on the ammonium assimilation enzymes in peanut at 40 DAP differ from other legumes such as lentil where GS and GOGAT activities were much higher in nodules than in roots from 40 to 90 DAP (Chopra et al., 2002). It is possible to suggest that the higher availability of fixed N may modulate the enzyme gene expression in the nodule, considering that nodule GS and GOGAT are plant gene products whose expression can be influenced by the stage of the nodule development and effectiveness (Vance et al., 1988; Suganuma et al., 1999). In contrast to the result found in peanut nodules by acid stress, in faba bean nodules the GS and GOGAT activities were inhibited by salinity (Cordovilla et al., 1999), indicating that the response of GS and GOGAT activities in nodules are also subjected to the influence of different environmental stresses.

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Fig. 9.5. Glutamine synthetase specific activity of roots, stems, leaves and nodules of peanut plants. Data are means ± S.E. of four independent determinations. Different letters in each column indicate significant differences (P < 0.05), according to the Duncan’s test.

Fig. 9.6. Glutamate synthase specific activity of roots, stems, leaves and nodules of peanut plants. Data are means ± S.E. of four independent determinations. Different letters in each column indicate significant differences (P < 0.05), according to the Duncan’s test

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Field experiments were conducted to investigate the effect of slow release sulphur (S) nutrition on crop growth, enzyme activities and yield attributes in peanut cultivars. Two combinations of sulphur (in kg/ha): 0S (–S) and 20S (+S) were used. In +S treatment, S was applied as a single basal dose in the field of peanut crop. The results showed that application of S (+S) significantly (P < 0.05) increased the biomass accumulation in both the genotypes at all the growth stages compared with without S (–S). Rapid increases of nitrate reductase (NR) and ATP-sulphurylase were observed up to 45 days after sowing (DAS), and thereafter a decline was observed (Fig. 9.7). Nodule weight and nitrogenase activity was increased up to 75 DAS and thereafter, these parameters declined. S fertilization favourably influenced NR, ATPsulphurylase, nodule weight and nitrogenase activity. Seed yield, biological yield and harvest index were also enhanced by slow S-release fertilization.

Fig. 9.7. Effect of SGF on nitrate reductase activity in the leaves of peanut cultivar at various growth stages

Analysis of proteins is a direct approach to define the function of their associated genes. Proteome analysis linked to genome sequence information is critical for functional genomics. However, the available protein expression data is extremely inadequate. Proteome analysis of the peanut leaf was conducted using two-dimensional gel electrophoresis in combination with sequence identification using MALDI/TOF to determine their identity and function related to growth, development and responses to stresses. Peanut leaf proteins were resolved into 300 polypeptides with pI values between 3.5 and 8.0 and relative molecular masses from 12 to 100 kDa. A master leaf polypeptide profile was generated based on the consistently expressed protein pattern. Proteins present in 205 spots were identified using GPS software and Viridiplantae database (NCBI). The identity of some of these proteins included RuBisCO, glutamine synthetase, glyoxisomal malate dehydrogenase, oxygen evolving enhancer protein and tubulin. Bioinformatical analyses showed that there are 133 unique protein identities. They were categorized into 10 and 8groups according to their cellular compartmentalization and biological functionality, respectively. Enzymes necessary

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Fig. 9.8. Peanut leaf proteome reference map generated by PD Quest (BioRad). Gel image with pI and molecular weight indicators. Proteins were separated by 2-DE. Three hundred protein spots were detected and numbered.

for carbohydrate metabolism and photosynthesis dominated in the set of identified proteins. The reference map derived from a drought-tolerant cv. Vemana should serve as the basis for further investigations of peanut physiology such as detection of expressed changes due to biotic and abiotic stresses, plant development. Furthermore, the leaf proteome map will lead to the development of protein markers for cultivar identification at the seedling stage of the plant. Amino Acids Total amino acid was as follows: (g/100 g crude protein, cp): 83.5 (raw seeds, Rs), 85.9 (cooked seeds, Cs) and 66.8 (roasted seeds, Rt.s) with corresponding essential amino acids as: 39.4 or 47.2% (Rs), 38.3 or 44.6% (Cs) and 30.0 or 44.9% (Rt.s). Predicted protein efficiency ratios were 2.55 (Rs), 3.00 (Cs) and 2.31 (Rt.s) and essential amino acid index of 1.18 (Rs), 1.08 (Cs) and 0.83 (Rt.s). Cooking enhanced the amino acid levels of Asp, Ser, Glu, Pro, Arg, Ala, Cys, Val, Leu and Phe. The following essential amino acids were reduced by both cooking and roasting: Lys (15.9-27.6%), His (4.2316.5%), Thr (40.1-60.6%), Met (38.0-63.4%) and Ile (13.3-31.8%). All the parameters between Rs/Cs and most of the parameters between Rs/Rt.s were significantly different at r = 0.05 (Table 9.5). Germins and germin-like proteins (GLPs) are plant exclusive cupin subfamily water-soluble glycoproteins. Germin was first identified during wheat germination (Thompson and Lane, 1980) and later was found to be oxalate oxidases (OXOs) (Lane et al., 1993). Germins and germin-like protein subfamilies are characterized

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Physiology of the Peanut Plant Table 9.5. Amino acid profiles of raw, cooked and roasted groundnut seeds (g/100 g crude protein) dry matter Amino acid

Lysine (Lys)a Histidine (His)a Arginine (Arg)a Aspartic acid (Asp) Threonine (Thr)a Serine (Ser) Glutamic acid (Glu) Proline (Pro) Glycine (Gly) Alanine (Ala) Methionine (Met)a Cystine (Cys) Valine (Val) Isoleucine (Ile) Leucine (Leu) Phenylalanine (Phe) Tyrosine (Tyr) Protein, g/100 g a

Raw seeds 4.34 2.36 5.92 9.09 4.24 3.38 14.9 3.53 4.64 3.69 1.42 1.39 4.60 4.28 7.54 4.70 3.51 29.0

Cooked seeds 3.65 2.26 6.52 10.7 2.54 3.54 15.7 4.31 3.48 4.70 0.88 1.68 4.72 3.71 8.45 5.59 3.51 28.4

Roasted seeds

Mean

S.D.

3.14 1.97 4.89 8.43 1.67 2.66 13.2 2.77 2.85 2.99 0.52 1.09 3.90 2.92 6.77 4.20 2.80 26.9

3.71 2.20 5.78 9.41 2.82 3.19 14.6 3.54 3.66 3.79 0.94 1.39 4.41 3.64 7.59 4.83 3.27 28.1

0.60 0.20 0.82 1.17 1.31 0.47 1.28 0.77 0.91 0.86 0.45 0.30 0.44 0.68 0.84 0.70 0.41 1.08

An essential amino acid

by the presence of germin boxes (PHIHPRATEI) and a conserved cupin super family derived-motif (Dunwell et al., 2000; Zimmermann et al., 2006). This motif is a conserved beta-barrel protein with a metalion binding ability (Chakraborty et al., 2002). According to their sequence similarities and other characters, Germins and the GLP gene family are divided into two distinct group proteins. The first group named

Fig. 9.9. The free amino acid content in organs in different growing periods. AA: Amino Acid

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“the true germins” is only identified in “true cereals”, which contain barley, corn, oat, rice, rye and wheat. Members in this group have relatively homogeneous protein sequences (Lane, 2000) and always carry OXO enzyme activity. The second group is designated as germin like proteins (GLPs), whose members show relatively high sequence divergence. Their amino acid sequence similarity to wheat germin varies from 30% to 70%. The second group contains more members than the first group and only a few of the second group members possess OXO activity. It was identified that three new AhGLP members (AhGLP3b, AhGLP5b and AhGLP7b) have distinct but very closely related DNA sequences. The spatial and temporal expression profiles revealed that each peanut GLP gene has its distinct expression pattern in various tissues and developmental stages. This suggests that these genes all have their distinct roles in peanut development. Subcellular location analysis demonstrated that AhGLP2 and 5 undergo a protein transport process after synthesis. The expression of all AhGLPs increased in responding to Aspergillus flavus infection, suggesting AhGLPs’ ubiquitous roles in defence to A. flavus. Each AhGLP gene had its unique response to various abiotic stresses (including salt, H2O2 stress and wounds), biotic stresses (including leaf spot, mosaic and rust) and plant hormone stimulations (including SA and ABA treatments). These results indicate that AhGLPs have their distinct roles in plant defence. Moreover, in vivo study of AhGLP transgenic Arabidopsis showed that both AhGLP2 and 3 had salt tolerance, which made transgenic Arabidopsis grow well under 100 mM NaCl stress (Fig. 9.10).

Fig. 9.10. Expression patterns of peanut AhGLPs in various tissues/organs and developmental stages. Expression patterns of AhGLPs were examined in (A) developing pods at 1, 4, 15 and 35 days after penetration into soil, (B) seeds at 1 hour, 1, 3 and 5 days after germination, (C) roots, stems and leaves of 14-day-old seedlings, and (D) roots, flower buds and flowers during flowering. Transcript abundance detected using qRTPCR was normalized to the expression of actin gene. Numbers (1–8) inside boxes correspond to AhGLP1, AhGLP2, AhGLP3b, AhGLP4, AhGLP5b, AhGLP6, AhGLP7b and AhGLP8, respectively. Expression levels are colour-coded on the bottom bar. Green colour indicates the down-regulation of gene expression, red colour indicates the up-regulation of gene expression, black indicates the RNA levels unchanged

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9.2.

Physiology of the Peanut Plant

Nitrogen Fixation

Nitrogen fixation is a dynamic and high energy demanding process (Rosenblueth et al., 2018). The pathway for the biological reduction of inert N2 into the reactive compound NH 3(ammonia) under micro-aerobic conditions is as follows: N2 + 8H+ + 8e− + 16Mg-ATP → 2NH3 + H2 + 16Mg-ADP + 16P As mentioned earlier, a protein complex called nitrogenase (composed of enzymes with metal co-factors) makes nitrogen fixation possible in plants. The first one is dinitrogenase and the second one is dinitrogenase reductase (Bulen and LeComte, 1966). According to the active site co-factor binding metal, there exist three types of dinitrogenase in nature. (a) Molybdenum (Mo) nitrogenase; it is most abundant and carries the most significance in the nitrogen-fixing bacterial and archaeal niche and the alternative vanadium (V) and iron-only (Fe) nitrogenases (Bishop and Joerger, 1990). The molybdenum dependent dinitrogenase is formed by nifD and nifK gene products and dinitrogenase reductase is a homodimer of the nifH gene product (Buren and Rubio, 2017; Shah and Brill, 1977). It is well documented that molybdenum nitrogenase is produced in all diazotrophs in nature, while some produce vanadium or iron nitrogenase addition to Mo-nitrogenase (Dos Santo et al., 2012; McGlynn et al., 2013). The rhizobium bacteria residing in nodules fix atmospheric nitrogen gas to NH3, which plants can assimilate via glutamine synthase to form glutamine. Bacterial nif genes are well known to encode the components of the nitrogenase enzyme complex. nifH, nifD, and nifK genes encode the structural subunit of di-nitrogenase reductase and the 2 subunits of di-nitrogenase, respectively. Many rhizobial genes have been fully sequenced, for instance, Mesorhizobium loti, Sinorhizobium meliloti, and Bradyrhizobium japonicum (Galibert et al., 2001; Giraud et al., 2007; Kaneko et al., 2000). These proteins have similar sequences and common structures and functions in many diazotrophs, for instance, Azotobacter vinelandii, Herbaspirillum seropedicae, Pseudomonas stutzeri, and Bradyrhizobium japonicum (Adams et al., 1984; Jacobson et al., 1989; Pedrosa et al., 1997; Yan et al., 2008). Furthermore, genetic and biochemical analyses revealed that many additional nif genes, including nifE, nifN, nifX, nifQ, nif W, nifV, nifA, nifB, nifZ, and nifS, play roles in the regulation of nif genes and maturation processes of electron transport and FeMo-cofactor biosynthesis and assembly (Masepohl et al., 2002; Lee et al., 2000). In addition, the fixABCX genes first identified in Rhizobium meliloti (Kallas et al., 1985; Earl et al., 1987) and subsequently in other diazotrophs were reported to encode a membrane complex participating in electron transfer to nitrogenise (Edgren and Nordlund, 2004). As the nodule begins to form, the bacteria become surrounded by a plant-derived membrane and are released inside plant cells forming the nodule. The bacteria subsequently lose their cell walls and undergo a profound change in cell morphology to form large, irregularly shaped branching cells called bacteroides. They then are entirely dependent on the host plant for their energy needs. In return, the bacteria fix nitrogen for the plant.

9.3.

Biological Nitrogen Fixation

An examination of the history of BNF shows that interest generally has been focused on the symbiotic system of leguminous plants and rhizobia, because these associations

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have the greatest quantitative impact on the nitrogen cycle. A tremendous potential for contribution of fixed nitrogen to soil ecosystems exists among the legumes (Brockwell et al., 1995; Peoples et al., 1995; Tate, 1995). There are approximately 700 genera and about 13,000 species of legumes, only a portion of which is about 20% (Sprent and Sprent, 1990) have been examined for nodulation and shown to have the ability to fix N2 (Fig. 9.11). Estimates are that the rhizobial symbioses with the somewhat greater than 100 agriculturally important legumes contribute nearly half the annual quantity of BNF entering soil ecosystems (Tate, 1995).

Fig. 9.11. Rod shaped Bradyrhizobium

Legumes are very important both ecologically and agriculturally because they are responsible for a substantial part of the global flux of nitrogen from atmospheric N2 to fixed forms such as ammonia, nitrate, and organic nitrogen. Whatever the true figure, legume symbioses contribute at least 70 million tonnes of N per year, approximately half deriving from the cool and warm temperature zones and the remainder deriving from the tropics (Brockwell et al., 1995). Increased plant protein levels and reduced depletion of soil N reserves are obvious consequences of legume N2 fixation. Deficiency in mineral nitrogen often limits plant growth, and so symbiotic relationships have evolved between plants and a variety of nitrogen-fixing organisms (Freiberg et al., 1997) (Fig. 9.12). Several environmental conditions are limiting factors to the growth and activity of the N2-fixing plants. A principle of limiting factors states that “the level of crop production can be no higher than that allowed by the maximum limiting factor” (Brockwell et al., 1995). In the Rhizobium-legume symbiosis, which is a N2­ fixing system, the process of N2 fixation is strongly related to the physiological state of the host plant. Therefore, a competitive and persistent rhizobial strain is not expected to express its full capacity for nitrogen fixation if limiting factors (e.g., salinity, unfavourable soil pH, nutrient deficiency, mineral toxicity, temperature extremes, insufficient or excessive soil moisture, inadequate photosynthesis, plant diseases, and grazing) impose limitations on the vigour of the host legume (Brockwell et al., 1995; Peoples et al., 1995; Thies et al., 1995).

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Fig. 9.12. Comparison of peanut plants with and without Bradyrhizobia. Plants are (left to right), uninoculated with Bradyrhizobium, inoculated with Bradyrhibium, non-nodulating mutant peanut inoculated with Bradyrhizobium, and non-nodulating mutant peanut uninoculated with Bradyrhizobium.

Typical environmental stresses faced by the legume nodules and their symbiotic partner (Rhizobium) may include photosynthate deprivation, water stress, salinity, soil nitrates, temperature, heavy metals, and biocides (Walsh, 1995). A given stress may also have more than one effect, e.g., salinity may act as a waters tress, which affects the photosynthetic rate, or may affect nodule metabolism directly. The most problematic environments for rhizobia are marginal lands with low rainfall, extremes of temperature, acidic soils of low nutrient status, and poor water-holding capacity (Bottomley, 1991). Populations of Rhizobium and Bradyrhizobium species vary in their tolerance to major environmental factors; consequently, screening for tolerant strains has been pursued (Keyser et al., 1993). Biological processes (e.g., N2 fixation) capable of improving agricultural productivity while minimizing soil loss and ameliorating adverse edaphic conditions are essential. The Rhizobium-legume symbiosis is superior to other N2­ fixing systems with respect to N2 fixing potential and adaptation to severe conditions. Several symbiotic systems of legumes which are tolerant to extreme conditions of salinity, alkalinity, acidity, drought, fertilizer and metal toxicity were identified. These associations might have sufficient traits necessary to establish successful growth and N2 fixation under the conditions prevailing in arid regions. In fact, the existence of Rhizobium-tree legume symbioses, which are able to fix appreciable amount of N2 under arid conditions, is fascinating. These symbioses represent the best source of the “ideal” fertilizer in arid regions. Seven peanut genotypes were planted in a randomized complete block design with 4 replications under rainfed conditions in 2001 and 2002. Nitrogen fixation parameters were recorded as leaf colour scores, nodule dry weights, shoot dry weights, total dry weights, total nitrogen and fixed nitrogen while agronomic data were recorded as pod number per plant, pod weight

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per plant, seed number per plant, seed number per pod, seed weight per plant, 100­ seed weight, shelling percentage and harvest index. The difference between years was significant for most traits except for shoot dry weight and 100-seed weight. Variety × year interactions were also significant for most traits except for shelling percentage. KKU 72-1 was the best genotype for nitrogen fixation parameters in 2001. KKU 1 did not perform well in nitrogen fixation traits, but did for most agronomic traits, especially in 2002 except for 100-seed weight. Leaf colour score and shoot dry weight may be useful as alternative means for determining nitrogen fixing ability of peanut under field evaluation. High association among nitrogen fixation parameters was found in both years. However, correlations between nitrogen fixation parameters and agronomic traits were not strong in 2002 (Table 9.6). Table 9.6. Mean comparison of seven peanut genotypes for N2-fixation parameters evaluated for two years at Khon Kaen University’s agronomy farm in 2001 and 2002 Line

Nodule wt. (g.plant-1)

Shoot dry wt. (g.plant-1)

Total dry wt. (g.plant-1)

Total N (mg.plant-1)

Year 2001 PI 268 770 PI 269 109 PI 152 133 KKU 1 KKU 72-1 KK 60-3 Non-nod Mean

0.470a 0.477a 0.418ab 0.334b 0.460a 0.418ab 0.430

Year 2002 PI 268 770 PI 269 109 PI 152 133 KKU 1 KKU 72-1 KK 60-3 Non-nod Mean

0.476a 0.399ab 0.284c 0.376b 0.388ab 0.398ab 0.387

Total fixed N (mg. plant-1)

53.07b 49.84b 38.28c 24.88d 73.82a 55.15b 21.08d 45.16

60.49b 59.10b 49.79c 29.82d 86.22a 64.77b 25.96e 53.74

116.20b 105.70b 71.04c 48.37cd 162.40a 119.10b 30.18d 93.28

86.00b 75.54b 40.87c 24.14c 132.20a 88.93b 74.61

46.10bc 49.80ab 32.33d 43.58bcd 47.99ab 58.74a 34.86cd 44.77

63.83ab 69.11a 43.58c 72.54a 70.24a 77.62a 50.41bc 63.90

101.60a 98.57a 54.93b 68.41b 103.90a 120.30a 47.67b 85.05

53.91ab 50.90b 7.26c 20.74c 56.20ab 72.63a 43.61

Means in the same column followed by the same letter are not significantly different at 0.05 probability level by DMRT. Non nodes were deleted from the data set.

Although a response was not always obtained, Robut 33-1, a cultivar of Andhra Pradesh, gave substantial increases in pod yield when inoculated with a strain NC 92. Strain NC 92 was obtained from NCSU and isolated from nodules collected in South America (Fig. 9.13). During early growth stages of nodules, the acetylene reduction activity per unit nodule fresh weight is proportional to nodule size, and the colour of the rhizobium­ infected area changed from white to dark red with the increase in nodule diameter. The red colour of nodule may be responsible for the high nitrogen-fixing activity because the colour reflects the condition of leghaemoglobin, an important enzyme for nitrogen fixation in rhizobia (Vikman and Vessey, 1993). In the later growth stages of nodules,

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Fig. 9.13. Nodule formation by different Rhizobium strains

the acetylene reduction activity per unit fresh weight of nodule decreased remarkably, and the rhizobium-infected area became greenish, which suggests senescence of the enzyme in the infected area. Root nodules in peanut are thereby classified into three growth phases based on nodule size and interior colour: small nodules with white infected areas; medium-size nodules with red infected areas; and larger, older nodules with greenish infected areas. Among these, the medium-size nodules may have the highest nitrogen-fixing activity (Figs. 9.14 and 9.15). Seasonal pattern of acetylene reduction (AR) and shoot nitrogen accumulation was studied in nine groundnut cultivars. Shoot N accumulation by all the cultivars was maintained until shortly before maturity and it occurred faster over the reproductive growth phase than over the earlier phases. In all cultivars plant AR (PAR) did not reflect this pattern of N accumulation, being greater over the vegetative and pod initiation phases. This suggests that the commonly observed low PAR values for groundnut over the reproductive growth phase may be the result of factors other than sink competition. There were significant interactions of cultivars with stage of crop growth for PAR, nodule mass, and specific nitrogenase activity (SNA). Virginia types generally showed better nodulation, higher N2‐fixing capacity (both PAR and SNA) than Valencias, and significant differences were observed between cultivars within a botanical type.

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Fig. 9.14. Means of cultivars for nitrogenase activity (μ mol C2H4 per plant per hr) over harvest dates (Clayton, 1978)

Fig. 9.15. Relationship between rhizobium-infected area and acetylene reduction activity at 70 DAS (A), 113 DAS (B) and 133 DAS (C)

276

9.4.

Physiology of the Peanut Plant

Rhizobium Strains

Peanut is a promiscuous crop nodulating with a wide range of rhizobia isolates affiliated to the Rhizobium (Taurian et al., 2006) and Ensifer (Yang et al., 2005) genera. However, the main peanut micro-symbiont partners are those classified within the Bradyrhizobium genus (Chen et al., 2003; Yang et al., 2005). Bradyrhizobium is the genus type of the Bradyrhizobiaceae family, described in 1982 to receive the slow-growing strains formerly classified as Rhizobium japonicum (Jordan, 1982). In September 2017 there were 36 described and validated Bradyrhizobium species in the List of Prokaryotic Names with a standing in Nomenclature (LPSN, available at http://www.bacterio.net/ bradyrhizobium.html) (LPSN, 2017). Among this species, B. lablabi (Chang et al., 2011), B. arachidis (Wang et al., 2013), B. guangdongense and B. guangxiense (Li et al., 2015) had strains, their type, isolated from peanut nodules in China, whilst B. subterraneum and B. kavangense type strains were isolated from peanut root nodules in Namibia (Grönemeyer et al., 2015a, b). In the last few years, new species of Bradyrhizobium have been described by means of the polyphasic characterization of Brazilian bacteria (da Silva et al., 2014; Delamuta et al., 2015; Michel et al., 2017; Zilli et al., 2014), but none of them originated from peanut nodules or isolated in a Semi-Arid region. The characterization of peanut bradyrhizobia applying a molecular approach with the concomitant evaluation of housekeeping and symbiotic gene sequences was recently carried out in countries such as China (Chen et al., 2016) and South Africa (Jaiswal et al., 2017), revealing a large diversity of bacterial isolates. Jaiswal et al. (2017) showed that the bacterial isolates evaluated from two different regions of South Africa were classified as B. kavangense (2), B. stylosanthis (2), B. elkanii (1), B. manauense (1) along with four bacterial isolates less close to B. diazoefficiens. Chen et al. (2016) showed that among 36 bacterial isolates, 11 of them were closely related to B. japonicum and 21 others to B. guangxiense in addition to four other bacteria related to B. iriomotense. The evaluation of symbiotic genes sequences is important to indicate the range of hosts nodulated by the bradyrhizobial isolates due to the determination of their “symbiovar” affiliation (Hungria et al., 2015). In addition, the concomitant evaluation of the sequences of housekeeping and symbiotic genes lead to a better understanding of the taxonomic position and phylogenetic relationships of the bacterial isolates (Delamuta et al., 2012; Hungria et al., 2015). Despite their importance, only the sequence analyses cannot indicate the bacterial symbiotic efficiency (Muñoz et al., 2011; Ribeiro et al., 2015). Recent reports showing the peanut Bradyrhizobium diversity based in gene sequences are available in the literature (Chen et al., 2016; Jaiswal et al., 2017). Moreover, studies reporting more detailed symbiotic characteristics of peanut rhizobia do not achieve a better understanding of their phylogenetic relationships (Santos et al., 2017; TorresJúnior et al., 2014). Biological nitrogen fixation in rhizobia occurs primarily in the root or stem nodules and is induced by the bacteria present in legume plants. This symbiotic process has fascinated researchers for over a century, and the positive effects of legumes on soils and their food and feed value have been recognized for thousands of years. Symbiotic nitrogen fixation uses solar energy to reduce the inert N2 gas to ammonia at normal temperature and pressure, and is thus today, especially, important for sustainable food production. Increased productivity through improved effectiveness of the process is seen as a major research and development goal. The interaction

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between rhizobia and their legume hosts has thus been dissected at agronomic, plant physiological, microbiological and molecular levels to produce ample information about processes involved, but identification of major bottle necks regarding efficiency of nitrogen fixation has proven to be complex. Photosynthesis and biological nitrogen fixation are two vital biochemical processes for the growth, development and yield of a leguminous crop. The plants can require large amounts of phosphorus (P) for effective N2-fixation. Under deficient conditions, P fertilization will result in improved nodulation and enhanced N2-fixation. A field trial was conducted in 2015 cropping season at the Institute for Agricultural Research Experimental Field, Ahmadu Bello University, Zaria, Nigeria. The study determined the symbiotic nitrogen fixation potential of groundnut genotypes grown on an inherently P-deficient soil. The treatments consisted of 16 groundnut genotypes and 3 P sources. The P sourced was in the main plot with a genotype in the sub-plot of a split plot design that was replicated four times. More chlorophyll content was recorded by ICGV-IS 07815. The lowest chlorophyll content was recorded by ICIAR 7B. All the genotypes were at par in terms of number of nodules, but statistically different in their nodule weights. Both P sources, Sokoto rock and single super phosphates (SRP and SSP) contributed to nodule weights that were statistically better than the other 0 kg P2O5 ha-1 source. There was, however, no statistical difference between all the P sources in terms of nodule number. It was concluded that ARRORS ICGX-SM 00017/5/P15/P2 had the highest potential for symbiotic nitrogen fixation, as it outperformed all other genotypes in terms of nodule number, weight and chlorophyll content (Fig. 9.16).

Fig. 9.16. Genotypes and phosphorus level on nodule weight in groundnut

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Physiology of the Peanut Plant

9.5. Abiotic Stress Although the root nodule-colonizing bacteria of the genera Rhizobium and Bradyrhizobium are more salt tolerant than their legume hosts, they show marked variation in salt tolerance. Growth of a number of rhizobia was inhibited by 100 mM NaCl (Yelton et al., 1983), while some rhizobia, e.g., Rhizobium meliloti, were tolerant to 300 to 700 mM NaCl (Embalomatis et al., 1994; Helemish et al., 1991; Mohammad et al., 1991; Sauvage et al., 1983). Strains of Rhizobium leguminosarum have been reported to be tolerant to NaCl concentrations up to 350 mM NaCl in broth cultures. Soybean and chickpea rhizobia were tolerant to 340 mMNaCl, with fast-growing strains being more tolerant than slow growing strains (El Sheikh, 1990). Rhizobium strains from Vigna unguiculata were tolerant to NaCl up to 5.5%, which is equivalent to about 450 mM NaCl (Mpepereki et al., 1997). It has been found recently that the slow growing peanut rhizobia are less tolerant than fastgrowing rhizobia (Ghittoni and Bueno, 1996). Drought-tolerant, N2-fixing legumes can be selected, although the majority of legumes are sensitive to drought stress. Moisture stress had little or no effect on N2 fixation by some forage crop legumes, e.g., M. sativa (Keck et al., 1984), grain legumes, e.g., groundnut (Arachis hypogaea) (Venkateswarlu et al., 1989), and some tropical legumes, e.g., Desmodium intortum (Ahmed and Quilt, 1980). One legume, guar (Cyamopsis tetragonoloba), is drought tolerant and is known to be adapted to the conditions prevailing in arid regions (Vankateswarlu et al., 1983). Several mechanisms have been suggested to explain the varied physiological responses of several legumes to water stress. The legumes with a high tolerance to water stress usually exhibit osmotic adjustment; this adjustment is partly accounted for by changing cell turgor and by accumulation of some osmotically active solutes (Ford, 1984). The accumulation of specific organic solutes (osmotica) is a characteristic response of plants. High root temperatures strongly affect bacterial infection and N2 fixation in several legume species, including soybean (Munevar and Wollmer, 1982), guar (Atkin et al., 1984), peanut (Kishinevsky et al., 1992), cowpea (Rainbird et al., 1983), and beans (Hungria and Franco, 1993; Piha and Munnus, 1987). Critical temperatures for N2 fixation are 30°C for clover and pea and range between 35 and 40°C for soybean, guar, peanut, and cowpea (Michiels et al., 1994). Nodule functioning in common beans (Phaseolus spp.) is optimal between 25 and 30°C and is hampered by root temperatures between 30 and 33°C (Piha and Munnus, 1987). Nodulation and symbiotic nitrogen fixation depend on the nodulating strain in addition to the plant cultivar (Atkin et al., 1984; Munevar and Wollmer, 1982). Temperature affects root hair infection, bacteroid differentiation, nodule structure, and the functioning of the legume root nodule (Roughley and Dart, 1970). Heat shock proteins have been found in Rhizobium (Aarons and Graham, 1991) but have not been studied in detail (Graham, 1992). The synthesis of heat shock proteins was detected in both heat-tolerant and heat sensitive bean-nodulating Rhizobium strains (Michiels et al., 1994) at different temperatures. The fast-growing strains of rhizobia have generally been considered less tolerant to acid pH than have slowly growing strains of Bradyrhizobium (Graham et al., 1994), although some strains of the fast-growing rhizobia, e.g., R. loti and R. tropici, are highly acid tolerant (Cooper et al., 1985; Cunningham Munns, 1984; Graham et al., 1994; Wood et al., 1988). The failure of legumes to nodulate under acid-soil conditions is common, especially in soils of pH less than 5.0. The inability of some rhizobia to persist under such conditions is one

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cause of nodulation failure (Bayoumi et al., 1995; Carter et al., 1994; Graham et al., 1982), but poor nodulation can occur even where a viable Rhizobium population can be demonstrated (Graham, 1992; Graham et al., 1983). The basis for differences in pH tolerance among strains of Rhizobium and Bradyrhizobium is still not clear (Correa and Barnes, 1997; Graham et al., 1994), although several workers have shown that the cytoplasmic pH of acid-tolerant strains is less strongly affected by external acidity (Chen et al., 1993; Chen et al., 1992; Goss et al., 1990; O’Hara et al., 1989). Aarons and Graham (1991) reported high cytoplasmic potassium and glutamate levels in acidstressed cells of R. leguminosarum bv. phaseoli, a response which is similar to that found in osmotically stressed cells. Differences in LPS composition, proton exclusion and extrusion (Chen et al., 1992, 1993), accumulation of cellular polyamines (Fujihara and Yoneyama, 1993), and synthesis of acid shock proteins (Hickey and Hirshfield, 1990) have been associated with the growth of cells at an acidic pH. Temperatures in the assay bottle greater than 25° C decreased nitrogenase activity of nodulated roots of groundnut cv Kadiri 71-1 (Tables 9.7 and 9.8). Excess or insufficient moisture also decreased acetylene reduction activity (Fig. 9.17). It was observed that shading causes a rapid decrease in nitrogenase activity. When 109-day old Kadiri 71-1 plants were shaded to 60% of ambient light intensity, nitrogenase activity was reduced within a day by 30%. Table 9.7. Influence of RMxoblum Inoculum level on nodulation and nitrogen fixation by groundnut Level of Rhizobium applied as broth (number/seed)

Shoot dry wt* (g/plant)

Nodule dry wt* (g/plant)

3.2 × 109

3.38*

0.13a

5.5 × 107

2.38*

0.12a

4.8 × 104

1.08*

0.03*

6.1 × 102

0.97*

0.02*

4.34

0

Nitrate control

* Data in each column followed by the same letter are not significantly different at the 0.05 level.

Note: Kadiri 71-1 plants inoculated with strain NC-92 were grown under semi sterile

conditions watered with nitrogen-free nutrient solution and harvested 57 days after

planting.

Table 9.8. Symbiotic characters in groundnut germplasm entries (days after planting) Range Nodule number Nodule weight Nitrogenase activity μmol C2H4 plant-1h-1 μmol C2H4/g.dry wt.nodule/hr

4.8-85 247-628 0.30-0.75 g/plant–1 36-176 95-386

Rhizobia’s attachment to groundnut roots is dependent on the bacteria growth stage, and the optimal attachment was observed for the bacteria at late log to early stationary phase. Further, Dardanelli et al. (2003) reported the involvement of cell

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Physiology of the Peanut Plant

Fig. 9.17. Effect of temperature on acetylene reduction activity of peanut nodules

surface proteins of Bradyrhizobium sp. during RNS. This protein appeared to be a calcium-binding adhesion because cells treated with EDTA were found to be able to bind to adhesin-treated roots, as this protein has similar properties to those reported for rhicadhesin. Most probably, this rhizobia adhesin is involved in the attachment process of rhizobia to the root surface for the establishment of an effective symbiosis during the crack entry of groundnut. Rhizobial infection for nodule development requires bacterial exo polysaccharides (EPS) in legumes.This indicated that rhizobial EPS play an important role in establishing an effective symbiosis through crack entry in groundnut. Another invasion mode that occurs via natural wounds caused by splitting of epidermis and the emergence of young lateral or adventitious roots is known as crack entry. This invasion occurs in a few sub-tropical legumes like Arachis sp., Neptunia sp., Sesbania sp., Aeschynomene sp. and Stylosanthes sp. A. hypogaea, Stylosanthes sp. and Aeschynomene sp., the structures similar to infection threads have never been observed (Spaink, 2000). Bradyrhizobia enters the root through the middle lamella between two adjacent axillary hair cells, a place where the cell wall seems to be loosely constructed. After penetration, rhizobial dissemination occurs intercellularly by separating cortical cells at the middle lamella. At the first signal of this molecular communication, leguminous roots start to release flavonoids that accumulate in the rhizosphere. These compounds activate the bacterial transcriptional regulator protein NodD, which, in turn, induces the transcription of other nodulation genes (nod, nol, and noe genes), whose products are involved in the synthesis and secretion of main rhizobial nodulation signals called Nod factors (NF) or lipo-chito oligosaccharides (LCOs) (Spaink, 2000). Groundnut was predominantly reported to be nodulated with nodABC-bearing Bradyrhizobium strains, but was also found to be nodulated by nodABC lacking Bradyrhizobium strain Btai1 (Noisangiam et al., 2012), which suggests that NFs might not be indispensable in the RNS of groundnut (Ibanez and Fabra, 2011; Guha et al., 2016). Therefore,

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groundnut may harbour two different modes of RNS: (i) NF-dependent and (ii) NFindependent. In the NF-dependent mode of RNS, Nod factor molecules are required to trigger the cellular divisions for nodule primordium development (Ibanez and Fabra, 2011). Nodules show bacteroids tightly packed in the cortical cell cytoplasm, most of which show a central vacuole and peripherally delimited nucleus. The bacteroids are enclosed singly in peri-bacteroidal membrane sacs (Sen et al., 1986). Furthermore, ultra-structurally, four types of inclusion bodies are exclusively present in the groundnut nodule: microbodies, oleosomes, electron-dense bodies, and proteinaceous inclusions. Oleosomes (lipid accumulations) and microbodies are found outlining the peribacterial membrane and dense bodies are situated at the peribacteroid space, adjacent to bacteroids (Bal et al., 1989) (Fig. 9.18).

Fig. 9.18. Rhizobia inside the cell

In this study, Bradyrhizobium sp. strain, Lb8, was isolated from peanut root nodules and sequenced using PacBio long reads. The complete genome sequence was a circular chromosome of 8,718,147 base-pair (bp) with an average GC content of 63.14%. No plasmid sequence was detected in the sequenced DNA sample. A total of 8,433 potential protein-encoding genes, one rRNA cluster, and 51 tRNA genes were annotated. Fifty-eight per cent of the predicted genes showed similarity to genes of known functions and were classified into 27 subsystems representing various biological processes. The genome shared 92% of the gene families with B. diazoefficens USDA 110T. A presumptive symbiosis island of 778 Kb was detected, which included two clusters of nif and nod genes. A total of 711 putative protein-encoding genes were in this region, among which 455 genes have potential functions related to symbiotic nitrogen fixation and DNA transmission. Of 21 genes annotated as transposase, 16 were located in the symbiosis island. Lb8 possessed both Type III and Type IV protein secretion systems. However, when comparing DEGs between nod– genotypes and nod+ genotypes, it was apparent that most of the genes normally involved in nodulation had no response to rhizobia infection in nod– E4 and E7. Among these DEGs were several important

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symbiotic signalling pathway genes, including orthologs of NFR5, NSP2, NIN, and ERN1. Specifically, the two peanut orthologs of NIN were up-regulated in all four nod+ genotypes, but were not regulated in either E4 or E7. In M. truncatula, the NIN mutants were blocked in infection and not able to form nodules. This strongly indicated that the symbiosis signalling pathway was activated in nod+ genotypes but not in nod– E4 or E7. The two nod– mutants are blocked either at the symbiotic signalling stage in NIN or upstream of NIN, or even earlier processes of nodulation. Based on our analysis, E4 and E7 probably have different genetic defects due to several reasons. Firstly, only 11 (4.7%) DEGs were shared between E4 and E7 out of the 233 DEGs identified in these two nod− genotypes, indicating that E4 and E7 had a different response to rhizobia infection. Secondly, we have developed two F2 mapping populations derived from two crosses (E4 and E5; E6 and E7). The segregation ratios of nod+ to nod– are different (unpublished data), which is a clue that the inheritance of nodulation in these two populations is different or the genetic defects of E4 and E7 are different. A further investigation based on results from this study is needed to uncover the genetic mechanisms of non-nodulation. In summary, as a member of the legume family, A. hypogaea with a “crack entry” infection mode shared a considerable number of nodulation related genes with model legumes with ‘root hair entry’. To reveal the mechanisms specific to the “crack entry” mode of infection, additional studies are needed to focus on the 122 peanut-specific DEGs, including the eight gene families and the 102 unassigned genes that shared no homology with genes from the two model legumes, as well as the 21 DEGs showing similar differential expression in both nod– and nod+ genotypes.

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CHAPTER

10

Lipid Metabolism The mobilization of storage lipid during seed germination begins with the decomposition of the triacylglycerols accumulated in oil bodies into free fatty acids and glycerol. Mostly lipases are active at this stage of the process (Barros et al., 2010). Next, the fatty acids undergo β-oxidation, which occurs in peroxisomes (glyoxysomes). The following step is the glyoxylate cycle, which partially occurs in the peroxisome and partially in the cytoplasm. In the peroxisome are located three of the five enzymes of the glyoxylate cycle (citrate synthase, isocitrate lyase and malate synthase), while two other enzymes (aconitase and malate dehydrogenase) operate in the cytoplasm (Pracharoenwattana and Smith, 2008). Succinate synthesized in the peroxisome, after transport into the mitochondrium, is converted to malate through a part of the Krebs cycle. This metabolite, in turn, after transport to the cytoplasm, is converted to oxaloacetate. The last step of storage lipid breakdown is gluconeogenesis and the synthesis of sugars, which are a form of carbon transport in plants (Quettier and Eastmond, 2009; Borek and Ratajczak, 2010).

10.1. Seed The comparison of seed chemical composition of several agriculturally important plants (including lupins) is presented in Table 10.1. Table 10.1. Protein, lipid and total carbohydrates in seeds of some species Species or genus Protein (% seed DW) Lipid (% seed DW)

Total carbohydrates %

Andean lupin

40-50

20

Yellow lupin

45

6

-

36

White lupin

38

7-14

-

Narrow-leaf lupin

31

6-10

-

Soybean

37-44

12-26

30-35

Pea

24-30

3-6

56-60

Beans

23-32

1

56

Lentil

30

3

62

Peanut

31

48-50

12-14

Sunflower

25

49

1

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Physiology of the Peanut Plant

Fig. 10.1. Schematic representation of storage lipid breakdown in plants, based on Borek and Ratajczak (2010), Borek et al. (2013b) and literature cited

Fatty acid composition of peanut seed oil in four varieties cultivated in Tunisia showed that linoleic (C18:2), oleic (C18:1) and palmitic (C16) acids account for more than 84% for Chounfakhi and Massriya and for more than 85% of the total fatty acids of Trabilsia and Sinya seed oil respectively (Table 10.2). Seed oil contents were significantly different (P ≤ 0.05) and did not exceed 48%.

291

Lipid Metabolism Table 10.2. Fatty acid compositions (% of total fatty acid) of peanut cultivars Fatty acids

Chounfakhi

Trabilsia

Sinya

Massriya

C14

1.19

0.77

0.79

1.56

C16

12.11

17.45

13.34

11.89

C16:1

3.43

1.93

2.15

3.17

C18

4.01

4.12

4.1

4.59

C18:1

32.63

27.16

30.31

32.12

C18:2

39.65

41.38

41.85

40.06

C18:3(w3)

1.63

1.27

1.31

1.41

C22

0.58

1.1

0.98

0.39

C22:1

2.73

2.22

2.35

2.57

C24

2.04

2.6

2.82

2.24

SFA

19.93

26.04

22.03

20.67

MUFA

38.79

31.31

34.81

37.86

PUFA

41.28

42.65

43.16

41.47

All values given are means of three determinations.

SFA: Saturated fatty acids; MUFA: Monounsaturated fatty acids; PUFA: Polyunsaturated

fatty acids.

In the research on mechanisms of regulation of storage lipid accumulation and breakdown, sucrose is considered primarily. It is the substrate for storage lipid biosynthesis in developing seeds (Weber et al., 2005; Baud et al., 2008) and is one of the main end-products of storage lipid breakdown during seed germination (Graham, 2008; Quettier and Eastmond, 2009). The fatty acid profile of the peanut has a large bearing on the quality of the peanut, which can be used as cooking oil, in snack foods, peanut butter, and in confectionaries. The fatty acid profile affects the shelf-life and flavor of snacks and peanut butter, and the stability of the cooking oil. The typical fatty acid profile for peanut (estimated) is: 10% palmitic, 3% stearic, 45% oleic, 35% linoleic, and 2% behenic, with the other 5% comprising small amounts of possibly seven different fatty acids, which include linolenic, arachidic, and eicoseonic (Ahmed and Young, 1982). The ratio of oleic to linoleic acid (O/L) is a measure of oil stability (Holley and Hammons, 1968; Worthington and Hammons, 1977), and it is a critical factor in determining peanut oil quality (Fore et al., 1953; Sanders et al., 1992). The majority of commercial peanuts have an O/L ranging from 1:1 to 2.5:1, with Spanish types usually having a lower O/L (Fig. 10.2). In oilseeds, the majority of the lipids are stored in the cotyledons and endosperm. The enzyme lipase is used to break down lipids to produce glycerol and free fatty acids. The free fatty acids are then broken down by α-oxidation or β-oxidation. α-oxidation involves the successive loss of one carbon atom and CO2 by the aid of fattyacid peroxidase and aldehyde dehydrogenase enzymes (Copeland and McDonald, 1995).

292

Physiology of the Peanut Plant

Fig. 10.2. The unsaturated to saturated (U/S) ratio of each entry in order with increasing oleic to linoleic (O/L) ratio

The fatty acids may also be broken down by β-oxidation, which results in the cleaving of two carbon units in the form of acetyl CoA, which can enter the tricarboxylic acid cycle (Mayer and Paljakoff-Mayber, 1975). It can also go through the glyoxylate cycle to be converted into sucrose. β-oxidation is the main source of fatty acid breakdown during the germination process, although α-oxidation does play a minor role. In oil-bearing seeds metabolism of storage lipid provides the main source of energy for the early cellular development and in general the onset of germination is concomitant with a large increase in lipase activity in the storage tissue resulting in the release of free fatty acids which then become available for the catabolic processes by which energy is released (Hitchcock and Nichols, 1971). It states that the unsaturated fatty acids, such as oleic and linoleic, allow the lipids to remain functional at lower temperatures compared to the saturated fatty acids, such as palmitic and stearic. It is also theorized that polyunsaturated fatty acids, such as linoleic, are metabolized easier at lower temperatures than the monounsaturated oleic acid. Willing and Leopold (1983) proposed that low-temperature injury during imbibition interfered with membrane expansion, possibly by lowering elasticity and hindering incorporation of lipid material into the expanding cell membrane. According to Voet and Voet (1995), unsaturated fatty acids have a lower melting point than saturated fatty acids, which allows for increased fluidity at lower temperatures. This study analyzed the carbohydrate, lipid, nitrogen, and protein contents and mineral elemental compositions of germinating groundnut (Arachis hypogaea L.) seeds and seedlings. The result showed that there was a progressive decrease in the lipid contents from the time of germination to the seedling age of 14 days (Table 10.3). There was also a decrease in the protein contents of the seedlings from the day of germination till the fourteenth day of the seedling development. The assessment also shows an increase in the carbohydrate’s contents of the seedlings during germination and development. The magnesium contents of the seeds and seedlings decreases with

293

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the age of the plant until the twelfth day, and it increased on the fourteenth day. There is a trend in the increase and decrease in the calcium, potassium and sodium contents of the seed and seedling as the groundnut germinates. Table 10.3. Lipids contents analysis of germinating seeds and seedlings of A. hypogaea L. S/n Days

Wt. of sample before ext. (g)

Wt. of sample After ext. (g)

Wt. loss (g)

1

0.20

0.15

9.45

2

0.18

0.13

8.19

3

0.15

0.10

6.30

4

0.19

0.14

8.82

5

0.13

0.08

5.04

6

0.21

0.16

10.08

7

0.16

0.11

6.93

8

0.22

0.17

10.71 12.60

9

0.25

0.20

10

0.20

0.15

9.45

11

0.26

0.21

13.23

12

0.28

0.23

14.50

13

0.27

0.22

13.86

14

0.30

0.25

15.75

15

0.28

0.23

14.49

16

0.32

0.27

17.00

Av. % of lipids

8.82±0.63 7.56±1.26 7.56±2.52 6.66±1.89 5.03±1.58 4.88±0.64 4.80±0.95 3.75±1.26

The castor-bean endosperm—the best-studied material of reserve lipid hydrolysis in seed germination—was previously shown to have an acid lipase and an alkaline lipase having reciprocal patterns of development during germination. Oil seeds from 7 species, namely castor bean (Ricinus communis L.), peanut (Arachis hypogaea L.), sunflower (Helianthus annus L.), cucumber (Cucumis sativus L.), cotton (Gossypium hirsutum L.), corn (Zea mays L.) and tomato (Lycopersicon esculentum Mill.) were studied. The storage tissues of all these oil seeds except castor bean contained only alkaline lipase activity which increased drastically during germination. The pattern of acid and alkaline lipases in castor bean does not seem to be common in other oil seeds. The alkaline lipase of peanut cotyledons was chosen for further study. On sucrose gradient centrifugation of cotyledon homogenate from 3-d-old seedlings, about 60% of the activity of the enzyme was found to be associated with the glyoxysomes, 15% with the mitochondria, and 25% with a membrane fraction at a density of 1.12 g cm-3. The glyoxysomal lipase was associated with the organelle membrane, and hydrolyzed only monoglyceride whereas the mitochondrial and membrane-fraction enzymes degraded mono-, di- and triglycerides equally well. Thus, although the lipase in the glyoxysomes had the highest activity, it had to cooperate with lipases in other cellular compartments for the complete hydrolysis of reserve triglycerides. Peanut seeds have a high oil content making them an important oil crop. During development and germination, seeds undergo complex dynamic and physiological

294

Physiology of the Peanut Plant

changes. Changes in lipid metabolism and underlying mechanisms during seed development have been studied extensively by DNA and RNA sequencing; however, there are few studies on dynamic changes of proteomics during peanut seed development and germination. In a study, proteomic analyses were carried out 20, 40, 60, and 80 days after pollination and 5, 10, 20, and 30 days after germination using isobaric tags for relative and absolute quantitation (iTRAQ) technology to determine the protein profiles of lipid dynamics during peanut seed development and postgermination. A total of 5712 of 8505 proteins were identified, quantified, and divided into 23 functional groups, the largest of which was metabolism-related. Further analyses of the proteins and their pathways revealed initiation of fatty acid accumulation at early stages after flowering, while lipid degradation occurred largely through the lipoxygenase-dependent pathway. Protein expression patterns related to lipid accumulation and degradation were also verified at transcript levels using quantitative real-time polymerase chain reaction. Results suggest that lipids can be used as a source of carbon for respiration in germinating oilseeds. The byproducts of fatty acid catabolism can pass from the peroxisome to the mitochondrion independently of the glyoxylate cycle. However, an additional anaplerotic source of carbon is required for lipid breakdown and seedling establishment. This source can be provided by the glyoxylate cycle or, in its absence, by exogenous sucrose or photosynthesis.

10.2. Plant To achieve photoautotrophism, seedlings must adapt both developmental and metabolic programs to the prevailing environmental conditions (Holdsworth et al., 1999). Seed storage reserves fuel this process. In oilseeds, a massive conversion of triacylglycerol to sugar occurs after germination (Kornberg and Beevers, 1957). β-oxidation of fatty acids derived from triacylglycerol produces acetyl-CoA, which ultimately is converted to sucrose through the glyoxylate cycle and gluconeogenesis. The sucrose produced is transported throughout the seedling, where it supports growth and development (Kornberg and Beevers, 1957; Canvin and Beevers, 1961; Beevers, 1980). In addition to its established role in the post germinative growth of oilseeds, the glyoxylate cycle has also been reported to operate as a salvage pathway in senescing plant tissues (Gut and Matile, 1988) and to have an anaplerotic role in carbohydratestarved tissue (Graham et al., 1994), similar to the role that was described in microorganisms (Kornberg and Krebs, 1957). Furthermore, plant lipids are thought not to be a quantitatively important respiratory substrate even during the period of massive lipid mobilization that occurs in germinating oilseeds (ap Rees, 1980) (Fig. 10.3). Together with sugars and proteins, lipids constitute the main carbon reserves in plants. Lipids are selectively recycled and catabolized for energy production during development and in response to environmental stresses. Autophagy is a major catabolic pathway, operating in the recycling of cellular components in eukaryotes. Although the autophagic degradation of lipids has been mainly characterized in mammals and yeast, growing evidence has highlighted the role of autophagy in several aspects of lipid metabolism in plants. To summarize recent findings focusing on autophagy functions in lipid droplet (LD) metabolism it was further provided novel insights regarding the relevance of autophagy in the maintenance and clearance of

Lipid Metabolism

295

mitochondria and peroxisomes and its consequences for proper lipid usage and energy homeostasis in plants. Macroautophagy is a versatile mechanism involved in lipid metabolism, operating both in the turnover of membrane components and in various aspects of reproduction, like pollen and seed metabolism. A mechanism resembling yeast microlipophagy was recently proposed in plant cells and is specifically active under starvation.The interplay between microlipophagy and lipolysis in plants seems to drive the efficient usage of lipid reserves. The autophagic maintenance of mitochondria and peroxisomes is essential for the optimal operation of these organelles, further enabling proper lipid turnover. LDs are formed of apolar lipids (i.e. TAGs and sterol esters) surrounded by a monolayer of phospholipids containing specific proteins. The LDs of the seeds are also referred to as oleosomes, as they are coated by a specific family of proteins called oleosins. The presence of oleosins at the surface of the seed LDs prevents the phospholipids from adjacent LDs coming into contact and fusing with them (Schmidt and Herman, 2008; Miquel et al., 2014). Although LDs mainly accumulate in the embryo or endosperm of oleaginous seeds, they can also be found in the leaves, especially during senescence and under stressful conditions (Slocombe et al., 2009; Pyc et al., 2017; Coulon et al., 2020). LDs isolated from leaves and oleaginous fruit tissues of Arabidopsis differ from those isolated from the seeds; instead of oleosins they are coated with a family of proteins called LD-associated proteins (LDAPs, also

Fig. 10.3. Total fatty acid content of etiolated wild-type and iclmutant seedlings grown on media in the presence (A) and absence (B) of 1% (wt/vol) sucrose. DAI, days after imbibition. Fatty acid content is expressed as percentage of the seed dry weight. Values are shown as means, SE of measurements on three batches of 20 seedlings

296

Physiology of the Peanut Plant

termed small rubber-particle proteins, SRPs) (Horn et al., 2013; Gidda et al., 2016; Brocard et al., 2017; Coulon et al., 2020). Interestingly, whereas the expression of oleosins is developmentally controlled and restricted to seed and pollen, the expression of LDAPs is induced in response to stress (Miquel et al., 2014; Gidda et al., 2016). The formation of LDs occurs at the membrane of the endoplasmic reticulum (ER) and is facilitated in leaves by the interaction between LDAP and LDAP-interacting protein (LDIP) (Pyc et al., 2017; Coulon, et al., 2020). In plants, de novo biosynthesis of fatty acids (FAs) takes place in the chloroplast, while TAG assembly occurs at the ER. Newly synthesized FAs are exported from the plastids and eventually modified by desaturation after incorporation into phosphatidylcholine (PC) in the ER. The assembly of FAs into TAGs occurs through different pathways that take place in the ER (reviewed by Li-Beisson et al., 2013). Indeed, both the diacylglycerol acyl transferase DGAT1 that is involved in the last step of the Kennedy pathway and the phospholipid:diacylglycerol transferase PDAT1 that acylates the diacylglycerol (DAG) using PC as the acyl donor reside in the ER (Kaup et al., 2002; Chapman et al., 2013; Brocard et al., 2017). The TAGs accumulate between the two leaflets of the ER membranes to form LDs that bud from the ER and are eventually released into the cytosol. Both membrane and storage lipids are used to fulfil the β-oxidation pathway and produce energy in the mitochondria of mammals, and in the peroxisomes of yeast and plants. During leaf senescence, peroxisome β-oxidation is enhanced (Charlton et al., 2005; Yang and Ohlrogge, 2009; Watanabe et al., 2013; Zhang et al., 2020) and the accumulation of LDs suggests that the FAs released from the membranes are first interconverted as TAGs in droplets to prevent FA-induced toxicity before being used for β-oxidation (Kaup et al., 2002; Lin and Oliver, 2008; Yang and Ohlrogge, 2009). The TAGs and lipid-esters can also accumulate in the chloroplasts during senescence, forming plastoglobules. These are formed from the FAs released from the degradation of the galactolipids and phytols (Lippold et al., 2012) (Fig. 10.4). The dynamics of the formation and degradation of LDs in vegetative plant tissues are yet to be determined. According to Fan et al. (2019) and Havé et al. (2019), autophagy may be involved in the recycling of lipids that originate from the plasma membrane and the endomembrane of several organelles, except for those of chloroplasts. While the stroma material of the chloroplasts is degraded through the macro-autophagy pathway, chlorophagy occurs through the micro-autophagy pathway (Izumi et al., 2019). The influence of two salts of sodium on the lipid metabolism of the groundnut during a ten day period of germination was studied. The amount of total lipids, total fatty acids and triglycerides in the water soaked cotyledons fell markedly by the eighth day of germination. Chloride and sulphate treatments delayed these changes and inhibited the lipase activity. Similar studies in the embryonic axes indicated lowered amounts of total lipids, total and free fatty acids and total phospholipids in salt treated seedlings, indicating a depressing effect of salts on lipid synthesis. Low incorporation rates of 14C-acetate into total lipids of salt treated seedlings, lends support to the inhibiting effect of salts on lipid synthesis. Higher lipase activity in salt treated embryonic axes might possibly indicate degradation of lipids synthesized. Analysis of fatty acids revealed synthesis and/or accumulation of unsaturated fatty acids and higher ratios for unsaturated to saturated fatty acids in salt treated embryonic axes, which might be considered as the plant’s response to the adverse influence of salts in the medium.

Lipid Metabolism

297

Fig. 10.4. Schematic representation of the potential roles of autophagy in lipid degradation in Arabidopsis leaves. Arrows indicate autophagy-dependent lipid degradation pathways; double-ended arrows indicate that autophagy may be involved in both the degradation and replenishment of lipid droplets and fatty acids. LDs, lipid droplets; TAGs, triacylglycerols; FAs, fatty acids; SDP1, SUGARDEPENDENT1 triacylglycerol lipase; PDAT-1, phospholipid: diacylglycerol transferase, VLCFA, very-long-chain fatty acid; ER, endoplasmic reticulum.

Oil producing plants showed a certain degree of salinity tolerance. The accumulation of lipids in these plants under stress condition could be regarded as a means of osmo regulation (Ahmed et al., 1977, 1979, 1987; Younis et al., 1987; Abdel-Rahman and Hassanein, 1988). It is well known that during germination of seeds rich in lipids, the fatty acids derived from triglycerides undergo β-oxidation in the glyoxysome (Devlin and Witham, 1986). The ability of peanut (Arachis hypogaea L.) to grow at high concentrations of NaCl may be due to the alteration in gene expression. SDS-PAGE analysis has revealed that plants grown under NaCl showed induction (127 and 52 kDa) or repression (260 and 38 kDa) in the synthesis of few polypeptides. In addition, nine different esterase isoenzymes were detected in embryos of seeds germinated in 105 mM NaCl, whereas only five of them were detected in the embryos of untreated seeds. On the other hand, in the cotyledons, the esterase patterns of both stems and leaves were less influenced by NaCl in comparison to those of roots. The lipid contents, and fresh and dry masses were increased up to 45 NaCl and decreased at higher concentrations (Figs. 10.5 and 10.6). The cold signal is transduced from the extracellular to intracellular regions after being sensed by the plasma membrane and causes a series of physiological and biochemical changes. The ability to tolerate cold in peanut is based on the signal transduction by various factors in plant cells. However, how cold signals are perceived by plasma membranes and how cold signals transduce into intracellular through membranes is poorly understood. The diversity in plasma membrane composition, structure, and function is determined by the membrane lipids and membrane proteins. The interaction between membrane lipids and membrane proteins with different

298

Physiology of the Peanut Plant

Fig. 10.5. Esterase isoenzyme pattern (on 7.5% polyacrylamide gel) in cotyledons of peanut seeds germinated for 1d (lane 1), 2d (lane 2), 4d (lane 3), 6d (lane 4), 8d (lane 5), and 15d (lane 6)

Fig. 10.6. Esterase isoenzyme pattern in embryos of germinated seeds treated with 15 mM

NaCl (lane 2), 30 mM NaCl (lane 3), 45 mM NaCl (lane 4), 75 mM NaCl (lanes 5, 6), 90 mM

NaCl (lane 7) and 105 mM NaCl (lane 8), compared with untreated plant (lane 1)

299

Lipid Metabolism

structures leads to differences in plasma membrane function. It is the key for analysis of cold signal transduction and elucidation of cold tolerance mechanism in peanut to understand the dynamic changes of plasma membrane structure and identify the function of the key protein. The unsaturation of membrane lipids is closely related to cold tolerance in peanut. The proportion of unsaturated fatty acids has been regarded as an important index to measure the cold tolerance. Changing the ratio of saturated fatty acid to unsaturated fatty acid to improve cold tolerance peanut has become the research direction in recent years. However, the fatty acid composition of various lipids is variable, and it is not enough to analyze the fatty acid composition of membrane lipids in isolation to understand the physiological mechanism of membrane lipids. The main phospholipid molecules that make up the cell membrane and the main glycolipid molecules forming the chloroplast thylakoid membrane are also important in studying the physical phase transition of the membrane system at low temperature. The synthesis of lipids in plants is a unique process with a number of steps identified by years of research. In cellular systems, the compartmentization of the lipids synthesis process had been described by numerous workers (Fig. 10.7).

Fig. 10.7. Compartmentization of lipid synthesis

300

Physiology of the Peanut Plant

Mature peanut seeds consist of approximately 52% oil, mostly in the form of triacylglycerols (TAGs) within oil bodies. KEGG was used to automatically annotate peanut Unigenes that coded for orthologues involved in lipid metabolism. About 1,500 Unigenes were found to be involved in lipid metabolism (Table 10.4). The lipid Unigenes covered most of the known cellular activities of acyl lipid metabolism, including many that were poorly characterized or recently described. There were nine cellular activities of lipid metabolism. This is one of the most complete and extensive efforts for lipid genes annotation of peanut plants. Table 10.4. Unigenes involved in lipid metabolism in peanut Passway

Symbol

Fatty acid biosynthesis

DESA1

Acyl-[acyl-carrier-protein] desaturase

8

fabl

Enoyl-[acyl-carrier protein] reductase I

3

fabZ

Hydroxymyristoyl ACP dehydrase

3

FATA

Fatty acyl-ACP thioesterase A

4

FATB

Fatty acyl-ACP thioesterase B

5

accC

Acetyl-CoA carboxylase, biotin carboxylase subunit

12

FAD2

Omega-6 fatty acid desaturase (delta-12 desaturase)

13

PPT

Palmitoyl-protein thioesterase

6

PAAG

Enoyl-CoA hydratase

3

ACADM

Acyl-CoA dehydrogenase

2

DCI

Methylglutaconyl-CoA hydratase

1

ATOB

Acetyl-CoA C-acetyltransferase

6

ACAA1

Acetyl-CoA acyltransferase

7

ACSL

Long-chain acyl-CoA synthetase

33

ACOX

Acyl-CoA oxidase

28

PDAT

Phospholipid:diacylglycerol acyltransferase

10

DGAT1

Diacylglycerol O-acyltransferase

5

GPAT3_4

Glycerol-3-phosphate O-acyltransferase

8

GPAT

Glycerol-3-phosphate acyltransferase

7

OLE

Oleosin

Fatty acid metabolism

Glycerolipid metabolism

Enzyme

Doi: 10.1371/journal.pone.0073767.t001

Unigenes

13

Lipid Metabolism

301

The effect of thiocarbamates (S-ethyldipropylthiocarbamate and diallate), substituted ureas (monuron and diuron), and uracils (bromacil and terbacil) on lipid metabolism in groundnut (Arachis hypogaea) leaves was investigated under nonphotosynthetic conditions. The uptake of [1-14C] acetate by leaf disks was inhibited by the thiocarbamates and marginally by the substituted ureas, but not by the uracil herbicides. The uptake of [methyl-14C] choline was inhibited to a lesser extent by thiocarbamates, while the other herbicides showed a slight stimulation. The thiocarbamates almost completely inhibited uptake of [32P] orthophosphate at 1.0 mM concentration, while diuron and terbacil showed significant inhibition. [1-P] ortho phosphate at 1.0 14C] Acetate incorporation into lipids was inhibited only by diallate. [methyl-14C] Choline incorporation into the choline phosphoglycerides was inhibited by diallate, diuron, and bromacil. The incorporation of [32P] orthophosphate mM into phospholipids was substantially inhibited (over 90% at 1.0) by the thiocarbamates, but not by the other herbicides. [35S] Sulfate incorporation into sulfoquinovosyl diglycerides was markedly inhibited only by the thiocarbamates. Fatty acid synthesis by isolated chloroplasts was inhibited 40–85% by thiocarbamates, substituted ureas, and bromacil, but not by terbacil. The inhibitory effect of the urea derivatives was reversible, but that of thiocarbamates was irreversible. sn-Glycerol-3-phosphate acyltransferase(s) of the chloroplast and microsomal fractions were profoundly inhibited by thiocarbamates, but not by the other two groups of herbicides. Phosphatidic acid phosphatase was insensitive to all the herbicides tested. As in other eukaryotic cells, lipids are found in membranes but are also stored as droplets in the plant cell. Triacylglycerols (TAGs) packed in lipid droplets (LDs) represent a major form of carbon storage. Oleaginous plants such as rapeseed, soybean, sunflower, and peanut accumulate numerous LDs in their seeds. However, other plants such as avocado and olive store lipids in the pericarp of their fruits. Bio-membranes consist of double layers of polar lipids that are mainly composed of glycerolipids (phosphor lipids and galacto lipids). Membranes also contain sterols and long-chain sphingolipids (Gomez et al., 2018). While galacto lipids in plants are located in the membranes of chloroplasts, sphingo lipids such as glycosyl inositol phosphorylceramide (GIPC) are preferentially localizedto the plasma membrane where they are highly enriched in microdomains, together with sterols (Cacas et al., 2016). To maintain high yields under an increasingly hotter climate, high temperature resilient peanut cultivars would have to be developed. Therefore, the mechanisms of plant response to heat need to be understood. A study was to explore the physiological and metabolic mechanisms developed by virginia-type peanut at early growth stages in response to high temperature stress. Peanut seedlings were exposed to 40/35°C (heat) and 30/25°C (optimum temperature) in a growth chamber. Membrane injury (MI), the Fv/Fm ratio, and several metabolites were evaluated in eight genotypes at four timepoints (day 1, 2, 4, and 7) after the heat stress treatment initiation. It was highlighted that some metabolites, e.g., hydroxyproline, galactinol, and unsaturated fatty acids, explain specific differential physiological (MI) responses in peanut seedlings, and overall data suggested general stress responses rather than adaptive mechanisms to heat (Table 10.5).

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Physiology of the Peanut Plant

Table 10.5. Fatty acid content (mg mg-1 leaf dry weight) and the saturated vs. unsaturated fatty acid ratio in the leaves of eight virginia-type peanut cultivars and breeding lines after 1, 2, 4, and 7 days of optimum (control) (30/25°C) and high temperature (40/35°C) under controlled conditions Genotype

Bailey CHAMPS NO4074FCI NO5006 NO5008 NO5024J Philips SPT06-07 Average

Saturated fatty acid

Unsaturated fatty acid

Saturated/unsaturated Control

Control

Heat

Control

Heat

1.60a 1.40ae 1.12de 1.53abc 1.38ae 1.24be 1.54ab 1.56ab 1.42A

1.25cde 1.39ad 1.15e 1.31be 1.24cde 1.25cde 1.20de 1.52ab 1.29B

0.28cf 0.26def 0.26ef 0.30bf 0.29cf 0.23f 0.29cf 0.25f 0.27B

0.32ae 0.36ab 0.33ae 0.35abc 0.35abc 0.33ad 0.32ae 0.37a 0.34A

5.7 5.4 4.3 5.1 4.8 5.4 5.3 6.2 5.9

Heat 3.9 3.9 3.4 3.7 3.5 3.8 3.8 4.1 3.8

Means followed by different lower-case letters are significantly different for genotype 3 temperature regime interaction (P#0.05 Tukey-HSD). b means followed by different capital letters between temperature regimes are significantly different (P#0.05 student’s t-test).

10.3. Synthesis of Plastid Lipids Eukaryotic DAGs and prokaryotic DAG structures are the precursors of glycolipid synthesis (SQDG, MGDG, and DGDG). There are therefore two types of glycolipids: prokaryotic glycolipids whose DAG backbone is of the C18/C16 type and which are desaturated exclusively in the plastid and eukaryotic glycolipids including DAGs of the type (C18:1/C18:1 and C16:0/C18:1) are derived from phosphatidylcholine and are desaturated in RE and plastid (Xu et al., 2006). The synthesis of glycolipids, being localized in the membranes of the plastid envelope, thus requires a mechanism for importing DAGs of eukaryotic structure. These differences in DAG structure are due to different specificities of the chloroplast and ER acyl transferases. The first step of the prokaryotic pathway is the transfer of the oleate to a glycerol-3-phosphate at position sn-1 by an acyl ACP-glycerol 3 phosphate acyltransferase (EC 2.3.1.1), soluble in the stroma of the plastid (Kim and Huang, 2004). Lysophosphatidic acid (LPA) is thus formed. A second plastid-related plastid acyltransferase, the LPAACP acyltransferase, catalyzes the esterification of palmitoyl-ACP at the sn-2 position (LPAAT1; EC 2.3.1.51) (Block et al., 1983). This results in the synthesis of 18:1/16:0-PA. Phosphatidic acid (PA) can either be converted to CDP-DAG by the action of a CTP-phosphatidate cytidylyltransferase (EC 2.7.7.41) which catalyzes the reaction between PA and CTP to form CDP-DAG and pyrophosphate or dephosphorylated to diacylglycerol (DAG) by phosphatidate phosphatase (PAP; EC 3.1.3.4). CDPDAG will be used for the synthesis of phosphatidyl glycerol (PG) of the plastid (Kim and Huang, 2004) and DAG can be used for the synthesis of galactolipids (MGDG, DGDG) or a sulfolipid (sulfoquinovosyldiacylglycerol). From the phyllogenetic point of view, the difference between so-called “C16:3” and “C18:3” plants is related to the presence of plastid phosphatidate phosphatase in “C16:3” plants, lost during evolution in “C18:3” plants. The chloroplast enzyme

Lipid Metabolism

303

is clearly different from other phosphatidate phosphatases in the cell because it is membrane-bound, strongly associated with the inner membrane of the envelope, has an optimum alkaline pH, and is inhibited by cations such as Mg2+ (Awai et al., 2001). The DAG thus produced (18/16 DAG) is at the origin of the glycolipids of the prokaryotic structure, SQDG, MGDG, and DGDG.

10.3.1. Synthesis of Monogalactosyl Diacylglycerol (MGDG) MGDG is synthesized in a single step by a 1,2-DAG 3-β-galactosyltransferase (or MGDG synthase) that transfers galactose from UDP-Gal to DAG via a β1 → 3 glycosidic linkages (Kobayashi et al., 2004). MGDG synthase 1 catalyzes the synthesis of eukaryotic and prokaryotic MGDG molecules in vitro with no apparent specificity for either structure and is at the origin of the majority of the MGDG synthesized in standard condition. In contrast, MGDG synthase 2 and 3 would be localized in the outer membrane. These two enzymes have a better affinity for eukaryotic DAG (C18:2/C18:2) (Kobayashi et al., 2004) and would likely be in the supply of MGDG for synthesis of DGDG (Kelly et al., 2003).

10.3.2 Synthesis of the DGDG A small proportion of MGDGs are again glycosylated by DGDG synthase (EC 2.4.1.241) to form DGDG. Two enzymes catalyze DGDG synthesis by adding Gal from UDP-Gal to MGDG via α1 → 6 glycosidic linkages (Sanda et al., 2001). DGDG synthase1 acts preferentially on MGDG C18/C18, whereas DGDG synthase 2 seems to have an affinity for MGDG with C16/18 (Yu et al., 2002). These two enzymes would be localized in plastids, presumably in the outer membrane of the envelope (Yu et al., 2002).

10.3.3 Synthesis of Sulfoquinovosyl-diacylglycerol (SQDG) Similarly, a sulfolipid synthase (EC 3.13.1.1) catalyzes the attachment of UDPsulfoquinovose (UDP-SQ) to the sn-3 position of DAG to form SQDG. The first step in the synthesis of SQDG or sulfolipid is the formation of UDP-SQ, a polar donor group (Wallis and Browse, 2002). The second reaction is catalyzed by of UDP-SQ, a polar donor group (Wallis and Browse, 2002). The second reaction is catalyzed by a sulfolipid synthase (EC 3.13.1.1) that transfers SQ from UDP-SQ to a DAG molecule (Gidda et al., 2009).

10.3.4 Synthesis of phosphatidylglycerol (PG) Phosphatidic acid (PA) is also a substrate for CDP-DAG synthase (EC 2.7.7.41) to form CDP-DAG, the precursor of PG synthesis. In chloroplasts, PG is generated in the inner membrane of the envelope where phosphatidylglycerol-phosphate synthase and phosphatidylglycerol-phosphate phosphatase (EC 3.1.3.27) activities have been detected (Xu et al., 2002). The fatty acids that make up the various glycerolipids formed in the plastid are characterized by a high degree of unsaturation introduced by the various fatty acid desaturases (FAD6, FAD7 and FAD8, EC 1.14.19) to generate polyunsaturated fatty acids (PUFA) necessary for the proper functioning of plastids (Xu et al., 2002).

304

Physiology of the Peanut Plant

10.4. Synthesis of Glycerophospholipids in the Endoplasmic Reticulum A major proportion of palmitic and oleic acids are transported as CoA esters outside the chloroplast to be incorporated at the endoplasmic reticulum (ER) into the phospholipids (PC, PE, PI, and PS). ER is the main site for the synthesis of phospholipids and triacylglycerol, which derive from lysophosphatidic acid (LPA) as for the prokaryotic pathway (Fig. 10.8). In plant, the glycerophosphate acyltransferase (GPAT) family is involved in the first reaction leading to LPA synthesis of the eukaryotic pathway (Kim and Huang, 2004). In the second reaction, cytosolic lysophosphatidic acid acyl transferase (LPAAT2, EC 2.3.1.23) specifically incorporates oleic acid at the sn-2 position of LPA, which is the specific signature glycerolipids from the eukaryotic pathway. Most of the flow of chloroplast-exported fatty acids are incorporated in phosphatidylcholine (PC) by a mechanism called “acyl editing” (Sanda et al., 2001). This mechanism consists of a deacylation-reacylation cycle of the PC which makes it possible to exchange acyls present on the PC with activated FAs taken from a cytosolic pool of free acyl CoA. The oleate exported from plastids, in the form of oleoyl CoA, is

Fig. 10.8. Synthesis of glycerophospholipids

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used as a substrate for the synthesis of polyunsaturated fatty acids which are inserted either in membrane lipids (PC, PE, and PI) or in storage lipids (triacylglycerols TAG). In general, the synthesis of phospholipids is separated into three pathways: the phospholipids derived from cytidine diphosphate (CDP)-DAG (PI, PS), those derived from DAG (PC, PE) (Fig. 10.9), and those from exchange of polar heads belonging to other phospholipids. The membrane lipids of peanut plants are mainly composed of phospholipids (PL), which include phosphatidyl choline (PC), phosphatidyl ethanolamine (PE), phosphatidyl inositol (PI), phosphatidyl glycerol (PG), phosphatidic acid (PA), glycolipids (GLs) that consist of mono-galactose diglyceride (MGDG) and di-galactose diglyceride (DGDG), and a small amount of sulfolipids (SLs) and neutral lipids (NLs), such as cholesterol (Jouhet et al., 2004). The biomembrane is a dynamic equilibrium system that adaptively adjusts the internal composition based on changes in external temperature. Changes in lipid components are closely related to peanut abiotic stress, and the distribution ratio of lipids on the biomembrane of different tolerant cultivars which change with different degrees under various stresses (Lauriano et al., 2000; Sui et al., 2018). Phospholipid content is positively correlated to cold tolerance in plants, and cold tolerance is weakened when PL synthesis is blocked (Saita et al., 2016). Phosphatidyl glycerol is the main factor determining the membrane lipid phase transition for containing highly saturated fatty acids, although it only accounts for 3–5% of thylakoid membrane lipids. The percentage of high-melting point molecules (C16:0/16:0 + C16:0/16:1t + C18:0/16:0 + C18:0/16:1t) in total molecular species or saturated fatty acids (C16:0 + C16:1t + C18:0) in total fatty acids in PG is significantly related to plant cold sensitivity, which is higher in cold-sensitive cultivars (Eriksson et al., 2011). The MGDG and DGDG are important components of thylakoid membrane lipids, which are closely related to photosynthesis, and their contents also change dynamically at low temperatures (Kobayashi, 2016) (Fig. 10.9).

Fig. 10.9. The pathway of membrane lipid biosynthesis in peanut

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The biomembrane system, including cell, nuclear and organelle membranes, is the initial site of injury, particularly in terms of its structure, function, stability, and enzyme activity, thereby resulting in substantial metabolic imbalance, especially involving respiration and photosynthesis. These changes in turn affect the plant growth and development and eventually incur damages at the whole-plant level, leading to the occurrence of chilling damage. Biomembrane is also the main repository of lipids for peanut plants (Yu, 2008), and fatty acids are the main component of the biomembrane, which has been used as the primary index to evaluate peanut quality. Recent studies have further shown that chilling tolerance in peanut is closely correlated with the composition and structure of the membrane lipids, particularly the saturation of membrane fatty acids (Tang, 2011). The complex physiological, biochemical, and molecular mechanisms between membrane lipid metabolism and cold tolerance is being continuously explored to improve cold tolerance by means of high-throughput gene identification, gene editing, and transgenic technology. Biological membranes are the first target of many abiotic stresses. It is generally accepted that the maintenance of integrity and stability of membranes under water stress is a major component of drought tolerance in plants (Bajji et al., 2002). Cell membrane stability, reciprocal to cell membrane injury, is a physiological index widely used for the evaluation of drought tolerance (Premachandra et al., 1991). Moreover, it is a genetically related phenomenon since quantitative trait loci for this have been mapped in drought-stressed rice at different growth stages (Tripathy et al., 2000). Dhanda et al. (2004) showed that membrane stability of the leaf segment was the most important trait to screen the germplasm for drought tolerance. Cell membrane stability declined rapidly in Kentucky blue-grass exposed to drought and heat stress simultaneously (Wang and Huang, 2004). In a study on maize, K nutrition improved the drought tolerance, mainly due to improved cell membrane stability (Gnanasiri et al., 1991). Tolerance to drought evaluated as an increase in cell membrane stability underwater deficit conditions was differentiated between cultivars and correlated well with a reduction in relative growth rate under stress (Premachandra et al., 1991). In holm oak (Quercus ilex) seedlings, hardening increased drought tolerance primarily by reducing osmotic potential and stomatal regulation, improved new root growth capacity and enhanced cell membrane stability. Among treated seedlings, the greatest response occurred in seedlings subjected to moderate hardening. Variation in cell membrane stability, stomatal regulation and root growth capacity was negatively related to osmotic adjustment (Villar-Salvador et al., 2004). The causes of membrane disruption are unknown; not withstanding, a decrease in cellular volume causes crowding and increases the viscosity of cytoplasmic components. This increases the chances of molecular interactions that can cause protein denaturation and membrane fusion. For model membrane and protein systems, a broad range of compounds have been identified that can prevent such adverse molecular interactions. Some of these are proline, glutamate, glycine betaine, carnitine, mannitol, sorbitol, fructans, polyols, trehalose, sucrose and oligosaccharides (Folkert et al., 2001). Another possibility of ion leakage from the cell may be due to thermal-induced inhibition of membrane-bound enzymes responsible for maintaining chemical gradients in the cell (Reynolds et al., 2001). Arabidopsis leaf membranes appeared to be very resistant to water deficit, as shown by their capacity to maintain polar lipid contents and the stability of their composition under severe drought (Gigon et al., 2004).

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A number of phospholipid systems are activated by osmotic stress, generating an array of messenger molecules, some of which may function upstream of the osmotic stress-activated protein kinases. Abscisic acid biosynthesis is regulated by osmotic stress at multiple steps. Both abscisic acid-dependent and -independent osmotic stress signaling first modify constitutively expressed transcription factors, leading to the expression of early response transcriptional activators, which then activate downstream stress tolerance effector genes (Zhu, 2002). Recently, Wan et al. (2007) reported that amongst the 29 calcium-dependent protein kinase genes identified so far, all contained multiple stress-responsive cis-elements up-stream in the promoter region (1 kb). Sucrose non-fermenting 1-related protein kinase 2 has also been reported to be capable of mediating signals initiated during drought stress, resulting in appropriate gene expression (Umezawa et al., 2005). In fact, various chemical signals transduced under drought stress activate an array of genes, leading to the synthesis of proteins and metabolites, conferring drought tolerance in a number of plant species. Based on the results of the transcriptomic and lipidomic analyses, a schematic diagram was proposed to illustrate the gene-metabolite network of lipids in peanut during adaptation to cold stress, which clearly demonstrated the molecular regulation mechanism of membrane lipid metabolism in peanut cold tolerance. In membrane lipid metabolism, the increased PA induced by a cold signal was immediately catalyzed by CDS1/2 and PAP1 to generate the lipid intermediates DAG and CDP-DAG, which inhibited the excessive accumulation of PA. Subsequently, the upregulation of CRLS and PIS1 accelerated the biosynthesis of CL and PI from CDP-DAG. The increased PI can not only increase the DAG level but also activates the Ca2+ signal transduction pathway through “double messenger system”. Moreover, transcriptomic analysis showed the upregulation of SQD2, MGD, and DGD1 that can contribute to the formation of SQDG and DGDG from DAG and CDP-DAG, compensated for the decreased lipid unsaturation caused by the reduced level of MGDG. In fatty acid metabolism, the genes involved in fatty acid elongation all significantly down-regulated and the genes involved in fatty acid β-oxidation all significantly up-regulated, which prevented the formation of very long chain fatty acids (VLCFA) (i.e. fatty acids having at least 20 carbon atoms), and promoted the generation of C16 and C18 fatty acids. The increased level of C18:3 and the significant upregulation of LOX3, AOS1/3, and AOC activated the α-linolenic acid metabolism pathway, which may have caused an increase in JA and activated the JA signal transduction pathway. Overall, transcriptional responses of peanut to cold stress were consistent with lipidomic changes, thus indicating the main regulations occurring at the transcriptional level. Lipids are associated with gravitropism and involved in the gravity perception through calcium signaling and auxin redistribution (Smith et al., 2013). The lipid derived inositol triphosphate (InsP3) plays a crucial role in the gravitropic response through Ca2+ (Smith et al., 2013). Arabidopsis transgenic plants constitutively expressing InsP-5-ptase enzyme had reduced levels of InsP3, which results in reduced bending of hypocotyls, roots and stems upon reorientation (Perera et al., 2008). InsP3 has a potential to trigger accumulation of intracellular Ca2+, which plays a significant role in gravitropic response (Yang and Poovaiah, 2003). The lipid-mediated auxin redistribution is a highly regulated complex process involving both auxin efflux (PIN/ABCD) and influx carrier (AUX/LAX) transporter proteins (Smith et al., 2013), which were found expressed in the developing peanut peg through transcriptomic and proteomic approaches. More recently, InsP3 and Ca2+

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mediated signaling has been demonstrated during PIN protein localization, and a defect in the inositol-5-P results in altered PIN2 localization (Zhang et al., 2011). A dual role of Ca2+ signaling has been elucidated during development of peanut peg and additionally also during gravitropism-related auxin distribution and amyloplast sedimentation (Moctezuman and Feldman, 1996; Moctezuma and Feldman, 1999a; Moctezuma and Feldman, 1999b; Yang et al., 2017). The development of peg and pod involves several genes involved in lipid metabolism and signaling such as phospholipid transporters, phospholipid-transporting ATPase, lipid kinase, sterol binding protein, calcium-dependent lipid-binding-like protein, phosphatidylinositol 3- and 4-kinase, myo-inositol transmembrane transporter, phosphatidylinositol phosphatase, inositol bisphosphate phosphatase, and more. (Xia et al., 2013). For ground-based studies, information of these components in detail and correlation of physiological studies by observing mutants could be useful to gain detailed knowledge of lipid mediated gravitropic responses in peanut peg.

10.5. Seed Development It is suggested that lipid synthesis commences at a very early stage of seed development. Radioactive tracer studies by Pattee et al. (1974) also support this observation. However, the conclusion that lipid synthesis is always dominates other storage processes is not supported. It was shown earlier that the starch content in stages 5 thru 7 is equal to or higher than the lipid content. Lipids represent 14 and 17 per cent of the total components at stages 5 and 6, respectively. At stage 7 the lipid content increased to 27 per cent; at this stage starch content is also 27 per cent. Beyond stage 7 lipid synthesis becomes the dominant reserve accumulation mechanism (Fig. 10.10).

Fig. 10.10. Influence of developmental stage on peanut seed lipid content. First harvest (-); Second harvest ( - ); Third harvest ( Δ-Δ); Fourth harvest (-)

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The growth of peanut seeds occurred over the 70 days post fertilization in peanut plants grown in field condition. The major storage reserves accumulated from 20 DAF-70 DAF, with maximum rates of accumulation between 40 and 60 DAF. The major accumulation of storage lipids started at about 30 DAF and was indicated by an increase in total fatty acid content. Seeds at approximately 20 DAF contained 20% oil and the oil content increased to over 40% by 60 DAF. The accumulation of total lipid of U606 showed similar trend to that of U12, but the oil content of U606 seeds was increased significantly by 13% compared to that of U12 seeds at the mature stage. It was rationalized that the mRNA changes and their regulating processes would precede the appearance of the enzymes and their products. This study focused on the period between 30 DAF and 50 DAF because this period preceded the rapid increase in storage product synthesis, and the seed biosynthetic pathways were also at their maximum activity. The development of oil bodies of two peanut cultivars was observed with the Nile Red confocal method. A few oil bodies were distributed in the center of the storage cells in the cotyledons at 30-40 DAF, and the volume and number of oil bodies per cell dramatically increased during 50-60 DAF. At 60 DAF, all oil bodies showed a globular shape, and localized in the middle of the cells. As compared with the high-oil cultivar U606, the low-oil U12 had a smaller number of uniformly sized oil bodies in storage cells. In mature seeds, the oil bodies were distributed in the central region of the storage cells and were mostly with an elliptical or irregular shape. Storage lipids in developing seeds is synthesized mainly from sugars (mostly from sucrose and glucose) and to a lesser extent from amino acids provided by the mother plant (Weber et al., 2005; Baud et al., 2008). Sugars in lipid-storing cells undergo glycolysis, which occurs both in the cytoplasm and in the plastids. Within the plastids, acetyl-CoA, a direct substrate for fatty acid biosynthesis, is synthesized. In plastids, the most common fatty acids, such as palmitic acid, stearic acid and oleic acid, are mainly synthesized. Lengthening and other modifications of fatty acids already occur in the endoplasmic reticulum. In this organelle, through the Kennedy pathway, triacylglycerols (the main component of storage lipid) are synthesized from fatty acids and glycerol-3-phosphate. Finally, storage lipid is deposited in the cytoplasm in the form of oil bodies (oleosomes). KAAS (Ohlogge et al., 1991) and KOBAS (Block et al., 2007) were used to automatically annotate the peanut Unigenes coded for known orthologues of plant enzymes involved in fatty acid biosynthesis, fatty acid metabolism, glycerolipid metabolism and glyceropholipid metabolism pathways. These results suggested that fatty acid biosynthesis, fatty acid metabolism, glycerolipid metabolism, glyceropholipid metabolism and tricarboxylic acid cycle (TCA) were all activated in the seed development process. The FAs played a fundamental role in oil mobilization and their oxidation produced 2-carbon compounds, which ultimately provided substrates for the TCA cycle. Many of the enzymes involved in fatty acid biosynthesis, fatty acid metabolism, glycerolipid metabolism and glyceropholipid metabolism pathways were up-regulated or down-regulated. ACC, FatA, FatB, SAD and FAD2 which are specific and critical enzymes in fatty acid biosynthesis were up- or down-regulated more than 10 fold (P < 0.01) between the high- and low-oil peanut seeds. Peanut oil contains approximately 80% unsaturated fatty acids (USFAs) and fewer than 15% saturated fats. USFAs are mostly composed of 50% monounsaturated FA oleic acid (OA, C18:1) and 30% polyunsaturated FA linoleic acid (LOA, C18:2) (Chen et al., 2016). Edible peanut oils with high levels of OA have been confirmed to have positive effects on human health, including decreasing the risk of coronary heart

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diseases and decreasing cholesterol levels (Isleib et al., 2006). Therefore, with the increased attention on the health benefits of OA, there is a need to develop a marketoriented peanut cultivar with high levels of OA (Akhtar et al., 2014). OA is classified as a monounsaturated omega-9 fatty acid with the formula of C18H34O2, and the double bond is located at position δ9 (Barkley et al., 2013). The majority of vegetable OA is commonly present in the form of monoglycerides, diglycerides, and triglycerides. The de novo synthesis of OA in plants occurs in the plastid with initial carbon flux from pyruvate to acetyl-CoA, which is subsequently used for the synthesis of palmitoyl-ACP (C16:0-ACP) by 3-ketoacyl-ACP synthase (KAS) III and KAS I. KAS II catalyzes the further conversion of C16:0-ACP to stearoyl-ACP (C18:0-ACP) (Block and Jouhet, 2015). The biosynthesis of OA continues with the dehydrogenation of stearoyl-ACP to oleoyl-ACP (C18:1-ACP), catalyzed by stearoyl-CoA 9-desaturase 2 (SAD2). In fact, oleoyl-ACP is exported from the chloroplast into the endoplasmic reticulum to promote OA synthesis by the acetyl-CoA transport pathway (Joyard et al., 2010). In addition, OA can be converted into LOA by the crucial enzyme fatty acid desaturase 2 (FAD2), which catalyzes the dehydrogenation at the δ12 position of the hydrocarbon chain. Peanuts with high OA are usually obtained from FAD2 mutants (Dar et al., 2017). Currently, AhFAD2A (Patel et al., 2004) and AhFAD2B (Patel et al., 2004) are cloned from diploid ancestors A and B’s subgenomes, respectively. Both of them encode δ12 FA desaturase to predominantly modulate the critical conversion of OA to LOA. Moreover, the recessive mutant allele AhFAD2 also induces OA accumulation (Jung et al., 2000). To date, six candidate isoforms of AhFAD2 have been identified, which can facilitate further genetic manipulation of peanut oil quality (Wang et al., 2015). Nonetheless, a comprehensive illustration of the OA formation pathways and the molecular functions of the homologous gene AhFAD2 in peanut remains to be accomplished. From the sequencing data, approximately 1500 lipid metabolism-associated Unigenes were identified. The RT-PCR results quantified the different expression patterns of these triacylglycerol (TAG) synthesis-related genes in the early developmental stages of peanut pods. Based on these results and analysis, a novel construct of the metabolic pathways involved in the biosynthesis of TAG was proposed, including the Kennedy pathway, acyl-CoA-independent pathway and monoacylglycerol pathway. It showed that the biosynthetic pathways of TAG in the early developmental stages of peanut pods were much more complicated than a simple, unidirectional, linear pathway. Peanuts with high oleic acid content are usually considered to be beneficial for human health and edible oil storage. In breeding practice, peanut lines with high monounsaturated fatty acids are selected using fatty acid desaturase 2 (FAD2), which is responsible for the conversion of oleic acid (C18:1) to linoleic acid (C18:2). Here, comparative transcriptomics were used to analyze the global gene expression profile of high- and normal-oleic peanut cultivars at six time points during seed development. First, the mutant type of FAD2 was determined in the high-oleic peanut (H176). The result suggested that early translation termination occurred simultaneously in the coding sequence of FAD2-A and FAD2-B, and the cultivar H176 is capable of utilizing a potential germplasm resource for future high-oleic peanut breeding. Furthermore, transcriptomic analysis identified 74 differentially expressed genes (DEGs) involved in lipid metabolism in high-oleic peanut seed, of which five DEGs encoded the fatty acid desaturase. Aradu.XM2MR belonged to the homologous gene of stearoyl-ACP (acyl carrier protein) desaturase 2 (SAD2) that converted the C18:0

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into C18:1. Further subcellular localization studies indicated that FAD2 was located at the endoplasmic reticulum (ER), and Aradu.XM2MR was targeted to the plastid in Arabidopsis protoplast cells. To examine the dynamic mechanism of this finding, researchers focused on the peroxidase (POD)-mediated fatty acid (FA) degradation pathway. The FAD2 mutant significantly increased the POD activity and H2O2 concentration at the early stage of seed development, implying that redox signaling likely acted as a messenger to connect the signaling transduction between the higholeic content and Aradu.XM2MR transcription level. Taken together, transcriptome analysis revealed the feedback mechanism of SAD2 (Aradu.XM2MR) associated with FAD2 mutation during the seed developmental stage, which could provide a potential peanut breeding strategy based on identified candidate genes to improve the content of oleic acid. Furthermore, after removing the duplicative gene accession number, a total of 2138 DEGs in seeds of high-oleic cultivar H176 in comparison with the normal-oleic cultivar L70 at the same developmental stage was obtained. The DEG distribution statistics demonstrated that there were 777, 162, 442, 402, 884, and 995 DEGs from stages 1 to stage 6, respectively. A total of 1,308 up-regulated and 2,354 downregulated DEGs were identified at six stages in H176 vs. L176 cultivars, indicating that FAD2 mutation could induce the high-oleic phenotype and suppress generous gene expression. The quantities of identified DEGs in the initial (20–30 DAF) and mature (60–70 DAF) phases were obviously larger than in the middle developmental stage (40–50 DAF), suggesting the involvement of complex transcription regulation events during these stages, particularly the conversion of stearoyl-ACP into OA-ACP in the seed maturity phase. Among the six stages, the middle developmental stage harbored 174 specifically detected DEGs, and 565 DEGs were specifically detected in the stage of early seed swelling, whereas the largest number of specific DEGs (811) was only identified during the period of seed maturity. In total, 103 DEGs were shared across three periods. Peanut (Arachis hypogaea) plant introductions (732) were analyzed for fatty acid composition. Palmitate varied from 8.2 to 15.1%, stearate 1.1 to 7.2%, oleate 31.5 to 60.2%, linoleate 19.9 to 45.4%, arachidate 0.8 to 3.2%, eicosenoate 0.6 to 2.6%, behenate 1.8 to 5.4%, and lignocerate 0.5 to 2.5%. The eicosenoate was shown to be cis11-eicosenoate. In addition, epoxy fatty acids were found in many plant introductions in percentages ranging as high as 2.5%. These were tentatively identified as chiefly 9,10-epoxy stearate and coronarate with smaller amounts of vernolate. The percentage of palmitate was shown to be correlated positively with linoleate and negatively with oleate, eicosenoate, and lignocerate. Stearate was highly correlated with arachidate and negatively with eicosenoate and lignocerate. Oleate and linoleate, the two major fatty acids, were negatively correlated. Arachidate was negatively correlated with eicosenoate, and eicosenoate was positively correlated with lignocerate. Behenate and lignocerate were positively correlated. Epoxy esters were positively correlated with palmitate and negatively with oleate. The effect of an early-, mid-, or late-season planting date on the fatty acid chemistry of four high oleic acid, one mid oleic acid, and five normal oleic acid peanut (Arachis hypogaea L.) genotypes was evaluated over a three year period. Oleic acid was also compared to other fatty acids and to indices of oil quality. High-oleic genotypes included SunOleic 97R, UF98326, UF99621, and 88×1B-OLBC1-6-1-3-1b2-B with a mean oleic acid content between 77.8 and 82.5%. Florida MDR98, a mid-

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oleic cultivar, was intermediate in oleic acid chemistry (59.8−68.0%). The normal oil chemistry lines (Georgia Greene, Andru93, Florunner, 86×13A-4-2-3-2-b3-B, and UF97102) had an oleic acid content between 50.0 and 59.0%. The ratio of oleic to linoleic (O/L) was 18:1 to 51:1 for high-oleic lines and 1.7:1 to 3.5:1 for normal genotypes. When analyzed as a split–split plot in time, year had a highly significant effect (PNC7>Ex-Dakar≥Samnut-22. Oil body size and number were inversely related. This variation can be explained by the different amounts of acidic amino acids and the

Fig. 10.12. Variation in oleic acid-linoleic acid ratio with fresh seed weight by maturity colour (a) Runner market type-variety “68-17”, (b) Virginia market type-variety “Spain”, (c) Spanish market type-variety “Ole”, (d) Valencia market type-variety “Valencia 308”

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protein structure on the surface of the oil bodies of these cultivars. The distributions of protein and oil bodies seemed to be negatively correlated. Cultivar effect was significant, and variation was found in seed production and quality in addition to the size and number of oil bodies, oil productivity, and oil quality among cultivars. Seasonal effects were not evident due to the stability of the climatic conditions during the two seasons of the study (Fig. 10.13).

Fig. 10.13. (A) Picture of five cultivar seeds. (B) Transmission electron microscope (TEM) picture of five peanut cultivars: (1) Ismailia-1, (2) Ex-Dakar, (3) Samnut-22, (4) Samnut-26, and (5) NC-7. Bars represent 2 μm (P—protein bodies; L—oil bodies).

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CHAPTER

11

Plant Growth Regulators 11.1. Dormancy and Germination Ethylene is an important component of the gaseous environment, and regulates numerous plant developmental processes including seed germination and seedling establishment. Dormancy, the inability to germinate in apparently favourable conditions, has been demonstrated to be regulated by the hormonal balance between abscisic acid (ABA) and gibberellins (GAs). Ethylene plays a key role in dormancy release in numerous species, the effective concentrations allowing the germination of dormant seeds ranging between 0.1 and 200 μL L-1. Studies using inhibitors of ethylene biosynthesis or of ethylene action and analysis of mutant lines altered in genes involved in the ethylene signalling pathway (etr1, ein2, ain1, etr1, and erf1) demonstrate the involvement of ethylene in the regulation of germination and dormancy. Ethylene counteracts ABA effects through a regulation of ABA metabolism and signalling pathways. Moreover, ethylene insensitive mutants in Arabidopsis are more sensitive to ABA and the seeds are more dormant. Numerous data also show an interaction between ABA, GAs and ethylene metabolism and signalling pathways. It has been increasingly demonstrated that reactive oxygen species (ROS) may play a significant role in the regulation of seed germination interacting with hormonal signalling pathways. Germination, ethylene production, and carbon dioxide production by dormant Virginia-type peanuts were determined during treatments with plant growth regulators. Kinetin, benzylaminopurine, and 2-chloroethylphosphonic acid induced extensive germination above the water controls. Benzylaminopurine and 2-chloroethylphosphonic acid increased the germination of the more dormant basal seeds to a larger extent above the controls than the less dormant apical seeds. Coumarin induced a slight stimulation of germination while abscisic acid, 2,4-dichlorophenoxyacetic acid, and succinic acid 2,2-dimethylhydrazide did not stimulate germination above the controls. In addition to stimulating germination, the cytokinins also stimulated ethylene production by the seeds. In the case of benzylaminopurine, where the more dormant basal seeds were stimulated to germinate above the control to a larger extent than the less dormant apical seeds, correspondingly more ethylene production was induced in the basal seeds. However, the opposite was true of kinetin for both germination and ethylene production. When germination was extensively stimulated by the cytokinins, maximal ethylene and carbon dioxide evolution occurred at 24 and 72 hours, respectively. Abscisic acid inhibited ethylene production and germination of the seeds while carbon dioxide evolution was comparatively high. The crucial physiological event for

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germination of dormant peanut seeds was enhancement of ethylene production by the seeds (Figs. 11.1 and 11.2). Germination behaviour and seed dormancy in nine varieties of groundnut were studied. Bunch types were non-dormant while the spreading and semi spreading types had prolonged seed dormancy. The period of dormancy varied among the varieties. A tightly attached seed coat was found to be one of the factors that delayed seed germination. Its removal enhanced germination in certain dormant varieties. Application of GA caused further enhancement of germination. However, removal of seed coat and GA application were effective only when one week period of seed dormancy had elapsed. Application of CCC caused partial inhibition of germination in the non-dormant seeds which could be reversed by GA (Table 11.1). The effect of different growth retardants viz., MH at 5000, 10000, 15000 ppm, CCC at 1000, 2000, 3000 ppm and ABA at 250 and 500 ppm on inducing dormancy in non-dormant groundnut cv. TMV 7 was studied by foliar application at 70 and 80 days after sowing. It was evident from the results that ABA 500 ppm applied at 70

Fig. 11.1. The effect of kinetin, CEPA, coumarin, and abscisic acid applied for 96 hrs on the germination of dormant NC-13 peanut seeds. Control germination values for apical seeds at 96 hr of germination were 22±t0, 23±-1, 16±18, and I11±I% for kinetin. CEPA, coumarin, and abscisic acid, respectively. Similarly, control germination values for basal seeds were 5±f2, 4±+3, 13±15, and 8±+0%. The same control served for both concentrations of the growth regulator where applicable. Each point in this and subsequent figures represents the mean of duplicate samples each containing, c50 seeds

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Fig. 11.2. Ethylene production and germination relations in peanut seeds treated with 2,4-D

days after sowing was most effective in inducing dormancy in the resultant seeds as well as recorded the lowest in situ germination of pods. BARC-evolved a high yielding groundnut variety TG-1, having 60 days of dormancy (Fig. 11.3), is not suitable for immediate sowing, in a double cropping system. Therefore, seed pre-soaking for 16 hrs with PGRs (Benzyl Adenine and Etherel) not only terminated the seed dormancy but preponed the flowering by 7 days, resulting in increased pod yield and quality of seeds (inset). Similarly, seeds of another high yielding groundnut var, TAG-24, pre-soaked for 6 hrs in Bayletone (B, 10-5 M and 10-2 M) solutions, showed early germination, early flowering (2 days) and 15 days early maturity (cutting down completely the requirement of one irrigation). It was also accompanied with 20% high pod yield, without affecting the seed and oil qualities. Similar effects were also noted in other Groundnuts (TGS-1 & TG-26).

325

Plant Growth Regulators Table 11.1. Seed dormancy in different varieties and effect of GA (0.1 mg/1) Days after harvest

Treatment

TMV1

TMV3

% Germination TMV10 C-148

M-13

M-145

0-5

Control GA

0.00 0.00

0.00 0.00

0.00 0.00

0.00 0.00

0.00 0.00

0.00 0.00

15

Control GA

29.33 30.33

28.66 33.33

0.00 29.33

11.66 13.33

7.33 18.33

0.00 21.66

30

Control GA

69.00 70.00

39.66 78.00

26.66 33.66

31.66 73.66

44.00 71.00

38.33 39.66

40

Control GA

70.33 91.00

79.33 98.33

58.66 88.33

48.00 81.00

48.66 81.00

38.33 61.00

50

Control GA

99.00 100.00

99.00 100.00

98.33 99.33

97.66 98.00

97.00 100.00

88.66 90.66

65

Control GA

100.00 100.00

100.00 100.00

100.00 100.00

100.00 100.00

100.00 100.00

100.00 100.00

Note: Varieties TMV-2, J-1 I and Pol-l showed about 100% germination at 0 day stage after harvest. C.D at 5% Varieties 1.03 Days 0.94 Treatments 0.29

Fig. 11.3. Pattern of seed dormancy release in Groundnut var TG-1, under field conditions

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Seeds, immediately after harvest, were sown and germination was recorded up to 60 days (expiry of dormancy). Inset: Early release of dormancy by PGRs. Postharvest, 10 day old, air dried dormant seeds were pre-soaked for 16 hrs in various PGR solutions, which induced >90% germination in dormant seeds, within 24 hrs of treatment. Water soaked dormant seeds served as control. Abscisic acid (ABA) plays an important role in seed dormancy, embryo development and adaptation to environmental stresses. It was found that imbibition of exogenous ABA by peanut seeds led to a significant increase in the levels of both AhNCED1 gene [a key gene encoding nine-cis-epoxy carotenoid dioxygenase (NCED) involved in ABA biosynthesis in peanut] transcript and endogenous ABA in germinating seeds, and also led to a marked decrease in α-amylase activity, germination rate and viability index of germinating seeds. This was associated morpho genetically with inhibited plumule apex growth and reduced leaf primordium elongation, a decreased number and length of axial and lateral buds, and shorter length of compound leaves during germination. Imbibition by peanut seeds of naproxen (a potent ABA biosynthesis inhibitor specifically targeting to NCED) significantly decreased the levels of endogenous ABA and AhNCED1 gene transcript in germinating seeds, and markedly increased α-amylase activity, germination rate and viability index of germinating seeds. These observations suggest the involvement of a positive feedback regulation of ABA biosynthesis in ABA-mediated inhibition of seed germination in peanut (Fig. 11.4). Dormancy and germination are complex phenomena that are controlled by a large number of genes, which are affected by both developmental and environmental factors (Bewley, 1997b; Koornneef et al., 2002). These are only beginning to understand part of the intensive cross-talk between the hormones implicated in these processes (Brady and McCourt, 2003; Finkelstein, 2004). A crucial role for ABA has been identified in inducing seed dormancy. Factors determining spatial and temporal ABA content and sensitivity patterns in seeds positively regulate induction of dormancy and probably its maintenance, and negatively regulate dormancy release and germination. GA releases dormancy, promotes germination and counteracts inhibitory ABA effects, directly or indirectly. GA is required for embryo cell elongation, for overcoming coat restrictions to germination of non-dormant and dormant seeds, and for inducing endosperm weakening. BR and ethylene also counteract the inhibitory effects of ABA on seed germination, but in most species they appear to act after dormancy has been released by GA. Ethylene seems to counteract the inhibitory effects of ABA on seed germination by interfering with ABA signalling. This is completely different from the situation in seedling root growth, which is inhibited by both hormones. The ABA– ethylene antagonism in seeds shows hormonal interactions and responses. Use 50-100 mg/L paclobutrazol solution for seed dressing, soak the seeds, and stuff for 1 hr and dry after sown in the field. It can regulate the growth and development of peanut seedlings. The specific performance is to promote the internode shortening, plant height, branching, root system development, root activity increase; the leaf chlorophyll content and photosynthetic rate increased significantly. This is very beneficial for the robust growth of peanuts in the middle and later stages, and the reduction of the resulting parts and fruit needles into the soil. As the hypocotyl shortening after seed dressing is more, the sowing should not be too deep, so as not to affect the emergence or delayed emergence.

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Fig. 11.4. Effects of ABA and naproxen on the levels of AhNCED1 gene transcript (a), ABA (b) and the α-amylase activity (c) in peanut seeds during germination

100-200 mg of paclobutrazol solution was used to spray 5-6 leaf peanut plants. After 11 days, watering was stopped and the soil gradually became dry. After 40 days, the soil moisture content decreased to 41%. This is the determination of various indicators, compared with the control, paclobutrazol treatment can promote root growth, and increase root water absorption, absorption capacity; leaf inside the volume of water storage cells increased, transpiration rate decreased, leaf water content increased. The above anatomical and physiological features are conducive to enhancing cold resistance. Therefore, spraying paclobutrazol can improve the drought resistance of peanuts. With the concentration of 0.01 ~ 0.1 mg/L brassinolide soaking, can improve germination rate, promote the growth of peanut seedlings, and also can improve the cold resistance of seedlings, and the prevention of spring peanut often affected by late spring coldness impact caused by lack of seedling.

328

Physiology of the Peanut Plant

11.2. Plant Seedlings 75-300 mg/L CCC solution was used for dealing with the three periods of peanuts followed by dry processing. The experiments showed that the love-transplanted bone marrow could increase the content of endogenous abscisic acid in peanut seedlings under drought stress, and 150 mg/L treatment was the most significant. In addition, it can also increase the activity of the protective enzyme that protects oxygen free radicals from damage in the plant and leaves the cell membrane less vulnerable to drought. Therefore, the water content of the leaves is higher, which shows that the treatment of chloro choline can improve the anti-drought ability of peanut seedlings to some extent. Plants of groundnut with four leaves grown in pots under greenhouse conditions, were sprayed with chlormequat (2000 ppm), daminozide (4000 ppm), gibberellic acid (100 ppm), indolyl acetic acid 100 ppm, as a mid check treatment. Daminozide 4000 ppm reduced plant height, internode number and the length of the fourth internode. Daminozide increased the number of leaves, retarded flowering, increased the number of flowers and presented a tendency to increase the dry weight of stems. Chlormequat (2000 ppm) and indolyl acetic acid (100 ppm) reduced plant height and the length of the fourth internode of the groundnut plant stem. Different concentrations of the IAA and GA3 have shown significant increase in the length of roots. The average length of roots in control was 20 cm (Table 11.2). Application of GA3 increased length of roots up to 30 ppm concentration which decreased at higher concentrations of GA3 (40 and 50 ppm). There was a direct correlation between the concentration of IAA and length of the root. An increase in the root length was statistically significant at p=0.01 in all concentrations of IAA and in 30 ppm GA3. In 10 and 20 ppm IAA it was significant at p=0.05. The concentration of Gibberellic acid had shown a direct effect on the number of root nodules in the groundnut. In control the number of root nodules per plant was 28 which were increased up to 38 in plants treated with 50 ppm GA3 (Table 11.2). An increase in the number of nodules per plant in 10 ppm and 20 ppm GA3 was statistically non-significant while in 30 ppm, 40 ppm and 50 ppm GA3 increase in the number of nodules was statistically significant at p=0.001. Plants treated with different concentrations of IAA had shown a variable effect on the number of root nodules per plant. 10 and 20 ppm concentration of IAA showed increases but these are statistically nonsignificant. Root nodules decreased (26 per plant) compared to control (28/plant) in the plants treated with 40 and 50 ppm IAA but this decrease was statistically non-significant. Such increased nodulation was recorded by Dobert (1992), Lievens (2005) and Emonger and Ndambole (2011) due to the application of growth hormones. The effect of brassinolide, 24-epibrassinolide and 28-homobrassinolide on nodulation and nitrogenase activity of groundnut was studied. The tested brassinosteroids substantially increased both nodulation and nitrogenase activity. In greenhouse experiments, groundnuts cv. MH-2 seedlings were subjected to 0, 7 and 14 d waterlogging and treatment with water, or 10 or 100 mg gibberellic acid (GA)/litre after the stress period at vegetative, flowering and pod filling stages (35, 50 and 80 d after sowing, respectively). Waterlogging decreased the number, FW, DW, leghaemoglobin content and nitrogenase activity of nodules regardless of growth stage. The effect of waterlogging at pod filling was more deleterious than at other growth stages. 10 mg GA/litre significantly increased the number, FW, DW and nitrogenase activity of the nodules and alleviated some of the effects of waterlogging.

329

Plant Growth Regulators

Table 11.2. Influence of growth hormones morphological and yield parameters of groundnut Treatments Height of plant (cm)

Length of root (cm)

No. of Size of seed Number 100 seeds Yield root (seed/25 of pods weight (kg/ha) nodules ml) (gm)

Control

35

20.0

28.0

38.3

24.0

34.9

1500.0

GA 10 ppm

37.1**

20.3*

30.0

37.7

28.0*

35.0

1500.0

GA 20 ppm

37.2**

20.3*

30.0

37.3*

32.0**

35.5

1550.0

GA 30 ppm

38.1**

20.4**

33.0**

36.3**

33.0**

35.7

1510.0

GA 40 ppm

37.8**

19.9

34.0**

35.0**

34.0**

37.3**

1670.0*

GA 50 ppm

37.6**

19.8

38.0**

35.0**

38.0**

39.2**

2250.0**

IAA 10 ppm 35.8**

20.4**

30.0

38.3

26.0

34.8

1480.0

IAA 20 ppm 36.2**

20.6**

30.0

38.7

25.0

34.8

1510.0

IAA 30 ppm 37.4**

21.0**

28.0

38.3

25.0

35.0

1610.0

IAA 40 ppm 38.9**

21.0**

26.0

38.0

24.0

34.9

1520.0

IAA 50 ppm

21.1**

26.0

38.3

23.0

35.0

1490.0

40.2**

In addition, application of BRs affected nodulation and nitrogen fixation in groundnut (Arachis hypogaea), pea and soybean (Vardhini and Rao, 1999; Terakado et al., 2005; Shahid et al., 2011). In groundnuts, BRs enhanced the growth and yield of the plants, and the growth promotion was associated with enhanced levels of nucleic acids, soluble proteins and carbohydrates (Vardhini and Rao, 1998). The effect of BRs on nodulation and nitrogen fixation was also investigated. Exogenous application of BR increased in nodulation. Several reports of transcriptomic studies show the biosynthesis of hormones and the genes behind its activation or degradation being differentially expressed upon NF treatments. A summarized effects of phytohormones on nodulation is given in Table 11.3. Table 11.3. Summary of phytohormones and their regulatory effects on nodulation Plant hormones

Overall influence on nodulation

Mechanism of regulation

Cytokinin

Positive

• Ligand for HK1, essential for cortical signalling • Delimits polar auxin transport during growth of nodule primordia

Auxin

Positive

• Required in nodule meristem for cortical division • Regulated by NF signalling

Gibberellin

Negative

• Nod factor triggered negative feedback maintenance

Ethylene

Negative/ Positive

• Downregulates defense response gene Lj Pr-10 • Downregulates early genes of symbiotic pathway

Abscisic acid

Negative/ Positive

• Increases lateral root density (LDR) in legumes • Negatively regulates ENOD 11 and RIP1

330

Physiology of the Peanut Plant

11.2.1. Auxin Several findings link Nod Factor (NF) signalling to auxin transport, which is inhibited by flavonoids. NF application or S. meliloti infection inhibits auxin transport from shoot to root at 24 h, as well as regulating the expression of some MtPIN auxin efflux transporter genes in a MtCRE1-dependent manner (Plet et al., 2011; Ng et al., 2015). Moreover, MtCRE1-dependent pathways also control the accumulation of flavonoids in M. truncatula roots upon infection, and flavonoid application can rescue the MtCRE1 nodulation phenotype. These data suggest that NF-induced cytokinin signalling triggers flavonoid induction and the subsequent inhibition of polar auxin transport. The resulting accumulation of auxin initiates cortical cell division and nodule organogenesis. Available evidences suggest that NFs regulate the expression of auxin signalling genes. NF application induces transcription factors encoding genes MtARF16a and MtPLETHORA3 in root hairs related to auxin, while several ARF genes are downregulated in NF-treated root hairs after 24 h (Breakspear et al., 2014; Jardinaud et al., 2016). However, these genes were found to be induced by auxin in roots of M. truncatula (Herrbach et al., 2017). It indicates that the perception of NF could lead to the accumulation of auxin which, in turn, activates some specific auxin signalling genes which are reported to control cell divisions or IT formation in legumes (Breakspear et al., 2014). No information is available thus far in groundnut to provide clarity as to the role of auxin transport in groundnut nitrogen fixation.

11.2.2. Cytokinin Various studies documented the crucial role of cytokinins (CKs) as key regulators of nodule organogenesis (Gonzalez-Rizzo et al., 2000; Trichine et al., 2007) and IT formation (Jardinaud et al., 2016; Held et al., 2014). A comparative analysis between groundnut and the model legumes highlighted the predominance of CKs and ethylene signalling pathways during nodule formation. It was found that the two-component CK receptor Histidine kinase 1 (HK1) plays a central role in nodule organogenesis in both groundnut and other model legumes (Gonzalez-Rizzo et al., 2006; Murray et al., 2007; Plet et al., 2011; Kundu and Dasgupta, 2017). During nodule development, the AhHK1, LjHK1 (LHK1), and MtCRE1 have a similar pattern of expression, but AhHK1, a downstream effector, showed an altered pattern of expression during groundnut symbiosis. Expressions of CK-responsive TFs such as type-B RR like MtRR1, MtNSP1, and MtbHLH476 have a distinct expression pattern in groundnut in comparison to model legumes. The distinct role of CK signalling during groundnut nodulation is in accordance with the previous report, where the silencing of AhHK1 resulted in delayed nodulation associated with nodule differentiation (Kundu and Dasgupta, 2017). Rhizobia–legume interaction activates the common symbiosis pathway (SYM­ pathway) that recruits CK signalling for the induction of nodule primordia in the cortex (Frugier et al., 2008). CK-signalling causes local auxin accumulation at the site of incipient nodule primordia by modulating the expression of auxin transporters (Plet et al., 2011; Ng et al., 2015). These phytohormonal signals and the SYM-pathway together reprogram the cortical cells and regulate their division, ultimately building a nodule primordium for the endocytic accommodation of the symbionts (Mathesius et al., 2000; Suzaki et al., 2012). Loss-of-function mutations of LHK1 (lhk1-1 allele,

Plant Growth Regulators

331

formerly known as hit1)/MtCRE1 (cre1-1/2) led to a strong reduction in nodulation (Murray et al., 2007; Plet et al., 2011), suggesting that CK-dependent phosphorelay through these receptor histidine kinases is essential for nodule organogenesis. Further, gain-of-function mutation of LHK1 (snf2) is sufficient to induce nodule primordia in the absence of rhizobia, demonstrating that CK signalling is sufficient to initiate this developmental process (Tirichine et al., 2007). A recent study unravelled the role of CK signalling in groundnut (Kundu and Dasgupta, 2017). This study provided insight on the silencing (RNAi) of putative CK receptor Histidine Kinase1(AhHK1) resulting in a decrease in nodule number, indicating that CK signalling, mediated through this receptor, is important for the inception of nodule primordium. CK’s role in nodule differentiation was first reported by Plet et al. (2011), but whether this involves factors like retinoblastoma-related protein (RBR), which restrains proliferation to promote the differentiation of cells (Perilli et al., 2013), and the Wuschel-related homeobox5 (WOX5) meristem maintenance factor (Franssen et al., 2015), which are functionally conserved in root nodule meristems, is a subject for future investigation (Kundu and Dasgupta, 2017). Further, some evidence suggests that NFs induce CK production, which initially regulates nodule organogenesis, and then rapidly activates negative feedback on NF signalling and infection processes.

11.2.3. Gibberellins NFs can rapidly induce the biosynthesis of bioactive gibberellin (GA) in root hairs, which consequently triggers negative feedback leading to both downregulation of NF signalling and the activation of GA catabolic enzymes (Maekawa et al., 2009; Herrbach et al., 2017; Fonouni-Farde et al., 2017). Increasing evidence also shows that bioactive GAs negatively regulate nodulation in both determinate and indeterminate nodules (Maekawa et al., 2009; Fonouni-Farde et al., 2017). Thus, NF-induced GAs could help fine-tune NF signalling and rhizobium infection during symbiosis.

11.2.4. Ethylene The ethylene response factor (ERFs) plays a crucial role by regulating cell division and differentiation during nodule development (Asamizu et al., 2008; Vernie et al., 2008). Earlier genetic and physiological studies highlighted the key regulatory role of ethylene during early symbiotic processes (Penmetsa et al., 2008; Zaat et al., 1989; Heidstra et al., 1997; Oldroyd et al., 2001). Groundnut transcriptomics highlighted the upregulation of several AP2-domain and symbiotic orthologues of ERF1 during nodulation (Karmakar et al., 2019; Peng et al., 2017). In L. japonicus, LjERF1 is a positive regulator of nodulation and downregulates the expression of defence gene LjPR-10 during symbiosis (Asamizu et al., 2008). Intriguingly, the high expression of ERF1 in groundnut is associated with the high expression of PR-1s, indicating that the ethylene signalling network is differently recruited during symbiosis. The expression of the master regulator of ethylene signalling, EIN2, was significantly high in groundnut, and the expression of a negative regulator of nodulation, EFD, was distinctly different from model legumes. Vernie et al. (2008) observed that the differential role of ethylene signalling during crack-entry nodulation was because an EFD (for the ethylene response factor required for nodule differentiation) appears to be involved in an ethylene-independent feedback inhibition and regulates the expression of cytokinin response during nodulation.

332

Physiology of the Peanut Plant

11.2.5. Abscisic Acid Abscisic acid (ABA) is a methyl–pentadienoic compound involved in seed maturation, dormancy, and drought stress response pathways (Giraudat et al., 1994). ABA inhibits lateral root development in an auxin-independent manner and governs root architecture, taking cues from nutritional signals (De Smet et al., 2003). Interestingly, non-legumes like Arabidopsis respond to ABA by decreasing lateral root density (LRD) and legumes respond to ABA by elevating LRD. Furthermore, increasing LRD by ABA was found to be associated with the formation of root nodules in the non-legume actinorhizal plant Casuarina glauca. Hence, this suggests that increased lateral root formation to ABA response is pivotal in the case of nodule formation (Liang and Harris, 2005). Studies in M. truncatula have indicated that ABA can influence both nodule formation and bacterial infection. ABA negatively regulates early gene expressions of RIP1 and ENOD11, possibly through the regulation of the Nod factor signalling pathway. It was also shown that ABA drastically suppresses the cytokinin-orchestrated induction of ENOD40 and NIN in sta-1, a mutant sensitive to ABA (Ding et al., 2008). Thus, examination of the role of ABA in nodulation is important to enhance the groundnut nitrogen use efficiency in order to reduce the yield gap. A field experiment was conducted during the kharif 2017 at Cotton Research Station, Junagadh Agricultural University, Junagadh to study the effects of different plant growth regulators on dry matter production and yield attributes of groundnut cv. GJG-9. The investigation was carried out in RBD design three replications and foliar application of different concentrations of growth regulators such as GA3 (50, 100 ppm), NAA (40, 80 ppm), TRIA (2.5, 5.0 ppm), BR (10, 15 ppm) and water spray (control) at 40 and 55 DAS. The experiment results revealed that foliar application of PGRs increased the total dry matter production and the increase was more in GA3 @ 100 ppm treated plants. Among different treatments, significantly higher no. of filled pods per plant (14.87), shelling percentage (77.32 %) and yield (1732.36 kg ha-1) were observed in GA3 @ 100 ppm treated plants as compared to control (Table 11.4).

11.3. Physiological Aspects Different growth regulators are shown to influence different crop physiology parameters e.g. alter plant archetype, promote photosynthesis, alter assimilate partitioning, stimulate uptake of mineral ions, enhance nitrogen metabolism, promote flowering, uniform pod formation, increase mobilization of assimilates to defined sinks, improve seed quality, induce synchrony in flowering and delay senescence of leaves. The response of groundnut varieties to different growth regulators, aliphatic alcohols, phenolic compounds and more, varies and for details reference be made to the studies of Parmar et al. (1989, 2003), Sharma and Malik (1994), Verma et al. (2008, 2009). Malik (1995) in his presidential address had detailed plant growth regulators: software for plant development and crop productivity. Further Verma et al. (2008, 2009) had investigated the role of some PGR’s on crop productivity. Several studies earlier too have demonstrated the effect of growth regulators in altering several physiological traits and hence yield (Malik et al., 1990 and 1995). Menon and Srivastava (1984) had emphasized the importance of PGR’s in source and sink relationship leading to enhanced translocation of photo assimilates. Wang et al. (1995) and later Parmar et al. (2003) had demonstrated in their field studies that application of mepiquat chloride

Treatments

Dry weight of root (g plant–1)

Dry weight of lamina (g plant–1)

Dry weight of stem (g plant–1)

Dry weight of reproductive parts (g plant–1)

50 DAS

70 DAS

90 DAS

50 DAS

70 DAS

90 DAS

50 DAS

70 DAS

90 DAS

50 DAS

70 DAS

90 DAS

T1

GA3 @ 50 ppm

0.59

1.33

2.33

7.14

11.33

7.97

7.31

11.45

13.48

1.05

11.82

19.26

T2

GA3 @ 100 ppm

0.61

1.38

2.46

7.62

12.62

8.49

7.96

12.35

14.25

1.35

13.15

19.89

T3

NAA @ 40 ppm

0.54

1.17

2.34

6.88

10.97

7.48

5.85

10.70

12.43

0.91

10.95

16.95

T4

NAA @ 80 ppm

0.55

1.36

2.29

6.62

10.92

7.36

5.59

10.81

12.32

0.82

10.94

16.95

T5

TRIA @ 2.5 ppm

0.70

1.80

2.45

6.46

10.68

6.72

6.94

10.53

12.29

1.15

10.66

16.34

T6

TRIA @ 5.0 ppm

0.68

1.81

2.59

6.51

10.51

7.20

6.89

10.56

12.77

1.19

10.99

16.37

T7

BR@ 10 ppm

0.72

2.10

2.89

6.89

11.14

7.47

7.31

11.36

13.28

1.22

10.59

17.80

T8

BR © 15 ppm

0.76

2.23

3.11

7.09

11.21

7.55

7.16

10.98

13.40

1.17

11.38

18.07

T9

Water spray

0.49

1.03

1.72

5.16

8.55

5.69

5.17

8.98

10.58

0.71

8.53

14.08

T10

Control

0.46

0.97

1.65

5.10

8.47

5.09

5.11

8.97

10.72

0.69

8.16

13.69

S.Em.±

0.04

0.09

0.15

0.34

0.55

0.33

0.35

0.52

0.58

0.05

0.64

0.86

C.D. at 5%

0.11

0.26

0.43

1.00

1.65

0.99

1.04

1.54

1.74

0.16

1.91

2.55

C.V. %

10.30

10.05

10.56

8.92

9.03

8.13

9.34

8.42

8.07

8.84

10.39

8.79

Plant Growth Regulators

Table 11.4. Effect of growth regulators on dry weight of different parts of groundnut cv. GJG-9

333

334

Physiology of the Peanut Plant

decreased partitioning of photo assimilates to the main stem branches but increased the mobilization of assimilates into the reproductive sinks. Sharma and Malik (1994) used three PGR’s to investigate chemical regulation of carbon acquisition in groundnut. Regulation of the plant metabolism by exogenous growth regulating substances offers new possibilities in circumventing environmental limitations, relaxing genetic restrains, improving the quality and aiding production. The effect of foliar application of growth regulating substances, i.e. indole acetic acid (IAA) @ 5 and 7.5 ppm followed by a second spray of Ethrel @ 25 ppm, sequential spray of Mepiquat chloride @ 125 ppm and Mepiquatchloride @ 125 ppm+Ethrel @ 25 ppm was studied in groundnut (Arachis hypogaea L.) cultivars SG99 and M13 under field conditions. Chlorophyll content increased with the foliar application of IAA @ 5 ppm+Ethrel @ 25 ppm in both the cultivars. Chla, Chlb and total chlorophyll enhanced with the Mepiquat chloride @ 125 ppm+Ethrel @ 25 ppm in SG99. Nitrate reductase activity did not very much in SG99 with foliar applications; however, in cultivar M13, it increased in leaves with IAA @ 7.5 ppm+Ethrel 25 ppm. Both the concentrations of IAA increased the nitrate reductase activity in the nodules of SG99. Leghaemoglobin content increased with IAA @ 5 ppm+Ethrel 25 ppm in both the cultivars, whereas Mepiquat chloride @ 125 ppm+Ethrel 25 ppm was effective only in M13 as compared to control. Number of mature/developed pods, immature pods and gynophores increased with the foliar sprays. Sequential application of IAA @ 7.5 ppm+Ethrel 25 ppm increased the pod yield of SG99 and M13 to the tune of 10 and 8 per cent respectively over control. To avoid the wastage of resources to late formed flowers, and to retain flowers which could benefit from availability of sufficient days for completing the seed filling, it was essential to arrest production of new flowers after 60 DAS. Thus in the experiment, the foliar spray was taken up 60 DAS to prevent the flowers formed later from not having a sufficient period to complete seed filling in the crop which has a duration of 110 days. Among the foliar spray treatments, NAA 200 ppm and Ethrel 400 ppm recorded the lowest number of flowers plant-1 (45.68 and 112.65, respectively) after spraying, while control recorded the higher number of flowers (249.36). However, all other plant growth regulators irrespective of the concentration were effective in reducing the number of flowers produced in plant-1, compared to control (Table 11.5). The stage of fruiting subsequent to flowering of peanut plants is called pegging. Pegs naturally produce ethylene during the initial stages of growth. A study was done to test whether an exogenous source of ethylene, Ethrel [(2-chloroethyl) phosphonic acid], could affect growth, pegging, and yield of peanut plants. Spanishtype peanut (Arachis hypogaea L.) plants grown in the greenhouse were sprayed to runoff with different Ethrel concentrations. Dry weight of the shoots was not reduced. Concentrations below 5 × 10-4 M did not inhibit flowering, but all concentrations except 10-5 M reduced production of mature pods and seeds. Three applications of 5 × 10-4 M Ethrel at 2-week intervals beginning 2 and 3 weeks postemergence (PE) caused prolonged inhibition of the onset of flowering, and there was eventual cessation of flowering in treatments beginning at 4 weeks PE. All treatments reduced production of mature pods and seeds. Only a single application of 5 ×10-4 M Ethrel at 6 weeks PE did not reduce yield. In 1976 field trials, Ethrel was applied at 0, 0.14, and 0.28 kg active ingredient/ha in 374 litres of water/ha. Three applications of each rate were made to separate plots at 2-week intervals beginning 2 weeks PE. There was no significant effect on yield or value for ‘Start’, ‘Tanmut 74’, and ‘Florunner’ cultivars. In 1977, two

335

Plant Growth Regulators Table 11.5. Effect of foliar spray with plant growth regulators on flowering characteristics of groundnut var. TMV 7 Treatment

Flowers per plant when sprayed before flowering

Flowers per plant when sprayed after flowering

Ethrel 200 ppm

83.13

172.34

Ethrel 400 ppm

86.52

112.65

CCC 500 ppm

92.36

195.32

CCC 1000 ppm

74.68

199.68

MH 100 ppm

95.43

207.33

MH 200 ppm

103.02

215.66

NAA 100 ppm

78.71

142.00

NAA 200 ppm

84.66

45.68

Pix 500 ppm

89.67

216.34

Pix 1000 ppm

81.34

222.02

Control

90.68

249.36

Mean

87.29

179.85

S.Ed. CD 5%

6.3262 13.1963**

10.7306 22.3838**

separate field trials were conducted with single PE Ethrel treatments of 0, 0.28, and 0.56 kg active ingredient/ha at 4 weeks (early flowering). The 4-week application at both rates reduced yields, but did not affect value. However, treatment at 10 weeks did not affect yields, but reduced value, indicating delayed fruit maturation in addition to inhibition of late flowering. Although flowering of peanut plants was readily regulated by Ethrel treatment, it had either no effect or its effect(s) was deleterious to yield and value of peanuts over a range of concentrations and times of application. Waterlogging is reported to decrease growth and yield of legume crops (Cannell et al., 1979). The extent of injury due to waterlogging depends upon the genotype, environmental conditions, stage of development and the duration of stress (Orchard and Jessop, 1984; Choi et al., 1986). Water logging reduces root and shoot growth, dry matter accumulation and the final crop yield (Minchin et al., 1977; Scott et al., 1989). The deleterious effects of stress are due to anaerobic conditions and impaired respiratory metabolism with the resultant inhibition of water and mineral uptake and deranged hormonal metabolism (Hironand Wright, 1973; Pradet and Bomsel, 1978; Orchard et al., 1985). While the synthesis of gibberellin and cytokinin in the root is reduced (Burrows and Carr, 1969) that of ABA and ethylene is increased (Hiron and Wright, 1973). Therefore, some of the waterlogging symptoms may be partly because of deficiency of endogenous GA3 and cytokinins (Jackson and Campbell, 1979). Application of GA3 significantly increased the number of flowers and pods, pod weight and seed weight. The inhibitory effect of 7 days of water logging was completely alleviated by 100 mg-1 of GA3 given at the vegetative stage. However, the beneficial effect of GA1 was partial when the duration of waterlogging was increased to 14 days. At other stages, both concentrations of GA3 partially alleviated the deleterious effects of stress.

336

Physiology of the Peanut Plant

Many species of Arachis fail to produce seeds after self- or cross-pollination. A primary barrier to seed production is pegging for many genotypes; therefore, the effect of applying GA3 (gibberellic acid) to flowers was investigated. Species of Arachis were treated with 0, 88, 176, or 352 ppm GA3 daily for 30 days and the number of flowers and pegs recorded. The species A. chacoense Krap. et Greg. nom. nud., A. villosa Benth., A. correntina (Burk) Krap. et Greg. nom. nud., A diogoi Hoehne, A. stenosperma Greg. et Greg. nom. nud., and A. sp. coll. GK 30006 had a linear response in peg formation to increased levels of GA3. However, A. sp. coll. GKPSc 30108 had a quadratic response. Arachis cardenasii Krap. et Greg, nom. nud. had a cubic response to GA3 levels. The species A. helodes Mart. ex. Krap. et Rig., A. sp. coll. GK 30008, A. sp. coll. GK 30011, A. sp. coll. GK 30017, A. glabrata Benth and A. hypogaea did not have a significant peg response to the application of GA3. Flowering was suppressed on all species by 352 ppm GA3. Application of either 88 or 176 ppm GA3 resulted in an increased numbers of pegs for all species except A. hypogaea, A. sp. coll. GK 30017 and A. sp. coll. GK 30011. In another experiment, plants of A. chacoense, A. cardenasii, A. villosa, A. helodes, and A. diogoi were treated with 176 ppm GA3 and pegs were allowed to mature but no seeds were recovered. A crossing program using NC 4 in reciprocal with five species resulted in a significant increase in seeds when GA3 applications were applied, but only for hybrid combinations which are normally successful without GA3. Parthenocarpic development is believed to account for an increased numbers of pegs. Because pegging is mandatory before seeds can be obtained in Arachis, applications of GA3 will add significantly toward overcoming a reproductive barrier in Arachis. However, application of additional growth regulators will be necessary to stimulate development of the embryo. Gynophore elongation and pod formation were studied in peanut plants (Arachis hypogaea L.) under light and dark conditions in vivo. The gynophores elongated until pod formation was initiated. Pod (3–20 mm length) development could be totally controlled by alternating dark (switched on) and light (switched off) conditions, repeatedly. Gynophore elongation responded conversely to light/dark conditions, compared to pods. The light/dark effects had been correlated with endogenous growth substances. The levels of endogenous growth substances were determined in the different stages of pod development. Gynophores shortly after penetration into the soil, ‘white’ gynophores, released twice the amount of ethylene as compared to the aerial green ones, or to gynophores bearing pods. Ethylene inhibitors that had no effect on the per cent of gynophores that developed pods, but affected pod size were smaller compared to the control. A similar level of IAA was extracted from gynophore tips of green gynophores, ‘white’ gynophores and pods. ABA levels differed between the three stages and were highest in the green gynophores and lowest in the pods. In peanut, development of pegs involves an influx of auxin corroborated with slow evolution of ethylene (Shlamovitz et al., 1995). Notably, ethylene levels tend to become elevated during peg burying or post-burying, while auxin levels become depleted (Shlamovitz et al., 1995). This phase includes friction with soil, peg reorientation towards the horizontal (a gravitropic movement) and the swelling of pegs and pod enlargement. After this phase, ethylene production declines (Shlamovitz et al., 1995). Further, when the peg bends sideways during early fruit formation (pod), IAA accumulation increases at the lower portion to facilitate its growth as well as upward bending (Moctezuma and Feldman, 1999a; Peng et al., 2013). Ethylene is known to enhance basipetal auxin transport by inducing AUX1 and PIN2 transcripts

Plant Growth Regulators

337

which reduces root gravitropism (Růžička et al., 2007; Negi et al., 2008). Additionally, treating roots with ACC (a precursor for ethylene in its biosynthesis pathway) suppresses root gravitropism through an ETR1 and EIN2-dependent pathway by regulating basipetal auxin transport (Buer et al., 2006). GA and Cytokinin Have Major Effects on Cell Elongation and Division The developing peanut peg also produces a significant amount of GAto promote the cell elongation that ultimately facilitates peg elongation. Afterwards, GAs concentration declines once the peg penetrates the soil and buries (Shushu and Cutter, 1990). It was demonstrated that a combination of GA3 and auxin could restore peg growth in excised pegs as compared to the intact ones (Shushu and Cutter, 1990). However, unlike auxin, which affects only young peg cells, GA3 can promote cell elongation across the entire length of peg (Shushu and Cutter, 1990). In contrast, cytokinin regulates cell division of the juvenile peg structure. During peg development cytokinin accumulates at the early stages to facilitate cell division (Moctezuma, 2003). Interestingly, kinetin (a type of cytokinin)-induced peg elongation in the dark, does not occur in decapitated pegs, suggesting that auxin and cytokinin interact during peg elongation (Shushu and Cutter, 1990). In contrast, application of GA3 restores elongation of kinetin treated decapitated peg, suggesting that GA3 may act by elongating the newly dividing cell, facilitated by kinetin application. Ethylene and Triple Response Phenotype of Peg It is evident that ethylene levels tend to become elevated during peg burying or postburying, and the white peg developed after soil penetration demonstrates the “triple response” phenotype shown by etiolated pea seedlings exposed to higher ethylene: thick and short hypocotyl, radial swelling, and horizontal growth habit (Shaharoona et al., 2007). Ethylene also regulates cell division by regulating a gene encoding microtubule-stabilizing protein WAVE-DAMPENED2-LIKE5 (WDL5), a member of the WAVE-DAMPENED2 (WVD2) protein family, which reorganizes cortical microtubules during cell elongation (Sun et al., 2015). Interestingly, overexpression of WVD2 in Arabidopsis also results in “triple response” phenotype in seedlings (Yuen et al., 2003). In peanut, the role of the microtubule-associated protein has been reported through a combined transcriptome and proteome approach (Zhao C. et al., 2015). Additionally, Shlamovitz et al. (1995) demonstrated that application of the ethylene inhibitors aminooxy acetic acid and silver thiosulphate significantly affect the pod growth without altering the percentage of total pod formation (Shlamovitz et al., 1995). It is, thus, plausible that the peanut peg maintains a lower ethylene level in both the aerial peg and subterranean pod to facilitate cell division and elongation suggesting that a basal level of ethylene might be required to maintain normal cell division and elongation of developing pegs and pod swelling and elongation. ABA Directs the Embryo Growth of Developing Peg: Embryo Development and Abortion Abscisic acid (ABA) controls cell division and elongation of a developing embryo (Da Silva et al., 2008). Therefore, ABA-deficient mutants have an increased seed size and weight due to increased cell numbers in embryo (Cheng et al., 2014). In peanut, the aerial peg exclusively produces a high level of ABA, which progressively declines

338

Physiology of the Peanut Plant

after the peg penetrates the soil and as the pod develops (Shlamovitz et al., 1995). Ziv and Kahana (1988) have evaluated the response of the excised embryo in the dark and found that the embryo development was arrested by the exogenous application of ABA. Therefore, it cannot be ruled out that high levels of ABA found in the aerial peg arrests the cell division of the developing embryo. Therefore, it is possible that in light conditions, the aerial peg maintains a high level of ABA to arrest embryo growth. However, in the dark, a signal is perceived for resuming embryo growth via suppressing ABA levels. Interestingly, the discovery of high levels of ABA also supports the high levels of anthocyanin content of the aerial peg, which decline in the subterranean conditions correlating with low ABA levels because ABA is known to regulate the anthocyanin content (Ferrara et al., 2015). As described earlier, desiccation/loss of soil moisture can induce formation of the aerial pod, which provides another correlation with ABA levels, as ABA production is induced during drought stress (Kumar et al., 2017; Kumar et al., 2018). Further, an increase of ABA beyond a certain level can induce swelling of the root tip in sorghum (Kannan and Shaikh, 1986), suggesting that it is likely to play a similar role in the aerial pod developed in peanut. Together, combining two previous observations we conclude: (a) a high level of ABA can cause tip swelling, and (b) ABA negatively regulates the embryo development, suggesting that a shift in the ABA level in the developing peg is required for embryo maintenance and proper development of the pod or else it can induce aerial pod formation. Recently, a comprehensive analysis of aerial and subterranean pod transcriptome reported three candidate genes, which could be responsible for embryo abortion in the aerial peg (Zhu et al., 2014). Among them, two were putative senescence-associated genes while the third was the late embryogenesis-abundant (LEA) gene all of which were dramatically upregulated in the aerial young pod. Additionally, LEA genes are highly expressed during late embryogenesis in Arabidopsis, wheat and cotton (Battaglia and Covarrubias, 2013), and are required for normal seed development (Manfre et al., 2006; Manfre et al., 2008). Senescence-associated genes are known to be involved in leaf senescence and fruit yield via hormone homeostasis (Gepstein et al., 2003; Kim et al., 2011; Lira et al., 2017). LEA proteins are highly conserved (Gao and Lan, 2016) and provide tolerance against drought/desiccation, heat and salt stress (Shih et al., 2008; Gao and Lan, 2016). Promoters of LEA encoding genes contain abscisic acid response elements (ABREs) and/or low temperature response (LTRE) cis-acting regulatory elements (Hundertmark and Hincha, 2008). Studies that explored the legume genome sequence identified LEA genes in Phaseolus vulgaris, Glycine max, Medicago truncatula: Lotus japonicas, Cajanus cajan, and Cicer arietinum (Varshney et al. 2012; Varshney et al. 2013b), and their role in legumes are anticipated to be the same (Battaglia and Covarrubias, 2013). Basuchaudhuri (1987) noted that foliar application of CCC @ 500 ppm reduced the height of the peanut plants appreciably irrespective of variety, which is important in heavy rainfall areas to reduce the excessive vegetative growth of peanut plants so that more photosynthates are translocated to the developing pods and source sink balance is maintained. With the application of CCC there is an increase in the yield mainly attributed to the increased weight of seeds. The investigation was directed to assess the impact of the foliar spray with plant growth regulators on flowering characteristics of groundnut (Arachis hypogaea L.) var. TMV 7 and their corresponding influence on pod and seed attributes. The plants were

339

Plant Growth Regulators

sprayed with plant growth regulators such as Ethrel, Chloro choline chloride (CCC), Maleic Hydrazide (MH), Naphthalic acetic Acid (NAA) and Mepiquat chloride (PIX) in different concentrations at 60 days after sowing (DAS). The weight of pod and seed yield plant -1 obtained in the plants sprayed with plant growth regulators recorded statistically significant variation. In all the treatments, the number of flowers in the later stages were reduced which reduced the number of immature pods and increased the number of double seeded pods, eventually contributing to higher seed yield than the untreated (control) plants. Among the treatments, the highest pod yield per plant was recorded in NAA 200 ppm (29.33 g) followed by Ethrel 400 ppm (24.88 g) when compared with the control (15.79 g) in which the lowest yield was obtained. These treatments indicated an increasing efficiency of pod filling thereby increasing the seed yield in groundnut (Table 11.6). Table 11.6. Effect of foliar spray with plant growth regulators on the number of pods plant -1 in groundnut var. TMV 7 Treatment (ppm)

Pods plant-1

Mature

Immature

Ethrel 200

42.05

22.65

7.98

3.33

1.89

6.21

Ethrel 400

38.85

25.63

6.21

2.40

1.40

3.20

CCC 500

42.87

20.06

9.14

3.47

2.53

7.67

CCC 1000

43.87

19.89

9.89

3.58

2.65

7.86

MH 100

43.25

18.13

10.11

3.73

3.13

8.12

MH 200

44.11

17.46

11.15

3.87

3.36

8.27

NAA 100

40.88

24.02

6.39

3.01

1.73

5.73

NAA 200

35.98

28.31

3.65

1.82

0.60

1.60

Pix 500

44.58

16.71

11.45

4.16

3.62

8.62

Pix 1000

45.14

16.12

11.87

4.35

3.84

8.96

Control

50.16

14.28

14.69

4.99

4.47

11.73

Mean

42.89

20.30

9.32

3.52

2.66

7.09

SEd CD 5%

2.4719 5.1563**

Double seeded pods plant-1

Single seeded pods plant-1 Mature

Immature

Ill filled pods plant-1

1.7823

0.4245

0.4619

0.1577

0.5914

3.7178**

2.9715**

0.9635**

0.3290**

1.2337**

Foliar spray of 300 ppm TIBA, twice 15 days before first and second flushes, enhanced pod yield due to enhancement in the number of branches, pods and total biomass. TIBA reduced the leaf area but accumulated more nutrients as well as dry matter (Basuchaudhuri et al., 1990).

11.3.1. Drought Experiments were carried out to investigate the physiological (dry weight of root, stem, peg, flowers plant-1, fruit set percent, pod yield (kg plot-1), 100-kernel weight, days to flowering and maturity) and biochemical (endogenous proline level) traits of groundnut cultivar Swat Phalli-96 under drought stress. The result showed that drought stress significantly (P>0.05) effect on various parameters under drought

340

Physiology of the Peanut Plant

stress conditions. Foliar application of ABA (10-4 M) partially reduced the adverse effect of drought stress on growth and yield components. Foliar application of ABA to plants when subsequently exposed to drought stress resulted in elevated levels of endogenous shoot and root proline (Fig. 11.5).

Fig. 11.5. Effect of drought stress and foliar application of ABA on flowers plant-1 of groundnut variety Swat Phalli-96. The bars show ±1 LSD at p < 0.05

The study was carried out to investigate the physiological (relative water content) pod dry weight, pods plant-1, pod yield (kg plot-1), shelling (%), plant height and biochemical (endogenous ABA level) traits of peanut cultivar Swat Phalli-96 under drought stress. The result showed that drought stress significantly (p < 0.05) reduced relative water content (RWC), pod dry weight, pods plant-1, pod yield (kg plot-1), shelling (%) and plant height. GA and IAA applied as seed treatment or foliar spray had no significant (p>0.05) effect on various parameters under drought stress conditions. However, foliar application of ABA (10-4 M) partially ameliorated the adverse effects of drought stress on growth and yield components. Foliar application of ABA to plants when subsequently exposed to drought stress resulted in elevated levels of endogenous ABA. The endogenous ABA levels in shoots increased earlier in response to the applied ABA compared to that of the root (Fig. 11.6). Adrought negatively affects the growth and yield of terrestrial crops. Seed priming, pre-exposing a seed to a compound, could induce improved tolerance and adaptation to stress in germinated plants. To understand the effects and regulatory mechanism of seed priming with brassinosteroid (BR) on peanut plants, seeds were treated with five BR concentrations and examined physiological and biochemical features, and transcriptomic changes in leaves under well-watered and drought conditions. It was found that optimal 0.15 ppm BR priming could reduce inhibitions from drought and increase the yield of peanut, and priming effects are dependent on the stage of

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341

Fig. 11.6. Effect of drought stress and foliar application of ABA on shoot and root endogenous ABA level (μg g-1 fresh weight). The bar show ±1 LSD at p < 0.05.

plant development and duration of drought. BR priming induced fewer differentially expressed genes (DEGs) than in its absence under well-watered conditions. Drought with BR priming reduced the number of DEGs than drought only. These DEGs were enriched in varied gene ontologies and metabolism pathways. Down regulation of DEGs involved in both light perceiving and photosynthesis in leaves is consistent with low parameters of photosynthesis. Optimal BR priming partially rescued the levels of growth promoting auxin and gibberellin which were largely reduced by drought, and increased levels of defence associated abscisic acid and salicylic acid after a long-term drought. BR priming induced many DEGs which function as kinase or transcription factors for signal cascade under drought. It was proposed that BR priming-induced regulatory responses will be memorized and recalled for fast adaptation in later drought stress (Fig. 11.7).

Fig. 11.7. Effects of BR priming on peanut yield components under well-watered and drought conditions

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Physiology of the Peanut Plant

The foliar administration of plant growth regulators and nutrients were found to have a profound impact on increasing the productivity of groundnut. The application of ethephon and mepiquat chloride altered the flower production there by pod yield and improved the partitioning efficiency of the translocating assimilates to the sink organs in groundnut (Table 11.7). Table 11.7. Effect of foliar application of PGRs and nutrients on yield in groundnut Treatment

Pod yield (kg.plot-1)

Pod yield (kg.ha-1)

100 kernel wt. (g)

T1

2.15

1890

32.8

T2

2.52

2313

40.3

T3

2.05

1912

38.3

T4

3.20

2553

44.2

T5

2.61

2389

34.6

T6

2.32

1980

36.2

T7

2.41

2127

38.1

Mean

2.47

2166.29

32.8

SEd

0.0272

13.6373

0.2376

CD 5%

0.0593

29.7135

0.5177

T1 – Control, T2 – Ethephon @ 50 ppm at 25 DAE + Ethephon @ 50 ppm at 60 DAE, T3 – Ethephon @ 50 ppm+Mepiquat chloride @ 125 ppm at 25 DAE+ at 60 DAE, T4 – Ethephon @ 50 ppm at 25 DAE+Mepiquat chloride @ 125 DAE+Ethephon @ 50 ppm at 60 DAE, T5 – T2+ MAP (1%) at 40 DAE, KCl (1%) at 70 DAE, T6 – T3+MAP (1%) at 40 DAE, KCl (1%) at 70 DAE, T7 – Ethephon @ 50 ppm at 60 DAE.

Paclobutrazol (PB), an inhibitor of endogenous gibberellin synthesis, was applied to peanut plants altered dry-matter distribution and increased seed yield. A PB solution at a concentration of 100, 200 or 400 ppm was sprayed on the foliage at the beginning of the pod formation stage (BPFS), the early pod filling stage (EPFS) and the middle pod filling stage (MPFS). The height of the plants treated with PB at BPFS and EPFS was shorter than that of the control plants by more than 10 and 5 cm, respectively. The pod number of plants treated with 100 or 200 ppm PB at any developmental stage was higher than that of those treated with 0 or 400 ppm PB. The seed yield was increased by PB applied at any stage, and the yield after the treatment with 100 or 200 ppm PB at BPFS or EPFS was approximately 370 g m-2. From the studies on the yield components, PB is considered to increase seed yield mainly by increasing the number of pods and seeds. The percentage of early blooming flowers produce mature seeds might be increased by PB treatment (Table 11.8).

11.3.2. Organogenesis Intact peanut (Arachis hypogaea L.) seeds, incubated on media containing N6­ benzylaminopurine (BAP) or thidiazuron (TDZ) exhibited de novo regeneration at the hypo cotyledonary notch region. Regeneration was observed when seeds were cultured on either TDZ or BAP but the optimal level of media supplementation was 10 μmol.L−1 for TDZ and 50 μmol.L−1 for BAP. Light microscopic observations revealed that the regenerants induced by TDZ were somatic embryos while those induced by

343

Plant Growth Regulators Table 11.8. Yield and yield components in peanut Treated stage

Conc. (ppm)

Pod.m-2

Seeds.m-2

Seed wt. (g)

Pod wt. (g.m-2)

Seed wt. (g.m-2)

BPFS

0 ppm 100 ppm 200 ppm 400 ppm

265.4 290.0 274.6 260.0

366.7 441.3 449.2 420.8

0.86 0.85 0.83 0.86

448.7 517.5 515.4 500.4

317.1 372.8 374.4 360.6

EPFS

0 ppm 100 ppm 200 ppm 400 ppm

285.0 308.3 307.9 292.9

387.1 450.0 440.4 389.2

0.80 0.83 0.85 0.85

441.8 536.2 539.5 510.9

308.5 372.5 373.9 333.0

MPFS

0 ppm 100 ppm 200 ppm 400 ppm

269.2 300.8 284.6 267.5

371.7 380.8 376.7 385.0

0.83 0.86 0.88 0.83

437.8 488.8 477.1 452.0

308.0 328.8 330.5 320.6

ns ns ns

* ** ns

** ** ns

** * ns

Significance Conc. (C) Stage (T) C×T

ns ns ns

BPFS: Beginning of pod formation stage. EPFS: Early pod filling stage. MPFS: Middle pod filling stage. *, **: at 5%, 1% level and ns: Not significant.

BAP were shoots. An alternative approach of exposing the seeds to TDZ was through vacuum infiltration followed by culture on basal media but BAP did not induce regeneration by this method. Although TDZ has often been classified as a synthetic cytokinin, results clearly demonstrate that seedlings treated with TDZ undergo a different morphological route of development than that induced by purine cytokinins. The effect of culture temperature on the morphogenetic response of Arachis hypogaea was studied. Cotyledons were cultivated on MS medium supplemented with 110 µM 6-benzyladenine. Leaf explants were cultivated in the presence of the same growth regulator at a concentration of 22 µM. Cultures were incubated at temperatures of 25, 28, and 35±5° C. Both direct organogenesis from cotyledons and development of organo genic calluses from leaves showed optimal rates at 35±5° C. The highest frequency of elongation of buds into shoots from leaf-derived calluses occurred in the presence of 5 µM AgNO3. At the best culture temperature, an average of 95% of shoots formed roots on the growth-regulator-free MS medium. Plants were successfully transferred to soil, showing normal phenotypes. A study was designed to assess the regeneration response of cotyledonary node explant taken from the germinating seeds of peanut (Arachis hypogaea L.). Complete plants were regenerated from in vitro cultured sectioned cotyledonary nodes. Multiple shoots arose on 6-benzylaminopurine (BAP) supplemented Murashige and Skoog medium (1-50 mg L-1), with maximum production occurring at 15 mg L-1 in most varieties. 11 Lower concentrations of BAP proved to be effective in inducing multiple shooting whereas higher concentrations proved to be inhibitory in all the genotypes. Shooting potential of all the genotypes varied under the influence of BAP that might be due to genotypic variations among the varieties. Flowering was observed in PBS24030 genotype on 1 mg L-1 NAA. On 5 mg L-1 BAP, extensive flowering was

344

Physiology of the Peanut Plant

obtained in 11 RG-141 genotypes. Rooting of isolated shoots in four genotypes (HNG­ 10, PBS24030, M-335 and M-13) was observed on MS supplemented with 1 mg L-1 NAA, while a combination of 1 mg L-1 NAA and 0.5 mg L-1 IBA was required for rooting in RG-141. Cytokinins are used in in vitro protocols singly or in combination with auxins to induce cell proliferation and to promote shoot regeneration. A protocol was reported for efficient regeneration of immature leaf explants from groundnut (Arachis hypogaea L.) var. Kadiri-6 and K-134 using a combination of NAA and BAP. A maximum of 90% regeneration with more than 7 shoots per explant was obtained from explants cultured on MS medium with 4 mg/L BAP and 1 mg/L NAA with subsequent substitutions of NAA with AgNO₃ for shoot induction and AgNO₃ by GA3 for elongation of the shoots. The levels of BAP in the culture medium significantly influenced the frequency of regeneration. This protocol of indirect regeneration from the immature leaves may be used in genetic transformation protocols of groundnut with a higher efficiency of recovery of plantlets. In a majority of the regeneration protocols reported in peanut, cotyledons have been used as explant to get direct organo genesis. In callus mediated regeneration from leaf explants, the regeneration frequency reported has been very low (Radhakrishnan et al., 2004). Differences in regeneration frequencies among different explants are due to their differences in the physiological state, endogenous level growth regulators and /or in their response towards growth regulators (Radhakrishnan, 1996; Pawar et al., 2012). Supplementation of chemicals in the culture medium has been found to modulate the level and availability of both exogenous and endogenous growth regulators to the tissues in culture. In order to develop high frequency regeneration protocols in peanut exploiting ethylene modulators, influence off our ethylene modulators viz. ethrel, silver nitrate, cobalt chloride and putrescine were studied on the regeneration behaviour of immature leaves of peanut. Ethrel was found to inhibit the in vitro development. The addition of silver nitrate had enhanced the number of shoot buds regenerated per explant and hence could be used as a regular culture medium additive in peanut. Cobalt chloride was also found to increase the frequency of regeneration in peanut. The variation in the response induced by putrescine was also wider than the other additives. It was concluded that silver nitrate is the best ethylene modulator among the four additives studied and its use in the culture medium at a level 0.01 mM enhanced the regeneration frequency from the immature leaves of the peanut irrespective of the genotype used. In the new method of acclimatization developed by combining the rooting and hardening steps, prolonged growing of the plants on culture media could be avoided. In this method no mortality was observed during the process of acclimatization (Fig. 11.8). Tissue culture is a necessary tool in the genetic modification of peanut for the improvement of its agronomic and nutritional attributes. Since the genotype can affect tissue culture responses, research was undertaken to determine the optimum concentration of auxins and cytokinins in the basal media needed for organogenesis from Arachis hypogaea L. cv. Florman INTA. The first two leaves (2–5 mm in length) were dissected from aseptically germinated seeds and cultivated on Murashige and Skoog (MS) media supplemented with 16 combinations of α-napthalene acetic acid (NAA) (0.01 and 1 mg/l) and benzyladenine (BAP) or kinetin (KIN) (1 to 10 mg/l) during the initiation stage. Bud regeneration occurred in all growth regulator combinations, but the maximum number of buds per explant (1.2) was regenerated at 1 mg/l NAA with 3 mg/l BAP. Development of buds into shoots was readily

Plant Growth Regulators

345

Fig. 11.8. (a) Callus with shoot buds induced from the immature leaves of peanut in the culture initiation medium containing 3 mg/L BAP and 1 mg/L NAA. (b) Shoot buds regenerating on callusing explants sub-cultured in a medium containing the ethylene modulator silver nitrate as additive. (c) The shoot buds opened up and elongated in subcultures with a medium containing GA3 and ready for root induction. (d) The elongated shoots transferred to the Hoagland’s solution containing 1 mg/L NAA for induction of roots. (e) The plantlets hardened in Hoagland’s medium showing the development of secondary roots also

Fig. 11.9. Effect of NAA and BAP concentrations on elongation of shoots after 30 days: right axis: probability of success; left axis: growth in height (mean 6 SD) and tendency of the means (dotted line). Bars with different letters are significantly different (P>0.05)

346

Physiology of the Peanut Plant

achieved by transferring regenerated buds onto a fresh medium containing 0.01 mg/l NAA (without BAP). Roots were induced to grow when shoots were transferred to a medium containing 3 mg/l of NAA. The vigorous root system allowed for a high survival rate of the plantlets post transplantation. The overall efficiency of the system was 15%. Plants transplanted into the soil were completely normal and capable of producing seeds (Fig. 11.9).

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CHAPTER

12

Abiotic Stresses Peanut is an allotetraploid (AABB, 2n = 4x = 40) originated from a single hybridization event between Arachis duranensis (AA genome) and Arachis ipaensis (BB genome), and subsequently underwent spontaneous genome duplication (Bertioli et al., 2016; Kenta et al., 2013). Additionally, peanuts often cultivated in the semiarid tropical regions, are often exposed to water stress (mid-season and end-season) and high temperature (> 34°C) during the critical stages of flowering and pod development (Musingo et al., 1989). An attempt was made to apply low-salinity fields to peanut cultivation to gain more crop production in China in recent years (Cui et al., 2018; Singh et al., 2008), which made peanuts suffer salt stress. Due to the still incomplete assembly of the genome, a number of initially identified loci had incorrectly assigned junctions. These assemblies were then corrected by aligning the genomic sequences to the corresponding transcripts by focused reverse transcription (RT)-PCR analyses with total peanut RNA. A total of 40 genes encoding the core components of the SUMO pathway were identified from two wild peanut species, and the gene structure was analyzed. The list showed that each of the AA and BB genomes contain four SUMO genes, designated as AdSUMO1 to AdSUMO4 and AiSUMO1 to AiSUMO4, respectively. The SUMO genes which have the homologous loci between AA and BB genomes had consistent amino acid sequences. To further understand the evolutionary relationship between these SUMO isoforms, a phylogenetic tree was constructed together with homologues from other plant genomes. The phylogenetic analysis revealed a highly conserved SUMO group called as canonical group including AdSUMO1/2/3, AiSUMO1/2/3, GmSUMO1/2/3, AtSUMO1/2, ZmSUMO1a/b and OsSUMO1/2. In contrast, the “noncanonical” group included AdSUMO4, AiSUMO4, GmSUMO4/5/6, AtSUMO3/5 and shared its lower amino acid identity to the canonical group. Amino acid sequence alignment found that non-canonical members also had the C-terminal di-Gly motif necessary for conjugation, which indicated their potential ability of covalently attaching to the target proteins. As the SUMO and SCE1 genes show higher levels of expression during the peanut pod developmental stage based on RNA-seq data, it was detected SUMO conjugates in two stages of pegs (aerial peg and subterranean no-swelling peg) and five distinct stages of pod development. Compared to the aerial pegs, the SUMO conjugates showed reduction in the subterranean no-swelling pegs. Also, several signature conjugates were not found in the subterranean no-swelling pegs. After the soil penetration of the peg, the amount of SUMO conjugates sharply decreased, which might have resulted from the darkness and mechanical stimuli. During pod development, the SUMO conjugate profiles showed a gradual rise during seed expansion stages, then decreased

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Physiology of the Peanut Plant

Fig. 12.1. SUMOylation profiles among peanut pod development. Total protein extracts were subjected to immune blot analysis with anti-AtSUMO1 from various tissues including aerial pegs, subterranean no-swelling pegs and pods of five distinct developmental stages. SUMO conjugates are highlighted by the brackets. The arrowheads highlight specific SUMO conjugates, Ponceau S stained with high abundant protein showed equal loading of protein samples. The asterisk denotes a non-inducible immune reactive product, subterranean.

at pod maturation (Fig. 12.1). These results suggested that SUMOylation plays an active role in promoting pod development. Few germplasm accessions of cultivated groundnut were identified as tolerant to salt stress conditions (Nautiyal et al., 2000). Such lines are being used directly in crop improvement programs globally (Mungala et al., 2008). Linkage drag and minor quantitative trait loci (QTLs) were identified in peanut for improved water use efficiency (Holbrook et al., 2011). However, the polygenic nature of drought and salinity stresses restricts the conventional and molecular methods of breeding to develop tolerant varieties in peanut (Venkatesh et al., 2014). The constraints include the low genetic diversity, highly conserved genome, cross incompatibilities, limited range of polymorphism, genetic isolation of the tetraploid peanut from its wild diploid ancestors, difficulty in foreground and background selections as pointed out by Varshney et al. (2005), Feng et al. (2012) and Janila et al. (2013). Conventional methods of plant breeding for developing drought tolerant varieties are laborious, time-consuming and yet the final yields are limited introgression from the wild-type into the cultivated varieties will be difficult due to incompatibility barriers and laborious backcrossing methods. Therefore, utilization of newer technologies such as genetic engineering will be pivotal for increasing food production and the sustainability of peanut yields

353

Abiotic Stresses

under challenging environments. Initially, attempts have been made to regenerate and genetically transform peanut varieties in vitro using either β-glucuronidase or hygromycin resistant marker genes (Ozias-Akins et al., 1993; Eapen and George, 1994; Livingstone and Birch, 1995). Either Agrobacterium-mediated or biolistic methods have been used to generate transgenics but with a low frequency (0.3–10%) of transformation. Sharma and Anjaiah (2000) developed a producible transformation protocol with 55% frequency of transformation. Peanut cotyledon and mesocotyl Table 12.1. List of genes used for developing transgenic peanut Name of gene

Function

Promoter used

Peanut variety/ cultivar

References

DREB1A

Water-limited conditions (drought avoidance)

rd29A

JL-24

Bhatnagar­ Mathur et al. (2004)

DREB1A

Water-limiting conditions (transpiration efficiency)

rd29A

JL-24

Bhatnagar­ Mathur et al. (2007)

AtNHXl

Drought tolerance

CaMV35S

Golden and BARI-2000

Asif et al. (2011)

IPT

Drought tolerance in the lab and field

Senescence associated receptor protein kinase [P(SARK)]

New Mexico Valencia A

Qin et al. (2011)

CBF3

Drought stress tolerance

rd29A

Smruti

Rana and Mohanty (2012)

AtNHXl

Salt stress tolerance

CaMV35S

Flavor runner458

Banjara et al. (2012)

AVP1

Drought and salt stress

CaMV35S

New Mexico Valencia A

Qin et al. (2013)

mtlD

Drought tolerance

CaMV35S

GG-20

Bhauso et al. (2014)

AtDREBlA

Drought/salinity tolerance

CaMV35S

GG-20

Sarkar et al. (2014)

AtDREB2A, AtHB7, AtABF3

Drought tolerance

CaMV35S

TMV-2

Pruthvi et al. (2014)

SbNHXLP

Salt stress tolerance

CaMV35S

JL-24

Venkatesh (2016)

SbVPPase

Drought and salt stress

CaMV35S

JL-24

Amareshwari (2017)

AtHDGll

Drought and salt stress

rd29A

JL-24

Banavath et al. (2018)

MuWRKY3

Drought stress tolerance

CaMV35S

-

Kiranmai et al. (2018)

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Physiology of the Peanut Plant

explants of four genotypes were used for genetic transformation studies using the hpt and GUS genes by Agrobacterium tumefaciens-mediated method and their phenotypic and genotypic inheritance studies showed Mendelian inheritance patterns (Chen et al., 2015). A new cost-effective and rapid peanut transformation protocol was standardized using silicon carbide whiskers with callus explants with a transformation efficiency of 6.88% (Hassan et al., 2016). Since then, attempts have been made to transfer genes into many varieties of peanut like Valencia, and Runner types for drought and salt stress tolerance using Agrobacterium as a vehicle. However, transformation frequency in peanut is still considered as low and often, the protocol cannot be extended to all the cultivars (Table 12.1).

12.1. Drought In the tropical and subtropical regions, peanut is cultivated under rain-fed conditions by resource poor farmers where it is exposed to abiotic stresses such as drought and salinity (Bhauso et al., 2014; Sarkar et al., 2014). A major part of global peanut cultivation takes place in areas exposed to water shortage. Over six million tons annual loss of productivity of this important oil-seed crop occur due to drought (BhatnagarMathur et al., 2013; Gautami et al., 2011). Drought mediated increase in top soil hardness can restrict or delay peg penetration into the soil and therefore restrict the pegging stage resulting in limited pod set and subsequent seed numbers (Haro et al., 2010, 2011). However, in contrast, if the drought affected pegs are re-watered, viable pegs starts penetrating the soil again (Haro et al., 2008). The resumption of pegging upon re-watering is an adaptive trait in peanut plants that ensures long-term survival of the fertilized embryo compared with other grain-crops like maize and soybean where embryo viability is rapidly lost (Westgate and Boyer, 1986). Overall, drought is the major abiotic stress-limiting peanut crop yield (Boote et al., 1976; Haro et al., 2008). Furthermore, owing to a lack of genotypic variability for better water use efficiency among peanut plants (Gautami et al., 2011), the development of drought-tolerant peanut genotypes is a major focus of plant breeders (Sarkar et al., 2016). Stressed plants have lower RWC than non-stressed plants. For example, relative water content of non-stressed plants range from 85 to 90%, while in drought stressed plants; it may be as low as 30% (Babu and Rao, 1983). A number of studies have reported significant genotypic variability in maintenance of leaf water potential and stomatal conductance under diverse water availability situations (Gautreau, 1977; Clavel et al., 2004; Nautiyal et al., 2008). Clavel et al. (2004) identified two different plant–water relation strategies to cope with water deficit in groundnut. The first strategy was characterized by delayed stomatal closure and low cell membrane damage during drought. Genotypes with these characters had ability to maintain high water uptake even under water deficit conditions. In the second strategy, genotypes maintained higher RWC by early stomatal closure. However, genotypes with these two divergent strategies produced similar yield. Groundnut crop under semiarid environments can experience stomatal closure very frequently (usually during mid day) when saturation vapour pressure deficit can often exceed 3 kPa and thus potentially limiting period of active photosynthesis during the growing cycle. Prolonged water deficits coupled with high temperatures can progressively reduce the duration of active gas exchange through stomata affecting plant growth and developmental processes. However, the rapid and complete recovery

355

Abiotic Stresses

to normal stomatal conductance after severe stress following renewal of water supply has been widely reported in groundnut (Puangbuti et al., 2009; Devries et al., 1989). This capacity to recover rapidly to normal transpiration and CO2 assimilation represents an important mechanism underpinning the adaptive response of groundnut. Growth and nodulation as well as some physiological and biochemical stress indicators in response to drought stress and subsequent rehydration in the symbiotic association peanut-Bradyrhizobium sp. SEMIA6144 revealed followings. Drought stress affected peanut growth reducing shoot dry weight, nodule number, and dry weight as well as nitrogen content, but root dry weight increased reaching a major exploratory surface. Besides, this severe water stress induced hydrogen peroxide production associated with lipid and protein damage; however, the plant was able to increase soluble sugar and abscisic acid contents as avoidance strategies to cope with drought stress. These physiological and biochemical parameters were completely reversed upon rehydration, in a short period of time, in the symbiotic association peanut-Bradyrhizobium sp. (Table 12.2). Table 12.2. Influence of drought stress and rehydration on peanut nodulation and nitrogen content Treatment

Nodule No.

Nodule DW (mg)

NNW

SNC (mg.pl-1)

Control

41.85±2.92

22.24±2.62

0.018±0.003

23.52±2.84a

Drought stress

29.15±1.50b

15.83±1.08b

0.019±0.001a

10.14±1.88b

Rehydration

33.57±2.14

16.65±1.37

0.023±0.003

10.42±0.43b

a

b

a

b

a

a

NNW: Normalized nodule weight; SNC: Shoot nitrogen content. Values are means ± S.E. (n=12). Different letters in each column indicate significant differences at P < 0.05 according to LSD Fisher’s test.

A number of studies have reported that drought severely reduced nitrogen fixation (Serraj et al., 1999; Reddy et al., 2003; Pimratch et al., 2008). However, comparative studies of groundnuts with other legume species subjected to drought, nitrogen fixation of groundnut was found to be relatively insensitive (DeVries et al., 1989; Venkateswarlu et al., 1989; Sinclair and Serraj, 1995). Several mechanisms have been proposed to describe effects of drought on N fixation by legumes: (a) reduced carbon supply, (b) shortage of oxygen, (c) feedback regulation of nitrogen accumulation, and (d) oxidative stress damage at cellular level. These processes individually or collectively affect the nitrogenase activity in the nodule (Serraj et al., 1999; Marino et al., 2007). Nitrogenase activity has been shown to be severely reduced as leaf and nodule water potentials decreased below –1.4 MPa (De vries et al., 1989). In a recent study by Padmavathy and Rao (2013) demonstrated that drought tolerant groundnut genotype ICGV 91114 accumulated the high concentrations of total soluble sugars and reducing sugars up to 15 days of waters deficit conditions, indicating their role as typical osmo protectants in stabilizing the cellular membranes and maintaining their turgor thus providing tolerance against drought stress up to 15 days (Fig. 12.2). Drought can significantly limit yield and quality in peanut (Arachis hypogaea L.), depending on its timing, duration and severity. The objective of this study was to identify potential molecular mechanism(s) utilizing a candidate-gene approach in five peanut genotypes with contrasting drought responses. An early season drought stress

356

Physiology of the Peanut Plant

Fig. 12.2. Soluble sugar content in leaves and nodules of peanut plants exposed to drought stress and rehydration. Values are means ± SE (n=10). Different letters indicate significant differences at P < 0.05 according to LSD Fisher’s test

treatment was applied under environmentally controlled rain-out shelters. When water was completely withheld for three weeks, no physical differences were observed for treated plants compared with their fully irrigated counterparts as indicated by relative water content; however, yield, grades (total sound mature kernel, TSMK), specific leaf area, and leaf dry matter content showed significant differences. Comparing expression levels of candidate genes, ‘C76–16’ exhibited significantly higher levels for Cu Zn SOD, NsLTP and drought protein one week earlier compared to the other genotypes, followed by significantly lower levels for the same genes. This suggested an early recognition of drought in C76–16 followed by an acclimation response. Cultivar ‘Georgia Green’ showed different patterns of gene-expression than C76–16. AP-3, a susceptible genotype, showed generally lower levels of gene-expression than C76–16 and Georgia Green. Myo-inositol phosphate synthase gene-expression showed high levels in irrigated treatment, ranging from 4-fold for 08T-12 to 12-fold for Georgia Green, but were significantly inhibited in drought treatment after two weeks of drought and after recovery. Very recently to get an insights into the drought induced changes in antioxidative mechanism of groundnut, Akcay et al. (2010) used two contrasting groundnut genotypes cv. Florispan (tolerant) and cv. Gazipasa (sensitive) and two different concentrations of polyethylene glycol (PEG) to imitate different levels of water stress. The results showed, two different cultivars respond in slightly different ways to the imposed stress in terms of individual components. In both cultivars, POX activity appeared not to be directly responsible from remarkable protection against oxidative injury, whereas APX activity might be important for Gazipasa, which is possibly equipped with mechanisms that only provide protection during mild drought conditions. Florispan appeared to be more resistant in terms of physiological and anatomical parameters and can be considered as a more tolerant cultivar. Unlike Gazipasa, it revealed significant increases in proline, CAT and APX levels on higher magnitudes of stress.

Abiotic Stresses

357

Peanut genotypes from the US mini-core collection were analyzed for changes in leaf proteins during reproductive stage growth under water-deficit stress. Oneand two-dimensional gel electrophoresis (1- and 2-DGE) was performed on soluble protein extracts of selected tolerant and susceptible genotypes. A total of 102 protein bands/spots were analyzed by matrix-assisted laser desorption/ionization–time-of­ flight mass spectrometry (MALDI–TOF MS) and by quadrupole time-of-flight tandem mass spectrometry (Q-TOF MS/MS) analysis. Forty-nine non-redundant proteins were identified, implicating a variety of stress response mechanisms in peanut. Lipoxygenase and 1L-myo-inositol-1-phosphate synthase, which aid in inter- and intracellular stress signalling, were more abundant intolerant genotypes under waterdeficit stress. Acetyl-CoA carboxylase, a key enzyme of lipid biosynthesis, increased in relative abundance along with a corresponding increase in epicuticular wax content in the tolerant genotype, suggesting an additional mechanism for water conservation and stress tolerance. Additionally, there was a marked decrease in the abundance of several photosynthetic proteins in the tolerant genotype, along with a concomitant decrease in net photosynthesis in response to water-deficit stress. Differential regulation of leaf proteins involved in a variety of cellular functions (e.g. cell wall strengthening, signal transduction, energy metabolism, cellular detoxification and gene regulation) indicates that these molecules could affect the molecular mechanism of water-deficit stress tolerance in peanut (Fig. 12.3).

Fig. 12.3. Functional categorization of proteins detected from both one-dimensional (1-D) and two-dimensional (2-D) gels and identified by tandem mass spectrometry (MS/MS), matrixassisted laser desorption/ionization (MALDI) and quadrupole time-of-flight (Q-TOF) analyses

Qin et al. (2011) introduced a cytokinin biosynthetic gene IPT (iso pentenyl transferase) under the control of a maturation- and stress-induced promoter into groundnut. The PSARK : IPT-transgenic groundnut plants performed much better than wild-type plants under reduced irrigation conditions in greenhouse, growth chamber, and field conditions. Transgenic plants produced much larger root systems under reduced irrigation in greenhouse conditions, which allowed them to use water more efficiently. They also maintained higher photosynthetic rates and stomatal conductance, produced significantly more biomass under reduced irrigation conditions in greenhouse and field conditions. The yields of PSARK : IPT-transgenic groundnuts

358

Physiology of the Peanut Plant

plants were 30-35% higher than that of wild-type plants based on two years of field data. Furthermore, PSARK : IPT-transgenic peanuts plants appeared to produce larger and more multiple- seed pods than wild-type groundnut plants do (Qin et al., 2011). Same group generated the transgenic plants overexpressing a Arabidopsis gene AVP1 encoding an H+-pyrophosphatase. Transgenic groundnut plants showed more biomass and maintained higher photosynthetic rates under drought and salt stress conditions in greenhouse and growth chamber conditions. In the field, AVP1-overexpressing groundnuts also out performed wild-type plants by having higher photosynthetic rates and producing higher yields under low irrigation conditions (Qin et al., 2013). Drought negatively affects the growth and yield of terrestrial crops. Seed priming, pre-exposing seed to a compound, could induce improved tolerance and adaptation to stress in germinated plants. To understand the effects and regulatory mechanism of seed priming with brassinosteroid (BR) on peanut plants, seeds treated with five BR concentrations and examined dozens of physiological and biochemical features, and transcriptomic changes in leaves under well-watered and drought conditions. It was found optimal at 0.15 ppm BR priming could reduce inhibitions from drought and increase the yield of peanut, and priming effects are dependent on stage of plant development and duration of drought. BR priming induced fewer differentially expressed genes (DEGs) than no BR priming under well-watered condition. Drought with BR priming reduced the number of DEGs than drought only. These DEGs were enriched in varied gene ontologies and metabolism pathways. Down regulation of DEGs involved in both light perceiving and photosynthesis in leaves is consistent with low parameters of photosynthesis. Optimal BR priming partially rescued the levels of growth promoting auxin and gibberellin which were largely reduced by drought, and increased levels of defence associated abscisic acid and salicylic acid after long-term drought. BR priming induced many DEGs which function as kinase or transcription factor for signal cascade under drought. It was proposed that BR priming-induced regulatory responses will be memorized and recalled for fast adaptation in later drought stress. These results provide physiological and regulatory bases of effects of seed priming with BR, which can help to guide the framing improvement under drought stress (Fig. 12.4). A multi-seasonal phenotypic analysis of 10 peanut genotypes revealed C76­ 16 (C-76) and Valencia-C (Val-C) as the best and poor performers under deficit irrigation (DI) in West Texas, respectively. In order to decipher transcriptome changes under DI, RNA-seq was performed in C-76 and Val-C. Approximately 369 million raw reads were generated from 12 different libraries of two genotypes subjected to fully irrigated (FI) and DI conditions, of which ~329 million (90.2%) filtered reads were mapped to the diploid ancestors of peanut. The transcriptome analysis detected 4,508 differentially expressed genes (DEGs), 1554 genes encoding transcription factors (TFs) and a total of 514 single nucleotide polymorphisms (SNPs) among the identified DEGs. The comparative analysis between the two genotypes revealed higher and integral tolerance in C-76 through activation of key genes involved in ABA and sucrose metabolic pathways. Interestingly, one SNP from the gene coding F-box protein (Araip.3WN1Q) and another SNP from gene coding for the lipid transfer protein (Aradu.03ENG) showed polymorphism in selected contrasting genotypes. These SNPs after further validation may be useful for performing early generation selection for selecting drought-responsive genotypes (Fig. 12.5).

Abiotic Stresses

359

Fig. 12.4. The proposed mechanism of BR priming inducing drought tolerance. The diagram shows the BR priming increases plant growth, photosynthesis, and defense via gene regulation, and the response could be memorized. Once the drought stress occurs, the memorized regulation could be recalled in a fast-adaptive way, which rescues the adverse effects of drought stress. Finally, the seed priming with BR results in an improved drought tolerance in peanut plants. BRI: Brassinosteroid Insensitive 1; BZR: Brassinazole Resistant 1; PIFs: Phytochrome Interacting Factors; FLS2: Flagellin Sensitive 2; ABA: Abscisic Acid; SA: Salicylic Acid; GA: Gibberellins

Fig. 12.5. An overview of differentially expressed genes in drought stress-responsive pathway mechanism. Heat maps demonstrating the expression profiles of differentially expressed drought stress-responsive genes during fully irrigated (FI) and deficit irrigation (DI). Dip black colour scale represents normalized FPKM values. Corresponding reference gene ids (with suffix XLOC) falling have been given on the right side. The colour scale on the top represents normalized FPKM values

360

Physiology of the Peanut Plant

12.2. Salinity Salt in soil or water is one of the major stresses which can severely limit plant growth and productivity. Salt stress impairs plant growth by reduction in yield, leaf area, fruit size and weight, photosynthesis and finally leads to necrosis and death. A better understanding of the mechanism, by which, plants respond to salinity stress may be crucial in developing more tolerant varieties. The property of salinity tolerance is not a simple attribute, but it is an outcome of various features that depend on different physiological interactions, which are difficult to determine. The morphological appearance presented by the plant in response to salinity, may not be enough to determine its effect, so it is important to recognize other physiological and biochemical factors. Plants develop various physiological and biochemical mechanisms in order to survive in soils with high salt concentration. Principle mechanisms include, but are not limited to, (1) ion homeostasis and compartmentalization, (2) ion transport and uptake, (3) biosynthesis of osmo protectants and compatible solutes, (4) activation of antioxidant enzyme and synthesis of antioxidant compounds, (5) synthesis of polyamines, (6) generation of Nitric Oxide (NO), and (7) hormone modulation. Like other legumes, peanut plants are susceptible to soil salinity (Greenway and Munns, 1980) and genotypic difference for salinity tolerance exists within the species (Sun et al., 2013). Peanut plants are glycophytes indicating their vulnerability. In this context, marked salinity-mediated reduction in the peanut pod production has been reported (Meena et al., 2016). Salinity can reduce peanut seed germination, seedling establishment, and the dry weight of plants (Meena et al., 2016; Parida and Jha, 2013). Moreover, salinity induced disruption in the photosynthetic apparatus and disturbance in nutrient uptake are believed to contribute to peanut yield losses (Qin et al., 2011a). Salinity induced downregulation of genes associated with photosynthetic light harvesting complex proteins and phenylalanine metabolism, and subsequent production of terpenoids, phenylalanine, tyrosine, and plant hormones has been reported (Chen et al., 2016). At the molecular level, salinity induced downregulation of 36 peanut genes along with upregulation of seven genes associated with the ROS network after 48 hr of salinity treatment has been reported (Chen et al., 2016). Efforts are required to develop salt tolerant peanut genotypes using transgenic approaches (Chen et al., 2010) so as to increase the yield of cultivated peanut. The use of good quality seed and salinity management practices could contribute to improvement in peanut yield under salinity stress (Meena et al., 2016). To evaluate the emergence, growth, biomass accumulation and tolerance of peanut genotypes under salt stress the experiment was conducted in a protected environment (greenhouse), evaluating six peanut genotypes (Tatuí, L7151, Caiapó, IAC8112, IAC881 and Havana), which were subjected to two levels of irrigation water salinity (0.5 [control] and 3.5 dS m-1), arranged in a 6 × 2 factorial scheme, in a randomized block design, with five repetitions, with two plants per plot. Plants were cultivated for 30 days after sowing in lysimeters with capacity for 0.5 dm3, filled with a mixture of non-saline, non-sodic soil and commercial substrate in 1:1 proportion on volume basis. During this period, plants were evaluated for emergence, growth, biomass accumulation, tolerance to salinity and dissimilarity. The genotypes Tatuí and L7151 are the most sensitive to salt stress in the emergence stage. Irrigation with high-salinity

Abiotic Stresses

361

water reduced the growth and biomass accumulation of the peanut genotypes, and Caiapó and IAC8112 were the least affected. The classification of salinity tolerance had the following sequence: Caiapó > IAC8112 > Havana > Tatuí > IAC881 > L7151 (Fig. 12.6).

Fig. 12.6. Salinity tolerance index – STI (D) of peanut genotypes subjected to irrigation with saline water (S1 = 0.5 and S2 = 3.0 dS m-1) at 30 days after sowing

Peanuts an important edible oilseed crop in the world. Salinity is one of the important abiotic stresses affecting peanut productivity by hampering germination, arresting vegetative and reproductive growth and affecting seed quality. Ten lines developed through introgressive hybridization along with 23 popular varieties were screened in vitro for their germinability under high salt concentration (250 mM NaCl) in test tube. Interspecific derivatives NRCGCS-296 (J11 × A. duranensis) and NRCGCS-241 (GG 2 × A. cardenasii) had high germination stress tolerance index (GSTI) and promptness index (PI) and were regarded as tolerant whereas susceptible genotype (TMV-2) had low GSTI and PI. Tolerant, moderately tolerant and susceptible genotypes where evaluated further for biomass accumulation and salt uptake. Root length, shoot length, chlorophyll a, b and carotenoid contents reduced under high salt conditions. Transportation of the absorbed Na+ ions from roots to leaves was more in susceptible plant (TMV-2) compared to tolerant genotypes (NRCGCS-241 and NRCCS-296). These four genotypes were further screened with gene specific primers (Na+/H+ antiporter, NAC, WRKY and PR10) synthesized from the sequences available in the NCBI database. WRKY and Na+/H+ antiporter gene specific primers discriminated tolerant and susceptible genotypes and amplified about 350 bp and 1000 bp amplicons, respectively in NRCGCS-241 which were absent in rest of the three genotypes. Besides, Na+/H+ primer amplified a separate amplicon of about 900 bp in all genotypes except NRCGCS-296. Rest two primers were monomorphic among these genotypes and did not differentiate these four genotypes. WRKY and Na+/H+ genes might be responsible for imparting tolerance to salinity stress in peanut (Fig. 12.7). As a glycophytic plant, groundnut growth is very sensitive to salt. To improve the yield and quality of groundnut under high salt conditions, AtNHX1 was introduced under the control of a constitutive promoter into groundnut plants (Banjara et al.,

362

Physiology of the Peanut Plant

Fig. 12.7. Effect of NaCl induced salt stress on Na/K ratio in root and shoot of four peanut genotypes. Each value is the mean of ten replicates

2012). AtNHX1 that encodes the vacuolar membrane-bound sodium/proton (Na+/H+) antiporter in Arabidopsis could 377 improve salt tolerance in transgenic plants (Apse et al., 1999). The AtNHX1-overexpressing groundnut plants displayed increased tolerance of salt at levels up to 150 mM NaCl. When compared to wild-type plants, AtNHX1 transgenic groundnut plants suffered less damage, produced more biomass, contained more chlorophyll, and maintained higher photosynthetic rates under salt conditions (Banjara et al., 2012). Groundnut yields have been reported to be severely affected with an increase in soil and water salinity (Girdhar et al., 2005; Nithila et al., 2013). It can affect seed germination and inhibit root and shoot growth. It interferes with water absorption leading to osmotic stress; it enhances accumulation of Na+ and Cl- ions which at higher concentration may lead to cytotoxicity, impaired enzymatic function and imbalance of other elements. The cellular metabolism, biochemical as well as photosynthetic activities are all adversely affected by salt stress (Abogadallah, 2010; Cokkizgin, 2012). One of the most effective ways to overcome salinity problems is the introduction of salt-tolerant crops. It has been reported that differences in salt tolerance exist, not only among different species, but also within certain species (Murillo-Amador et al., 2001). Success of selection depends upon the amount of genetic variation present in the population. Evidence collected from various species suggests that salt tolerance is a developmentally regulated, stage-specific phenomenon, so that tolerance at one stage of development may not be correlated with tolerance at other developmental stages (Shannon, 1986). The effects of salt stress on the level of osmolyte accumulation in two different cultivars (K-134 and JL-24) of groundnut seedlings were studied. Seeds were grown at different concentrations of NaCl stress: 50, 100 and 150 mM and their respective controls (0.0% NaCl) for nine days. Salt stress resulted in a significant modification in the level of osmolyte accumulation in the two cultivars of groundnut.

Abiotic Stresses

363

The accumulation level of osmolytes such as proline, glycine betaine, soluble sugars, free amino acids and polyamines were increased significantly in both cultivars with increasing stress severity and duration when compared with their controls. However, the per cent increase of osmolyte accumulation was higher in cv. K-134 and lower in cv. JL-24. The study indicated that cv. K-134 is salt tolerant than cv. JL-24 based on osmolyte accumulation and growth parameters (Fig. 12.8). In the laboratory screening study, ten varieties were subjected to three levels of salinity stress, viz., 50 mM, 100 mM, 125 mM NaCl and three levels of sodicity stress viz., 25 mM, 50 mM and 75 mM NaHCO3. Based on mean stress tolerance index (STI) TMV7, CO5, JL24 and BSR1 recorded lesser STI of 13 to 19 under high

Fig. 12.8. Metabolite changes in leaves of peanuts under varying salinity

364

Physiology of the Peanut Plant

salinity and 15 to 24 under high sodicity levels. Therefore, by rejecting these four varieties, the other six varieties were further evaluated under pot culture condition were subjected to two levels of salinity stress (50 mM and 100 mM NaCl) and two levels of sodicity stress (25 mM and 50 mM NaHCO3). The groundnut variety CO4, identified as tolerant variety and ALR3, as susceptible variety, through pot culture experiment. To assess the influence of different plant growth regulating chemicals on alleviating the adverse effects of sodicity stress, field study was conducted using CO4 and ALR3 under sodic soil condition. Brassinolide 1 ppm sprayed at pre flowering, pegging and pod formation stages was highly effective in overcoming the adverse effects of salt stress through enhancing the overall physiological efficiency of the crop and in improving the pod yield even in the varieties sensitive to salt stress (Nithila et al., 2013). A laboratory experiment was carried out to evaluate 26 groundnut genotypes for salt tolerance as well as to assess physiological basis for salt tolerance. Twenty seeds of each genotype were allowed to germinate for 72 hours at 28±1°C and then transferred to plastic beakers containing neutral sand. Seven days old seedlings were then subjected to different treatments viz. control (plants receiving Hoagland nutrient solution adjusted to pH 6.3) and salinity treatment (plants receiving Hoagland nutrient solution plus 200 mM NaCl adjusted to pH 6.3). The treatments were repeated on every third day and data was collected on 35-day old plants for different growth parameters. The tolerance index (expressed as Stress Responsive Index for total plant dry weight) of the genotypes had ranged from 47.57% to 96.40%. Out of all the genotypes KDG-197 (TI 96.40%) was found to be the most tolerant under a salinity followed by R2001-2 (TI 87.92%), VG-315 (TI 84.05%), TCGS1157 (77.59%) and TG 51 (73.67%) (Fig. 12.9). While the genotypes Girnar 3 (TI 47.57%), OG52-1 (TI 49.09%), TVG-0856 (TI 49.28%) and J86 (TI 50.66%) were the most susceptible genotypes based on their relative performance under stress in respect of total dry weight. The extent of reduction in sugar content in the five tolerant genotypes varied from 2.70%-49.82% over that of control while it was 51.83-70.32% for the four susceptible genotypes. Increase in leaf proline content over control was recorded from tolerant and susceptible genotypes were 531.52-780.16% and 76.14-449.43% respectively. The extent of increase in protein content in the tolerant genotypes ranged from 34.25-144.01% over control. Among all the genotypes, Girnar 3 registered maximum increase (31.48%) in Electrolytic leakage (EL) of the leaf under stress (Table 12.3 and Fig. 12.9). The obtained results cleared that, all studied characters of growth, total water content, free water content, relative water content, leaf water potential, photosynthetic pigments, total carbohydrates and nonsoluble carbohydrates showed a significant decrease by increasing salinity levels. Moreover, data recorded a significant increase in total soluble sugars, leaf water deficit, bound water content, membrane integrity, proline and total free amino acids concentrations and enzyme activity as a result of increasing salt stress conditions. Gregory variety had the best growth and yield, and also it was more stable in its physiological and chemical component under salt stress conditions compared to variety Giza 6. So, Gregory variety was more tolerance to salinity stress compared to Giza 6 variety. The obtained data also show that, the yield of both straw and seeds and also its content of N, P and K were decreased with the increase the levels of soil salinity. Such decrease was found with oil and protein content of seeds. In two growth seasons, the previous decrease found with Gregory cultivar was lower than that found with Giza 6 cultivar (Tables 12.4 and 12.5).

365

Abiotic Stresses Table 12.3. Effect of salinity stress on total soluble sugar, proline, soluble protein and electrolyte leakage in tolerant and susceptible genotypes Genotype

KDG197 R2001-2 VG315 TCGS1157 TG512 J861 TVG08563 OG5212 Girnar33

Sugar (mg.g1 DW) Con 148 136 122.4 63.20 26.40 31.20 20.00 95.20 54.40

Tr 144 104.8 97.6 56.80 113.60 63.20 127.20 124.80 105.20

Protein (mg.g-1 FW) Con 79.99 80.36 94.76 125.59 106.76 169.72 103.62 125.22 152.36

Tr 195.19 179.87 220.30 168.61 196.30 200.92 147.75 164.18 154.76

Proline (µM.g-1 FW) Con 146.64 126.20 144.08 46.64 89.16 133.86 109.59 154.30 325.46

Tr 1166.97 1110.76 1149.08 1166.97 563.05 735.49 566.88 666.25 573.27

EL (%) Con 69.67 65.10 71.74 74.95 76.44 57.96 68.63 75.05 70.92

Tr 74.18 67.05 82.64 86.55 84.97 69.19 80.75 94.67 93.24

Fig. 12.9. Effect of salinity stress on seedling growth of tolerant genotypes of groundnut

Level soil salinity**

Peanut variety*

2008 season

366

Table 12.4. Total free amino acids, proline concentrations, enzyme activity and leaf salinity hazard coefficient (L.S.H.C) of two peanut cultivars as affected by different salinity levels during 2008 and 2009 seasons 2009 season

Total free amino acids mg/g dwt.

Proline ug/g dwt.

Peroxidase activity (O.D/g f.wt)

Phenoloxidase activity (O.D/g f.wt)

L.S.H.C.

Total free amino acids mg/g dwt.

Proline ug/g dwt.

Peroxidase Phenolactivity oxidase (O.D/g f.wt) activity (O.D/g f.wt)

L.S.H.C.

0.62 0.57

6.95 6.84

26.52 33.21

180.4 200.3

0.65 0.59

0.54 0.48

6.56 6.46 8.80

V1 V2

27.42 32.48

162.1 175.2

0.82 0.71

S2

V1

35.08

270.3

1.00

0.83

7.69

37.27

304.8

0.87

0.70

V2

43.20

385.4

0.91

0.68

7.40

42.81

358.3

0.75

0.62

8.54

S3

V1 V2

41.28 49.62

492.6 554.5

1.23 1.10

0.92 0.85

8.97 8.81

40.66 47.91

475.1 510.4

0.93 0.84

0.83 0.74

12.32 11.95

Mean Mean

V1 V2 S1

34.59 41.76 29.95

308.3 371.7 168.6

1.01 0.90 0.76

0.79 0.70 0.59

7.87 7.68 6.89

34.82 41.33 29.86

320.1 356.3 190.3

0.82 0.73 0.62

0.61 0.69 0.51

9.23 8.98 6.51

Mean

S2 S3

39.14 45.45

327.8 523.5

0.95 1.16

0.75 0.89

7.54 8.89

40.07 44.28

331.5 492.7

0.81 0.88

0.66 0.78

8.67 12.13

LSD. 5% A

0.50

3.58

0.01

0.008

0.012

0.79

7.06

0.01

0.02

0.03

LSD. 5% B

0.52

2.31

0.01

0.013

0.013

0.19

5.67

0.02

0.009

0.05

LSD. 5% A×B

-

0.86

3.99

0.02

0.014

0.021

0.33

12.23

0.020

0.016

* V1: Variety Giza 6; V2: Variety Gregory ** S1: Soil salinity at 7.5 mmoh; S2: Soil salinity at 10.20 mmoh; S3: Soil salinity at 12.5 mmoh

Physiology of the Peanut Plant

SI

Peanut variety

Soil salinity*

N

P

Straw Conc. %

Uptake g/plant

Seed Conc. %

K

Straw

Uptake g/plant

Conc. %

Seed

Straw

Protein (%)

Oil (%)

Seed

Uptake Conc. Uptake Conc. Uptake Conc. g/plant % g/plant % g/plant %

Abiotic Stresses

Table 12.5. Macronutrients, protein and oil content in two cultivars of crop among two growth seasons in relation to soil salinity

Uptake g/plant

First season (2008) Giza

Gregory

S1

2.72

0.887

3.63

1.246

0.34

0.111

0.48

0.161

2.12

0.691

1.43

0.479

22.7

40.0

S2

2.60

0.731

3.47

0.916

0.25

0.070

0.42

0.111

2.05

0.576

1.38

0.364

21.7

39.8

S3

245

0.617

3.15

0.633

0.22

0.055

0.25

0.050

1.96

0.494

1.33

0.267

19.7

36.9

S1

2.74

0.951

3.66

1.303

0.34

0.118

0.52

0.185

1.45

0.503

2.18

0.756

22.9

41.5

S2

2.63

0.855

3.51

0.958

0.31

0.101

0.49

0.134

1.39

0.452

2.14

0.696

21.7

40.0

S3

2.50

0.703

3.20

0.742

0.25

0.070

0.35

0.098

1.34

0377

2.08

0.584

20.0

38.5

L5D. 5%A

0.02

0.001

0.02

0.01

0.01

0.004

0.013

0.003

0.01

0.01

0.01

0.005

0.13

0.24

LSD. 5%B

0.05

0.002

0.01

0.003

0.02

0.007

0.013

0.0!

0.013

0.00!

0.02

0.002

0.13

0.33

LSD. 5% A×B

N.S

0.002

N.S

0.01

0.02

0.007

0.023

0.005

0.02

0.002

N.S

0.01

N.S

0.41

Second season (2009) Giza 6

1.75

0.924

3.70

1.262

0.36

0.121

0.52

0.177

2.15

0.722

1.50

0.512

23.2

40.6

S2

2.65

0.766

3.47

0.920

0.28

0.081

0.44

0.117

2.05

0.592

1.40

0.371

21.7

40.0

S3

2.48

0.632

3.20

0.666

0.25

0.064

0.28

0.058

2.00

0.51

1.33

0.277

20.0

37.8 (Contd.)

367

S1

Peanut variety

Soil salinity*

N

368

Table 12.5. (Contd.) P

Straw

Seed

K

Straw

Seed

Conc. %

Uptake g/plant

Conc. %

Uptake g/plant

Conc. %

S1

2.80

0.986

371

1.351

0.37

0.130

0.55

0.201

S2

2.65

0.875

3.60

1.004

0.31

0.102

0.50

S3

2.60

0.741

3.30

0.776

0.28

0.080

0.38

LSD. 5% A

0.01

0.001

0.01

0.02

0.013

0.01

LSD. 5% B

0.01

0.001

0.01

0.07

0.013

LSD. 5% A×B

0.03

0.001

0.02

N.S

N.S

Gregory

Straw

Protein (%)

Oil (%)

Seed

Uptake Conc. Uptake Conc. Uptake Conc. g/plant % g/plant % g/plant %

Uptake g/plant

1.50

0.528

2.23

0.814

23.2

42.7

0.140

l.46

0.482

2.14

0.597

22.5

41.2

0.089

1.36

0.388

2.10

0.494

20.6

39.0

0.01

0.01

0.01

0.013

0.04

0.001

0.03

0.25

0.001

0.02

0.001

0.013

0.047

0.07

0.001

0.04

0.33

0.002

0.02

0.002

0.016

0.023

N.S

0.002

0.05

0.43

Physiology of the Peanut Plant

Abiotic Stresses

369

One of the scientifically important applications of plant tissue culture is in the study of ROS homeostasis in plants. In vitro culture has been known to be a useful and rapid method to evaluate salt resistance and it provides a controlled and stable medium for studying physiological and biochemical pathways in plants, especially at the molecular level under different salt concentration levels. In vitro culture techniques are the perfect means to study biochemical and molecular responses of plants at cellular and molecular level in a uniformly controlled environment. Plant tissue culture can be used as an effective model to understand oxidative stress physiology under controlled conditions with different stress inducing agents and also for the subsequent screening of plants with better tolerance capacities. In the present study, it was observed that the response was delayed in explants grown on salt containing medium compared to those grown on salt-free medium. No visibly perceptible variation was observed in the growth patterns of plants at 100 mM and 200 mM NaCl under in vitro conditions (Fig. 12.10). The overall growth pattern of groundnut seedlings in the soil was found to be similar in both the normal and stressed plants (Fig. 12.11). Peanut (Arachis hypogaea L.) is considered as a moderately salt-sensitive species and thus soil salinity can be a limiting factor for peanut cultivation. To gain insights into peanut plant physiology in response to salt stress and alleviation, it was comprehensively characterized leaf relative electrolyte leakage (REC), photosynthesis, leaf transpiration, and metabolism of plants under salt stress and plants that were subjected to salt stress followed by salt alleviation period. As expected,

Fig. 12.10. Groundnut plantlets raised under tissue culture (in vitro) conditions, (A) Normal, (B) 100 mM NaCl treated and (C) 200 mM NaCl treated

370

Physiology of the Peanut Plant

Fig. 12.11. Groundnut plantlets raised under potted (in vivo) conditions, (A) Normal, (B) 100 mM NaCl treated and (C) 200 mM NaCl treated

it was found that REC levels were higher when plants were subjected to salt stress compared with the untreated plants. However, in contrast to expectations, REC was even higher compared with salt treated plants when plants were transferred from salt stress to standard conditions. To decipher REC variation in response to salt stress, especial during the recovery, metabolite, and transcript variations were analysed by GC/MS and RNA-seq method, respectively. Ninety two metabolites, among total 391 metabolites identified, varied in response to salt and 42 metabolites responded to recovery specially. Transcriptomics data showed 1,742 in shoots and 3,281 in roots transcript varied in response to salt stress and 372 in shoots and 1,386 transcripts in roots responded specifically to recovery, but not salt stress. Finally, 95 transcripts and 1 metabolite are indicated as candidates involved in REC, photosynthesis, transpiration, and Na+ accumulation variation were revealed by using the principal component analysis (PCA) and correlation analysis.

12.3. Heat Stress The cultivation of peanut is largely confined to warmer regions of the globe (Selvaraj et al., 2011). However, in such regions, water stress in combination with heat stress (HS) poses a serious threat to crop production. Peanut growth is best favoured at optimum temperatures ranging between 25 and 30°C and the pod yield substantially decreases when temperature exceeds 33°C (Prasad et al., 2003). Therefore, HS disturbs plant growth and development resulting in considerable peanut productivity losses (Gillooly et al., 2001). In some reports, HS negatively affected physio-biochemical processes in peanut genotypes affecting dry mater partitioning, fruit set, and overall yield (Srinivasan et al., 1996; Vara Prasad et al., 2001). Several studies reported genotypic variation for HS tolerance among peanut genotypes (Awal et al., 2003; Prasad et al., 2003; Selvaraj et al., 2011; Vu, 2005). To maintain high yields under an increasingly hotter climate, high temperature resilient peanut cultivars would have to be developed. Therefore, the mechanisms

Abiotic Stresses

371

of plant response to heat need to be understood. A study was undertaken to explore the physiological and metabolic mechanisms developed by virginia-type peanut at early growth stages in response to high temperature stress. Peanut seedlings were exposed to 40/35°C (heat) and 30/25°C (optimum temperature) in a growth chamber. Membrane injury (MI), the Fv/Fm ratio, and several metabolites were evaluated in eight genotypes at four time-points (day 1, 2, 4, and 7) after the heat stress treatment initiation. Even though these were able to highlight some metabolites, e.g., hydroxyproline, galactinol, and unsaturated fatty acid, explaining specific differential physiological (MI) responses in peanut seedlings, overall data suggested general stress responses rather than adaptive mechanisms to heat. Apart from individual metabolites, a combination of several metabolites better explained (41 to 61%) the MI variation in heat stressed peanut seedlings. The genotype SPT 06-07 exhibited lower MI, increased galactinol, reduced hydroxyproline, and higher saturated vs. unsaturated fatty acid ratio under heat stress compared to other genotypes. SPT 06-07 was also separated from the other genotypes during hierarchical clustering and, based on this and previous fieldwork, SPT 06-07 is proposed as a potential source for heat tolerance improvement of virginia-type peanut (Fig. 12.12).

Fig. 12.12. Biplot from principal component analysis (PCA) of eight virginia-type peanut cultivars and breeding lines based on physiological characteristics and relative metabolite levels measured at day 1, 2, 4, and 7 of heat stress (40/35°C under controlled conditions

At start of flower bud initiation (21 d after planting, DAP) plants of the cvs. ICGV 86015 and ICGV 87282 were grown either at 28/22°C (optimum temperature, OT) or at 38/22°C (high temperature, HT) or were reciprocally transferred at 3‐d intervals between the OT to HT regimes and vice versa, until 46 DAP. Transferred plants remained in the new temperature regime for 6 d before being returned to their original regime. All plants were harvested at 67 DAP. In cv. ICGV 86015, transfers between 6 d before and 15 d after flowering (DAF) significantly (P < 0.001) affected total number of pegs (i.e., pegs and pods) and reproductive (peg and pod) dry weight, with the greatest effect occurring at 9 DAF. In cv. ICGV 87282, number of pegs and

372

Physiology of the Peanut Plant

reproductive dry weights were also significantly reduced by transfers at 9 and 12 DAF. Heat stress had no effect on flower production or the proportion of pegs forming pods, but did significantly reduce the proportion of flowers producing pegs. Data suggest that it is heat stress during floral bud development that determines peg number. Plants were grown at optimum and ambient soil temperature from planting until start of podding at 45 d after planting (DAP) in Experiment 1, and until start of flowering at 28 DAP in Experiment 2. Thereafter, plants of each cultivar were exposed to a factorial combination of two air temperatures (optimum: 28°/22°C and high: 38°/22°C) and two soil temperatures (ambient: 26°/24°C and high: 38°/30°C) until final harvest at 90 DAP. The effects of high air and high soil temperatures imposed from start of flowering or podding were similar. Exposure to high air and/or high soil temperature significantly reduced total dry matter production, partitioning of dry matter to pods, and pod yields in both the cultivars. High air temperature had no significant effect on total flower production but significantly reduced the proportion of flowers setting pegs (fruit-set) and hence fruit numbers. In contrast, high soil temperature significantly reduced flower production, the proportion of pegs forming pods and 100 seed weight. The effects of high air and soil temperature were mostly additive and without interaction. Tolerance to high soil and air temperature during the reproductive phase is an important component of adaptation to arid and semi-arid cropping environments in groundnut. Between 10 and 22 genotypes were screened for tolerance to high air and soil temperature in controlled environments. To assess tolerance to high soil temperature, 10 genotypes were grown from start of podding to harvest at ambient (28°C) and high (38°C) soil temperatures, and crop growth rate (CGR), pod growth rate (PGR) and partitioning (ratio PGR:CGR) measured. To assess tolerance to high air temperature during two key stages microsporogenesis (3±6 days before ̄flowering, DBF) and flowering, fruit-set was measured in two experiments. In the 1st experiment, 12 genotypes were exposed to short (3±6 days) episodes of high (38°C) day air temperature at 6 DBF and at flowering. In the second experiment, 22 genotypes were exposed to 40°C day air temperature for 1 day at 6 DBF, 3 DBF or at flowering. Cellular membrane thermostability (relative injury, RI) was also measured in these 22 genotypes. There was considerable variation among genotypes in response to high temperature, whether assessed by growth rates, fruit-set or RI. Pod weight at high soil temperature was associated with variation in CGR rather than partitioning. Flowering was more sensitive to high air temperature than microsporogenesis. Genotypes tolerant to high air temperature at microsporogenesis were not necessarily tolerant at flowering, and nor was tolerance correlated with RI. Six genotypes (796, 55-437, ICG 1236, ICGV 86021, ICGV 87281 and ICGV 92121) were identified as heat tolerant based on their performance in all tests. These experiments have shown that groundnut genotypes can be easily screened for reproductive tolerance to high air and soil temperature and that several sources of heat tolerance are available in groundnut germplasm (Fig. 12.13). Two hundred sixty eight (268) groundnut genotypes were evaluated in four trials under both intermittent drought and fully irrigated conditions, two of the trial being exposed to moderate temperature while the two other trials were exposed to high temperature. The pod yield decrease due to drought stress was 72% at high temperature and 55% at moderate temperature. Pod yield under well-watered (WW) conditions did not decrease under high temperature conditions. Haulm yield decrease

Abiotic Stresses

373

Fig. 12.13. Effects of temperature combinations on floral components

due to water stress (WS) was 34% at high temperature and 42% under moderate temperature. Haulm yield tended to increase under high temperature, especially in one season. Correlation analysis between pod weight and traits measured during plant growth showed that the partition rate, i.e. the proportion of dry matter partitioned into pods was contributing in heat and drought tolerance and could be a reliable selection criterion for groundnut breeding program. Groundnut sensitivity to high temperature stress was in part related to the sensitivity of reproduction. An investigation on impact of heat stress on yield and yield components of groundnut (Arachis hypogaea L.) genotypes with 4 different dates of sowing and 4 genotypes under factorial RBD was undertaken in 2015 (kharif ). The obtained results revealed that 23rd standard week (D1 temperature regime) recorded higher value in pod yield (3,504 kg ha-1) and yield components viz., number of pods per plant (15.75), number of seeds per plant (27.25), pod weight (14.02 g), seed weight (10.67 g), haulm weight (3.35 g), shelling per cent (76.21%), test weight (35.46 g), harvest index (51.97%) were reduced gradually with delayed sowing (D2, D3 and D4 temperature regimes, respectively). Among the 4 genotypes G2-52 and Dh-86 were found to be better performer at heat temperature. Various metabolites were analyzed in groundnut genotypes grown under varying temperature regimes (based on date of sowing). Four contrasting groundnut genotypes viz. ICGS44 (high-temperature tolerant), AK159 and GG7 (moderately­ high-temperature tolerant), and DRG1 (high-temperature sensitive) were grown at

374

Physiology of the Peanut Plant

three different temperature regimes i.e., low (early date of sowing), normal (normal date of sowing) and high temperature (late date of sowing) under field conditions. Untargeted metabolomic analysis of leaf tissue was performed by GC–MS, while targeted metabolite profiling was carried out by HPLC (polyamines) and UPLCMS/MS (phenolics) at both the pegging and pod filling stages. It was noted that untargeted metabolomic profiling revealed exclusive expression/induction of betad-galactofuranoside, l-threonine, hexopyranose, d-glucopyranose, stearic acid, 4-ketoglucose, d-gulose, 2-o-glycerol-alpha-d-galactopyranoside and serine in ICGS44 during the pegging stage under high-temperature conditions. During the pod filling stage at higher temperature, alpha-d-galactoside, dodecanedioic acid, 1-nonadecene, 1-tetradecene and beta-d-galactofuranose were found to be higher in both ICGS44 and GG7. Moreover, almost all the metabolites detected by GC–MS were found to be higher in GG7, except beta-d-galactopyranoside, beta-d-glucopyranose, inositol and palmitic acid. Accumulation of putrescine was observed to be higher during low-temperature stress, while agmatine showed constitutive expression in all the genotypes, irrespective of temperature regime and crop growth stage. Interestingly, spermidine was observed only in the high-temperature tolerant genotype ICGS44. In the study, it was found a higher accumulation of cinnamic acid, caffeic acid, salicylic acid and vanillic acid in ICGS44 compared to that of other genotypes at the pegging stage, whereas catechin and epicatechin were found during the pod filling stage in response to high-temperature stress, suggesting their probable roles in heat-stress tolerance in groundnut. Doubling of CO2 increased leaf photosynthesis and seed yield by 27% and 30%, respectively, averaged across all temperatures. There were no effects of elevated CO2 on pollen viability, seed-set, seed number per pod, seed size, harvest index or shelling percentage. At ambient CO2, seed yield decreased progressively by 14%, 59% and 90% as temperature increased from 32/22 to 36/26, 40/30 and 44/34°C, respectively. Similar percentage decreases in seed yield occurred at temperatures above 32/22°C at elevated CO2 despite greater photosynthesis and vegetative growth. Decreased seed yields at high temperature were a result of lower seed-set due to poor pollen viability, and smaller seed size due to decreased seed growth rates and decreased shelling percentages. Seed harvest index decreased from 0.41 to 0.05 as temperature increased from 32/22 to 44/34°C under both ambient and elevated CO2. It was concluded that there were no beneficial interactions between elevated CO2 and temperature, and that seed yield of peanut would decrease under future warmer climates, particularly in regions where present temperatures are near or above optimum (Table 12.6 and Fig. 12.14). The effect of exogenous calcium nitrate [Ca(NO3)2] (6 mM) on the dissipation of excess excitation energy in the photosystem II (PSII) antenna, especially on the level of D1 protein and the xanthophyll cycle in peanut plants under heat (40°C) and high irradiance (HI) (1200 µmol m−2 s−1) stress were investigated. Compared with the control plants [cultivated in 0 mM Ca(NO3)2 medium], the maximal photochemical efficiency of PSII (Fv/Fm) in Ca2+-treated plants showed a slighter decrease after 5 h of stress, accompanied by higher non-photochemical quenching (NPQ), higher expression of antioxidative genes and less reactive oxygen species (ROS) accumulation. Meanwhile, higher content of D1 protein and higher ratio of (A+Z)/ (V+A+Z) were also detected in Ca2+-treated plants under such stress. These results showed that Ca2+ could help protect the peanut photosynthetic system from severe

375

Abiotic Stresses

Table 12.6. P values of the effects of temperature, CO2 and interaction between temperature and CO2, based on statistical analyses on various measured parameters of peanut Temperature

CO2

Leaf photosynthetic rate

Parameter

0.2858

0.0172*

0.2997

Stomatal conductance

0.0159

0.0092

**

0.0826

Transpiration

0.0020**

0.0040**

0.0779

Pollen viability

***

0.0005

0.9088

0.6508

Seed set

0.0002***

0.5119

0.8788

**

*

Interaction

Vegetative dry matter

0.0139

0.0025

0.1523

Total dry matter

0.0135*

0.0025**

0.1523

*

Pod harvest index

0.0002

1.000

0.4677

Seed harvest index

0.0002***

0.8361

0.4310

Seed growth rate

*

0.0367

0.1114

0.2123

Seed-filling duration

0.2797

0.3243

0.9188

*

0.4663

***

Pod yield

0.0006

0.0311

Seed yield

0.0006***

0.0376*

0.4308

Pod number

0.0096**

0.0621

0.9616

Seed number

0.0010

*

0.0316

0.5010

0.0181*

0.9336

0.7135

0.0344 0.0010***

0.9336 0.8114

0.7204 0.5379

Shelling percentage Seed number per pod Seed size

***

***

*

***,**,* Significant at P 0.001, 0.01 and 0.05 respectively.

Fig. 12.14. Relations between daytime maximum/night time minimum temperature (a) pod yield; (b) seed yield; (c) pod number; and (d) seed number at ambient ( 350 mmol mol-1) and elevated ( 700 mmol mol-1) CO2

376

Physiology of the Peanut Plant

photoinhibition under heat and HI stress by accelerating the repair of D1 protein and improving the de-epoxidation ratio of the xanthophyll cycle. Furthermore, EGTA (a chelant of Ca ion), LaCl3 (a blocker of Ca2+ channel in cytoplasmic membrane), and CPZ [a calmodulin (CaM) antagonist] were used to analyze the effects of Ca2+/CaM on the variation of (A+Z)/(V+A+Z) (%) and the expression of violaxanthin de-epoxidase (VDE). The results indicated that CaM, an important component of the Ca2+ signal transduction pathway, mediated the expression of the VDE gene in the presence of Ca to improve the xanthophyll cycle (Fig. 12.15).

Fig. 12.15. Characterization of NPQ and qf of CK and CA plants were monitored under normal and heat and HI stress.

Heat shock transcription factors (Hsfs) are important transcription factors (TFs) in protecting plants from damages caused by various stresses. The released whole genome sequences of wild peanuts make it possible for genome-wide analysis of Hsfs in peanut. In a study, a total of 16 and 17 Hsf genes were identified from Arachis duranensis and A. ipaensis, respectively. It was identified 16 orthologous Hsf gene pairs in both peanut species; however HsfXs was only identified from A. ipaensis. Orthologous pairs between two wild peanut species were highly syntenic. Based on phylogenetic relationship, peanut Hsfs were divided into groups A, B, and

Abiotic Stresses

377

C. Selection pressure analysis showed that group B Hsf genes mainly underwent positive selection and group A Hsfs were affected by purifying selection. Small scale segmental and tandem duplication may play important roles in the evolution of these genes. Cis-elements, such as ABRE, DRE, and HSE, were found in the promoters of most Arachis Hsf genes. Five AdHsfs and two AiHsfs contained fungal elicitor responsive elements suggesting their involvement in response to fungi infection. These genes were differentially expressed in cultivated peanut under abiotic stress and Aspergillus flavus infection. AhHsf2 and AhHsf14 were significantly up-regulated after inoculation with A. flavus suggesting their possible role in fungal resistance.

12.4. Iron Deficiency The factorial pot experiment was comprised of two major factors: (i) soil-Fe status [normal-Fe, deficit-Fe], and (ii) genotypes [five] with differential IDC response, constituting 10 treatments. They were assessed for five morpho-physiological parameters associated with IDC resistance across five crop growth stages and also yield and its related traits. Associations between these traits were also estimated. Under deficit-Fe conditions, IDC resistant genotypes recorded significantly lower visual chlorosis rating (VCR), higher SPAD values, active Fe, chlorophyll content, peroxidase activity, and high yield compared to susceptible ones. Between normal- to deficit-Fe soils, resistant compared to susceptible genotypes showed no change in VCR scores; a lower reduction in SPAD, chlorophyll, active Fe, peroxidase activity, and pod yield. Under deficit-Fe conditions, high yield among resistant genotypes could be attributed to higher seed weight, number of pods and haulm yield, while contrasting reduction in main stem height and number of primaries. The results indicate that for initial large-scale screening of groundnut genotypes for IDC resistance, SPAD values are most ideal while active Fe could be utilized for confirmation of identified lines. A significant fraction of peanut crops cultivated on calcareous soils in Northern China exhibited symptoms of leaf chlorosis due to Fe-deficiency resulting in about one third of oil-seed productivity losses (Kong et al., 2014; Zuo et al., 2000; 2007). Peanut is a strategy-I plant that is vulnerable to Fe-deficiency on calcareous soils (Song et al., 2016). Under such conditions, peanut plants respond to Fe-insufficiency in a three-step process characteristic of strategy-I plants (Marschner et al., 1987; Santi and Schmidt, 2009). In strategy-1 plants, the first step is rhizosphere acidification that increases Fe3+ solubility (Palmgren and Harper, 1999; Santi and Schmidt, 2009). This is followed by catalytic reduction of Fe3+ chelate to Fe2+ by ferric chelate reductase (Ding et al., 2009; Robinson et al., 1999) and a third step of iron uptake by plasmalemma IRT1 (metal transporter) also termed as Fe-regulated transporter in root cells (Ding et al., 2010; Eide et al., 1996). Recently, it has been reported that the exogenous application of salicylic acid, sodium nitroprusside, and epibrassinolide ameliorated Fe-deficiency in peanut (Dong et al., 2016; Song et al., 2016). Furthermore, drainage of the calcareous soils improved Fe-availability (Zuo et al., 2007). For this study, sodium nitroprusside (SNP) was used to supply NO for hydroponic peanut plants. After 18 days, the peanut seedlings growing without iron exhibited significant leaf interveinal chlorosis, and this iron-deficiency induced symptom was completely prevented by NO. An increased content of chlorophyll and active iron was observed in NO-treated young leaves, suggesting an improvement of iron availability in plants. In addition, the improved rhizosphere acidification and increased secretion

378

Physiology of the Peanut Plant

of organic acids by root in NO-treated plants suggesting that NO is effective in modulating iron uptake and transport inside the peanut plants. Furthermore, NO treatment alleviated the increased accumulation of superoxide anion (O-2(center dot-)) and malondialdehyde (MDA), and modulated the antioxidant enzymes. However, the SNP with a prior sunlight treatment that does not release NO had no significant effect on the chlorophyll levels in iron-deficient plants. Therefore, these results support a physiological action of NO on the availability, uptake and transport of iron in the plant. It is due to its geocarpic nature of growth that the overall yield performance of groundnut is hindered by several biotic and abiotic stress factors. Multidimensional attempts were undertaken to combat these factors by developing superior groundnut varieties, modified with integral mechanism of tolerance/resistance; however this approach proved to be futile, owing to inferior pod and kernel quality. As a superior alternative, biotechnological intervention like transformation of foreign genes, either directly (biolistic) or via Agrobacterium, significantly aided in the development of advanced groundnut genotypes equipped with integral resistance against stresses and enhanced yield attributing traits. Several genes triggered by biotic and abiotic stresses, were detected and some of them were cloned and transformed as major parts of transgenic programmes. Application of modern molecular biological techniques, in designing biotic and abiotic stress tolerant/resistant groundnut varieties that exhibited mechanisms of resistance, relied on the expression of specific genes associated to particular stress. The genetically transformed stress tolerant groundnut varieties possess the potential to be employed as donor parents in traditional breeding programmes for developing varieties that are resilient to fungal, bacterial, and viral diseases, as well as to draught and salinity (Fig. 12.16).

Fig. 12.16. Characteristics of transgenic peanut plants

Abiotic Stresses

379

References Abogadallah, G.M., M. Serag and W.P. Quick. 2010. Fine and coarse regulation of reactive oxygen species in the salt tolerant mutants of barnyard grass and their wild type parents under salt stress. Physiol. Plant., 138: 60-73. Akcay, U.C., O. Ercan, M. Kavas, L. Yildiz , C. Yilmaz et al. 2010. Drought-induced oxidative damage and antioxidant responses in peanut (Arachis hypogaea L.) seedlings. Plant Growth Regul., 61: 21-28. Apse, M.P., G.S. Aharon, W.A. Snedden and E. Blumbald. 1999. Salt tolerance conferred by overexpression of a vacuolar Na+/H+ antiports in Arabidopsis. Science, 285: 1256-1258. Awal, M.A., T. Ikeda and R. Itoh. 2003. The effect of soil temperature on source‐sink economy in peanut (Arachis hypogaea). Environmental and Experimental Botany, 50: 41-50. Babu, V.R. and D.V.M. Rao. 1983. Water stress adaptation in the groundnut (Arachis hypogaea L.) foliar characteristics and adaptation to moisture stress. Plant Physiol. Biochem., 10: 64-80. Banjara, M., L. Zhu, G. Shen, P. Payton and H. Zhang. 2012. Expression of an Arabidopsis sodium/proton antiporter gene (AtNHX1) in peanut to improve salt tolerance. Plant Biotechnology Reports, 6: 59-67. Bertioli, D.J., S.B. Cannon, L. Froenicke, G. Huang, A.D. Farmer et al. 2016. The genome sequences of Arachis duranensis and Arachis ipaensis, the diploid ancestors of cultivated peanut. Nat. Genet., 48(4): 438. Bhatnagar‐Mathur, P., J.S. Rao, V. Vadez, S.R. Dumbala, A. Rathore et al. 2013. Transgenic peanut overexpressing the DREB1A transcription factor has higher yields under drought stress. Molecular Breeding, 33: 327-340. Bhauso, T.D., T. Radhakrishnan, A. Kumar, G.P. Mishra, J.R. Dobaria et al. 2014. Over‐ expression of bacterial mtlD gene confers enhanced tolerance to salt‐stress and water‐ deficit stress in transgenic peanut (Arachis hypogaea) through accumulation of mannitol. Australian Journal of Crop Science, 8: 413-421. Boote, K.J., R.J. Varnell and W.G. Duncan. 1976. Relationships of size, osmotic concentration and sugar concentration of peanut pods to soil water. Proceedings of the Soil and Crop Science Society of Florida, 35: 47-50. Chen, M., Q. Yang, T. Wang, N. Chen, L. Pan et al. 2015. Agrobacterium-mediated genetic transformation of peanut and the efficient recovery of transgenic plants. Canadian Journal of Plant Science, 95: 735-744. Chen, N., M. Su, X. Chi, Z. Zhang, L. Pan et al. 2016. Transcriptome analysis reveals salt-stressregulated biological processes and key pathways in roots of peanut (Arachis hypogaea L.). Genes & Genomics, 38(6): 493-507. Clavel, D., B. Sarr, E. Marone and R. Ortiz. 2004. Potential agronomic and physiological traits of Spanish groundnut varieties (Arachis hypogaea L.) as selection criteria under end-of­ cycle drought conditions. Agron. Sustain. Dev., 24: 101-111. Cokkizgin, A. 2012. Salinity stress in common bean (Phaseolus vulgaris L.) seed germination. Not. Bot. Horti. Agrobo., 1(40): 177-182. Cui, F., N. Sui, G. Duan, Y. Liu, Y. Han et al. 2018. Identification of metabolites and transcripts involved in salt stress and recovery in peanut. Front Plant Sci., 9: 217. DeVries, J.D., J.M. Bennett, S.L. Albrecht, K.J. Boote et al. 1989. Water relations, nitrogenase activity and root development of three grain legumes in response to soil water deficits. Field Crops Res., 22: 215-226. Ding, H., L. Duan, H. Wu, R. Yang, H. Ling et al. 2009. Regulation of AhFRO1, an Fe(III)‐ chelate reductase of peanut, during iron deficiency stress and intercropping with maize. Physiologia Plantarum, 136: 274-283. Ding, H., L.H. Duan, F. Li, H.F. Yan, M. Zhao et al. 2010. Cloning and functional analysis of the peanut iron transporter AhIRT1 during iron deficiency stress and intercropping with maize. Journal of Plant Physiology, 167: 996-1002.

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CHAPTER

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Source–Sink Relationships Plants assimilate carbon dioxide during photosynthesis in chloroplasts. Assimilated carbon is subsequently allocated throughout the plant. Generally, two types of organs can be distinguished, mature green source leaves as net photoassimilate exporters, and net importers, the sinks, e.g. roots, flowers, small leaves, and storage organs like tubers. Within these organs, different tissue types are developed according to their respective functions, and cells of either tissue type are highly compartmentalized. Photo assimilates are allocated to distinct compartments of the tissues in all organs, requiring a set of metabolite transporters mediating these compartmental transfers. The general route of photo assimilates can be briefly described as follows. Upon fixation of carbon dioxide in chloroplasts of mesophyll cells, triosephosphates either enter the cytosol mainly for sucrose formation or remain in the stroma to form transiently stored starch which is degraded during the night and enters the cytosol as maltose or glucose to be further metabolized to sucrose. In both cases, sucrose enters the phloem for long distance transport or is transiently stored in the vacuole, or can be degraded to hexoses which also can be stored in the vacuole. In the majority of plant species, sucrose is actively loaded into the phloem via the apoplast. Following long distance transport, it is released into sink organs, where it enters cells as a source of carbon and energy. In storage organs, sucrose can be stored, or carbon derived from sucrose can be stored as starch in plastids, or as oil bodies, or in combination with nitrogen as protein in protein storage vacuoles and protein bodies. Here, the focus was on transport proteins known for either of these steps, and the implications of yield increase in plants upon genetic engineering of the respective transporters (Fig. 13.1). Members of the Fabaceae family store protein in their seeds, which represent the main plant source for human nitrogen nutrition. However, soybean (G. max) is primarily grown to provide oil. Pea (Pisum sativum) seeds contain more starch than protein. Other crop members of the family are bean (Phaseolus vulgaris), chickpea (Cicer arietinum), and peanut (Arachis hypogaea). There are species of amilies other than Fabaceae which contain reasonable amounts of storage protein in their harvested organs. However, these are mainly grown for their starch (e.g. potato, corn) or oil (e.g. canola). In order to store protein ample nitrogen must be available. Consistently, members of the Fabaceae family living in symbiosis with rhizobial bacteria store protein in seeds as the symbiotic bacteria fix atmospheric molecular nitrogen and supply the host plant with sufficient reduced nitrogen. In legumes, protein is stored in the embryo in structures known as protein storage vacuoles. Storage proteins are translated in the ribosomes of the rough ER and are cotranslationally imported into the ER. The main route from the ER to the protein storage vacuoles is via the Golgi bodies

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Fig. 13.1. Overview of carbon transport proteins in source leaves. Transport steps in mesophyll cells (MCs), parenchyma cells (PCs), companion cells (CCs), and sieve elements are depicted indicating their respective mode of action. Squares represent antiporters, circles describe facilitators, pentagons depict symporters, and triangles H+-ATPases/PPases. 1: TPT; 2: pGlcT; 3: MEX; 4: TMT and VGT; 5: SUC4/SUT4; 6: ESL1; 7: SWEETs; 8: SUC2/SUT1; 9: SUC3. TP: Triose phosphates; Malt: Maltose; Glc: Glucose; Suc: Sucrose; Frc: Fructose. Please refer to the text for transporter abbreviations

(Herman and Larkins, 1999). However, in the alternative pathway, protein bodies budding off the ER or remaining in the cytosol or fusing with the protein storage vacuole (as in cereals) also seem to exist in legumes (Fig. 13.2; Hermanand Larkins, 1999; Vitaleand Ceriotti, 2004; Abirached-Darmencyetal, 2012). Sucrose (and amino acid) unloading from the phloem in legume seeds occurs symplastically. Analyses with fluorescent dyes indicate that sucrose is symplastically transported to the maternal release site, the ground-or thin-walled parenchyma (Tegeder et al., 1999; van Dongen et al., 2003). Sucrose is either simply or through specialized transfer cells released into the apoplast dependent on the legume species (Patrick and Offler, 2001; van Dongen et al., 2003). At the younger stages, legume seeds contain a liquid endosperm which acts as a buffer for sucrose and glutamine for the developing embryo (Melkus et al., 2009). At the expense of embryo growth, the endosperm is substantially degraded during development (Patrick and Offler, 2001). At later developmental stages, sucrose is released from the maternal tissue to the apoplast and is taken up by embryo epidermal transfer cells (Weber et al., 1997; Tegeder et al., 1999). With the discovery of a new class of sucrose facilitators from pea and bean, called SUFs, expressed at the maternal release site, sucrose release into the apoplast is probably mediated by these transporters (Zhou et al., 2007). In these same cells, the sucrose/H+-symporter SUT1 was found to be expressed and believed to likely function in seed coat sucrose retrieval (Tegeder et al., 1999; Zhou et al., 2007). At the filial side, uptake of sucrose by the embryo epidermis is mediated by SUT1 (Fig. 13.2), and uptake is energized by P-type H+ATPases which are both co-expressed in embryo transfer cells (Tegeder et al., 1999).

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Fig. 13.2. Overview of carbon transport proteins in sink organs. Transport steps in sieve elements and sink cells are depicted according to their respective mode of action. Squares represent antiporters, circles describe facilitators, pentagons depict symporters, hexagons represent ABC transporters, and triangles H+-ATPases/PPases. 2: pGlcT; 3: MEX; 4: TMT and VGT; 7: SWEETs; 10: GPT; 11: BASS2; 12: PPT; 13: DiT2; 14: NTT; 15: ABCA9; 16: Gly3P permease; 17: HT; 18: BT-1; 19: SUF1. (r)ER: (rough) Endoplasmic reticulum; TAG: Triacyl glycerol; PB: Protein body; PEP: Phosphoenolpyruvate; Pyr: Pyruvate; Mal: Malate; Glc6P: Glucose 6-phosphate; Pi: Orthophosphate; ADP: Adenosine diphosphate; ADP-G: ADP-glucose; Glc: Glucose; Malt: Maltose; Frc: Fructose. Please refer to text for transporter abbreviations

The embryo communicates the demand for sucrose uptake from the apoplast by the internal sugar level. Sucrose uptake fluxes have been found to be negatively correlated with pool sizes of intra cellular sugars, while SUT1 transcripts were sensitive to sugar levels (i.e. when the embryo contains sufficient sugars, SUT1 is transcriptionally down-regulated), which in turn led to decreased sucrose uptake (Zhou et al., 2009). To increase the uptake of sucrose into pea embryos, potato SUT1 was over-expressed using a storage parenchyma-specific promoter which resulted in increased sucrose uptake and accelerated growth rates of the embryo. However, final seed weight of transgenic plants was similar to the wild-type. It is speculated that overexpression of sucrose uptake at the primary site, the epidermal transfer cells, might have led to more significant results (Rosche et al., 2002). In a different transgenic approach, seed protein content in mature seeds was increased. Rolletschek et al. (2007) antisense inhibited the Vicia narbonensis GPT1 (Fig. 13.2) using a seed-specific promoter. As expected, flux of carbon into plastids and thus into starch and (mainly structural) lipids was found to decrease. A simultaneous increase in seed protein, however, was not predictable. This finding with increased expression of the amino acid permease AAP1 in transgenic embryos was potentially due to lower glutamine concentrations during the main protein storage phase. Glutamine was previously described to repress AAP1

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transcriptionally (Miranda et al., 2001). Indeed, when AAP1 was overexpressed in seeds of Vicia narbonensis and pea, sink strength for amino acids was increased and the amounts of total nitrogen and protein (mainly storage globulins) were increased (Rolletschek et al., 2005). The extent to which increased photosynthesis (the source of plant carbon), will translate into improved yields depends crucially on the adequate development of sinks (e.g. the grain) which sequesters the carbon and other resources such as nitrogen. The balance between the source and sink is complex and under tight genetic control. Furthermore, the delicate relationship is influenced by multiple environmental factors and the changing climate. Increasing photosynthesis can increase crop yield potential (Zhu et al., 2010; Long et al., 2015) when photosynthate partitioning and factors influencing sink growth remain unchanged (Long et al., 2006b). Unfortunately, although much effort has been made in exploring the source–sink relationship, we are still far from fully understanding source–sink interaction and even further from rational manipulation of the source–sink relationship. As a reflection of this, breeders usually struggle with the low grain setting rate and low-efficiency remobilization of stem and sheath reserves (thus a low harvest index and/or low grain filling rate) in hybrid rice cultivars with a high yield potential (Yang et al., 2002a; Yang and Zhang, 2010). Similarly, the optimal partitioning of photosynthate for root growth differs between cultivars and in the same cultivar under differen