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Climate change and agricultural ecosystems : current challenges and adaptation
 9780128164839, 0128164832, 9780128175224

Table of contents :
Content: 1. Agriculture in the Era of Climate Change: Consequences and Effects 2. Sustainable Agricultural Practices Using Beneficial Fungi Under Changing Climate Scenario 3. Climate Change and Soil Dynamics: Effects on Soil Microbes and Fertility of Soil 4. Agrochemicals: Harmful and Beneficial Effects of Climate Changing Scenarios 5. Climate Change and Secondary Metabolism in Plants: Resilience to Disruption 6. Impact of Xenobiotics Under a Changing Climate Scenario 7. Impact of Climate Change on Plant-Microbe Interactions Under Agroecosystems 8. Medicinal Plants Under Climate Change: Impacts on Pharmaceutical Properties of Plants 9. Air Pollution: Role in Climate Change and Its Impact on Crop Plants 10. Cyanobacteria and Their Role Under Elevated CO2 Conditions 11. Rising Atmospheric Carbon Dioxide and Plant Responses: Current and Future Consequences 12. Climatic Resilient Agriculture for Root, Tuber, and Banana Crops Using Plant Growth-Promoting Microbes 13. Understanding Soil Aggregate Dynamics and Its Relation with Land Use and Climate Change 14. Climate Change: A Challenge for Postharvest Management, Food Loss, Food Quality, and Food Security 15. Impact of Climate Change on Soil Carbon Exchange, Ecosystem Dynamics, and Plant-Microbe Interactions 16. Bioinformatics as a Tool to Counter Climate Change: Challenges and Prospects 17. Developing Adaptive Capability of Agricultural Societies in the Context of Climate Change

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Climate Change and Agricultural Ecosystems

Climate Change and Agricultural Ecosystems Current Challenges and Adaptation

Edited by

KRISHNA KUMAR CHOUDHARY AJAY KUMAR AMIT KISHORE SINGH

Woodhead Publishing is an imprint of Elsevier The Officers’ Mess Business Centre, Royston Road, Duxford, CB22 4QH, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States The Boulevard, Langford Lane, Kidlington, OX5 1GB, United Kingdom Copyright © 2019 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress ISBN: 978-0-12-816483-9 (print) ISBN: 978-0-12-817522-4 (online) For information on all Woodhead Publishing publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Charlotte Cockle Acquisition Editor: Nancy Maragioglio Editorial Project Manager: Ruby Smith Production Project Manager: Debasish Ghosh Cover Designer: Christian J Bilbow Typeset by MPS Limited, Chennai, India

LIST OF CONTRIBUTORS Mohd Aamir

Laboratory of Mycopathology and Microbial Technology, Centre of Advanced Study in Botany, Institute of Science, Banaras Hindu University, Varanasi, India Madhoolika Agrawal

Laboratory of Air Pollution and Global Climate Change, Department of Botany, Institute of Science, Banaras Hindu University, Varanasi, India Shashi Bhushan Agrawal

Laboratory of Air Pollution and Global Climate Change, Department of Botany, Institute of Science, Banaras Hindu University, Varanasi, India Arif Ahamad

School of Environmental Sciences, Jawaharlal Nehru University, New Delhi, India Naushad Ansari

Laboratory of Air Pollution and Global Climate Change, Department of Botany, Institute of Science, Banaras Hindu University, Varanasi, India Rahul Bhadouria

Ecosystems Analysis Laboratory, Department of Botany, Institute of Science, Banaras Hindu University, Varanasi, India; Natural Resource Management Laboratory, Department of Botany, University of Delhi, New Delhi, India; Department of Botany, Centre of Advanced Study, University of Delhi, New Delhi, India Nitish Rattan Bhardwaj

ICAR-Indian Grassland and Fodder Research Institute, Jhansi, India Anwesha Borthakur

Centre for Studies in Science Policy, Jawaharlal Nehru University, New Delhi, India Kamal Kumar Chaudhary

Department of Bioscience, Institute of Management Studies Courses Campus, Ghaziabad, India

University

Krishna Kumar Choudhary

Department of Plant Sciences, School of Basic and Applied Sciences, Central University of Punjab, Bathinda, India Kumari Divyanshu

Laboratory of Mycopathology and Microbial Technology, Centre of Advanced Study in Botany, Institute of Science, Banaras Hindu University, Varanasi, India Manish Kumar Dubey

Laboratory of Mycopathology and Microbial Technology, Centre of Advanced Study in Botany, Institute of Science, Banaras Hindu University, Varanasi, India

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List of Contributors

Akanksha Gupta

Department of Botany, Center of Advanced Study in Botany, Institute of Science, Banaras Hindu University, Varanasi, India Pooja Kannojia

National Centre of Organic Farming, Ministry of Agriculture and Farmers Welfare, Ghaziabad, India Manoj Kaushal

International Institute of Tropical Agriculture, Ibadan, Nigeria Pushpendra Koli

ICAR-Indian Grassland and Fodder Research Institute, Jhansi, India Ajay Kumar

Department of Botany, Center of Advanced Study in Botany, Institute of Science, Banaras Hindu University, Varanasi, India Chandramohan Kumar

Department of Botany, Center of Advanced Study in Botany, Institute of Science, Banaras Hindu University, Varanasi, India Gaurav Kumar

Department of Environmental Studies, PGDAV College, University of Delhi, New Delhi, India Sonu Kumar Mahawer

ICAR-Indian Grassland and Fodder Research Institute, Jhansi, India Amit Kumar Mishra

Texas A&M AgriLife Research and Extension Center, Texas A&M University, Uvalde, TX, United States Virendra Kumar Mishra

Institute of Environment and Sustainable Development, Banaras Hindu University, Varanasi, India Bhanu Pandey

Natural Resources and Environmental Management, CSIR-Central Institute of Mining & Fuel Research, Dhanbad, India Akhilesh Singh Raghubanshi

Institute of Environment and Sustainable Development (IESD), Banaras Hindu University, Varanasi, India Krishna Kumar Rai

Laboratory of Mycopathology and Microbial Technology, Centre of Advanced Study in Botany, Institute of Science, Banaras Hindu University, Varanasi, India; Division of Crop Improvement and Biotechnology, Indian Institute of Vegetable Research, Indian Council of Agricultural Research (ICAR), Varanasi, India Kshama Rai

Laboratory of Air Pollution and Global Climate Change, Department of Botany, Institute of Science, Banaras Hindu University, Varanasi, India

List of Contributors

xv

Swarnmala Samal

Laboratory of Mycopathology and Microbial Technology, Centre of Advanced Study in Botany, Institute of Science, Banaras Hindu University, Varanasi, India Kusum Sharma

ICAR-Indian Institute of Pulses Research (IIPR), Kanpur, India P.K. Sharma

ICAR-National Bureau of Agriculturally Important Microorganism (NBAIM), Mau Nath Bhanjan, India Awadhesh Kumar Shukla

Department of Botany, K.S. Saket P.G. College, Ayodhya, Faizabad, India Reetika Shukla

Department of Environmental Science, Indira Gandhi National Tribal University, Amarkantak, India Amit Kishore Singh

Botany Department, Kamla Nehru P.G. College, Raebareli, India Anand Vikram Singh

Department of Botany, Center of Advanced Study in Botany, Institute of Science, Banaras Hindu University, Varanasi, India Gurudutt Singh

Institute of Environment and Sustainable Development, Banaras Hindu University, Varanasi, India Hema Singh

Ecosystems Analysis Laboratory, Department of Botany, Institute of Science, Banaras Hindu University, Varanasi, India Kalpna Singh

Department of Obstetrics Gynaecology, Institute of Medical Science, Banaras Hindu University, Varanasi, India Monika Singh

Department of Botany, Center of Advanced Study in Botany, Institute of Science, Banaras Hindu University, Varanasi, India Pardeep Singh

Department of Environmental Studies, PGDAV College, University of Delhi, New Delhi, India Prem Pratap Singh

Department of Botany, Center of Advanced Study in Botany, Institute of Science, Banaras Hindu University, Varanasi, India Rishikesh Singh

Institute of Environment and Sustainable Development (IESD), Banaras Hindu University, Varanasi, India

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List of Contributors

Sandeep Kumar Singh

Department of Botany, Center of Advanced Study in Botany, Institute of Science, Banaras Hindu University, Varanasi, India Savita Singh

Department of Botany, Babu Shivnath Agrawal College, Mathura, India Suruchi Singh

Laboratory of Air Pollution and Global Climate Change, Department of Botany, Institute of Science, Banaras Hindu University, Varanasi, India Vipin Kumar Singh

Department of Botany, Center of Advanced Study in Botany, Institute of Science, Banaras Hindu University, Varanasi, India Pratap Srivastava

Department of Botany, Shyama Prasad Mukherjee Post-graduate College, University of Allahabad, Allahabad, India Yashi Srivastava

Department of Applied Agriculture, Central University of Punjab, Bathinda, India Sachchidanand Tripathi

Department of Botany, Deen Dayal Upadhyaya College, University of Delhi, New Delhi, India Yashoda Nandan Tripathi

Laboratory of Mycopathology and Microbial Technology, Centre of Advanced Study in Botany, Institute of Science, Banaras Hindu University, Varanasi, India R.S. Upadhyay

Laboratory of Mycopathology and Microbial Technology, Centre of Advanced Study in Botany, Institute of Science, Banaras Hindu University, Varanasi, India Sudheer Singh Verma

Department of South and Central Asian Studies (Including Historical Studies), School of Global Relations, Central University of Punjab, Bathinda, India Andleeb Zehra

Laboratory of Mycopathology and Microbial Technology, Centre of Advanced Study in Botany, Institute of Science, Banaras Hindu University, Varanasi, India

PREFACE Agricultural ecosystems are highly dependent on weather and climate for the production of necessary foods to sustain human life. Despite leading, cutting-edge tools and technologies for developing improved varieties, genetically modified organisms, and irrigation systems, climate change is still a major constraint for agricultural productivity. Climate variability and weather extremes such as droughts, floods, global warming, and severe storms which are prime consequences of climate change, will make agriculture more susceptible and difficult for ensuring food security for the ever-expanding global population. Therefore, with the help of contributing experts, this book summarizes the different aspects of vulnerability, adaptation, and amelioration of climate change in respect to plants (crops and medicinal plants), soil, and microbes for the sustainability of the agricultural sector and, ultimately, the issue of food security under futuristic scenarios. This book will also focus on the understanding and utilization of information technology for the sustainability of the agricultural sector along with the capacity and adaptability of agricultural societies under climate changing scenarios. Most importantly, the compiled information within the book will be useful for students, teachers, and researchers at universities and research institutes, especially those working in the study fields of global climate change, environmental sciences, soil sciences, agricultural microbiology, plant pathology, and agronomy.

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Agriculture in the Era of Climate Change: Consequences and Effects Rahul Bhadouria1, Rishikesh Singh2, Vipin Kumar Singh3, Anwesha Borthakur4, Arif Ahamad5, Gaurav Kumar6 and Pardeep Singh6 1

Department of Botany, Centre of Advanced Study, University of Delhi, New Delhi, India Institute of Environment and Sustainable Development (IESD), Banaras Hindu University, Varanasi, India 3 Department of Botany, Centre of Advanced Study in Botany, Institute of Science, Banaras Hindu University, Varanasi, India 4 Centre for Studies in Science Policy, Jawaharlal Nehru University, New Delhi, India 5 School of Environmental Sciences, Jawaharlal Nehru University, New Delhi, India 6 Department of Environmental Studies, PGDAV College, University of Delhi, New Delhi, India 2

Contents 1.1 Introduction 1.2 Climate Change and Sustainable Agriculture 1.2.1 Need for Integration of Mitigation and Adaptation Frameworks Into Sustainable Development Planning 1.3 Dependence on Subsistence Agriculture 1.4 Impacts of Climate Change on Agricultural Production Around the Globe 1.4.1 Effects of Climate Change on Indian Agriculture 1.5 Effects of Climate Change on Food Security 1.6 Effect of Climate Change on Geographical Distribution of Crop Species 1.7 Effect of Climate Change on Insect Pest and Disease Development 1.8 Effect of Climate Change on Ontogeny of Crops 1.8.1 Phenology 1.8.2 Flowering 1.8.3 Pollination 1.8.4 Seed Quality 1.9 Impact of Climate Change on Abiotic Environmental Factors Affecting Crop Productivity 1.10 Strategies for Adaptation 1.10.1 Changes in Seed Production Locations 1.10.2 Changing Sowing Date 1.10.3 Plant Breeding

Climate Change and Agricultural Ecosystems DOI: https://doi.org/10.1016/B978-0-12-816483-9.00001-3

Copyright © 2019 Elsevier Inc. All rights reserved.

2 4 5 5 5 6 8 8 9 10 10 11 11 12 15 17 17 18 18

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1.11 Conclusion and Recommendations Acknowledgment Conflict of interest References Further Reading

19 19 20 20 23

1.1 INTRODUCTION Agricultural production is extremely sensitive to climate change and weather conditions. Agricultural production is affected by climate change in various ways, such as changes in average monthly or annual temperature, rainfall variability, insect pests and plant pathogens, alterations in atmospheric carbon dioxide (CO2), atmospheric nitrogen (N) deposition, changes in the concentration of various gases available in the atmosphere, and changes in soil nutrient availability. Production under changing climate conditions can be enhanced by improving the current understanding of the reactions of certain plant species against environmental change (Bita and Gerats, 2013). The world population surpassed seven billion in 2011. Approximately 80% of the population lives in less-developed or developing countries. It is estimated that by 2050, the population that resides in less-developed or developing countries will be 90% (CGIAR-ISPC, 2010). The demand for food supply will increase with the ever-increasing population and if food production cannot keep up the population growth, there may be chronic food shortage. Globally, most of the food supply comes from cereals and these crops are highly sensitive to changing climatic and weather conditions. At present, approximately, 1600 million hectares of land of the total geographic area is utilized for crop production (FAO, 2008a). Studies have suggested that approximately 50 1600 ha land is suitable for agricultural expansion (Delft, 2008). Dry lands account for approximately 41% of the total geographic area of the world (Safriel and Adeel, 2005). In dry lands most of the rainfall evaporates due to high temperature, while tropical storms cause surface runoff and soil erosion. Rainfall is highly variable in dry lands both within the same year and also between years; also variable in short distance. Rainfall variability is the main cause of loss in agricultural production (Bray et al., 2000). Good seed is crucial for the livelihood of farmers. Agricultural production is also affected by the availability of quality seeds in a sufficient amount at affordable prices for farmers. New and improved varieties of crops with a wider range of adaptation under changing climate conditions

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may be a good move for combating food scarcity. The seed industry is, therefore, playing a vital role in increasing agricultural productivity. The increase in the mean global temperature and uneven precipitation will have a significant impact not only on human lives, but also on livelihoods and crop productivity. Moreover, considerable effect will be posed for the organisms having low population. Farmers working on low land areas, thus, having lower crop produce, will be severely affected. The rapid increase of greenhouse gas concentrations, elevated soil and water temperatures of marine and freshwater ecosystems, raised incidence and degree of severe events causing large-scale hazards to several trade and industry-based processes, and the decline of potable as well as irrigation water availability will severely hamper the sustainability of agriculture and food availability, particularly in developing countries. Fertile land is already extensively being used for agricultural productivity. Since no considerable increase in cultivable agricultural fields is expected in the future, humanity will suffer from many food-related difficulties. Under these conditions, fertile agricultural fields will be under severe stress to enhance productivity, keeping in mind the necessity of maintaining agricultural sustainability. The capability to effectively manage our precious natural resources, managing overall sustainability will be more challenging for developing countries. Approximately 80% differences in crop productivity are associated with changes in environmental conditions. In most developing countries, agricultural productivity, economy, and increased food security is intimately linked with rainfall. Natural calamities such as very low rainfall, the spread of plant diseases, and abiotic stresses such as floods and drought would have negative consequences in the form of crop loss, famine, changes in human habitat, livelihood, and the nation’s economy. According to an estimate (Hinrichsen and Tacio, 2002), up to the year 2025 nearly 30% of the global population will suffer from water paucity. The next 20 years after 2025 will experience an even worse situation. By 2045, there will be approximately 17% increase in the demand for water for agricultural purposes with an overall increase in water demand by 40%. Nearly 70% of freshwater reserves in developing countries is utilized for agricultural irrigation. The physicochemical characteristics of the soil and differences in crops grown are important for processes such as transfer of water vapor and heat from soil to ambient environment, which in turn are responsible for environmental changes. Incidences of environmental disasters such as flooding and drought have a severe impact on crop productivity, particularly at low latitude.

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Worldwide, crop productivity has been assumed to be increased due to the rise in mean local temperature from 1°C to 3°C; however, an increase beyond this range would reduce overall productivity. In contrast, a smaller rise in temperature (1°C 2°C) in dry tropical areas is expected to significantly reduce crop productivity leading to an increase in the frequency of mass starvation. Agricultural yield is expected to enhance up to some extent at mid to high latitudes due to an average increase in temperature of between 1°C and 3°C, and decrease after that. Rise in mean temperature would facilitate the heavy increase in insect-pests population with damaging effects on natural ecosystem. Crop productivity is also speculated to decrease in arid and semiarid regions of the globe. By the middle of the 21st century, East and South-East Asia are expected to experience 20% yield enhancement, whereas Central and South Asia are expected to face approximately 20% fall in overall productivity. Already dry regions such as Latin America face the overwhelming problem of increased soil salinity and land desertification. Climate changes will have detrimental consequences not only on the productivity of several important crop plant species, but also on the population of animals and their productivity. In this chapter, the effects of climate change on various components of agricultural production are considered.

1.2 CLIMATE CHANGE AND SUSTAINABLE AGRICULTURE The negative effects of climate change on agricultural systems can be considerably minimized by carbon sequestration phenomena. Another important potential strategy is based on the processes responsible for lowering the emission of CH4 and N2O from agricultural systems. Reduced emission can be achieved by adding organic material derived from crop plants, however, high market value has limited its applicability as a viable option. Several strategies can be implemented in agriculture in order to mitigate the impacts of climate change. An important approach involves the raising of different varieties of crop plants, the advancement and encouragement of substitutes for commonly employed crops, the release of stress tolerant varieties through genetic engineering and breeding techniques, frequent raising of mixed crops, application of biofertilizers and tillage practices, effective management of crop residues, application of biological techniques for weed control, development of new techniques of rain water harvesting, development of disease and pest resistant varieties, innovations in irrigation techniques, and development of reliable weather forecasting techniques.

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1.2.1 Need for Integration of Mitigation and Adaptation Frameworks Into Sustainable Development Planning Gaining a thorough understanding of the deleterious impacts of climate change is one of the most crucial segments of sustainable environment development programs. Many of the policies pertaining to environmental changes and sustainable progress generally act in an additive manner. Managing heavy losses through the assistance of local communities and low-cost sustainable strategies to minimize habitat destruction and anthropogenic forest clearing would provide direct advantages not only for the maintenance of biodiversity, but also on terrestrial and aquatic ecosystems. Moving much closer toward sustainable progress will help promote the currently practiced mitigation strategies and adaptability to minimize emissions and alleviate the risks of climate change.

1.3 DEPENDENCE ON SUBSISTENCE AGRICULTURE The mean increase in GDP for farming from 2000/1 to 2005/6 was determined to be 2.8%. This observed increase is definitely small as compared to the rate of population increase. Interestingly, the observed increment was quite variable in different years. For instance, the rate of GDP increase was 5.5% in 2001/2 but fell to 2.2% during the next year. Similarly, GDP was only 3.9% in 2003/4 and decreased further in the next year. Drastic alterations in 2005/6 were attributed to harsh environmental conditions leading to the reduced productivity of rice, barley, and wheat.

1.4 IMPACTS OF CLIMATE CHANGE ON AGRICULTURAL PRODUCTION AROUND THE GLOBE Although changes in environmental conditions have various impacts on agriculture, some important challenges associated with crop productivity will be discussed here, in addition to changes in water availability, alterations in mean sea level, changes in biodiversity, and most importantly human health, which are all important concerns pertinent to developing nations such as India. Changes in environmental conditions, such as rainfall, average temperature, and water produced from melting icebergs, have considerable impact on farming. Climate change affects crop productivity in multiple ways. Climate change will severely affect the accessibility of food around the world. Significant losses in agricultural productivity would, thus, not only affect food security but also the business economy.

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Nearly 39% of the world’s population is dependent on agricultural practices for their employment. Approximately half of the total population residing in Asia and the Pacific and 63% of the population of Sub-Saharan Africa are estimated to find employment solely from agriculture. The debilitating effects of climate change will exert huge pressure on the economic status of many developing nations; posing multiple risks to the livelihoods of a large portion of the population in addition to greatly increasing the threat to food accessibility. Reduced crop productivity is attributed to variations in water requirements for irrigation as well as in light intensities leading to changes in the rates of photosynthesis of crops as well as to crop damages due to pest attack. Increases in temperature resulting from a rise in the concentration of greenhouse gases (GHGs) in the environment has been expected to pose differential responses in plants from varying geographical conditions. According to an estimate, even a slight increase in temperature of between 1°C and 3°C may impose positive effects on agricultural productivity in the temperate regions of the globe, while dry tropics are assumed to face a reduced productivity in several important cereal yielding crops under the influence of a slight increase in temperature of between 1°C and 2°C. Generally, a rise in temperature of more than 3°C has been speculated to minimize crop productivity in all parts of the globe. The reports of the third meeting held by the IPCC in 2001 summarized that the changes in global environmental conditions would highly worsen the status of economically weak countries by hindering agricultural productivity. The reduction in agricultural productivity would be observed in both the tropical and subtropical regions of the world; most probably due to poor accessibility of irrigation water and alterations in degree of insect pest infestations and their population sizes. Different effects of climate change on agriculture are represented in Fig. 1.1.

1.4.1 Effects of Climate Change on Indian Agriculture Farming is the backbone of Indian economy which provides food and employment to a major portion of the population (Srivastava et al., 2016). The multidimensional, deleterious, effects of changes in environmental conditions on agricultural productivity have been monitored many times. Since most of the agricultural practices performed in India are mainly based on rainfall, changes in spatial and temporal rainfall patterns (as influenced by changes in climatic conditions) will drastically modify crop

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Figure 1.1 An illustrative representation of impact of climate change on different natural resources and activities affecting agricultural production.

productivity. Even a moderate shift in the rainfall pattern would disturb overall productivity, and thus the economy, food availability, and employment. Around 70% of the yearly rainfall in India is contributed by summer rainfall, thus, having a major impact on overall crop productivity. Field researches have forecast a substantial decrease in annual rainfall during summer by the year 2050. With increases in the mean global temperature, the West will get comparatively higher rainfall as opposed to parts of central India, which will face a reduction in winter rainfall ranging between 10% and 20% by the year 2050. Therefore, even a slight change in climatic conditions would pose great risks to water availability in arid and semiarid parts of India. One of the most productive Indo-Gangetic regions would also suffer from decrease in crop productivity due to reduced water availability caused by changes in climatic conditions. Rising temperatures will not only increase expenses on crop production by rising the required amount of fertilizer, but will also increase the emission of GHGs and release of ammonia into the atmosphere from agricultural lands. Changes in environmental conditions would aggravate the occurrences of natural events, like drought, rainstorms, and inundations of large areas, thus, leading to reduced crop productivity. Hence, environmentalists and policy makers should consider safeguarding both human life and valuable life sustaining processes.

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1.5 EFFECTS OF CLIMATE CHANGE ON FOOD SECURITY Estimations by FAO have shown that the overall reduction in crop productivity may reach up to 25% caused by elevations in temperature of between 3°C and 4°C (FAO, 2008b). As expected, an approximately 280 million ton decline in important grain productivity would put the economy of Africa and Asia at extreme danger (FAO, 2005). The productivity of rainfall dependent crops, such as pulses, which constitute around 60% of the fertile land in India, will be greatly influenced (Basu et al., 2009). Crop productivity is expected to be reduced by between 4% and 10% in Asian nations because of alterations in environmental conditions (Fischer et al., 2005). A report by IRRI (2004) demonstrated that for every 1°C rise in temperature, a reduction of 10% in productivity of rice crops may occur. A critical assessment of available worldwide data with regard to crop productivity between 1981 and 2002 indicated a reduced productivity of wheat, maize, and barley accounting for an annual US$5 billion loss (Lobell and Field, 2007). An increase in mean temperature was observed to exert great losses on wheat productivity during 2003 and 2004 (Gahukar, 2009). A reduced productivity was also recorded for sorghum, rice, and maize. A significant reduction in rice yield is expected, reaching up to 6.7%, 15.1%, and 28.2% in 2020, 2050, and 2080, respectively (Agrawal, 2011). An overall decrement of crop productivity higher than or equivalent to 30% has been expected for India and Africa (Cline, 2007) combined. Since agriculture is the main generator of economy in developing nations, they are greatly influenced by changes in climatic conditions, including changes in temperature and precipitation. Furthermore, changing environmental conditions would lead to more severe impacts in developing countries due to the lack of sophisticated and advanced tools and methodologies.

1.6 EFFECT OF CLIMATE CHANGE ON GEOGRAPHICAL DISTRIBUTION OF CROP SPECIES Increases in mean temperature are intimately linked to altered crop yields and early commencement of vital reproductive phenomena, which are specifically determined by geographical conditions and the crop plant under investigation, along with habitat displacement to elevated latitudes (Hedhly et al., 2008). Significant modifications in different stages of life cycle under the influence of changing environmental conditions may be

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capable of promoting the simultaneous occurrence of different plant species leading to enhanced competition and culminating in complete modifications in existing community structures (Chuine and Beaubien, 2001; Post et al., 2001). It is also expected that different climatic zones would shift towards the poles up to 550 km by 2100 causing considerable changes in prevalent ecological conditions and agricultural output (Agrawal, 2011).

1.7 EFFECT OF CLIMATE CHANGE ON INSECT PEST AND DISEASE DEVELOPMENT Crops growing in a given habitat are continuously affected by direct and indirect interactions with different environmental factors. Quite frequently agricultural crops struggle to harness natural resources against other coexisting crop plants as well as wild plants. Crop plants together with several other wild plant species may be influenced positively or negatively by different soil microorganisms, insect pests, plant pathogens, and herbivores. Insect mediated destruction happens through intensive feeding and sucking of the sap of plant leaves and stems. Furthermore, many plant viral diseases are also spread by insects. Worldwide, approximately 70,000 different pest species, consisting of 9000 insect and mite species, 50,000 crop disease causing agents, and 8000 undesirable competitor plant, that is, weeds, have been estimated (Pimentel, 2009). Around 10% of the expected pests are major risks to agricultural productivity. Modifications in global environmental conditions have exerted multifold alterations in the rate of multiplication, distribution, and survival of several plant pathogens as well as insect pests. An increase in the average temperature of a given area would induce rapid multiplication of pathogens and pests transmitted by air currents (Bouma, 2008). Temperature variations have considerable impacts on the behavior, distribution, and disease transmission of insects, while plant diseases caused by particular agents are greatly influenced by moisture content, temperature, and precipitation (Coakley et al., 1999). High temperatures increase the rate of multiplication of insect pests. Although, greater increases in temperature inhibit their multiplication, temperature increases during winter facilitate their population increase on the commencement of the next crop. Drought has been demonstrated to exert changes in the normal physiological processes of numerous host plants upon which insect pests rely for food, hence, causing crucial changes not only in the population density of insect pests but also to beneficial arthropods, predators, and

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birds (Rosenzweig et al., 2001). Yamamura and Kiritani (1998) reported that each 2°C rise in temperature may lead to rapid completion of life cycle of insects (extra one to five life cycles per period). Changes in the mean temperature of a geographical region has a greater influence on insect behavior than plant and animal species. Agricultural crops are continuously under the risk of heavy loss due to diseases caused by pathogenic fungi, bacteria, nematodes, viruses, and viroids. The occurrence and distribution of several plant pathogens are determined by changes in environmental conditions and farming processes. Several pathogens, including viruses and phytoplasmas, are frequently transported through vectors such as insects (Weintraub and Beanland, 2006). Environmental variables, such as moisture content, light intensity, precipitation, and temperature have great influence on the life cycle and transfer of pathogenic fungi and bacteria (Patterson et al., 1999). Some environmental conditions that exert significant impacts on disease development include the concentration of ozone, the amount of UV-B radiation (Manning and von Tiedemann, 1995) reaching the Earth’s surface, and soil nutrient status (particularly nitrogen and phosphorus) (Thompson et al., 1993). In addition, the existence, transmission, and multiplication of pathogens in an environment are also affected by climatic conditions, such as heat, rainfall, moisture content, sunlight intensity, and air current movement and patterns. Enhanced temperature and moisture availability in an environment facilitates the germination of dormant spores of pathogenic fungi and bacteria leading to rapid disease development.

1.8 EFFECT OF CLIMATE CHANGE ON ONTOGENY OF CROPS 1.8.1 Phenology Changes in environmental conditions have detrimental impacts on crops (Cleland et al., 2007). Different life cycle events related to reproduction are extremely responsive to nonbiological environmental factors, including irradiation, heat, and available water, therefore, it is imperative to recognize fruiting and flowering behavior under flexible environmental conditions. High temperature environments would drastically alter plant phenology in tropical parts of the globe (Abrol and Ingram, 1996). Kudo et al. (2004) demonstrated that elevated temperatures may modify different life cycle events, like blossoming, bud opening, seed formation, and vital physiological activities. Changes can also be observed in terms of leaf morphology, their numbers, and their arrangement. Elevated temperatures

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are considered to accelerate the different phases of the life cycles of several crops and most remarkably, can cause severe effects on reproductive organs because of the high susceptibility of certain crops to even moderate changes in temperature culminating in decreased seed formation, and thus, agricultural productivity (Porter, 2005). Hence, the specific durations and magnitude of temperature increases would put more detrimental impacts on agriculture ecosystem (White et al., 2006).

1.8.2 Flowering Flowering in a plant’s life cycle is significantly regulated by the prevailing temperature and solar irradiation, and the consequences on floral physiology of temperatures beyond the suboptimal are well established (Wallace and Yan, 1998). Currently, technical advancements in molecular biology have enhanced the current understanding of the genes governing floral development processes (Baurle and Dean, 2006; Tsuji et al., 2008). Crop plants start flowering after a time interval, and thus, simultaneously occurring multiple vital physiological processes including floral development under stress condition may result into retarded growth and development (Prasad et al., 2008). The prevalence of high temperature during blossoming has been reported to result in a decline in pollen viability, and thus, reduced crop yields in legumes, including groundnut (Prasad et al., 2000); Phaseolus vulgaris L. (Prasad et al., 2002), Vigna unguiculata (L.) (Ahmed et al., 1992), soybean (Salem et al., 2007; Djanaguiraman et al., 2010, 2011) and cereals, including rice (Jagadish et al., 2007; Prasad et al., 2006; Satake and Yoshida, 1978) and wheat (Saini et al., 1983). Remarkable reduction in number of flower is also associated with the declined seed number and seed productivity in Brassica napus (Angadi et al., 2000), Brassica rapa (Morrison and Stewart, 2002), and Brassica juncea (Gan et al., 2004).

1.8.3 Pollination Changes in environmental conditions have led to significant impacts on the occurrence and distribution of a number of plant, animal, microbe, insect, and bird species worldwide. Important insects, birds, and animals that facilitate pollination events may react by restricting or extending their area of distribution under varying climate change conditions. Hence, great population reductions in numerous insects or other pollinating organisms as well as considerable differences in the distances between pollinating

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individuals and crop plants pose significant risks to sustainable agricultural activities. Furthermore, shifts in temperature will modify the behavior and specific time regime of the interactions between different organisms of agricultural ecosystems. For instance, floral processes and the appearance of pollinating insects are highly sensitive to changes in temperature in their vicinity, which would ultimately introduce destructive consequences on the normal functioning of several plant species and agricultural ecosystems (Harrington et al., 1999). The importance of pollination in terms of economy has been estimated to be equivalent to US$ 224 billion accounting for 9.5% of worldwide agricultural productivity in 2005 (Gallai et al., 2008). Most importantly, many agricultural products of high economic importance are dependent on specific pollinating organisms. The Food and Agriculture Organization (FAO) has evaluated that amongst the more than one hundred crop plants supplying around 90% of the food demands of 146 nations, 71 are pollinated mainly by wild bees and the rest by different arthropods, including moths, wasps, insects, etc. Moreover, reduction in pollination activities would result in the decline of both the quality as well as the quantity of important fruits (Klein et al., 2007) in addition to the loss of approximately 75% of agricultural produce (such as vegetables, fruits, legumes, cereals, etc.). Therefore, changes in environmental conditions would lead to alterations in the behavior and diversity of pollinator species and would lead to destructive effects on agricultural productivity.

1.8.4 Seed Quality 1.8.4.1 Seed Size Abiotic factors, such as heat stress, have a destructive impact on seeds. An elevation of heat stress has been demonstrated to enhance the number of desiccated seeds together with a reduction in seed dimensions (Prasad et al., 2003). Seed protein content in soybean was observed to increase due to rising temperatures beyond 25°C (Piper and Boote, 1999). Heat stress caused contrasting effects on the oil and protein contents of the seeds (Piper and Boote, 1999). Contrary to this, the effect of heat stress on the oil and protein contents of laboratory propagated soybean seeds had similar consequences, which were also determined using the total dry matter content of the seeds (Pipolo et al., 2004). Hence, the effect of increased temperature on seed dimensions is probably due to alterations in the rate of seed development, which is largely determined by the translocation of carbon and nitrogen into developing seeds (Pipolo et al., 2004).

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1.8.4.2 Dormancy Seed dormancy is an important attribute from an agricultural point of view. In many events, dormancy is not only a limitation to plant growth and development but also a crucial hindrance in the determination of seed quality. The dormant behavior of seeds is affected by abiotic factors, including solar irradiation, water, temperature, moisture content, nutrient status, and altitude. Since the effect of a particular altered environmental factor depends on the type of dormancy, few general observations can be made. For instance, increased temperature, water scarcity for long periods, and low light duration are generally expected to promote germination and minimize seed dormancy. Elevated temperature at the time of seed formation facilitates reduced formation of abscisic acid and enhanced production of hormones, like gibberellins, leading to a reduction in the dormant nature of seeds. But, contradictory reports indicating an enhancement in dormancy after temperature increases have also been described. Furthermore, the nature of the reactions exhibited by developing seeds under limited water supply is also affected by type of dormancy. In the case of seed coat associated dormancy, continuous drought conditions promote the formation of considerably thickened seed coats leading to minimized seed germination. Contrary to this, in a seed relying on the synthesis of a specific inhibitory or promoting compound, seed dormancy is markedly reduced under drought conditions. Moreover, seed germination ability is also influenced by solar irradiation, duration of sunlight, as well as light quality of maternal plants from which seeds are produced (Contreras et al., 2008). 1.8.4.3 Seed Mass The impacts of increasing concentrations of CO2 on seed weight are quite inconsistent between and within species. Enhancements are higher in leguminous crops as compared to nonleguminous crops, and rise in seed mass may be associated with reduced seed nitrogen content in nonleguminous crops. Rising temperatures may lead to reductions in seed mass due to increased growth rates and rapid seed development. However, a low seed mass does not indicate a decline in seed germination ability and vigor index (Ambika et al., 2014). 1.8.4.4 Germination Similar to seed mass, seed germination under increasing CO2 concentrations is extremely conflicting and different among crop plants. The

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observed modifications in C/N proportions can result in diminished protein content and viability of seeds triggered by restricted supply of essential amino acids necessary for translation during embryonic development. In contrast, high concentrations of CO2 in the environment induces the synthesis of ethylene, which in turn stimulates germination efficiency in several plant species. In conclusion, temperature elevations prior to complete seed development at biochemical and physiological levels can cause (1) a reduction in seed germination by restricting a plant’s potential to provide the essential components required for the production of storage biomolecules helping in seed maturity, and (2) severe damage to seeds at biochemical and physiological levels culminating in significant loss of seed viability. 1.8.4.5 Seed Vigor No information is currently available pertaining to the impact of high concentrations of CO2 on seed vigor, but detrimental effects of increased temperature prior to as well as subsequent to seed maturity are inevitable. An increase in mean temperature accelerates the inhibition of the vital physiological processes of seeds severely (Akman, 2009); as supported by previous reports indicating a decline in seed vigor even in the presence of high temperature stress for extremely short durations at the crucial stage of seed development. 1.8.4.6 Yield Field experiments have demonstrated that the optimum temperature requirement for crop productivity in rice, peanut, and soybean was 25°C, beyond which there was a significant reduction in yield, whereas an absolute loss in crop yield in rice and sorghum was observed at 35°C and between 39°C and 40°C for peanut and soybean (Boote et al., 2005). In another experiment conducted on rice, Peng et al. (2004) demonstrated that in dry conditions a 1°C rise in temperature resulted in a 10% loss in productivity. They concluded that the loss in rice productivity was attributed to increased temperature during the night. The preceding century has experienced rapid increases in minimum temperatures as compared to maximum temperatures along with constant increments in GHG concentrations (Easterling et al., 1997). Enhancements in productivity were recorded for temperature increases up to 29°C, 30°C, 32°C for corn, soybean, and cotton, respectively; however, temperatures above the mentioned limits caused significant losses in productivity. Considering research

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reports, crop productivity is predicted to be reduced by 30% 46% prior to the completion of the century in the case of the slowest (B1) warming scenario, and to reach to a reduction of 63% 82% under the most rapid warming scenario (A1FI) according to the Hadley III model (Schlenker and Roberts, 2009). In the United States, for each 1°C rise in temperature, an approximately 17% loss in productivity in maize and soybean was observed due to the prevalence of the complicated actions of various inhibitory factors under such conditions (Lonbell and Asner, 2003). Lobell and Field (2007) described the effects of temperature and precipitation for the period of 1961 2002 and concluded that there was an 8.3% decline in productivity for each 1°C increase in temperature.

1.9 IMPACT OF CLIMATE CHANGE ON ABIOTIC ENVIRONMENTAL FACTORS AFFECTING CROP PRODUCTIVITY Plant reactions to increased CO2 concentrations are affected by abiotic factors, including solar irradiation, water availability, and essential nutrients. Increased CO2 contents enhance photosynthesis and crop productivity at varying solar irradiances. The most remarkable effects would be experienced by plants at compensation point. However, differential responses to elevated CO2 concentrations and solar irradiations may exist in different species. One experimental observation has shown that sciophytes benefit more to enhanced CO2 content as compared to heliophytes. Generally, it is assumed that plant physiological processes at low light intensities are significantly limited by carbon source availability, and thus, react positively to elevated CO2 levels. In general, high CO2 content leads to a reduction in stomatal conductivity, thus, resulting in a decline in transpiration, overall utilization of water, and enhanced water-use-efficiency (WUE). Hence, the longevity of plants facing a scarcity of water in a given habitat would be enhanced in an environment with high CO2 concentration. Although, the observed impacts would be low because of requital of water through slow rate of transpiration performed by leaves with high area and increased tissue temperature. Ordinarily, plants with a low nutrient pool react weakly against enhanced CO2 levels. Under poor nutrient conditions, plant developmental processes are not constrained by the presence of different carbon sources, as plants having a reduced nutrient pool possess sufficient quantities of starch as well as other carbon molecules. Thus, the enhancement of

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photosynthesis under the influence of elevated CO2 content, thus, causing an increase in the carbon pool is not expected to promote plant growth significantly unless plants are efficient in acquiring high contents of minerals and nutrients. Reports are also available that show an enhancement in the efficiency of nitrogen utilization under enhanced CO2 concentrations due to the induced rate of photosynthesis even under low leaf nitrogen content as compared to ambient CO2 levels. In addition, enhanced UV-B radiation may cause significant impacts on reproduction processes, and therefore, on the total number of seeds and seed weight. Usually, plant reproductive parts (pollens and ovules) are well enclosed by structures like sepals, petals, and the ovary wall. Pollen development in these plants is only possible after the pollen reaches receptive sites that is, stigma. Studies have demonstrated that UV-B irradiation diminishes pollen grain germination and the rate of pollen tube elongation by between 10% and 25% in crops, including rye (Secale cereale L.), maize (Zea mays), and tobacco (Nicotiana tobacum). Elevated UV-B irradiation has been reported to reduce the formation of pollen grains, pollen tube germination, and the rate of pollen tube growth (Koti et al., 2005). Therefore, changes associated with pollen grains exert negative impacts on fertilization phenomena in UV-B sensitive plants leading to the formation of a low number of seeds. Most importantly, after successful entry of pollen tube within the stigma, the negative effects are alleviated to a greater extent by protective nature of style and stigma against the UV-B exposure. Under the influence of UV-B irradiation, reduced growth in the leaves and shoots of many plant species in field and laboratory conditions has been shown (Kakani et al., 2003). The observed reductions in leaf and shoot growth are associated with a decline in cell proliferation as opposed to a reduction in cellular size. With respect to the effect of UV-B irradiation, most crop plants (60%) showed reduced dry matter synthesis, approximately 24% were not affected at all, and a mere 8% of crops showed enhanced total dry matter (Kakani et al., 2003). Reductions in crop productivity result from declines in the number of fruit (grains) caused by unsuccessful fertilization, destruction of floral organs, and reduced fruit size due to limited supply of essential assimilates for the development of fruit. Contrasting impacts of UV-B irradiation on crop productivity under both field and laboratory conditions have been presented by Kakani et al. (2003). They concluded that elevated UV-B radiation markedly decreased productivity in 50% of the plants that were

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sensitive. Rest of the grown plants were nonresponsive to UV-B irradiation. However, a few plants were reported to have positive effect (i.e., increase in productivity) under the exposure of UV-B. The observed discrepancies were due to the differences in intensity of UV-B light. Moreover, the altered reactions against UV-B irradiation could also be due to variations in cultivar response and cultivated plant species.

1.10 STRATEGIES FOR ADAPTATION Various strategies have been suggested to alleviate the effects of climate change. Important measures are presented in Fig. 1.2. Important methods adapted for the mitigation of climate change effects are discussed in Sections 1.10.1 1.10.3.

1.10.1 Changes in Seed Production Locations Models related to changes in environmental conditions expect an overall increase in agriculturally productive areas worldwide, most notably in North America, Northern Europe, and the Russian Federation, due to higher cropping seasons and more favorable climatic conditions with

Climate change

Improved farm wellbeing/ improved food security

Rainfall variability change in weather pattern

Flood drought water stress resource shortage

Disease/pest

Climate smart agriculture, mitigation, adaptation

Need adaptive measures

Change in date of showing

Reduce climate risk

Make farming productive and sustainable

Adaptation policies and strategies

Figure 1.2 Strategies for alleviating the climate change effects

1. Empower farmers 2. Formulation for agriculture 3. Enable agriculture innovation

Interer opping :potential and challenges

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increased temperatures (Fischer et al., 2005). For instance, it was speculated that the warming of the environment in parts of Europe would extend the agricultural lands for all cereals and crops of both low temperature (pea, canola) and high temperature seasons (soybean, sunflower) toward the northern regions, while productivity will be minimized in the Mediterranean region (Olesen and Bindi, 2002). Considerable reductions in fertile cultivated lands are expected for northern and Southern Africa because of a deterioration of cultivatable environments caused by elevated temperature as well as moisture stress (Fischer et al., 2005). Thus, it would be important to shift agricultural practices toward latitude or elevation for seed production to minimize negative consequences of changing environmental conditions on seed yield and quality (Farrar and Williams, 1991).

1.10.2 Changing Sowing Date An important strategy to escape the deleterious impacts of rising temperatures caused by changes in environmental conditions on floral organ development and seed formation is to variate the time of sowing (Singh et al., 2013). Changing the seed sowing time (sowing prior to normal date to escape the effect of heat stress in summer) has been recommended as an effective method for colder regions of Europe (Olesen et al., 2011), whereas sowing time may be extended further by a few days for every 1°C increase in environmental temperature (Kalra et al., 2008).

1.10.3 Plant Breeding Plant breeding techniques are extremely successful and have been widely used in agriculture to enhance the yield of several crop plants over the past five decades (Jaggard et al., 2010). In addition, under changing environmental conditions plant breeding is even more desirable for the development of crop varieties resistant to multiple environmental stresses (Ceccarelli et al., 2010). Important attributes of plant varieties (cultivars) that should be considered for development through plant breeding under changing environmental conditions include resistance against drought, high temperature, salinity, flooding, and insect pest infestation (Ceccarelli et al., 2010). Thus, plants with the aforementioned characteristics can be raised through the application of classical as well as advanced molecular biology and genetic engineering principles (Cairns, 2013).

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1.11 CONCLUSION AND RECOMMENDATIONS Research studies have demonstrated that variations in environmental conditions would have a more crucial impact on the economy of several small farmers either through yield enhancement or reduction. Reports have shown that most crops are negatively affected by changes in environmental conditions. In contrast, precipitation would have a positive influence on maize and other crops, whereas it would have detrimental consequences on the economy generated from tea plantations. Long-term impacts of changing temperatures have greater effects on crop yield as compared to short duration impacts, which advocates for the implementation of adaptive measures to eliminate the effects of climate change. There is a need to introduce modifications into agricultural activities to alleviate the threat imposed by varying environmental conditions. According to estimates, changes in environmental conditions would drastically change agricultural productivity by 2020, 2030, and 2040 if effective measures are not be taken into consideration, with the most visible risk being experienced by tea productivity in Kenya. Temperature is regarded as a good indicator of increases in mean global temperature as compared to rainfall in some countries. Therefore, it is imperative to consider the possible threats associated with climate variations in upcoming years and to incorporate them into national policies concerned with agriculture and the environment. Policies should be implemented to keep the natural environment protected to the maximum possible extent along with crop insurance for at least small-scale farmers belonging to different geographical regions in order to overcome the effects of climate change. Since anthropogenic activities are the most important factor linked to current changes in environmental conditions, it would be appropriate to put forward some fraction of capital at local, regional, national, and international levels. Furthermore, farmers should implement agricultural activities based on sustainable agricultural practices as well as implementing the application of cultivars resistant to different abiotic stresses, such as drought, high temperature, salinity, and water logging, along with greater inputs into farming.

ACKNOWLEDGMENT The authors are thankful to UGC and CSIR for providing fellowship. Rahul Bhadouria is thankful to UGC (BSR/BL/17-18/0067) for providing Dr. D. S. Kothari fellowship for Post-Doctoral Research.

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CONFLICT OF INTEREST Authors declare no conflict of interest

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Prasad, P.V.V., Boote, K.J., Allen Jr., L.H., Sheehy, J.E., Thomas, J.M.G., 2006. Species, ecotype and cultivar differences in spikelet fertility and harvest index of rice in response to high temperature stress. Field Crops Res. 95, 398 411. Prasad, P.V.V., Staggenborg, S.A., Ristic, Z., 2008. Impacts of drought and/or heat stress on physiological, developmental, growth, and yield processes of crop plants. Adv. Agric. Sys. Model. 1, 301 355. Rosenzweig, C., Iglesias, A., Yang, X.B., Epstein, P.R., Chivian, E., 2001. Climate change and extreme weather event: implications for food production, plant diseases, and pests. Glob. Change Hum. Health 2, 2. Safriel, U., Adeel, Z., 2005. Dryland systems. In: Hassan, R., Scholes, R., Ash, N. (Eds.), Ecosystems and Human Well-Being: Current State and Trends. Island Press, Washington, DC, pp. 625 656. Saini, H.S., Sedgley, M., Aspinall, D., 1983. Effect of heat stress during floral development on pollen tube growth and ovary anatomy in wheat (Triticum aestivum L.). Aust. J. Plant Physiol. 10, 137 144. Salem, M.A., Kakani, V.G., Koti, S., Reddy, K.R., 2007. Pollen-based screening of soybean genotype for high temperatures. Crop Sci. 47, 219 231. Satake, T., Yoshida, S., 1978. High temperature-induced sterility in indica rice at flowering. Jpn. J. Crop Sci. 47, 6 10. Schlenker, W., Roberts, M.J., 2009. Nonlinear temperature effects indicate severe damages to U.S. crop yields under climate change. Proc. Natl. Acad. Sci. U S A 106, 15594 15598. Singh, R.P., Prasad, P.V., Reddy, K.R., 2013. Impacts of changing climate and climate variability on seed production and seed industry, Advances in Agronomy, vol. 118. Academic Press, Oxford, pp. 49 110. Srivastava, P., Singh, R., Tripathi, S., Raghubanshi, A.S., 2016. An urgent need of sustainable thinking in agriculture—an Indian scenario. Ecol. Indic. 67, 611 622. Available from: https://doi.org/10.1016/j.ecolind.2016.03.015. Thompson, G.B., Brown, J.K.M., Woodward, F.I., 1993. The effects of host carbon dioxide, nitrogen and water supply on the infection of wheat by powdery mildew and aphids. Plant Cell. Environ. 16, 687 694. Tsuji, H., Tamaki, S., Komiya, R., 2008. Florigen and the photoperiodic control of flowering in rice. Rice 1, 25 35. Wallace, D.H., Yan, W., 1998. Plant Breeding and Whole-System Physiology. Improving Adaptation, Maturity and Yield. CABI, Wallingford. Weintraub, P.G., Beanland, L., 2006. Insect vectors of phytoplasmas. Annu. Rev. Entomol. 51, 91 111. White, M.A., Diffenbaugh, N.S., Jones, G.V., Pal, J.S., Giorgi, F., 2006. Extreme heat reduces and shifts United States premium wine production in the 21st century. Proc. Natl. Acad. Sci. U S A 103, 11217 11222. Yamamura, K., Kiritani, K., 1998. A simple method to estimate the potential increase in the number of generations under global warming in temperate zones. Appl. Entomol. Zool. 33, 289 298.

FURTHER READING Singh, R., Srivastava, P., Singh, P., Upadhyay, S., Raghubanshi, A.S., 2017. Human overpopulation and food security: challenges for the agriculture sustainability. Environmental Issues Surrounding Human Overpopulation. IGI Global, Hershey, Pennsylvania, pp. 12 39. United Nations, 2011. World demographic trends. Report of the Secretary-General.

CHAPTER 2

Sustainable Agricultural Practices Using Beneficial Fungi Under Changing Climate Scenario Vipin Kumar Singh, Monika Singh, Sandeep Kumar Singh, Chandramohan Kumar and Ajay Kumar

Department of Botany, Center of Advanced Study in Botany, Institute of Science, Banaras Hindu University, Varanasi, India

Contents Introduction Fungi as a Biofertilizer Fungi as a Biocontrol Agent Application of Fungi in Bioremediation of Contaminated Soils (Mycoremediation) 2.5 Conclusion References 2.1 2.2 2.3 2.4

25 26 31 34 37 38

2.1 INTRODUCTION In the recent past, chemical fertilizers and pesticides have been broadly used in the conventional agriculture system to enhance the yield and productivity of crops but the continuous application of chemical fertilizers directly or indirectly plays a significant role in the changing climatic conditions. Currently, it has been estimated that the population of the world could reach approximately 9 billion by 2050 (Béné et al., 2015). Therefore, to maintain the current status of food security for the extra population using the limited land available is a major challenge. Presently farmers use a huge amount of chemical fertilizers to achieve maximum yields, but these chemicals adversely affect the texture and productivity of plants and soil (Galloway et al., 2008; Youssef and Eissa, 2014). To overcome the problem of chemicals pesticides and achieve the food security of global rising population there is immediate need for sustainable approach of agriculture. Sustainable agriculture is considered as an advanced or Climate Change and Agricultural Ecosystems DOI: https://doi.org/10.1016/B978-0-12-816483-9.00002-5

Copyright © 2019 Elsevier Inc. All rights reserved.

25

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broad concept within agriculture, in which the maximum output is gained on the minimum cost without adversely affecting the needs of the future. For the past few decades, beneficial microbes, such as plant growth promoting bacteria, fungi, or cyanobacteria, have been used in sustainable agriculture as plant or soil inoculants to enhance crop production by acting as a biofertilizer, biopesticide, or by managing biotic or abiotic stress tolerance (Singh et al., 2017a,b, 2018; Kumar et al., 2018; Lengai and Muthomi, 2018; Glick, 2018). Fungus is an eukaryotic microbes comprising of various unicellular or multicellular species which could be either beneficial or pathogenic. Currently and for the past two decades fungi have been used, singly or in co-inoculation with bacterial genera, in sustainable agriculture as biofertilizer, biocontrol, or in the management of biotic or abiotic stress (Table 2.1, Fig. 2.1). Like other microbial biofertilizers, fungi have also been directly or indirectly involved in growth promotion or disease management via different mechanisms. One such mechanism involves the solubilization of phosphate, potash, zinc, etc. Some of the most common fungi used in agriculture include Aspergillus sp. Penicillium sp. Fusarium sp., Saccharomyces, Trichoderma, Mucor, etc. (Pradhan and Sukla, 2006; Khan et al., 2011; Zahoor et al., 2017; Fraceto et al., 2018).

2.2 FUNGI AS A BIOFERTILIZER Biofertilizers are ecofriendly, inexpensive, are an important source of essential nutrients for plants, and increase soil fertility as well as play a vital role in improving soil nutrient status and, thus, crop productivity. Biofertilizers are living formulations consisting of advantageous microorganisms, including fungi, bacteria, and actinomycetes, that can be applied successfully to seeds, seedlings, plant roots, or soil and which help in the mobilization as well as the accessibility of nutrients due to their inherent biological activities (Pal et al., 2015). Fungal biofertilizers, when applied in a natural field system either alone or in combination, are known to cause a direct or indirect beneficial impact on plant development, growth, and yield through several methods (Rai et al., 2013). The roots of different plant groups, such as herbs, shrubs, trees, aquatics, xerophytes, epiphytes, hydrophytes, and terrestrial plants, growing in natural conditions, have been reported to develop mycorrhizal associations when grown in conditions with a low

Table 2.1 Fungi as a biofertilizer and biocontrol agents in the field of sustainable agriculture S. No.

Name of fungus

Plant species

Properties

Reference

1.

Ustilago esculenta JYC070, Sporisorium reilianum YL-9, Hannaella coprosmaensis YL-10 Penicillium sp. RDA01, Penicillium sp. NICS01, Penicillium sp. DFC01, Kluyveromyces walti, Pachytrichospora transvaalensis, Sacharromycopsis cataegensis Lentinus connatus

Drosera indica L.

Biofertilizer

Sun et al. (2014)

Sesamum indicum

Radhakrishnan et al. (2014)

Beta vulgaris

Biofertilizer and biocontrol Biofertilizer

Arachis hypogaea

Biocontrol

Azadirachta indica A. Juss Zea mays

Biofertilizer

Lakshmanan et al. (2008) Hirose et al. (2001)

Biofertilizer

Wu et al. (2005)

Oryza sativa

Biofertilizer

Vicia faba

Biofertilizer

Pisum sativum

Biocontrol Biocontrol

Amprayn et al. (2012) Mohamed and Gomaa (2005) Nelson et al. (1988) Alam et al. (2011)

2.

3.

4.

5. 6.

Metarhizium anisopliae and Beauveria bassiana Glomus intraradices; Azotobacter chroococcum; Bacillus megaterium; (Bacillus mucilaginous) Candida tropicalis HY (CtHY) C. tropicalis

8.

Trichoderma koningii Trichoderma harzianum Penicillium sp. EU0013

9.

Penicillium citrinum VFI-51

Solanum lycopersicum; Brassica oleracea Sorghum bicolor

10. 11.

Penicillium adametzioides Saccharomycopsis schoenii

Vitis vinifera Citrus X sinensis

Biocontrol Biocontrol

12. 13. 14.

Malus domestica Malus domestica Capsicum annuum

Biocontrol Biocontrol Biocontrol

Helianthus annuus; Ricinus communis P. sativum

Biocontrol

Allium cepa

Biocontrol

18. 19.

Leucosporidium scottii At 17 Pichia angusta Trichoderma virens IMI-392430, T. pseudokoningii IMI-392431, T. harzianum IMI-392432, T. harzianum IMI-392433 T. harzianum IMI-392434 T. harzianum Th4d SC Trichoderma asperellum Tv5SC Streptomyces lydicus WYEC108 Penicillium roqueforti; Penicillium viridicatum Candida oleophila Pichia membranifaciens

Vitis vinifera Vitis vinifera

Biocontrol Biocontrol

20.

Pichia guilliermondii

Glycine max

Biocontrol

7.

15. 16. 17.

Biocontrol

Biocontrol

Agamy et al. (2013)

Sreevidya and Gopalakrishnan (2016) Ahmed et al. (2015) Pimenta et al. (2008) Vero et al. (2013) Fiori et al. (2008) Rahman et al. (2012)

Navaneetha et al. (2015) Yuan and Crawford (1995) Khokhar et al. (2012) Droby et al. (2002) Santos and Marquina (2004) Paster et al. (1993)

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Figure 2.1 Overview of plant growth promoting fungi in sustainable agriculture.

bioavailability of essential elements, including phosphorus, nitrogen, zinc, copper, iron, sulfur, and boron (Zhu et al., 2008). Phosphate solubilizing fungal biofertilizers are one of the most commonly employed biological agents for improving plant growth and development by facilitating phosphorus uptake in plants. Fungi possessing a phosphate solubilizing property contribute significantly to the availability of soil phosphates to plants. Seven phosphate solubilizing fungi, including Trichosporon beigelii, Rhodotorula aurantiaca A, Kluyveromyces walti, Saccharomycopsis schoenii Cryptococcus luteolus, Zygoascus hellenicus, Penicillium purpurogenum var. rubrisclerotium, Neosartorya fisheri var. fischeri, and Candida montana, have been reported from Teff rhizosphere soil (Gizaw et al., 2017) and to enhance phosphate availability to plants. In addition, phosphate solubilizing fungi belonging to genera Aspergillus, Penicillium sp., and Fusarium have also been reported to be found in the rhizospheric region of different plants (Elias et al., 2016). Penicillium, Aspergillus, and Chaetomium are fungal genera of widespread occurrence. One of the most commonly employed fungi for biofertilizer production is Trichoderma, which is usually present in agricultural soils. Trichoderma sp. inhabiting the rhizosphere can also interact with and

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parasitize other fungi. Trichoderma sp. have long been recognized for their ability to augment plant productivity by enhancing crop nutrition and nutrient acquisition. Furthermore, Trichoderma sp. are also known to produce metabolites that serve as a biofungicide against disease-causing fungal pathogens (Lindsey and Baker, 1967; Chang et al., 1986; Harman, 2000; Yedidia et al., 2001; Vinale et al., 2009; Pal et al., 2015). Soil inoculation with Trichoderma sp. and its utilization as a culture filtrate has been reported to enhance plant growth and biomass production. Its ease of cultivation under laboratory conditions, has made possible of the application of this fungus as a model organism not only for exhaustive evaluation of beneficial plantmicrobe interactions but also as a new tool for the enhancement of plant productivity (Varma et al., 1999). Currently, the application of yeast as a biological fertilizer has gained considerable interest because of its biological properties and safety to humans as well as to the natural environment (Agamy et al., 2013). Brewer’s yeast (Saccharomyces cerevisiae), a byproduct of the brewing industry, has been widely utilized as biofertilizer. The amendment of yeasts (either live or dead) to soil has been demonstrated to substantially enhance the nitrogen and phosphorus availability to the roots and shoots of Solanum lycopersicum L. and sugarcane plants. Moreover, yeast amendment to soil was also reported to enhance the root to shoot ratio in both plants and the induction of speciesspecific morphological alterations leading to enhanced tillering in sugarcane and higher shoot biomass in S. lycopersicum L. Brewer’s yeast is an inexpensive biofertilizer that enhances plant nutrient status and plant potential, thus, aiding plant growth and nutrient uptake (Lonhienne et al., 2014). Phytohormones are the small signal molecules responsible for the coordination of physiological processes and the regulation of plant growth and development. They play important roles in regulating plants’ adaptation to varying abiotic and biotic stress factors at low concentrations (Singh et al., 2017a,b). Of late the contribution of microbially synthesized indole-acetic-acid (IAA) in plantmicrobe interactions have gained considerable interest. Several bacteria (Radhakrishnan et al., 2013; Ali et al., 2009; Sachdev et al., 2009; Idris et al., 2007), fungi, and yeasts, capable of IAA synthesis can enhance plant growth; therefore, IAA synthesizing microbes have been recommended for their potential application as biofertilizer (Waqas et al., 2012; Ahmad et al., 2008; El-Tarabily, 2004; Sasikala and Ramana, 1997). Several yeast strains: Aureobasidium pullulans YL-11; Candida sp. JYC072; Cryptococcus flavus YL-2, YL-3, YL-12, JYC071, and JYC073;

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Pseudozyma aphidis YL-8 and YL-16, isolated from leaf samples of Drosera indica L., and all the strains significantly secrete Indole acetic acid during plant growth promotion activity test. These secretes showed their potential ability to act as bio fertilizer in sustainable agriculture (Sun et al., 2014). The synthesis of IAA by yeast under laboratory conditions was influenced by changes in the pH and temperature of the medium. Approximately 80% of plant species are reported to have mutualistic association with arbuscular mycorrhizal fungi (AMF). Partial or complete degeneration of AMF activity in soil can lead to significant changes in soil properties, that directly or indirectly helped in the enhancement of agriculture production. AMF confer resistance to plants against pathogens, heavy metals, environmental stresses, and facilitate plant growth by alleviating the harmful impact of disease-causing factors. AMF interaction with plants reduces the chances of disease development induced by different phytopathogens (Hildebrandt et al., 2007; Ene and Alexandru, 2008). Piriformospora indica, a member of the order Sebacinales, is highly variable in its mycorrhizal associations and its efficacy at enhancing plant growth. P. indica, a commonly found root endophyte, is known to infect several genera of flowering plants, mosses, and ferns (Varma et al., 2012). Rhizospheric interaction by P. indica enhances plant growth, early blossoming, seed production, synthesis of natural products, and adaptation to both abiotic and biotic stress factors. P. indica displays an inherent ability to colonize a wide spectrum of host plant species through alterations in the phytohormone synthesis signaling pathway during interaction. Host root interaction with P. indica enhances the performance of the host plant in many aspects, for instance, increased root production through indole-3-acetic acid synthesis results in improved nutrient uptake, and thus, increased crop growth and productivity. It can also stimulate systemic resistance in host plants against fungal and viral diseases by altering the signal transduction pathway. In addition, P. indica colonization may induce diverse antioxidant defense system responses and the expression of stress related genes responsible for plant stress tolerance. Hence, P. indica can help in the acclimatization of micropropagated plantlets, and eliminate the effect of transplantation stress. Furthermore, it can also take part in several complex symbiotic interactions, including three component interactions, and can improve the population dynamics of plant growth promoting rhizobacteria (PGPR). In conclusion, P. indica can be used for enhanced plant growth, improvement in soil fertility, resistance induction in host plants, and protection against environmental stresses, as well as against pathogenic microbes (Gill et al., 2016).

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2.3 FUNGI AS A BIOCONTROL AGENT Currently, “biocontrol” has emerged as a rapidly growing research area with a potential role in enhanced plant yield and food production. The phenomena may help to sustain the quality of food crops and to minimize the risks resulting from indiscriminate utilization of synthetic pesticides and hazardous chemicals. Plant diseases are one of the major factors responsible for 20% to 40% annual loss in global crop productivity. Although the development of disease resistant varieties, water management practices, and new techniques in agronomy and agricultural practices have greatly supported the effective management of plant diseases, there are still several pathogens for which synthetic chemicals are broadly being used for disease management. The use of biocontrol agents against various pathogens is an attractive choice because they have emerged as the most common and important natural factor responsible for large population insect deaths in nature (Villa et al., 2017). Several postharvest diseases can be managed biologically using wild yeast species having antagonistic properties. Research in this area has considerably attracted scientists worldwide. The development of effective strains as a substitute to synthetic fungicides can be helpful in effective management of postharvest losses of fruits, vegetables, and grains. A typical yeast-based biocontrol system comprises of three different trophic level interactions involving host, pathogen, and yeast species. All components of a biocontrol system are considerably influenced by various factors including temperature, pH, and UV light as well as biotic and abiotic stress factors. In addition, the preparation of biocontrol agents is severely affected by different abiotic factors leading to changes in their functionality. Therefore, thorough knowledge regarding the survivability of biocontrol agents under given environmental conditions and the development of procedures to make them stress tolerant are key factors for maintaining their effectiveness and commercial exploitation. Several experimental studies have been carried out to further understand the reaction of antagonistic yeasts under varied environmental factors in order to augment stress tolerance and efficacy, and the associated mechanisms that may result in enhanced adaptation to a given stress (Sui et al., 2015). Six isolates of S. cerevisiae were employed to control losses resulting from Colletotrichum acutatum. The biocontrol activity observed by these isolates was due to the synthesis and secretion of antifungal compounds, effective competing behavior for available nutrients, inhibition of

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pathogen growth, and the production of a cell wall degrading enzyme upon contact with fungal cells. The isolates were found to be equally effective in disease management even on detached flowers. This is the first study on the role of S. cerevisiae as a biocontrol agent against post bloom fruit drop, thus, advocating for the significant application of these strains in the control of citrus diseases (Lopes et al., 2015) Geotrichum citri-aurantii is reported to be responsible for Sour rot disease in citrus fruits during postharvest storage. Sour rot disease was markedly controlled due to the synthesis and secretion of hydrolytic enzymes after the application of yeast strains ACBL-23, ACBL-44, and ACBL-77. Strains ACBL-23, ACBL-44, and ACBL-77, controlled the disease effectively and were discriminated as Rhodotorula minuta, Candida azyma, and A. pullulans, respectively. Thus, C. azyma was firstly reported to possess biocontrol activity (Ferraz et al., 2016). Endophytic fungal strains procured from strawberry leaves, have been evaluated for their ability to control the activity of Duponchelia fovealis. A total of 517 fungi belonging to 13 genera were isolated. Among them, eight genera belonging to Aspergillus, Diaporthe, Paecilomyces, and Cladosporium were selected for biocontrol assay against larvae of D. fovealis. The fungus Paecilomyces exhibited the most larvicidal activity (Amatuzzi et al., 2018). The occurrence of tomato wilt disease caused by Fusarium, often reported in regions with repeated tomato cultivation, has resulted in huge loss of tomato productivity (Rangaswami, 1988). Fusarium oxysporum f.sp. lycopersici is a well-recognized tomato specific phytopathogen globally (Walker, 1969). Among other tomato phytopathogens are the fungal genera Botrytis cinerea and Alternaria solani (responsible for early blight in tomato). These fungal phytopathogens are quite common and are more harmful under environmental conditions such as high humidity and high precipitation with expression of disease symptoms in the form of damping off, rotting, blights as well as extensive leaf fall (de la Noval et al., 2007). The biocontrol activity of Verticillium leptobactrum HR1, in terms of nematicidal and fungicidal potential, has been described against the tomato pathogens Meloidogyne javanica and F. oxysporum f.sp. lycopersici under laboratory conditions. The tested biocontrol agent was found to be effective in reducing crop loss caused by F. oxysporum f.sp. lycopersici. Maximum nematicidal efficiency was also noticed in a greenhouse experiment. The tested biocontrol fungus markedly improved plant growth, i.e., total length and biomass. The loss in productivity caused by these plant pathogens was significantly reduced by inoculation of V. leptobactrum into the

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soil as compared to chemical treatment methods. The isolate designated as HR1 demonstrated strong biocontrol efficacy against wilt disease induced by F. oxysporum f.sp. lycopersici (Hajji-Hedfi et al., 2018). Alam et al. (2011) presented the effectiveness of plant growth promoting fungus, Penicillium sp. EU0013 against Fusarium wilt in S. lycopersicum L. and Brassica oleracea L. var. capitata. Similarly, Rhizoctonia strains (HBNR), G1, L2, W1, and W7 have also been reported for the management of Fusarium wilt in tomato (Muslim et al., 2003). Mushrooms have been exploited for medicinal as well as food purposes for a long time. The role of a balanced diet in the regulation of vital human physiological activities and thus overall health is now widely accepted. To date, several studies have declared the medicinal properties of mushrooms, which were traditionally used because of their antioxidant, antifungal, antibacterial, and antiviral nature in addition to their utilization as a food source (Wani et al., 2010). Mushrooms are known to contain a diverse array of biological molecules, including phenolics, flavonoids, steroids, glycopeptides, terpenes, coumarins, alkaloids, and phenyl propanoids, which impart different biological activities (Wang and Ng, 2004; Periasamy, 2005; Carbonero et al., 2006; Iwalokun et al., 2007). The antimicrobial responses of compounds from a few mushroom species, such as Amanita caesarea, Armillaria mellea, Chroogomphus rutilus, Clavariadelphus truncates, Clitocybe geotropa, Ganoderma sp., Hydnum repandum, Hygrophorus agathosmus, Lenzites betulina, Leucoagaricus pudicus, and Paxillus involutus, have been evaluated against a few bacterial and fungal species, including Escherichia coli, Enterobacter aerogenes, Salmonella typhimurium, Pseudomonas aeruginosa, Staphylococcus aureus, Staphylococcus epidermidis, Bacillus subtilis, Candida albicans, and S. cerevisiae. Cellular extract from H. agathosmus was demonstrated to possess inhibitory potential against both yeast and bacteria (Yamac and Bilgili, 2006). Cellular crude filtrate from Pleurotus eryngii var. ferulae was also reported to exhibit antimicrobial activity against selected bacterial and fungal strains (Akyuz et al., 2010). One can assume differing behavior of different strains of biocontrol agents under combination and individual treatment. The efficacy of two different strains of Trichoderma harzianum, that is, Th1 and Th2, against the phytopathogen A. mellea, alone and in combination was demonstrated by Raziq and Fox (2005). Strain Th2 alone demonstrated strong inhibitory action over the pathogen as compared to a combination treatment protecting 75% of plantations. The experiments were also conducted in order to further understand the behavior of different antagonistic biocontrol agents.

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The combination treatment was able to enhance the survivability of treated plants. Their interaction with the pathogen improved the activity of all fungal antagonists except Trichoderma hamatum and Chaetopelma olivaceum.

2.4 APPLICATION OF FUNGI IN BIOREMEDIATION OF CONTAMINATED SOILS (MYCOREMEDIATION) The rapid development of industries, indiscriminate use of agrochemicals, and high population growth has resulted in the degradation of natural ecosystems. These activities have introduced large amounts of contaminants into the environment, such as petroleum hydrocarbons and heavy metal/metalloids, which has emerged as a major calamity for forthcoming generations. Currently, numerous physicochemical treatment methods have been developed to solve the problem of environmental pollution but they have not gained much success at ground level (Akcil et al., 2015). Physicochemical methods suffer from limitations such as the production of large amounts of secondary products, their costly nature, and their hazardous impact on human health and the environment. Moreover, often the byproduct produced during treatment may be more harmful as compared to the parent compound. On the other hand, bioremediation techniques based on vital cellular activities are inexpensive, effective, efficient, environment friendly (Deshmukh et al., 2016) and produce negligible amounts of secondary sludge during the conversion of toxic compounds into nontoxic compounds. The process is based on the application of suitable microorganisms or plant systems as a whole that possess the specific biological activities required for efficient treatment of contaminated soil or water systems (Gillespie and Philp, 2013). Bioremediation techniques can be broadly categorized into in situ and ex situ methods. An in situ method would treat a given contaminant at the site of origin while ex situ methods are based on the treatment of contaminants at a different location from their original site, that is, the removal of contaminants from a site and their treatment at another site. Ex situ treatment of contaminated sites is, thus, more expensive in contrast to in situ treatment techniques (Rhodes, 2014). Due to their rapid adaptability conferred by diverse metabolic activities, fungi are widespread in nature with the soil system acting as a major reservoir. In soil systems, fungi can sustain under diverse environmental circumstances by producing different types of spores. Most importantly,

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fungi make a significant contribution to the maintenance of ecosystem functioning because of their decomposing behavior. Large habitat variability and the ability to synthesize different enzymes with unique features makes them a suitable candidate for the purpose of environmental cleanup. The remediation of hydrocarbon contaminated soil using different fungal strains is described by D’Annibale et al. (2006). Out of nine fungal strains isolated from contaminated sites, three fungal strains, namely Allescheriella sp. strain DABAC 1, Stachybotrys sp. strain DABAC 3, and Phlebia sp. strain DABAC 9, were employed for contaminant removal based on the synthesis of lignin modifying enzymes (LMEs) and efficient degradation of Poly R-478. The native fungal isolates were tested for their ability to degrade different hydrocarbons along with their survivability. The selected fungi synthesized the important enzymes, peroxidase and laccase, in the presence of soil with high metal content and subalkaline pH, even after 30 days of treatment indicating their suitability for the management of soils polluted with aromatic hydrocarbons. Interestingly, all the isolated strains demonstrated active removal of naphthalene, dichloroaniline isomers, o-hydroxybiphenyl, and 1,10 -binaphthalene. Stachybotrys sp. strain DABAC 3 was identified as the most efficient due to its ability to substantially remove the noxious compounds, 9,10-anthracenedione and 7H-benz[DE]anthracen-7-one. A germination assay and mortality test revealed the alleviation of soil toxicity conferred by fungal enzymatic activities. White rot fungi, so called because of the appearance of wood upon degradation of lignin through the action of fungal enzymes, are of immense importance in the treatment of contaminated systems. Lignin degrading enzymes, commonly referred to as LMEs, consist of three different enzymes, that is, laccase, Mn-peroxidase, and lignin peroxidase (Thurston, 1994). To date, several studies have been performed in order to design and develop biological treatment systems based on white rot fungi. Yateem et al. (1998) described the potential of three different white rot fungi (Phanerochaete chrysosporium, Pleurotus ostreatus, and Coriolus versicolor) for the decontamination of oil polluted soil. Different factors, such as inoculum density, presence/absence of nitrogen in media, and strains, were taken into consideration to observe the impact on pollutant degradation efficacy. C. versicolor was identified as the most effective candidate for the intended purpose as it was able to reduce the petroleum hydrocarbon content by up to 78%. The presence of nitrogen as well as an increase in

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the inoculum density improved the biodegradation potential of white rot fungi. A microcosm study revealed the growth of other soil inhabiting microbes under nutrient enriched conditions, thus, facilitating the removal of petroleum hydrocarbons. The study suggested the possible application of white rot fungi in the management of oil contaminated soils. The over-application of pesticides to control pest insects and plant diseases in agriculture has contaminated soil as well as agricultural produce, thus, causing serious health hazards. This over application of pesticides has resulted in changes in soil physicochemical and biological properties, thus, affecting crop productivity. The application of fungi for the treatment of pesticide contaminated soil in the current scenario is imperative. One of the most commonly used pesticides, that is, chlorpyrifos, was shown to undergo degradation by fungus (Verticillium sp. DSP) in culture medium, soil, and Brassica chinensis L. (Fang et al., 2008). The rate of chlorpyrifos degradation by fungi was found to be enhanced with increases in its concentration up until it reached 100 mg/L. Further increases in pesticide concentration had an inhibitory impact on fungal degradation characteristics. A higher rate of degradation was observed at neutral pH as compared to acidic and alkaline conditions. The rate of degradation was higher at 37°C in contrast to 15°C and 20°C. Chlorpyrifos degradation in contaminated soil inoculated with test fungi was significantly higher in comparison to untreated ones. The half-lives of chlorpyrifos in fungi treated soil and on B. chinensis L. were markedly reduced under both greenhouse and field conditions. Bhatt et al. (2002) demonstrated the potential of white rot fungi, including Irpex lacteus and P. ostreatus, for the treatment of polyaromatic hydrocarbon (PAH) contaminated industrial soils. The tested fungi were able to degrade soil contaminated with seven aromatic compounds containing, namely fluorene, phenanthrene, anthracene, fluoranthene, pyrene, chrysene, and benzo[a]anthracene. I. lacteus was more efficient as compared to P. ostreatus. The industrial soil system, after 15 days of fungal inoculation, was proven to be significantly less toxic in contrast to untreated soil as revealed by seed (Brassica alba) germination assay and bioluminescence tests. The potential ability of 160 fungal strains isolated from petroleum contaminated sites were evaluated by Marchand et al. (2017). Factors such as soil type, nutrient media composition, and strains were taken into consideration while determining their efficiency in contaminant degradation. The tolerance of isolated strains toward raw petroleum substrates were

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based on colorimetric test utilizing 2,6-dichlorophenolindophenol and p-iodonitrotetrazolium. Degradation potential was also checked in the presence of a mixture of four different PAHs. There were no significant differences in the ability of pollutant degradation by fungi isolated from less contaminated and highly contaminated soil. Furthermore, type of nutrient medium also did not affect the degradation potential. Phylogenetic relationships were observed to have strong impact on PAH degradation. Fungal genera belonging to Sordariomycetes had greater potential to mineralize the contaminant. The fungal strains competent in the degradation of the hydrocarbon mixture as compared to the control was designated as Trichoderma tomentosum and F. oxysporum. The treatment of sewage sludge containing large quantities of PAHs and hazardous heavy metals using fungi is an attractive technique to enhance the fertility of soil. Studies have been conducted to evaluate the potential of P. chrysosporium, its cell free extract, and commercial laccase enzyme to treat biosolids (Taha et al., 2018). Efficacy was determined in the presence of low (1 mg/g) as well as high (10 mg/g) PAH contents. The introduction of P. chrysosporium, cell free extract, and laccase into contaminated biosolid displayed significant enhancement in degradation in comparison to the untreated control. The cell free extract was able to achieve a nearly 80% degradation of hydrocarbon. Moreover, treatment with fungi, its cellular extract, and laccase enzyme had no impact on the diversity of microorganisms (both bacteria and fungi) present in the biosolids indicating their suitability for complex soil systems. Thus, the bioaugmentation of PAH contaminated soil by fungi is a promising approach to treat contaminated sites. Large scale treatment and the addition of treated biosolids into agricultural fields would enhance soil fertility as well as crop productivity.

2.5 CONCLUSION Currently changing climatic conditions are a major threat to the normal functioning of life because of the rise in temperature and uneven rainfall throughout the world. The utilization of a huge amount of chemical fertilizers and pesticides to achieve maximum agricultural productivity is partly responsible for changes in climatic conditions. In this context, the use of microbes, like fungi or PGPR, as biofertilizer, biocontrol, and for abiotic stress management is a safe, economic, and ecofriendly method toward sustainable agriculture. Currently, it has been estimated that the use of

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microbial biofertilizer reduces dependency on chemical fertilizer by up to 20%. Various microbial species alone or in combined (co-inoculation) have been used as plant or soil inoculants to enhance agricultural productivity as well as to reduce the growth of phytopathogens. Many fungal species have also been used in abiotic stress management and the remediation of xenobiotic compounds or heavy metal concentration through biological means. Fungi secretes a large number of bioactive compounds that are directly or indirectly involved in biological control. Furthermore, there is also a need to explore the hidden possibilities or uses of fungi microbes that can help in the enhancement of agricultural productivity, nano-agriculture, or metabolite production.

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CHAPTER 3

Climate Change and Soil Dynamics: Effects on Soil Microbes and Fertility of Soil Pooja Kannojia1, P.K. Sharma2 and Kusum Sharma3 1

National Centre of Organic Farming, Ministry of Agriculture and Farmers Welfare, Ghaziabad, India ICAR-National Bureau of Agriculturally Important Microorganism (NBAIM), Mau Nath Bhanjan, India 3 ICAR-Indian Institute of Pulses Research (IIPR), Kanpur, India 2

Contents 3.1 Microbial Response to Changes in Climate 3.1.1 Effect of Increase in Temperature 3.1.2 Impact of Increased Carbon Dioxide 3.1.3 Impact of Changing Soil Moisture 3.1.4 Effect of Climate Change on Fertility of Soil 3.2 Soil Management for Climate-Smart Agriculture 3.3 Conclusions References Further Reading

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Soil is the soul of infinite lives and comprises of minerals, organic matter, air, and water. Interactions between climate, geology, and vegetation determine the nature of habitat in which a community of organisms’ lives. It is predicted that drastic changes in the global climate will take place during the 21st century. This change will impact various parameters (Houghton et al., 2001). This applies to CO2 present in the atmosphere as its concentration is on the rise (IPCC Climate Change, 2007). Due to spikes in the level of CO2 in the atmosphere from natural and/or manmade causes, it has been predicted that global surface temperatures are likely to increase between 1.8°C and 3.6°C by the year 2100 (IPCC Climate Change, 2007). Aboveground and belowground, terrestrial ecosystems will be directly and indirectly affected by climate change (Fig. 3.1). The direct effects of climate change will be felt aboveground in the form of spikes in Climate Change and Agricultural Ecosystems DOI: https://doi.org/10.1016/B978-0-12-816483-9.00003-7

Copyright © 2019 Elsevier Inc. All rights reserved.

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Figure 3.1 Abiotic stress that affects plant growth and development.

atmospheric carbon dioxide and changes in temperature, rainfall pattern, and the availability of nitrogen. These changes will impact on the richness and distribution of plant species and in unmanaged ecosystems alterations in land cover will take place (Tylianakis et al., 2008). Various ecosystem scale factors, viz. climate change, increased carbon dioxide, deposition of nitrogen, or disturbances can bring about changes in the metabolic activities of microorganisms (Dhillion et al., 1996; Ajwa et al., 1999; Mayr et al., 1999). Various ecosystem processes, viz. storage of carbon, decomposition of organic material, and mineralization of nitrogen, may be changed due to changes in microbial activity (Carreiro et al., 2000; Sinsabaugh et al., 2002; Henry et al., 2005; Sowerby et al., 2005). In order to better understand the effects of changes in soil and ecosystem functioning at a global level, it is of paramount importance that the ways in which global changes will affect microbial metabolic responses are able to be predicted.

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3.1 MICROBIAL RESPONSE TO CHANGES IN CLIMATE 3.1.1 Effect of Increase in Temperature In response to climate warming, microorganisms either must acclimatize to the change or perish. Various short-term experiments have revealed an increase in the biomass of microorganisms. However, biomass is likely to decrease due to high temperatures over a long-term basis. This is because of changes in the growth efficiency of microorganisms at elevated temperatures (Schimel et al., 2007; Hyvonen et al., 2005). There is no direct correlation between biomass and decomposition; on the contrary organisms do not use labile carbon for the production of biomass. They use it for the production of energy (Contin et al., 2000; Zogg et al., 1997; Schimel et al., 2007). For example, alterations in the fluidity and permeability of the cell membrane takes place due to high temperatures. Due to this resynthesis of the membrane lipid is required (Petersen and Klug, 1994). Through this mechanism, which is high energy requiring, microbes respond to stress by using carbon for energy in lieu of biomass. Biomass may decrease at high temperatures when the microbial energy demand goes for long time (Balser, 2000; Balser and Firestone, 2005). If community of microorganisms gets the required labile carbon, at elevated temperature it will lead to a switch in assignment of carbon from growth to acclimation. At the same time, growth efficiency in terms of the increase in respiration per unit biomass is decreased (Schimel et al., 2007). In such a scenario, the biomass of microorganisms may not decline and its level may be maintained. The activity of microorganisms in soil, net nitrification rates, the mineralization rates of P and N, and total respiration in soil are stimulated due to the warming of soil (Andresen et al., 2010). However, these reactions to warming occasionally last for an extremely short time (Balser et al., 2006). Microbial communities in soil acclimatize to experimental warming. This may be due to extensive changes in the composition of the soil food web that may to lead to a reduction in the sensitivity of the whole soil community to temperature (Balser et al., 2006). Gutknecht (2007) in their study, observed a similarity in the reaction of microorganisms to increased temperature and to the addition of nitrogen. There was a decrease in the abundance of mycorrhizae, but there was an increase in bacterial and general fungal microbial indicators. Increased rainfall resulted in the lowering of the relative abundance of a mycorrhizal indicator. During the 6-year study period, only for 1 year the

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trend was found to be statistically significant. Climate manipulation resulted in significant effects. The impacts of elevated carbon dioxide or nitrogen in addition or alone were modified due to interaction of temperature. Arbuscular mycorrhizal (AM) colonization and development may be enhanced by increased temperatures (Pendall et al., 2004; Fitter et al., 2000; Gavito et al., 2003), or the effect may be contrary. Mycorrhizae make nutrients available to plants and also influence the responses of plants, communities, and ecosystems to change. Therefore, changes in the biomass of mycorrhizae may be an important factor in countering global climate change. Bardgett and Shine (1999) carried out a model ecosystem study wherein they looked at the response of a community of microorganisms to elevated temperature in three generations of plants. In the first plant generation, a significant increase in the biomass of the microorganisms was observed. The microbial biomass decreased in the third plant generation. The microbial biomass of microorganisms increased in the first generation because of the rapid growth of bacteria that reacted to the increased temperature. Actinomycetes and fungi which grew slowly were not affected predominately. The increase in temperature may cause shift in available substrates, thereby, increasing the relative abundance of Gram-negative and Gram-positive (Zogg et al., 1997). Increased temperatures may cause a decline in fungi and actinomycetes (Waldrop and Firestone, 2004). These investigations underline the response of microorganisms to temperature increases. A number of studies show that variations in temperature impact on the quantity and quality of the carbon pool that microbes use (MacDonald et al., 1995; Zogg et al., 1997; Tison and Pope, 1980; Linkins et al., 1984; Ellert and Bettany, 1992; Nicolardot et al., 1994). Models have been developed to find out how temperature affects decomposition or respiration rate coefficients (k). These models are based on the assumption of a constant size of substrate pool and a uniform preference for substrate (Ellert and Bettany, 1992). However, MacDonald et al. (1995) reported that temperature can cause substantial variation in the availability of carbon to a community of microorganisms. Various studies have highlighted that temperature affects the accessibility of microorganisms to the carbon pool and also causes alterations in the utilization of specific substrates by them (Zogg et al., 1997; Balser, 2000; Waldrop and Firestone, 2004; Nicolardot et al., 1994). However, some data indicate that the reaction is not invariably congruent with simple kinetics.

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An investigation on a community of microorganisms in the Californian annual grassland found that incubation temperature caused considerable variation in substrate-utilization (Balser and Wixon, 2009). It was predicted that higher temperatures would bring about the rapid degradation of polymeric carbon. On the contrary, the opposite effect was observed. Lower incubation temperatures favored the degradation of polymers (Balser and Wixon, 2009). The importance of this for soil carbon dynamics is yet to be understood. Several studies have shown that increased temperatures hasten microbial decomposition rates. This increases the emission of CO2 through soil respiration leading to global warming (Allison et al., 2010). Global warming would lead to the release of large amounts of carbon present in land soils into the atmosphere. As a result they will become a carbon source rather than a sink (Melillo et al., 2002). There are communities at the global level who want to change policies and research and also want to know whether climate warming will make soil a source or sink of carbon (Cox et al., 2000; Shaver et al., 2000; Rustad et al., 2001; Knorr et al., 2005). The concentration of organic carbon in terrestrial ecosystems is more than double the amount present in the atmosphere (Post et al., 1982; Schimel, 1995; Schlesinger, 1996). Minor alterations in flux can significantly affect the concentration of carbon dioxide in the atmosphere (Kirschbaum, 2000; Rustad et al., 2000; Schlesinger and Andrews, 2000). Global warming is adversely affecting soil health. This is evident from its effects on permafrost, which is a very important terrestrial carbon sink. Permafrost is permanently frozen soil. Its different layers hold huge quantities of carbon and organic matter. When permafrost thaws, microorganisms start decomposing the stored carbon and organic nutrients leading to the release of CO2 into the atmosphere. This adds to global warming (Davidson and Janssens, 2006). The vulnerability of older carbon to temperature is an important yardstick that can be relied upon for forecasting whether soil can give an indication about climate warming (Knorr et al., 2005; Giardina and Ryan, 2000; Luo et al., 2001; Fang et al., 2005). The response of younger (labile) carbon to temperature is predictable, however, the dynamics of older carbon remains to be deciphered (Kirschbaum, 2000; Ågren and Bossatta, 1996). It has been unpredicted trend that whether there will be a spike in recalcitrant carbon’s sensitivity to temperature rise (Knorr et al., 2005; Fierer et al., 2005), decrease (Wagai et al., 2008), or remain unchanged (Giardina and Ryan, 2000; Luo et al., 2001). This variation is because of interactions between myriad

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aspects that affect the stability of carbon in soil. Litter is transformed to “soil organic matter,” and it then matures into stable (humic) forms which become chemically changed (Balser, 2005) and associate with minerals in soils (Sollins et al., 1996). There are variations in the transformation of litter depending on chemical quality and the organisms available to decompose it (Balser, 2005). The sensitivity of older carbon does not depend on enzyme responses only, but is determined by a number of different interactions among the varying temperature responses of competing processes, such as activation energy (Bosatta and Agren, 1999), altered substrate diffusion (Mikan et al., 2002), adsorption or occlusion of minerals (Wagai et al., 2008; Thornley and Cannell, 2001), historical carbon input and use of land, and the acclimation of the decomposer community (Balser and Firestone, 2005; Waldrop and Firestone, 2004; Ågren and Bossatta, 1996).

3.1.2 Impact of Increased Carbon Dioxide Various studies have been conducted to investigate how increases in the concentration of carbon dioxide in the atmosphere affect the response of an ecosystem to carbon storage. Until now these studies have been based on the role played by the chemistry of tissue present aboveground and biomass production (Pan et al., 1998; Mooney et al., 1999). Predictions that an elevation in the concentration of carbon dioxide will lead to an increase in plant biomass and a decrease in the quality of litter present on the ground surface have turned out to be true (Norby and Cotrufo, 1998; Schlesinger and Lichter, 2001). It is now evident that mostly aboveground biomass does not change and it is difficult to tell the difference between the tissue chemistry of plants at senescence and of plants grown at ambient concentrations of carbon dioxide (O’Neill, 1994; Curtis et al., 1989; Franck et al., 1997). A plethora of information suggests that enhancement of carbon allocation below ground due to carbon dioxide enrichment along with increased requirements of plant nitrogen (O’Neill, 1994; Niinisto et al., 2004; Cardon et al., 2001). Mechanisms for nitrogen acquisition, including increased allotment to structural tissues of roots (Zak et al., 2000; Owensby, 1993; Kampichler et al., 1998), enhanced turnover of roots (Norby, 1994; Pregitzer et al., 2000), rhizodeposition (Cardon et al., 2001; Hungate et al., 1999), development of mycorrhizae (O’Neill, 1994; Treseder and Allen, 2000), and nitrogen fixation (Hungate et al., 1999; Montealegre et al., 2000), seem to determine the flow of carbon to belowground pools.

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There are few studies that have looked at the relative significance and long-term consequences of different mechanisms that cause increases in carbon present belowground and influence microbial communities and thus the dynamics of carbon in soil. Soil fungi are going to be more affected by increases in the concentration of CO2 in the atmosphere than bacteria. A number of studies suggest the possibility of increases in total soil bacterial biomass along with significant shifts in community composition in terms of its functions (Drigo et al., 2008). For instance, the enrichment of atmospheric CO2 has been found to stimulate Burkholderia and Pseudomonas, which are rhizosphere colonizers, while species of Bacillus found mainly in bulk soil were not stimulated (Drigo et al., 2009). Plant growth in high CO2 benefits endosymbiotic N2-fixing bacteria. As compared to non-N-fixers, N2-fixing plant species reacted more vigorously to the enrichment of CO2 in the atmosphere (Rogers et al., 2009). Atmospheric CO2 enrichment enhances rhizobial populations in the rhizosphere (Schortemeyer et al., 1996), the size of the population of organisms that fix N in bulk soil (Allen, 1990), and the size and number of root nodules formed (Serraj et al., 1998). In plant systems that are well managed and get supplemental fertilization, N fixation stimulation through the enrichment of CO2 invariably takes place. However, multiple resource limitations dampen rhizobial responses in natural systems (van Groenigen et al., 2006; Rogers et al., 2009). Because of a paucity of data, it is not yet clear how fungal and bacterial soil-borne diseases impact on the rise in atmospheric CO2. The impacts of enhanced CO2 on plant and canopy processes might be helpful in predicting the rise and fall of soil-borne diseases. For example, increased mycorrhizal symbioses might impart resistance to soil-borne and other diseases (van der Putten, 2009). This notion was buttressed by Drigo et al. (2009) who reported that there was a reduction in the density of Fusarium species in the rhizosphere of a mycorrhizal plant that was exposed to high CO2. The explanation for this may be that CO2-enriched conditions increased the competitive ability of the mycorrhizal fungus as compared to Fusarium, or the positive effects of CO2 enrichment on Trichoderma may decrease Fusarium density leading to a suppression of fungi thus causing plant diseases (Hagn et al., 2007), or it might be due to the impact of CO2 enrichment on bacteria-derived antibiotics. Plant responses are affected in several ways by increasing atmospheric carbon dioxide: there can be either an increase or a decrease in plant growth (Poorter, 1993; Joel et al., 2001), the use of nutrients and their

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allocation can change (Curtis et al., 1989; Cotrufo et al., 1998), and the patterns of biomass above- and belowground can be altered (Zak et al., 2000; Hungate et al., 1997). The competitive ability of different species of plants are affected by the level of carbon dioxide and this can result in alterations in the composition of plant communities (Zangerl and Bazzaz, 1984). Exotic, invasive species may be present in plant communities. A rise in carbon dioxide may change the competitive dynamics and growth of plants. This may favor either native species or exotic species. Many exotic, invasive species that belong to the C3 category of plants respond strongly to increases in carbon dioxide (Poorter, 1993). Concern over whether or not there will be an aggressiveness in the growth and establishment of invasive species under enriched atmospheric carbon dioxide is valid. Hungate et al. (1996) in their study reported that the effect of invasive grasses on nitrogen cycling in the annual grasslands of California was greater under enhanced carbon dioxide as compared to ambient levels. There was an increase in the uptake of pools of plant nitrogen and 15 NH41 by the invasive grasses under increased carbon dioxide. The native species showed lesser decreases or increases. The study shows that elevated carbon dioxide changes the dynamics of the growth of native and invasive species. This can lead to alterations at the ecosystem level. With plant senescence, such changes in primary productivity will influence the dynamics of soil microorganisms and carbon.

3.1.3 Impact of Changing Soil Moisture There is more variation in the relationship between soil moisture and microbial community as compared to temperature (Lavigne et al., 2004; Saiz et al., 2007). A negative correlation between temperature and the water content of soil has been shown (Luo et al., 2001). Empirical description of this relationship is difficult. There is a lack of unanimity regarding an equation that describes soil respiration and moisture (Emmett et al., 2004) or moisture and temperature (Lavigne et al., 2004). Clarity on temperature relationships is absent due to various interactions. There is a difference between the temporal and spatial scales of water change and temperature. Changes in moisture may be in terms of wet dry cycles, drought, flooding, or smaller shifts. Such changes affect the community structure and function in different ways which are modified by a native regime of the community.

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Microbial communities are affected by various mechanisms or physical processes that change with changes in moisture content (Rodrigo et al., 1997). Precipitation generally slows down decomposition under very dry or very wet conditions. There is an interaction between oxygen and moisture present in soil. However, the effects of soil moisture are not confined to very wet conditions. For instance, taking aerobic respiration into account, it has been demonstrated that substrate diffusion in clay soils is the primary regulating process at low water levels (Schjonning et al., 2003). Soil moisture changes may change the communities of microbes. These effects are based on a few investigations that were carried out in dry summers (Hui and Luo, 2004; Rey and Jarvis, 2006). The effects of drought conditions produced experimentally have not been consistent (Emmett et al., 2004). Water is an important factor in global change. Peats and wetlands, which are wet soils, constitute huge carbon sinks. Soil moisture optima are 60% 80% water holding capacity. Above these optima, heterotrophic respiration is negatively related to water content. Waterlogged soils create physical and chemical conditions that impede aerobic respiration. In future, management strategies for sequestering carbon might be achieved by saturating soils (Sylvia et al., 2005). Studies show that when an enhancement in carbon dioxide is considered, global climate change may make anaerobic wetland conditions less of a carbon sink (Wolf et al., 2007). Just as there cannot be a universal temperature optimum, in the same way the adaptation of a community of microorganisms to a local rainfall regime cannot be generalized. The community has a life history which determines its reaction to conditions such as flooding. Some communities have more tolerance against drying, rewetting, and flooding. Their response to such changes is different (Fierer et al., 2003; Mentzer et al., 2006). Fierer et al. (2003) cited an instance of the adaptation of a community and its process response to moisture contents. They studied the composition of a community of bacteria in soils of an oak woodland that often faces moisture stress. The community remained unaffected in grassland soils. The moisture stress history of litter decomposers strongly impacts on their size and function (Schimel et al., 1999). Therefore, the adaptation of a microbial community may decide the way the ecosystem reacts to moisture level changes. Responses to various stresses, such as drought, flooding, irrigation, and rewetting, are likely to be different when compared to moderate water

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stress or moderate spikes in soil moisture (Schimel et al., 2007). The activity of microorganisms or their biomass may increase when the season is wet or when soils regain their moisture after the dry season (Fierer et al., 2003; Waldrop and Firestone, 2006). Studies have been done to understand the unique process implications of rewetting soils after drying (Reichstein et al., 2005; Wu and Brookes, 2005). For instance, there is an increase in the release of dissolved organic carbon consequent to the rewetting of dry soils (Marschner and Bredow, 2002), but these effects vary from community to community (Reichstein et al., 2005). There may be alterations in microbial activity as a result of an increase in soil moisture. Microbial biomass may remain unaffected. In one study (Waldrop and Firestone, 2004), it was found that an increase in soil water content does not affect the usage of soil carbon which is of different age of the composition of microbial community. Hydrolytic enzymatic activity decreased by 42% and there was a reduction in the activity of peroxidase enzyme. This had implications in complex carbon degradation when there was increases in soil moisture. In some situations, there is a likelihood that soil moisture at levels other than the extremes can have an influence. Structurally and biologically the moisture effect is complex. It becomes clear when the role of soil microsites, osmotic stress tolerances, ion concentrations, water films, and the differential water retention of different pore sizes is considered. Nonequilibrium conditions that influence the efflux rates of carbon dioxide can be created through slight changes in environment, viz. including small changes in moisture which restrict or increase the carbon dioxide diffusion from soils or the surface layer (Hanson and Douglas, 2000). Various investigations have revealed that modest drying has a significant effect on decomposition (Reichstein et al., 2005). Soil respiration may decrease by 25% 50% due to modest water stress (Lavigne et al., 2004). Lastly, there may be interactions between certain factors and soil moisture regimes which may either impact the structure of microbial communities or their activity or both at the same time. Chena et al. (2008) carried out a study based on two near-optimal moisture levels (60% and 80% field capacity) and two different plant covers with differing biomass. They reported that plant species had a greater effect on the structure of the community, which was estimated via phospholipid fatty acid analysis. Plant species and soil moisture impacted on the physiological profiles at a community level. Both physiological and acclimation mechanisms played their roles and different factors contributed to each mechanism.

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3.1.4 Effect of Climate Change on Fertility of Soil As per the Soil Science Society of America (SSSA), soil quality is “The capacity of a specific kind of soil to function, within natural or managed ecosystem boundaries, to sustain plant and animal productivity, maintain or enhance water and air quality and support human health and habitation.” Various potential factors are indicators of soil quality and health. These indicators are visual (e.g., plant response, weed species, and runoff), physical (e.g., bulk density, topsoil depth), chemical (e.g., pH, salinity), and biological (e.g., activity of micro- and macroorganisms) indicators. 3.1.4.1 Invasive Organisms and the Soil Food Web Currently, it is believed that increases in the concentration of atmospheric CO2, warming, and alterations in precipitation patterns will favor invasive organisms and the frequency of plant invasion will increase and the environment will be negatively impacted. The spread of plant species including invasive weeds is likely to take place over a large area (Kriticos et al., 2003). Wolfe and Klironomos (2005) reported that the functionality of native soil food webs may be altered due to the immigration of plant species into new habitats in different ways, such as through: (1) changes in the quality, quantity, and timing of litter input and rhizodeposition; (2) direct release of novel antimicrobial compounds; (3) significant alteration in nutrient relations by incorporating alternate modes of acquiring nutrients, such as N fixation; or (4) by changes in the structure of soil or its physical properties due to rooting habits that are either novel or dominant. Of late, studies have pointed out that the dissemination and success of a number of plant-pathogenic and beneficial fungi is being altered by exotic plants. To bring about this change there is release of toxins into the soil by plant species or their patterns of decomposition change due to changed detritus quality (van der Putten, 2009). For instance, diffuse knapweed (Centaurea diffusa) releases the antimicrobial compound 8hydroxyquinoline which has invaded western North America areas. Similarly, there is a release of glucosinolates from the roots of garlic mustard (Alliaria petiolata) that adversely affects AM fungi (Roberts and Anderson, 2001). Environmental changes cause the dispersal of organisms to new areas and this could have important consequences. Immigrants are generally favored over native species that are already well established (Chejara et al., 2010). The invasive species are successful in the new habitat probably due to less incidence of root pathogens in the habitat

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(van Grunsven et al., 2007; MacKay and Kotanen, 2008) or due to the conferment of benefits by symbionts, such as AM fungi in habitats that are newly colonized (van Grunsven et al., 2010). The performance of native species show a greater decrease due to local soil pathogens as compared to their effect on the performance and success of invasive plants (Eppinga et al., 2006; Engelkes et al., 2008). There could be a shift in the preponderance of some taxonomic groups in soil or these groups could be eliminated due to the spread of organisms into new habitats. Ecosystem diversity and function could be influenced by effects such as plant systems that are either natural or managed, are vulnerable to pathogens and invasive or may result in decline in belowground competition, permitting disease causing soil organisms to thrive. The productivity of natural and managed plant systems may be shifted significantly. This shift may be buffered by the functional redundancy of soil taxonomic groups which are present in many soils. This also means that groups of organisms in soil undergo important changes. These changes may counteract each other, thereby ensuring that the productivity of an ecosystem is not affected by the soil community (Bradford et al., 2002). Invasive plant species have indirect effects on soil organisms. Soil organisms have the potential for dispersal, which may be directly increased by environmental changes (Coleman, 2008; Pariaud et al., 2009). The invasive root pathogen, Phytophthora cinnamomi, which is endemic to Papua New Guinea and Sulawesi, has reached at least 76 countries and most of the hotspots of global biodiversity (Myers et al., 2000). P. cinnamomi has been responsible for massive mortality of Quercus spp. and Castanea in the United States and Eucalyptus spp. in Australia in vast areas (Davison and Shearer, 1989; Dunstan et al., 2010). Directional dissemination of earthworms, plant parasitic nematodes, and other soil organisms is likely to be caused by soil warming (Boag et al., 1991; Ghini et al., 2008).

3.2 SOIL MANAGEMENT FOR CLIMATE-SMART AGRICULTURE It is generally accepted that industrial agriculture that uses large swathes of monocultures, tillage, and injudicious application of inorganic fertilizers is unsustainable. To make it sustainable, ecofriendly approaches, such as notill, cover cropping, growing more genetically diverse crops, crop rotation, the use of organic fertilizers and integrated pest management strategies are

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Table 3.1 Microbial isolates listed for abiotic stress tolerance Stress

Microbes

Plant

References

Salinity

Azospirillum brasilense Pseudomonas fluorescen Bacillus amylolequifaciens Achromobacter piechaudii A. brasilense Bacillus megaterium Paenibacillus polymyxa Pseudomonas spp. Pseudomonas sp. AMK-P6 Glomus mosseae Enterobacter cloacae Rhizobacteria

Cicer arietinum

Omar et al. (2009)

Arachis hypogaea

Saravanakumar and Samiyappan (2007) Ashraf et al. (2004)

Salinity Salinity Salinity Drought Drought Drought Drought Heat Heat Flooding UV radiation Heavy metals Heavy metals Heavy metals Heavy metals Freezing Freezing

Triticum aestivum Lycopersicon esculentum Zea mays Trifolium Vigna radiata

Mayak et al. (2004)

Z. mays Sorghum bicolor

Sandhya et al. (2010) Ali et al. (2009)

Poncirus trifoliata L. esculentum Withania somnifera Brassica napus

Wu (2011) Grichko and Glick (2001) Rathaur et al. (2012)

Mastretta et al. (2009)

Methylobacterium oryzae G. mosseae

Nicotiana tabacum L. esculentum Piper nigrum

Abdel Latef (2011)

Rhizobacteria Burkholderia phytofirmans

Pisum sativum Arabidopsis thaliana

Kumar Meena et al. (2015) Su et al. (2015)

P. fluorescens Sanguibacter sp.

Casanovas et al. (2002) Marulanda et al. (2007) Figueiredo et al. (2008)

Sheng, et al. (2008)

Madhaiyan et al. (2007)

becoming more relied upon. Residue management practices and tillage affect the water holding capacity of soil, its physical properties, temperature, and microbial activity. Some microbial isolates showing tolerance against various abiotic stresses are listed in Table 3.1.

3.3 CONCLUSIONS The structure and activities of microbial communities are going to be affected both directly and indirectly by climate change. Direct impacts will include changes in the physical and chemical environment of soil. Indirect effects will include alterations in land use and cover. Most studies

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on climate change effects are based on isolating the different drivers of the changes and then manipulating them independently. Changes in the soil habitat do not take place in isolation, they take place in concert. The result may be unexpected due to the interaction of factors like temperature with changes in water or the availability of nitrogen and plant species cover. The impact of climate changes on soil processes of natural or managed ecosystems, such as disease intensity, will depend on the diversity of species existing within successive trophic levels. The more the species diversity, the less the disease pressure will be. The way a given soil food is regulated, that is, from the top down or from the bottom up, will depend on the effects of environmental change (Tylianakis et al., 2008). Therefore, it is important to carry out studies based on multiple factors in order to better understand climate change interactions. It has become clear that the way land is used can have effects that will be felt in the foreseeable future. Experiments to decipher how soil organisms are affected by warming and the associated environmental changes must be realistic in terms of duration and complexity. Soil microorganisms have the ability to cycle nutrients and process soil carbon. Therefore, they play an important part in determining the responses of agricultural ecosystems to changes in climate. It is of paramount importance to carry out experiments to investigate the effects of the interactions among multiple climate-change factors over a longer duration so as to be able to comprehend and manage the effects of climate change on soil communities.

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Andresen, L.C., Michelsen, A., Jonasson, S., 2010. Plant nutrient mobilization in temperate heathlands responds to elevated CO2, temperature and drought. Plant Soil 328, 381 396. Ashraf, M., Hasnain, S., Berge, O., Mahmood, T., 2004. Inoculating wheat seedlings with exo-polysaccharide-producing bacteria restricts sodium uptake and stimulates plant growth under salt stress. Biol. Fertil. Soils 40, 157 162. Balser, T.C., 2000. Linking Soil Microbial Communities and Ecosystem Functioning (doctoral dissertation). University of California, Berkeley. Balser, T.C., 2005. Humification. In: Hillel, D., et al., (Eds.), Encyclopedia of Soils in the Environment. Elsevier, Oxford, pp. 195 208. Balser, T.C., Firestone, M.K., 2005. Linking microbial community composition and soil processes in a California annual grassland and mixed-conifer forest. Biogeochemistry 73, 395 415. Balser, T.C., Wixon, D., 2009. Investigating biological control over soil carbon temperature sensitivity. Glob. Change Biol. Available from: https://doi.org/10.1111/j.13652486.2009.01946.x. Balser, T.C., McMahon, D.K., Bart, D., et al., 2006. Bridging the gap between microand macro-scale perspectives on the role of microbial communities in global change ecology. Plant Soil 289, 59 70. Bardgett, R.D., Shine, A., 1999. Linkages between plant litter diversity, soil microbial biomass and ecosystem function in temperate grasslands. Soil Biol. Biochem. 31, 317 321. Boag, B., Crawford, J.W., Neilson, R., 1991. The effect of potential climatic changes on the geographical distribution of the plantparasitic nematodes Xiphinema and Longidorus in Europe. Nematologica 37, 312 323. Bosatta, E., Agren, G.I., 1999. Soil organic matter quality interpreted thermodynamically. Soil Biol. Biochem. 31, 1889 1891. Bradford, M.A., Jones, T.H., Bardgett, R.D., 2002. Impacts of soil faunal community composition on model grassland ecosystems. Science 298, 615 618. Cardon, Z.G., Hungate, B.A., Cambardella, C.A., Chapin III, F.S., Field, C.B., Holland, E.A., et al., 2001. Contrasting effects of elevated CO2 on old and new soil carbon pools. Soil Biol. Biochem. 33, 365 373. Carreiro, M.M., Sinsabaugh, R.L., Repert, D.A., Parkhurst, D.F., 2000. Microbial enzyme shifts explain litter decay responses to simulated nitrogen deposition. Ecology 81, 2359 2365. Casanovas, E.M., Barassi, C.A., Sueldo, R.J., 2002. Azospirillum inoculation mitigates water stress effects in maize seedlings. Cereal Res. Commun. 30, 343 350. Chejara, V.K., Kriticos, D.J., Kristiansen, P., Sindel, B.M., Whalley, R.D.B., Nadolny, C., 2010. The current and future geographical distribution of Hyparrhenia hirta. Weed Res. 50, 174 184. Chena, M., Zhu, Y., Sua, Y., Chena, B., Fu, B., Marschner, P., 2008. Effects of soil moisture and plant interactions on the soil microbial community structure. Eur. J. Soil Biol. 43, 31 38. Coleman, D.C., 2008. From peds to paradoxes: linkages between soil biota and their influences on ecological processes. Soil Biol. Biochem. 40, 271 289. Contin, M., Corcimaru, S., Nobili, M.D., Brookes, P.C., 2000. Temperature changes and the ATP concentration of the soil microbial biomass. Soil Biol. Biochem. 32, 1219 1225. Cotrufo, M.F., Ineson, P., Scott, A., 1998. Elevated CO2 reduces the nitrogen concentration of plant tissues. Glob. Change Biol. 4, 43 54. Cox, P.M., Betts, R.A., Jones, C.D., Spall, S.A., Totterdell, I.J., 2000. Acceleration of global warming due to carbon cycle feedbacks in a coupled climate model. Nature 408, 184 187.

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FURTHER READING Davidson, E.A., Artaxo, P., 2004. Globally significant changes in biological processes of the Amazon basin: results of the large-scale biosphere-atmosphere experiment. Glob. Change Biol. 10 (5), 519 529.

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Fierer, N., Bradford, M.A., Jackson, R.B., 2007. Toward an ecological classification of soil bacteria. Ecology 88 (6), 1354 1364. Gentili, F., Jumpponen, A., 2006. Potential and possible uses of bacterial and fungal biofertilizers. In: Rai, M.K. (Ed.), Handbook of Microbial Biofertilizers. Haworth Press, Binghamton, NY, pp. 1 28. Zak, D.R., Pregitzer, K.S., Curtis, P.S., Teeri, J.A., Fogel, R., Randlett, D.L., 1993. Elevated atmospheric CO2 and feedback between carbon and nitrogen cycles. Plant Soil 151, 105 117.

CHAPTER 4

Agrochemicals: Harmful and Beneficial Effects of Climate Changing Scenarios Pushpendra Koli, Nitish Rattan Bhardwaj and Sonu Kumar Mahawer ICAR-Indian Grassland and Fodder Research Institute, Jhansi, India

Contents 4.1 Introduction 4.2 Classification of Agrochemicals 4.2.1 Plant Protection Chemicals 4.2.2 Plant Growth Regulators 4.2.3 Fertilizers 4.3 Impacts of Climate Change on Insect Pests and Pathogens 4.3.1 Impacts of Climate Change on Insect Pests 4.3.2 Impacts of Climate Change on Plant Pathogens 4.3.3 Relationship Between Pesticides and Climate Change for Crops 4.4 Climate Change and Agrochemical Use 4.4.1 Agrochemical Usage as Influenced by Climate 4.4.2 Beneficial Effects of Agrochemical Use 4.4.3 Harmful Effects of Agrochemical Use 4.5 Effects of Climate Change on Pesticide Pollution in Surface and Ground Water 4.5.1 Pollution of Pesticides in Surface Water 4.5.2 Pollution of Pesticides in Ground Water 4.6 Conclusion Acknowledgments References Further Reading

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4.1 INTRODUCTION Agrochemicals are chemical products used in agriculture and are comprised of plant protection chemicals or pesticides, plant growth hormones, and synthetic fertilizers. These agrochemicals have a significant role in the efficient and economic production of nutritious or healthy food and fiber Climate Change and Agricultural Ecosystems DOI: https://doi.org/10.1016/B978-0-12-816483-9.00004-9

Copyright © 2019 Elsevier Inc. All rights reserved.

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products, decreasing soil erosion, and maintaining the health of humans and livestock throughout the world. Over the past few decades, huge quantities of agrochemicals have been utilized in agriculture to enhance crop production in most parts of the world. In the view of environmental concern, plant protection chemicals are perhaps the most important among the agrochemicals. Pesticides are used to control insect pests, disease causing pathogens, nematodes, rodents, and unwanted plant species known as weeds. In countries like India, these agrochemicals played a very important role in the success of the green revolution, where a production level was reached that was almost double what it was before. The world population at present stands at 7.2 billion and is likely to reach 9.3 billion by 2050 (FICCI, 2016). This will definitely lead to a highly increased demand for food and feed. To meet the demand for food and to meet the nutritional requirements of a rising population a sustainable approach is required that enhances productivity against the existing scenario of lower yields and decreasing farm sizes. Approximately 25% of the global crop production is lost due to attacks by insect pests, weeds, and diseases, which do not bode well for farming given the vital challenges ahead and as such agrochemicals have an escalating role to play. Climate change is probably going to affect the nature and severity of pathogens and chemicals in the environment and their exposure, fate, and transport. Climatic factors are likely to affect disease and pest scenarios in addition to agrochemicals in the environment. In addition, variations in the climate are likely to influence the types of pathogens, insect pest occurrences as well as the amounts and range of chemicals used for a variety of scenarios (Boxall et al., 2009). The future risks posed by pests and chemicals could, therefore, be very diverse compared to those of currently. In this chapter, information on the effects of climate change in relation to agrochemicals is compiled.

4.2 CLASSIFICATION OF AGROCHEMICALS Agrochemicals are substances used to facilitate the control of agricultural ecosystems or communities of organisms in farming areas. Broadly, they include pesticides, plant regulators, fertilizers, soil conditioners, and chemicals used in animal husbandry, such as antibiotics and hormones (shown in Fig. 4.1). The application of agrochemicals is significant during the various stages of plant growth or crop production. However, some of these

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Agrochemicals Pesticides Insecticides Fungicides Herbicides Rodenticides Nematicides Acaricides Mollucides

Plant growth regulators Growth promoters Growth retardents

Fertilizers Simple/straight Complex/multinutrients

Soil conditioner Chemicals used in animal husbandry Pesticides Antibiotics Hormones

Figure 4.1 Classification of agrochemicals.

chemicals cause substantial ecological and environmental harm, greatly reducing their benefits.

4.2.1 Plant Protection Chemicals Plant protection chemicals are those chemicals that protect crops from insect pests, diseases, and other biological factors. Generally, pesticide is a term is used for plant protection chemicals in the agricultural sector. A pesticide is defined as a substance used to decrease the abundance of pests; any living thing that may cause injury or disease to crops. 4.2.1.1 Insecticides Insecticides are those substances that prevent, destroy, or kill insects (belonging to class Insecta, Phylum Arthropoda). Inorganic Insecticides Inorganic insecticides were used in homes in ancient Greece around 1000 BC. The very first observation of pesticides use was the burning of sulfur as a fumigant; fumigants are pesticides that enter the insect through respiration and affect their nervous systems. Besides this, sulfur was also used as an insecticide and fungicide in the form of dusts and sprays and it is still used against powdery mildew. Sulfur dioxides (SO2) and hydrogen cyanide gas have also been utilized in the form of a fumigant. To control

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domestic ant populations, the use of inorganic fluorides were well known; even sodium fluoride and boric acid were also used to kill domestic cockroaches. A variety of oils have found use for several years as insecticides and as “dormant sprays” to kill insect eggs. Before the modern era of organic chemicals, even arsenic compounds found widespread use as pesticides (Niu and Yu, 2009). Even though inorganic insecticide usage has decreased over time, their contamination from former use remains an environmental problem in some areas of the world. Organic Insecticides Organic insecticides are chemically classified as organochlorines, organophosphates, and carbamates. Regrettably, at the dosage levels, inorganic and organic metallic pesticides are generally quite toxic pesticides. Since World War II, many organic insecticides have been developed that have largely displaced these inorganic and organic metallic substances. It was believed that organic insecticides are biodegradable, whereas inorganic compounds commonly used in pesticides are not biodegradable. After being released into the environment; they will remain indefinitely in all matrices (water, soil, air) and may enter the food chain if liberated from these sites. Usually low doses of organic compounds are required to be effective against the target pests, and thus, minor amounts of chemicals enter the environment. Given a dose of each large enough to act as a pesticide, organic substances are generally much less toxic to humans than inorganic pesticides are. 4.2.1.2 Fungicides Fungicides are chemicals that prevent, destroy, or inhibit the growth of fungi/diseases in crops. The word “fungicide” originated from two Latin words, viz., “fungus” and “caedo.” The word caedo means “to kill.” Thus, a fungicide is any agency/chemical that has the tendency to kill fungi. As per this meaning, physical agents, like ultra violet light and heat, should also be considered as fungicides. However, in common usage, the meaning is restricted to chemicals only. Hence, fungicide is a chemical which is proficient in killing fungi. Classification of Fungicides Fungicides are mainly classified based on their (1) mode of action, (2) general use, and (3) chemical composition (Anonymous, 2018).

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1. Classification based on mode of action: • Eradicant: Eradicants are those fungicides that remove pathogenic fungi from an infection court (area of the host around a propagating unit of a fungus in which infection could possibly occur), for example, organic mercurials, lime sulfur, dodine, etc. These chemicals eradicate dormant or active pathogens from the host. They can remain effective on or in the host for some time. • Protectant: Protectant fungicides are prophylactic in their behavior. Fungicide which is effective only if applied prior to fungal infection is called a protectant, for example, zineb and sulfur. • Therapeutant: A fungicide which is capable of eradicating a fungus after it has caused infection and thereby curing the plant is called a chemotherapeutant, for example, carboxin and oxycarboxin antibiotics like aureofungin. Usually chemotherapeutants are systemic in their action and affect deep seated infections. 2. Classification based on general uses: Fungicides can also be classified based on the nature of their use in managing diseases. 1. Soil fungicides (preplant): For example, bordeaux mixture, copper oxychloride, chloropicrin, formaldehyde, vapam, etc. 2. Soil fungicides: For example, bordeaux mixture, copper oxychloride (for growing plants), captan, pentachloronitrobenzene (PCNB), thiram, etc. 3. Seed protectants: For example, captan, thiram, organomercurials, carbendazim, carboxin, etc. 4. Eradicants: For example, organomercurials, lime sulfur, etc. 5. Foliage and blossom: For example, captan, ferbam, zineb, and protectants mancozeb and chlorothalonil, etc. 6. Fruit protectants: For example, captan, maneb, carbendazim, mancozeb, etc. 7. Tree wound dressers: For example, boreaux paste, chaubattia paste, etc. 8. Antibiotics: For example, actidione, griseofulvin, streptomycin, streptocycline, etc. 3. Classification based on chemical composition: The most important group of fungicides consists of salts of toxic metals and organic acids, organic compounds of mercury and sulfur, and quinines and heterocyclic nitrogenous compounds. Copper, mercury, zinc, tin, and nickel are some of the metals that are used as a base for inorganic and

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organic fungicides, while the nonmetal substances include chlorine, phosphorous, sulfur, etc. 4.2.1.3 Herbicides Herbicides are substances used for preventing or inhibiting the growth of plants or for killing weeds in crop fields. Classification of Herbicides 1. Classification based on mode of action The mode of action of herbicides depends on the physiological principles involved in the herbicidal action during the killing process of weeds. They are two types (Fig. 4.2), namely contact herbicides and translocated herbicides. a. Contact: These chemicals kill weed plants while contact takes place with herbicides. b. Translocated: This type of chemical kills plants after their absorption by accelerating or retarding the metabolic activities of weed plants.

Figure 4.2 Mode of actions of herbicides in plants. Adapted from: www.weedawareness.org.

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2. Classification based on their relative time of application to weed emergence a. Preemergence herbicides: herbicides that are applied before the emergence of weeds. b. Preplant herbicides: herbicides that are applied on the field before planting the crop. c. Postemergence herbicides: herbicides that are applied after the emergence of weeds. 3. Chemical classification of herbicides Chemical classification refers to the grouping of various herbicides according to their chemical composition. Compounds having chemical affinities are grouped together. This is useful in listing and characterizing herbicides. a. Inorganic herbicides: Inorganic herbicides do not contain any active carbon in their molecules. These were the first chemicals used for weed control before the introduction of organic compounds like: i. Salts: borax, copper sulfate, ammonium sulfate, Na chlorate, Na arsenite, and copper nitrate. ii. Acids: arsenic acid, arsenious acid, arsenic trioxide sulfuric acid. b. Organic herbicides: Mostly hydrocarbon containing compounds like oils and non-oils: i. Oils: diesel oil, standard solvent, xylene-type, aromatic oils, polycyclic, aromatic oils, etc. ii. Amides: propanil, butachlor, alachlor, CDAA, diphenamide, naptalam, propachlor. iii. Aliphatics: dalapon, TCA, acrolein, glyphosate methyl bromide. iv. Bipyridyliums: paraquat, diquat. v. Benzoics: 2,3,6 TBA, dicamba, tricamba, chloramben, fenac. vi. Carbamates: propham, chloropham, barban. vii. Thiocarbamates: butylate, dilate, triallate, EPTC, molinate, pebulate, vernolate, benthiocarb, aslum, cycolate. viii. Dithiocarbamates: CDEC, metham. ix. Phenoxy: 2,4-D, 2,4, 5-T, MCPB, 2,4-DB, 2,4-DP, 2,4, 5TP (silvex) x. Ureas: monuron, diuron, fenuron, neburon, flumeturon, mothabenzathiazuron- buturon, chlorbromuron, chloroxuron, norea siduson, metoxuron. xi. Uracils: bromacil, terbacil, lenacil.

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xii. Nitralin (benzonitrates): dichlobenil, bromoxynil, ioxynil. xiii. Ditroanilines (toluidines): benefin, nitralin, trifluralin, butralin, dinitramine, fluchlorine, oxyzalin, penoxalin. xiv. Triazines: atrazine, simazine, ametryne, terbuteryne, cyprazinc, metribuzin, prometryn, propazine. xv. Organic arsenicals: cacodylic acid, MSMA, DSMA. xvi. Diphenyl ethers: nitrogen, flurodifen. xvii. Others: bentazon, piclaram, pyrazon, pyrichlor, endothall, bensulphioe, MH, DCPA. 4.2.1.4 Rodenticide Rodenticides are chemicals or substances that inhibit the growth of, destroy, or kill rodents (belonging to class Mammalia, order Rodentia). This includes not only rats and mice, but also other household pests, like chipmunks, squirrels, woodchucks, and other animals. Even though rodents have important roles in nature, they sometimes may require control. They can potentially damage crops, violate housing codes, be vectors for pathogens, and in some cases cause ecological damage. Rodenticides are broadly categorized into two groups: anticoagulants and nonanticoagulants. The first anticoagulant rodenticide was discovered in the 1940s and has since become the most widely used ingredients for commercial rodent control. Rodents poisoned with anticoagulants die from internal bleeding, in combination with the results of loss of blood clotting and damage to the capillaries. Prior to death the animal exhibits increasing weakness due to blood loss, though appetite and body weight are not specifically affected. Because of the slow action of anticoagulant baits, the target animal is unable to associate its illness with the bait eaten. Therefore, bait shyness does not occur. This delayed action also has a safety advantage because it provides time to administer an antidote (vitamin K1) to save pets, livestock, and people who may have accidentally ingested the bait. Anticoagulant rodenticides are also classified into two groups: 1. First-generation anticoagulants or multiple-feed rodenticide: These are chronic in action, requiring multiple feeds over several days to a week or more to induce death. 2. Second-generation anticoagulants: Resistance to the lethal effects of firstgeneration baits led to the development of second-generation anticoagulants: difenacoum, bromadiolone, brodifacoum, and other active

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ingredients. These compounds are much more potent than the firstgeneration anticoagulants, making them effective in the control of rats and mice. As one feed can induce death if a sufficient amount of bait is consumed, they are often referred to as single feed anticoagulants.

4.2.2 Plant Growth Regulators Plant growth regulators are defined as small, simple substances produced naturally by plants to regulate their growth and developmental processes. If a substance is produced within the plant it is called a plant hormone. The term “hormone” is derived from a Greek word meaning “to arouse, stimulate, or enhance an activity” (Avery, 1937). These hormones or chemicals are synthesized in one part of the plant and then travel to another part where they effect growth and development. There are five main groups: 1. Auxins or indole-3-acetic acid (IAA) 2. Gibberellins or gibberellic acid (GA) 3. Cytokinins (Ck) 4. Ethylene (C2H4) 5. Abscisic acid (ABA) 4.2.2.1 Growth Promoting Hormones (Mahajan, 2014) 1. Auxins: It is the first plant hormone or growth substance that was discovered in growth hormone history. It is derived from the Greek word, “auxein” meaning “to grow.” Compounds are generally considered as auxins if they are synthesized by the plant and have similar activity to IAA. Functions: • Stimulates differentiation of phloem and xylem • Initiation of adventitious roots on stem cuttings • Stimulates cell loosening, expansion, and elongation • Lateral root development in tissue culture • Stimulates cell division in tissue culture in combination with cytokinins • Stimulation or delay of abscission (young fruit) • Mediates the tropistic response of bending in response to gravity and light 2. Gibberellins: Gibberellins are synthesized in the young parts of shoots, uncertainly in roots, and also in developing seeds. There are also some evidences that leaves may be the source of some biosynthesis.

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Functions: • Delay senescence in leaves and also in citrus fruits • Physiological processes stimulated by active gibberellins are dependent on type • Stimulate cell elongation in stems • Breaks seed dormancy in some plants which require light for germination • Stimulates alpha-amylase production in germinating cereal grains • Induces maleness in dioecious flowers • Plays a role in the development of seedless fruit (parthenocarpic) • Stimulates bolting/flowering in response to long days 3. Cytokinins: In the word cytokinin, “cyto” means cell and “kinin” means division, that is, meaning cell division. Cytokinin is also called cytokine. These are compounds that resemble adenine (aminopurine) which stimulates cell division. Functions: • Responsible for cell division • Induction of apoptosis (at high concentrations) • Activates metabolite attraction (sink effect) • Stimulates morphogenesis in tissue culture • Retardation of senescence (at low concentrations) • Enhances stomatal opening in some plants • Stimulates the growth of lateral buds and leaf expansion through cell division and enlargement • Stimulation of chlorophyll synthesis which causes the conversion of etioplasts into chloroplasts 4.2.2.2 Growth Inhibiting Hormones 4. Ethylene: Ethylene has the simplest structure among all the plant hormones. It is a gaseous substance that has effects similar to abscisic acid. Functions: • Stimulates the growth or release of dormancy • Induces shoot and root growth as well as differentiation • Stimulates Bromeliad flower induction • Stimulates leaf and fruit abscission • Stimulates fruit ripening • Induction of femaleness in dioecious flowers • Stimulates flower senescence, leaf senescence, and flower opening 5. Abscisic acid: ABA controls numerous physiological processes in plants and is mostly known for its regulatory role in abiotic stresses like drought and high salinity. It promotes stomata closing through which

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plants are able to adapt to water stress (desiccation tolerance). It also controls seed germination, vegetative growth, and bud dormancy Functions: • Stimulates the closure of stomata • Induces seed and bud dormancy • Stress responses, especially to water deficiency • Inhibits shoot growth but does not have much effect on roots • Induces seeds to synthesize and facilitates the storage proteins • Induces some effects on the induction and maintenance of dormancy • Inhibits the synthesis of alpha-amylase stimulated by gibberellins

4.2.3 Fertilizers Fertilizers are inorganic materials with high analytical value and definite composition which can supply nutrients and trace elements, usually applied to the soil to encourage the growth of crops. Examples: • Nitrogenous fertilizers (urea, ammonium sulfate); • phosphate fertilizers (single/triple super phosphate); • potassic fertilizers (muriate of potash); and • macronutrients (Ca, Mg, O, C) and micronutrients (Zn, Mn, Cu, Fe, Mo, S, etc.). Classification of fertilizers (see Fig. 4.3) 1. Straight fertilizers: Straight fertilizers only supply one primary plant nutrient, namely nitrogen or phosphorus or potassium. For example: urea, ammonium sulfate, potassium sulfate, and potassium chloride. 2. Complex fertilizers: Complex fertilizers contain two to three primary plant nutrients of which two primary nutrients are in chemical combination. These fertilizers are usually produced in granular form. For example: DAP, nitrophosphate, and ammonium phosphate.

Figure 4.3 Classification of fertilizers composition-wise.

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Figure 4.4 Classification of fertilizers based on physical form.

3. Mixed fertilizers: These are physical mixtures of straight fertilizers. They contain more than two primary plant nutrients. These are prepared through systematic manual mixing of ingredients. Fertilizers can also be classified based on physical form: (see Fig. 4.4) 1. Solid fertilizers are found in several forms: • Crystals (ammonium sulfate); • powder (single superphosphate); • prills (urea, diammonium phosphate, superphosphate); • briquettes (urea briquettes); • granules (Holland granules); or • supergranules (urea supergranules). 2. Liquid fertilizers: • Liquid form fertilizers are applied with irrigation water or through direct application. • Their ease of handling, low labor requirement, and the possibility of mixing with herbicides have made liquid fertilizers more acceptable to farmers.

4.3 IMPACTS OF CLIMATE CHANGE ON INSECT PESTS AND PATHOGENS Crop plants are attacked by fungi, viruses, bacteria, mites, insects, and nematodes. Globally, pests destroy almost one third of crop products and are, thus, a serious constraint to crop production, in spite of advancements made in disease and pest management techniques over the past four to five decades. Global warming, as perceived by many, would increase the pressure of pests and diseases on crops. Disease and insect pest infestations often coincide with aberrant climatic conditions, such as erratic rainfall, drought, and fluctuations in humidity, which affect crop yields. Climate change might also affect the use of pesticides, due to an increased

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frequency of pest and disease appearances. Briefly, climate changes can result in: • Increased geographical distribution of pests and pathogens; • increased risk due to invasion by migrant pests; • increased resistance of pests to pesticides; • extension of the growing season of crops, thus, providing more time for pest/pathogen buildup; • increased pesticide use, with a 2.4 to 2.7-fold predicted increase by 2050 (Tilman et al., 2001).

4.3.1 Impacts of Climate Change on Insect Pests Worldwide, insect pests cause substantial losses to agricultural production. Due to climate change, pest populations may become unbalanced, leading to sudden outbreaks in a particular locality and causing major crop losses; whereas in others losses may decline. Insects respond to climate change in several ways, such as: 1. Alterations in their geographical distribution; 2. fluctuations in the temperature limits of pests and natural enemies; 3. development of strains with enhanced virulence; 4. pest shifts due to changes in food availability. This might result in the loss of some pest species and extensive distribution of remaining species (Sharma, 2014; Boullis et al., 2015; Tripathi et al., 2016). Climate change affects the geographic spread of insect pests, which is governed more by low temperatures than by high temperatures (Bale et al., 2002; Menendez, 2007; Boullis et al., 2015). Insect pests respond to climate change in numerous ways, ranging from phenology changes to influencing community composition and dynamics (Menendez, 2007; Moore and Allard, 2008). Forests occur over a wide range of climatic conditions and thus, have the ability to persist under new climatic conditions. However, pest migration to new territories without any natural enemies, might result in sudden pest outbreaks, tree mortality, and reduced forest growth. Increased pest incidence in crops will result in substantial economic losses, and climate change can have a significant effect on the intensity of insect pest populations (Sharma, 2014: Dhaliwal et al., 2010). Climate change could lead to greater population shifts in Helicoverpa armigera (Hub.) and Maruca vitrata (Fab.) to temperate regions, which will have drastic effects on the hosts of these pests (Sharma, 2005, 2010).

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Spodoptera litura (Fab.) is emerging as a serious threat under increased CO2 levels (Kranthi et al., 2009). In Brazil and North America, H. armigera, and in Maharashtra (India), woolly aphid (Ceratovacuna lanigera), have been recognized as invasive and serious pests due to climate change (Czepak et al., 2013; Tay et al., 2013). Pigeon pea sown in August showed higher incidence of insect pests than those sown in the month of September. Similarly in chickpea, greater leaf damage caused by H. armigera and Spodoptera exigua was observed in crops sown in October, while the lowest was observed in crops sown in January (Sharma et al., 2016). Many insect species might have their diapause activities interrupted due to alteration in temperatures or moisture regimes and day lengths (Parmesan, 2006; Tripathi et al., 2016). The extent to which insects withstand changing climatic conditions, will depend on their life history characteristics. Fast growing and nondiapausing insect pests will expand geographically, whereas diapausing insect pests will show reduced expansion and will be more prone to extinction due to changes in climatic conditions.

4.3.2 Impacts of Climate Change on Plant Pathogens A plant disease results through interactions between a susceptible host plant, a virulent pathogen, and favorable environmental conditions. Environmental changes are strongly associated with the level of loss caused by a disease (Anderson et al., 2004). Changes associated with increased temperature, precipitation patterns, and drought, etc., may affect the severity of disease and coevolution of plants and pathogens (Chakraborty, 2005; Crowl et al. 2008; Eastburn et al., 2011). Pathogen biology can be directly influenced by environmental factors. The production and germination of plant propagules have been strongly dependent on temperature, relative humidity, and leaf wetness (in the case of foliar pathogens) (Colhoun, 1973; Huber and Gillespie 1992). Overwintering and oversummering of pathogens in the absence of a host can be affected by temperature and relative humidity. Pathogens that have evolved at high latitudes might tolerate a wide range of temperatures; therefore, climate change is expected to enhance their fitness and the risk of disease epidemics (Deutsch et al., 2008). Less rainfall may lead to decreased downy mildew incidence in grapes, however, the temperature increase compensates for the reduction in leaf wetness duration because infections starting earlier in the growing season allow more time for epidemics to develop

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(Salinari et al., 2006). Reductions in frost due to increased average minimum temperatures favor pathogens, such as Fusarium circinatum (pine pitch canker), particularly in Europe (Watt et al., 2011). Conversely, there is a reduction in disease occurrence in wounded hosts when frost conditions are reduced in the case of frost virulent pathogens (e.g., Seiridium cardinale on cypress species; Garbelotto, 2008). Therefore, environmental factors, such as high temperatures, that accelerate tissue death, may favor necrotrophs. On the other hand, factors such as higher levels of CO2, increased temperature, or drought, cause changes in the physiology of a host, which alter the colonization of host tissues by biotrophic pathogens (Chakraborty and Datta, 2003). Dry conditions have direct effects on pathogens, as reported in the case of an invasive exotic species Heterobasidion irregulare in central Italy, which is better adapted to the dispersal in the Mediterranean climate than the native Heterobasidion annosum species (Garbelotto et al., 2010). Temperature and moisture influence the reproduction rate of many pathogens (Caffarra et al., 2012). Most pathogens will have an advantage over plants because of their shorter generation periods and their ability to be quickly dispersed by the wind, thus, helping them in the migration of and adaptation to climate change (Davis et al., 2005). Climate change appears to have been associated with shifts in plant hosts for some fungi (Gange et al., 2011). Climate change is not only going to threaten plant health, but in some cases, it may enhance pest and disease incidence also. There have been reports of more frequent infections of Phytophthora infestans in Finland due to climate change (Hannukkala et al., 2007). In Canada and the United States, high temperatures have been associated with large-scale outbreaks of bark beetles (Bentz et al., 2010; Woods et al., 2010; Woods, 2011). In Scotland, models predict a lesser impact of oilseed rape diseases caused by Leptosphaeria maculans and Pyrenopeziza brassicae (Fitt et al., 2011). However, in Northern Germany, oilseed rape pathogens, such as Alternaria brassicae, Sclerotinia sclerotiorum, and Verticillium longisporum, are predicted to be favored by warmer average temperatures, in the long-term (2071 2100) (Siebold and von Tiedemann, 2012). High temperatures may possibly make it easier to deploy biological control in some cases, although there is still little information available on the impacts of climate change on plant disease management by biological means (Ghini et al., 2008; Compant et al., 2010).

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4.3.3 Relationship Between Pesticides and Climate Change for Crops Pesticides are associated with risks to human health and the environment. However, these are also responsible for benefits, particularly in agricultural production. Pesticide overuse results in significant decline in the populations of birds, predators like Chrysoperia carnea, ladybird beetles, spiders, Apanteles spp., Trichogramma spp., Cheloanus, blackburni, earthworms, etc. (IPCC, 2001a). An important indirect effect of climate change might be the appearance of late seasonal agricultural pests, which require pesticide applications after their population reaches economic injury levels, usually in mid-summer. Under high temperatures such pests may require early spraying in summer, adding additional pesticides into the environment. Several scientists have investigated the effects of climate change on pest populations (Patterson et al., 1999; Porter et al., 1991). They predicted that climate change will help some pests to survive winters and will accelerate the development of summer-active species. Therefore, this might lead to increased pesticide use, and later on the negative impacts on the environment may be amplified. Information on climate change pesticide-use environment interactions is quite limited. Temperature and rainfall patterns, as influenced by changing climatic conditions, are expected to have the largest influence on the partitioning of chemical toxicants (Henriksen et al., 2013). Koleva et al. (2009) studied the impact of climate change on pesticide applications in the United States. They found that weather and climate differences significantly influence the application rates of most pesticides. Fahim et al. (2011) studied the impacts of climate change on tomato diseases in Egypt and found that recent climate changes, such as increased duration of night and winter temperatures, may contribute to greater incidence of tomato diseases. The ranges of several important tomato diseases in Egypt, like tomato late blight, have expanded since the early 1990s, possibly in response to climate changes. Based on the analysis of plant disease climate relations, late blight epidemic onset in tomatoes was 1 2 weeks earlier than usual, meaning 2 3 additional sprays are necessary to achieve sufficient control of late blight. Accordingly, 1 3 more sprays will be applied in the coming decades. It is expected that changes in the climate and the accompanying emergence of diseases, insect pests and the influence of faster degradation on pesticide behavior due to higher temperature, increased precipitation, decreased activity under dry conditions, loss due to runoff by rainfall, etc., will increase the use of pesticides for certain crops.

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4.4 CLIMATE CHANGE AND AGROCHEMICAL USE The use of agrochemicals, especially chemical fertilizers, is a major contributor to climate change. The International Panel on Climate Change (IPCC) estimated that the agricultural sector accounts for up to 24% of the global greenhouse gas emissions (FAO, 2014). Among chemical fertilizers, the use of synthetic nitrogen fertilizer is the biggest contributor to climate change in agriculture owing to the potent greenhouse gas N2O (nitrous oxide). In continuation, the second largest source of greenhouse gas is enteric fermentation; methane from cows and sheep as their digestive tracts produce CH4 through anaerobic fermentation (Hopkins and Del Prado, 2007).

4.4.1 Agrochemical Usage as Influenced by Climate Agricultural pesticides have great impact on ecology, the environment, and human health. This impact is susceptible to climate change because due to the pressure of pests and diseases, optimal pesticide application rates differ with differing conditions of weather and climate. As the climate changes, the scenarios of pest populations also change, therefore, to control the pest population alteration in the patterns and types of agrochemicals is required which leads to cost variability. Pesticides are toxic to many forms of life. If pesticide dosages increase to a higher range, the risk to beneficial insects, such as ladybugs and honeybees, will increase, which could lead to worse insect problems in the future (Aktar et al., 2009). These factors are directly influenced by climate change: • Crop characteristics • Pesticide efficiency • Pest occurrence and severity The activities of pesticides are affected due to increased numbers and varieties of pests, diseases, and weeds (Müller et al., 2010; Ntonifor, 2011). Because of changes in climatic conditions, the application rates and amounts of herbicides exceed those of insecticides or fungicides which shows the existence of weed resistance and the declining efficacy of herbicides (Delcour et al., 2015). When temperatures increase, the volatilization of pesticides and fertilizers from soil and vegetation occurs causing atmospheric contamination. (Yeo et al., 2003) and takes place when a liquid or solid substance transfers to the gaseous phase. A higher volatilization rate of organochlorine pesticides during warmer weather was seen in the United States and Korea (Nations and Hallberg, 1992; Yeo et al.,

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2003). The risk of diffuse losses of pesticides into the environment also increases due to climate change. There are four main reasons for this (Maksymiv, 2015): 1. An increased amount and intensity of rainfall increases the risk of leaching in soil. An increase in temperature could counteract the leaching of pesticides by resulting in a higher degradation rate, but in certain regions higher temperatures could cause drought, which would inhibit pesticide degradation. Soil organic carbon content is very important for binding pesticides to the soil and it can be expected to decrease in a warmer climate, which is positive from a leaching perspective; although higher temperatures will mean that organic material is broken down more quickly in certain areas. The combined impact of these factors on the risk of pesticide leaching is difficult to predict and can be expected to vary between different regions. 2. Early application of pesticides can be beneficial in longer growing seasons if it is allowed because of the longer time available for degradation. However, autumn spraying can also be expected to be delayed owing to the greater area of winter crops grown, which may increase leaching, as precipitation in autumn/winter is predicted to increase. 3. Changes in crop rotations, land use, and the introduction of new varieties affect the requirements of pesticides. 4. Higher moisture content and temperature will enhance the pressure from pests and probably result in an altered weed flora, which is expected to increase the need for pesticides. Most pesticides used within agriculture today are herbicides, but climate change may result in a greater need for insecticides and fungicides in the future.

4.4.2 Beneficial Effects of Agrochemical Use Agrochemicals have various beneficial effects. These include supplementation of plant nutrients, crop protection, growth promoters, and growth retardants, preservation of food and materials, and prevention of vectorborne diseases. The most important benefits include the protection of human, animal, and crop health and protection of recreational turf. Table. 4.1 summarizes effects, primary and secondary benefits, and their interactions. The relationship between beneficial and harmful effects is quite complex and not easy to pursue.

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Table 4.1 The complexity of the effects, primary and secondary benefits of pesticides Primary benefits Secondary benefits

1. • • • •

Controlling pests and plant disease vectors Improved crop/livestock quality Reduced fuel use for weeding Reduced soil disturbance Invasive species controlled

2. • • • •

Controlling disease vectors and nuisance organisms Human lives saved Human disturbance reduced Animal suffering reduced Increased livestock quality

3. Prevent or control of organisms that harm other human activities and structures • Tree/bush/leaf hazards prevented • Recreational turf protected • Wooden structures protected

Community benefits • Nutrition and health improved • Food safety/security • Life expectancy increased • Reduced maintenance costs National benefits • National agricultural economy • Increased export revenues • Reduced soil erosion/ moisture loss Global benefits • Less pressure on uncropped land • Fewer pest introductions elsewhere • International tourism revenue

Source: Adapted from Cooper, J., Dobson, H., 2007. The benefits of pesticides to mankind and the environment. Crop Prot. 26 (9), 1337 1348.

4.4.3 Harmful Effects of Agrochemical Use Agrochemical use raises numerous environmental concerns, including human health and animal health hazards. Pesticides are ubiquitous in the environment and most are synthetic. Food toxicity due to pesticides is associated with severe effects on human health. Actually, the mode of action of pesticides involves targeting the systems or enzymes in pests which may be identical or similar to the systems or enzymes in human beings and they, therefore, pose risks to human and livestock health as well as to the environment. Pesticide toxicity can result from a high level of exposure, mishandling, ingestion, inhalation, or dermal absorption. Sustained exposure to

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such agrochemicals for a long period of time may result in various dangerous diseases, such as: • Hormonal imbalances, leading to infertility and breast pain; • neurological, psychological, and behavioral dysfunctions; • reproductive system defects; • immune system dysfunction; • blood disorders; • genotoxicity; • cancers. Pesticides can pollute soil, water, grasslands, and other vegetation. In addition to killing insect-pests, pathogens, or weeds, pesticides can be toxic to nontargeted organisms, like birds, fish, beneficial insects, and other plants. A number of toxicological studies in animals demonstrate that pesticides, to which the general population may be chronically exposed, are potential carcinogens, neurotoxins, reproductive toxins, and immunotoxins, besides the involvement of pesticides in the development of neurodegenerative diseases (Franco et al., 2010; Gupta, 2011). Several articles and reports evaluated toxicological and epidemiological evidences for various health effects associated with pesticides (Maksymiv et al. 2015; Lushchak et al., 2009; Kubrak et al., 2012). Pesticide contamination can effect aquatic fauna and flora, as well as human health when water is used for public consumption. Aquatic organisms are directly exposed to chemicals resulting from agricultural production via surface runoff or indirectly through trophic chains.

4.5 EFFECTS OF CLIMATE CHANGE ON PESTICIDE POLLUTION IN SURFACE AND GROUND WATER The degradation, sorption, and transport of agrochemicals are dependent on several factors, such as soil properties (clay, oxide, and organic matter content), agrochemical properties (cationic, anionic, and nonionic), different management practices (soil, crop, and plant protection), environmental concentrations, and different climatic factors, such as temperature, precipitation and freezing/thawing, etc. Climate change has both direct and indirect effects on pesticide exposure. Direct effects: • Changes in temperature will change the chemical degradation of pesticides.

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• • •

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Changes in the frequency of freezing and thawing will affect the equilibrium between sorbed and nonsorbed pesticides, thereby changing the availability of pesticides for degradation, leaching, and runoff. More frequent extreme events, such as heavy rainfall and storm events, will cause macropore flow and erosion in summer (increased runoff of organic matter) and remobilization, erosion and runoff events in winter and spring. Indirect effects: Changed microbial activity and changed microbial degradation of pesticides, extended and more frequent use of pesticides in existing agricultural regions because of more frequent attacks of pests and diseases, and changes in soil management and agricultural practices (Eklo, 2009).

There are three basic ways that properly applied pesticides may reach surface water and groundwater: runoff, run-in, and leaching. (Bicki, 1989). • Runoff is the physical transport of pesticides on the soil surface with rainwater that does not soak into the soil. Pesticides movement occurs from fields either through runoff water or through their attachment to the eroded segment. • Run-in is the physical transport of pesticides directly into groundwater. For instance, this can occur in parts of carbonate karst (limestone) aquifers that contain sinkholes and permeable or fractured bedrock. Irrigation or rainwater can carry pesticides directly into groundwater through sinkholes or fractured bedrock. • Leaching is the movement of pesticides through the upper layer of soil by irrigation or rainwater with the downward movement of water. Factors affecting the potential of pesticides to leach out into the soil include clay content, soil organic matter content, permeability, etc. In general, soils with moderate-to-high clay and organic matter contents, and slow or moderate permeability have less leaching potential for pesticides into groundwater. It is believed that macropores (principally root channels and wormholes) may contribute to the leaching of pesticides into fine-textured soils. Leaching of pesticides is more prone to occur in sandy, permeable soils along rivers, and run-in may occur in carbonate aquifers. Persistence and volubility are the main factors responsible for the ultimate fate of a pesticide applied to soil. Climate change affects both surface water agrochemical pollution and groundwater agrochemical pollution through different means:

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4.5.1 Pollution of Pesticides in Surface Water Water pollution is directly linked with human actions of industrial, urban, or agricultural origin, and as an indirect consequence of these actions, climate change could lead to degradation in surface water quality. Major climate change determinants that affect water quality include the ambient air temperature and increased extreme hydrological events. Other factors to be considered are soil drying rewetting cycles and increased solar radiation. Temperature must be considered as the main factor which affects almost all physicochemical equipoise and biological reactions. It is notable that all physicochemical constants vary with temperature, and frequently increasing endothermic reactions. According to Arrhenius relation, the kinetics of a given chemical reaction can be doubled with an increase of 10°C. Consequently, several effects or transformations related to water, such as degradation, solubilization, evaporation, complexation, dissolution, etc., will be favored by increased water temperature. (Delpla et al., 2009). Nutrients and pesticides cause pollution to surface water from different nonpoint sources such as agriculture. 4.5.1.1 Nutrients Nutrient loads are expected to increase under climate change (Bouraoui et al., 2002). According to Ducharne et al. (2007), an increased mineralization of nitrogen in the soil is expected because of an increased mean soil temperature. Enzymatic activities could be increased largely due to a moderate increment in soil temperature. Temperature has a positive correlation with nitrification processes by increasing the activity of phosphatase enzymes and P mobilization in soils. Changes observed in enzymatic activities are related to the direct effect of warming soil, which stimulates biological activities and increases nitrogen availability (Sardans et al., 2008). The soil extractable nitrate concentration increases due to warming in summer and autumn, while during winter the soil extractable ammonium concentration is increased. The rate of mineralization and release of carbon, nitrogen, and phosphorus from soil organic matter also increases. Further, there are chances of high runoff and elevated soil erosion due to increases in precipitation, which result in an increase in pollutant transport, especially after a drought period. For lakes in temperate countries, higher ammonium and phosphate concentrations

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in the hypolimnion are frequently observed during warm periods (Pettersson et al., 2003). According to a study by Mooij et al. (2005), climate change impacts on such ecosystems in various manners, such as changes in temperature, ice cover, wind, and precipitation. Higher ammonium concentrations were observed in rivers with a reducing dilution capacity caused by droughts (Zwolsman and van Bokhoven, 2007; Van Vliet and Zwolsman, 2008). Phosphorous loading exportation governed by discharges following heavy rainfalls, will tend to increase with climate change and consequently have an impact on lakes (Mooij et al., 2005). 4.5.1.2 Pesticides Surface waters are the main receptors of pesticides contamination from agricultural use. Main climatic factors which affect the surface water contamination by pesticides are changes in the rainfall seasonality and intensity, air temperatures for the changing pesticides fate and behavior, despite the fact these effects of climate change are likely to be variable and difficult to predict (Bloomfield et al., 2006). In a study on the physicochemical interactions between soil organic matter and herbicidal compounds after drying and rewetting cycles to see the impacts of climate induced soil water status variations, it was found that variations in soil water contents modify the structure of soil organic matter, which traps pesticides by hindering their diffusion (Lennartz and Louchart, 2007). An increase in extreme events due to climate change will probably inhibit pesticide reduction measures. Probst et al. (2005) simulated pesticides entries into streams and found that, in heavy rainfalls scenario (precipitation increase 10 20 mm day21), isoproturon and bifenox could potentially present a greater risk due to their eco-toxicity. Climate change has several impacts on factors affecting pesticide pathways responsible for surface water pollution. According to Bloomfield et al. (2006) these impacts are: • The volatilization and degradation of pesticides and their residues in soil and surface water will be increased with higher temperatures. • Wetter soils have higher hydraulic conductivities, so the movement of pesticide-rich water may be more rapid through these soils, although they will also show an enhanced rate of degradation. • More frequent precipitation with high intensity may lead to more bypass flow increasing the likelihood of rapid pesticide movement to drains, surface waters, and vertically below the soil layer.

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More frequent and greater intensity of rainfall may cause rapid and large increases in pesticide movement to drainage systems as well as other structures below soil layers. • In the case of shorter recharge periods, there will be less possibility for pesticide transport through the soil and loss of surface water. • Warmer, drier summers, may cause cracking of soils which increases the potential for bypass flow. • Dry soils have lower biodegradation potential than wet soils and pesticide residues from spring applications could persist throughout the summer. Increased use of chemical pesticides in agriculture could be attributed to a warmer climate that is more favorable for disease and pest outbreak. Some climatic extremes, such as heavy downpour, lead to higher runoff, which is responsible for the washing out of pesticides from soils to waterbodies and an increasing erosion of pesticide-rich soil particles from fields to drains, and surface waters may contaminate the water in waterbodies (Boorman., 2003). These pesticides can accumulate in food (fish, grain, or meat) because of their lipophilic/hydrophobic nature that may lead them to be bioaccumulated in the tissues (e.g., fat, milk) of living organisms. Pesticide receptors (waterbodies) are also influenced by climate change. Surface water concentrations could be significantly changed by small changes in mean river flows due to the dilution effect. In summer, there may be a significant reduction in dilution potential, which leads to a higher pesticide concentration in runoff water and ultimately in waterbodies. (Kibria, 2014).

4.5.2 Pollution of Pesticides in Ground Water Leaching is the major pathway of pollution into groundwater through soil, whereas reinfiltration of surface water is a minor cause of groundwater pollution (Balderacchi et al., 2013). The British Geological Survey has assessed the impacts of climate change on the fate and behavior of pesticides in the environment. The major approach was to analyze the source of pesticides, their movement through the subsurface (pathways), and their receptors. It was inferred that in the long run, changes in land-use systems driven by climate change may have a more significant effect on pesticides in surface and ground waters than the direct impacts of climate change on pesticide fate and transport. (Bloomfield et al., 2006)

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There are direct and indirect effects of climate change on the leaching of pesticides into ground and surface waters. Direct effects are the natural responses of the soil-ecosystem to changes in climatic variables (mainly temperature, precipitation, and potential evapotranspiration). Indirect effects may be defined as any changes in the use, fate, and movement of pesticides that are further aggravated by human activities in response to climate change. Some changes, such as changes in soil organic carbon content, may be considered either direct or indirect depending on the cause of the change Direct and indirect effects of climate changes on groundwater pollution: • Direct effects: • Higher turn-over of organic matter (less sorption sites), weaker sorption (for most compounds), and increased hydraulic conductivity. • Changes in freezing-thawing cycles that can lead to changes in soil structure (e.g., crack formation, aggregation). • Increased volumes of drain flow/percolation. • Reduced degradation of pesticides. • Preferential transport triggered more often because of high precipitation intensities. • Reduced degradation of pesticides and the cracking of clay soils affects macropore flow due to prolonged duration of drought. • Indirect effects: • Change in cropping patterns and the introduction of new crops due to shifting of seasons also requires newer pesticide application with different times and rates of spray • Warmer winters would favor the survival of insect pests and shift weed flora (Patterson et al., 1999), which would trigger the need for plant protection, which could result in an increased use of pesticides (larger area sprayed or more frequent sprayings). • Furthermore, each use of the pesticide increases the proportion of the less-susceptible individuals in a population which develop a resistance in insects against pesticides

4.6 CONCLUSION Changing climatic conditions definitely influence the use of pesticides. Therefore, several new factors get involved which are responsible for the

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usage pattern of pesticides, an amplified intensity of pesticide use is expected in the form of higher amounts, doses, frequencies at different types of applied formulations in the fields. In soil, the transport of pesticides is, thus, mainly determined by rainfall seasonality, intensity, and temperature inflation as well as land-use changes, which indicates an indirect impact of the long-term impacts of climate change. Therefore, they can be toxic to nontargeted organisms, like birds, fish, beneficial insects, and other plants. Adapted pesticide use will finally impact consumer exposure at the end of the food chain. Finally, there is need for government and non-governmental organizations (NGOs) to promote programs on education about basic pesticide use, the concept of ecofriendly farming, awareness of change in the context of agricultural, environmental, and health policies as well as global warming will ensure long-term food security and environmental safety. To a great extent, efforts should be made to understand the relationship between pesticide use and climate for crops toward protecting, sustaining, or restoring the health of people, communities, and ecosystems using integrated and comprehensive approaches and partnerships.

ACKNOWLEDGMENTS The authors sincerely acknowledge all the academicians, eminent scientists, students, and researchers engaged in the area agrochemicals study.

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Sharma, H.C. (Ed.), 2005. Heliothis/Helicoverpa Management: Emerging Trends and Strategies for Future Research. Oxford & IBH, and Science Publishers, New Delhi, 469 pp. Sharma H.C., 2010. Effect of climate change on IPM in grain legumes. In: 5th International Food Legumes Research Conference (IFLRC V), and the 7th European Conference on Grain Legumes (AEP VII), 26 30 April 2010, Antalya. Sharma, H.C., 2014. Climate change effects on insects: implications for crop protection and food security. J. Crop Improv. 28, 229 259. Sharma, H.C., Pathania, M., War, A.R., Pavani, T., Vashist, S., 2016. Climate change effects on pest spectrum and incidence in grain legumes. In: Dixit, G.P., Singh, G., Singh, N.P. (Eds.), Pulses: Challenges and Opportunities Under Challenging Climatic Scenario. ICAR-IIPR, Kanpur, pp. 124 137. Siebold, M., von Tiedemann, A., 2012. Potential effects of global warming on oilseed rape pathogens in Northern Germany. Fungal Ecol. 5, 62 72. Tay, W.T., Soria, M.F., Walsh, T., Thomazoni, D., Silvie, P., et al., 2013. A brave new world for an old world pest: Helicoverpa armigera (Lepidoptera: Noctuidae) in Brazil. PLoS ONE 8 (11), e80134. Available from: https://doi.org/10.1371/journal. pone.0080134. Tilman, D., Fargione, J., Wolff, B., Antonio, D., Dobson, A., Howarth, R., et al., 2001. Forecasting agriculturally driven global environmental change. Science 292, 281 284. Tripathi, A., Tripathi, D.K., Chauhan, D.K., Kumar, N., Singh, G.S., 2016. Paradigms of climate change impacts on some major food sources of the world: a review on current knowledge and future prospects. Agric. Ecosyst. Environ. 216, 356 373. Van Vliet, M.T.H., Zwolsman, J.J.G., 2008. Impact of summer droughts on the water quality of the Meuse river. J. Hydrol. 353 (1-2), 1 17. Watt, M.S., Ganley, R.J., Kriticos, D.J., Manning, L.K., 2011. Dothistroma needle blight and pitch canker: the current and future potential distribution of two important diseases of Pinus species. Can. J. For. Res. 41, 412 424. Woods, A., 2011. Is the health of British Columbia’s forests being influenced by climate change? If so, was this predictable? Can. J. Plant Pathol. 33, 117 126. Woods, A.J., Heppner, D., Kope, H.H., Burleigh, J., Maclauchlan, L., 2010. Forest health and climate change: a British Columbia perspective. For. Chron. 86, 412 422. Yeo, H.G., Choi, M., Chun, M.Y., Sunwoo, Y., 2003. Concentration distribution of polychlorinated biphenyls and organochlorine pesticides and their relationship with temperature in rural air of Korea. Atmos. Environ. 37 (27), 3831 3839. Zwolsman, J.J.G., van Bokhoven, A.J., 2007. Impact of summer droughts on water quality of the Rhine River—a preview of climate change? Water Sci. Technol. 56 (4), 45 55.

FURTHER READING Abadin, H.G., Chou, C.H.S.J., Llados, F.T., 2007. Health effects classification and its role in the derivation of minimal risk levels: immunological effects. Regul. Toxicol. Pharm. 47 (3), 249 256. Abd El-Aleem Saad Soliman Desoky, 2016. Rodenticides (anticoagulant). Int. J. Res. Stud. Zool. 2 (3), 1 6. Anonymous, http://www.groundwateruk.org/Default.aspx. U.S Environmental Protection Agency, Office of Prevention, Pesticides, and Toxic Substances, U.S. Government Printing Office, 2004. Analysis of rodenticide bait use. Washington, DC.

CHAPTER 5

Climate Change and Secondary Metabolism in Plants: Resilience to Disruption Suruchi Singh, Kshama Rai, Naushad Ansari and Shashi Bhushan Agrawal

Laboratory of Air Pollution and Global Climate Change, Department of Botany, Institute of Science, Banaras Hindu University, Varanasi, India

Contents Introduction Climate Change: A Dynamic Phenomenon Greenhouse Gases Aerosols Solar Radiation Volcanoes Pillars for Sustenance: Secondary Metabolites Terpene and its Role in Plant Defense Plant Phenolics and its Role in Plant Defense N-Containing Compounds Salicylic Acid and Jasmonic Acid Periodical Changes: Climate Change Versus Secondary Metabolites Ex Ante and Ex Post Investments in Secondary Metabolism: An Investment With a Return 5.14 Conclusion Acknowledgment References Further Reading 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 5.10 5.11 5.12 5.13

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5.1 INTRODUCTION Climate change has been recognized as any alteration in climate over time, whether due to natural changes or as a result of human activity. These changes include global warming, precipitation fluctuations, rising sea levels, and other extreme climatic events. Despite the amount of information that exists on global climate change, little is known on how these Climate Change and Agricultural Ecosystems DOI: https://doi.org/10.1016/B978-0-12-816483-9.00005-0

Copyright © 2019 Elsevier Inc. All rights reserved.

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changes may affect natural ecosystems, particularly interactions among living organisms. According to the phytochemical evolution theory, plants generate selective forces that lead to the evolution of plant defenses (i.e., plant secondary metabolites). These chemicals, although not required for primary plant metabolic processes, such as respiration or growth, have been recognized for their role in plant defense against herbivore or pathogen attack (Kliebenstein, 2004). Plant defense responses to climate change affect their other processes like their interaction with herbivores and pathogens. In general, plant defense responses to herbivores are dependent on their evolutionary history (past exposure to herbivores) as well as their physical environment, affecting plant insect association (Tollrian and Harvell, 1999). BidartBouzat et al. (2005) were the first to report that herbivory induced plant secondary chemicals (glucosinolates) can get modulated by changes in climatic factors like atmospheric carbon dioxide (CO2) concentration. The emergence of plants from their green algal ancestors dates back 500 million years (Kenrick and Crane, 1997). Plants have developed secondary metabolic pathways against the harsh environments they have been subjected to (Waters, 2003). So, these secondary metabolites act like a chemical interface between plants and their environment. Since, plants are sessile and cannot escape adverse conditions, they strategically utilize metabolites for defense. Evolved plants could withstand environmental stress and have shown adaptive evolution (Fig. 5.1). The details are discussed subsequently. Keeping in mind this tug of war between climate change and plant metabolic machinery, an attempt is made to gain an understanding on climate change, secondary metabolism, and the interface between the two.

5.2 CLIMATE CHANGE: A DYNAMIC PHENOMENON Earth’s climate is constantly being subjected to substantial changes due to natural and anthropogenic activities. Our understanding is limited owing to the challenges of complexities and interconnectedness of the components that is, the atmosphere, land, and water of the climate system. The basis of climate change is its physical drivers, which are fundamentally associated with atmospheric composition and cloud effects. Variations in the atmospheric concentrations of greenhouse gases (GHGs), aerosols, land cover, and solar radiation alter the energy budget of the Earth and are, thus, forces for climate change. Key drivers affect

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Climate change

Abiotic stress

Temperature

Low

Water

High Drought

Radiation

Flood

Low PAR

High PAR

Pollution

UV-B Water

Air

Edaphic

Soil Soil type Minerals Soil movement

ROS Oxidative stress

Adaptive evolution Adaptation Genotoxic

Damage

Ecosystem evolution

Figure 5.1 Image showing interface between environmental variables and plant responses as well as their role in inducing adaptive evolution.

the absorption, scattering, and emission of radiation within the atmosphere on the Earth’s surface. These drivers induce positive or negative changes in the energy budget which is expressed as radiative forcing (RF). The individual effect of these drivers is utilized to compare their warming and cooling influences on the Earth’s climate. The RF for different components is given in Table 5.1. Changes in their concentrations come about when their emissions and removal mechanisms are altered so that atmospheric concentrations are no longer in sync with the sources and sinks of these gases. The greenhouse effect occurs when solar energy that makes contact with the Earth’s surface is retransmitted into the atmosphere in the form of infrared thermal radiation. This reflected radiation has a lower frequency than solar energy itself. GHG molecules absorb this thermal radiation at low frequencies, causing these molecules to vibrate. These greenhouse molecules then emit energy in the form of infrared photons, many of which return to the Earth’s surface. Non-GHGs, such as oxygen and nitrogen, do not absorb thermal radiation. Earth’s average surface temperature has been rising since 1970. The global surface temperature has risen at an average rate of about 0.17°C

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Table 5.1 Radiative forcing of different climate components Human activities Gases

Long-lived GHGs

Ozone Water vapor Surface albedo Total aerosol Linear contralis Natural processes

Carbon dioxide Methane Nitrous oxide Halocarbons Stratospheric Tropospheric Land use Black carbon on snow Direct effect Total cloud effect Solar irradiance

Radiative forcing (positive or negative)

Positive Positive Positive Positive Negative Positive Positive Negative Positive Negative Negative Positive Negative

per decade. The average global temperature for 2016 was 1.10°C, above the 20th century average of 13.9°C exceeding the previous second warmest year 2015 by 0.04°C. Many factors affect the climate of the Earth (Fig. 5.2). These factors include output of energy from the sun (warming effect), volcanic eruptions (cooling effect), concentration of GHGs in the atmosphere (warming effect), and aerosols (cooling effect). Details about the contributing factors are provided here:

5.3 GREENHOUSE GASES Since the Industrial Revolution, the largest contributor to the increase in global warming is carbon dioxide (CO2) followed by methane (CH4). CO2 concentrations have increased from 278 ppm in 1960 to 401 ppm in 2015, a 44% increase. Water vapor has an important indirect effect on temperature increases, resulting from increasing GHG concentrations. The increase in global temperature due to GHGs also increases the capacity of the atmosphere to hold water vapor, thus, acting as a positive feedback as water vapor in return also produces a greenhouse effect. An increase in temperature by 1°C results in an approximately 7% increase in atmospheric water vapor. Therefore, although CO2 is the main contributor to climate change, water vapor is a strong and fast feedback that amplifies any initial forcing. Anthropogenic activity-induced increases in methane

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Figure 5.2 Picture depicting many factors affecting the climate of the Earth.

also increase stratospheric water vapor, thus, producing a positive RF (Solomon et al., 2010; Hegglin et al., 2014). Other anthropogenic sources of stratospheric water vapor include hydrogen oxidation (le Texier et al., 1988), aircraft exhaust (Morris et al., 2003), and volcanic eruptions (Löffler et al., 2016). In the troposphere, the amount of water vapor is controlled by temperature (Held and Soden, 2000). Atmospheric

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circulation, especially convection, limits the buildup of water vapor in the atmosphere that would otherwise evaporate from the surface. Direct implications on atmospheric water vapor are not much in comparison to the indirect alterations caused by temperature changes resulting from RF. As such, changes in tropospheric water vapor are considered feedback in the climate system. Ozone is another naturally occurring GHG in the troposphere and stratosphere, but is also produced and destroyed in response to a variety of anthropogenic emissions. Ozone concentrations vary spatially and temporally due to various reasons, like the nature of its production, loss, and transport processes, which regulate ozone abundances, thus, add complexity to ozone RF calculations. In the tropospheric region, ozone is photochemically formed from methane, NOx, carbon monoxide (CO), and non-methane volatile organic compounds (VOCs), both near and far downwind of these precursor sources, leading to regional and global positive RF contributions (Dentener et al., 2005). On the other hand, stratospheric ozone is destroyed photochemically in reactions involving halogen species; chlorine and bromine. The release of halogen into the stratosphere is mainly attributed to the decomposition of some halocarbons, which are emitted at the surface owing to natural processes as well as human activities. Stratospheric ozone depletion, which is most remarkable in the polar regions, has a net negative RF.

5.4 AEROSOLS Aerosols released from industrial sources neutralize about 26% of greenhouse warming by preventing solar radiation from reaching the Earth’s surface. However, there is large uncertainty regarding the degree of influence that aerosols can have on the climate. GHGs, like CO2, have a longer residence time in the atmosphere (B100 years) compared to aerosols (only 10 days), resulting in cooling due to the short-term effect of industrial pollution followed by long-term warming. In the near future, aerosols are believed to offset a lower percentage of warming potential of GHGs due to lower residence time, thus, there will be a possibility of acceleration in warming even without an acceleration of GHG concentrations. They usually consist of a mixture of components: sulfates, nitrates, ammonium, organic carbon, black carbon, sea salt, mineral dust, trace metals, and water. Thus, aerosol particles affect the climate both directly and indirectly, by scattering and absorbing solar radiation and by

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modifying the properties of clouds. Some aerosols like black carbon may cause warming, whereas most other aerosols cause cooling. The short residence time of aerosols (days/weeks) makes their effects more regional and less persistent than the long-lived GHGs. Their short lifetimes make aerosol concentrations highly flexible in space and time. This variability is an important factor in the relatively large uncertainty surrounding the effects of aerosols on the climate. This is the reason that atmospheric aerosols are probably the most complex and most unpredictable component of forcing (Myhre et al., 2013). The diverse natural and anthropogenic sources of aerosols interact in nonlinear ways (Masson-Delmotte et al., 2013). Aerosol particles that act as cloud condensation nuclei (CCN) or those that are scavenged by cloud droplets are removed by precipitation. The highly diverse sources and locations of aerosols combined with their short residence times lead to high spatial and temporal variability in global aerosol distribution and the associated forcing. Aerosols from anthropogenic activities influence RF in three primary ways: through aerosol radiation interactions, through aerosol cloud interactions, and through albedo changes from absorbing-aerosol deposition on snow and ice (Boucher et al., 2013). RF resulting from aerosol radiation interactions is a “direct effect” of aerosol, which involves the absorption and scattering of longwave and shortwave radiation. Whereas RF from aerosol cloud interactions is an “indirect effect” of cloud albedo, which results from changes in cloud droplet number and size due to changes in aerosol (CCN). The RF for global net aerosol radiation and aerosol cloud interactions is negative, however, RF is not negative for all aerosol types (Myhre et al., 2013). Light-absorbing aerosols like black carbon absorb sunlight, thus, producing a positive RF and resulting in warming of the atmosphere; on net, this response is assessed to increase cloud cover and therefore, increase planetary albedo. The climate is also affected by light-absorbing aerosols when the surface albedo gets lowered by aerosols present in surface snow, thus, yielding a positive RF (Flanner, 2009).

5.5 SOLAR RADIATION Solar irradiation is one among many other important factors that affect climate change, but the process is long term. Since solar irradiance is Earth’s primary energy source, any change in irradiance directly impacts on the climate system (Lean, 1997). Direct solar observations have been available

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since 1978, though indicators of solar cycles have been available from the early 1600s. Although these climate changes account for only 0.1% of the total solar output of about 1360 W m22 with relative variations in irradiance at specific wavelengths can be much accountable (tens of percent). Spectral alterations in solar irradiance are maximum at near-ultraviolet (UV) and shorter wavelengths (Floyd et al., 2003), which are also the most important wavelengths affected by changes in ozone (Ermolli et al., 2013; Bolduc et al., 2015). Affected ozone concentrations induce variations in total and spectral solar irradiance causing discernible changes in atmospheric heating and in circulation (Gray et al., 2010; Seppälä et al., 2014). The relationships between changes in irradiance and changes in atmospheric composition, heating, and dynamics, are such that changes in total solar irradiance are not directly correlated with the resulting radiative flux changes (Ermolli et al., 2013; Xu and Powell, 2013; Gao et al., 2015). The IPCC estimate of RF due to changes in total solar irradiance over the industrial era is 0.05 W m22 (ranging from 0.0 to 0.10 W m22). Solar radiance-induced changes in RF are small relative to RF from anthropogenic GHGs over the industrial era.

5.6 VOLCANOES Most volcanic eruptions are minor events with the effects of the emissions lasting for only weeks or months in the troposphere. On the other hand, explosive volcanic eruptions inject large amounts of sulfur dioxide (SO2) and ash into the stratosphere, which lead to significant short-term climate effects (Myhre et al., 2013). SO2 oxidizes to form sulfuric acid (H2SO4) which upon condensation forms new particles or adds mass to preexisting particles, thereby substantially enhancing the attenuation of sunlight transmitted through the stratosphere. Injected aerosols increase Earth’s albedo by scattering sunlight back into space, thus, creating a negative RF and thereby cooling the planet (Andronova et al., 1999). In the stratosphere, the RF of aerosols persist for a lifetime, which is a few years, thus, exceeding that in the troposphere. The oceans only respond through cooling to a negative volcanic RF, and thus, change ocean circulation patterns, which could last for decades after the event of a major eruption. In addition to the direct RF, volcanic aerosol heats the stratosphere, altering circulation patterns, and depleting ozone by enhancing surface reactions, which further changes heating and circulation.

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The impacts of advective heat transport can be greater than the temperature impacts of direct forcing. Aerosol from both explosive and nonexplosive eruptions also affect the troposphere through many alterations in diffuse radiation and aerosol cloud interactions. It has been put forth that major eruptions might fertilize the ocean with enough iron to affect phytoplankton production and, therefore, could enhance the ocean carbon sink (Langmann, 2014). Volcanoes, in addition to other emissions, also emit CO2 and water vapor. At present, estimates of annual CO2 emissions from volcanoes revealed that volcanic emissions constitute less than 1% of CO2 emissions from all anthropogenic activities (Gerlach, 2011). The magnitude of volcanic effects on the climate depends on the number and strength of eruptions, the latitude of injection, and for ocean temperature and circulation impacts, the timing of the eruption relative to the ocean temperature and circulation patterns. Volcanic eruptions represent the largest natural forcing within the industrial era. In the past millennium, many eruptions have led to several years of transient episodes of negative RF which went up by several W m22. The climate has been constantly subjected to change since time immemorial and this change acted as a selective force for plants. Plants, in response, developed certain traits and among these is the emergence of metabolic activity which evolved into a more complex metabolic nexus.

5.7 PILLARS FOR SUSTENANCE: SECONDARY METABOLITES It is unthinkable, the plethora of compounds synthesized by plants to fulfill the actions required for their survival throughout their life cycle. Photosynthesis is a vital process required for the production of energy, which governs and links all other processes such as ATP production, cell division, proliferation and differentiation, protein and lipid biosynthesis, and so on. The production and allocation of photosynthates and energy from one place to another is a well-known phenomenon that involves several pathways, such as glycolysis, Kreb’s cycle, electron transport chain, pentose phosphate pathway, and many more (Fig. 5.3). However, there are some other cyclic pathways involved in the synthesis of some uncommon compounds which are not harnessed for the growth and development of plants; but play vital roles in protecting plants and aid in their survivability, these include the phenylpropanoid pathway,

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Photosynthesis (Calvin cycle) Glycolysis Glyseraldehyde-3-phosphate Phosphoenolpyruvate Pyruvate

Acetate/malonate pathway

Oxidative pentose phosphate pathway Erythrose-4-phosphate Glyseraldehyde-3-phosphate

Shikimate pathway 3-Dehydroquinic acid

Acetyl-CoA

3-Dehydroshikimic acid Malonyl-CoA

Shikimic acid

Phlorotannins, polyketides

Hydrolysable tannin pathway

Phenylpropanoid precursors

Gallic acid

Phenylalanine Monogalloylglucose Cinnamic acid Pentagalloylglucose

p-Coumaric acid

Flavonoids, condensed tannins, stilbenes

Galloyl quinic acids and other simple gallic acid derivatives

p-Coumaroyl-CoA

Coumaric and caffeic acid esters, lignans

Simple phenolic acids

Gallotannins

Ellagitannins

Figure 5.3 Image showing several pathways, such as glycolysis, Krebs cycle, electron transport chain, pentose phosphate pathway, and their interconnectedness.

shikimic acid pathway, malonic acid pathway, mevalonic acid pathway, methyl-erythritol pathway, and the tricarboxylic acid pathway (Kreb’s cycle) as well. All these pathways are somehow connected to each other and are involved in the synthesis of a stupendous number of compounds which are classified into two major categories known as primary and secondary metabolites. All the compounds directly involved in the cell survival and propagation, growth and development, photosynthesis, and respiration of plants are termed primary compounds/metabolites (e.g., amino acids, carbohydrates, proteins, lipids) (Korkina, 2007); however, compounds that play vital roles in processes other than growth and development are termed secondary metabolites (e.g., phenolics, terpenoids, alkaloids). Although, the latter are not involved in the growth and development of plants directly, they are involved in several other functions like pollinator attraction, protection against herbivores, insect and pest attack, disease outbreaks, plant microbe interactions (Reichling, 2018), root-nodule formation, screening compound synthesis, and they also act as signaling compounds. The absence of secondary metabolites will not lead to the sudden

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death of plants, but rather they may cause impairment to the survivability of plants. Earlier researchers underestimated the importance of secondary metabolites, as they were under the influence of a misconception that these compounds were waste or excretory products of plants. However, further studies explained that secondary metabolites are equally as important as primary metabolites. So, after understanding the importance of secondary metabolites, researchers became curious about the myriads of chemical compounds synthesized by plants. Numerous studies have suggested that secondary metabolites include phenolics, alkaloids, terpenoids, nitrogen containing compounds, sulfur containing compounds, and enzymatic and nonenzymatic antioxidants. These compounds are the main building blocks of a huge web of compounds involved in several related actions in plants. However, the main energy sources for their synthesis are carbohydrates, such as glucose, fructose, sucrose, raffinose, etc. (primary metabolites). Secondary metabolites follow special pathways for their synthesis which involve the products of primary metabolites. Moreover, various products of TCA and the pentose phosphate cycle are directly utilized as precursors for the biosynthesis of many secondary metabolites. Therefore, it can be concluded that the biosynthesis of both primary and secondary metabolites are interrelated and are extremely important for plants to complete their life cycles. Secondary metabolites are a very diverse group of compounds which are classified broadly on the basis of their biosynthetic precursors and origins as terpenoids (carotenoids, cardiac glycosides, sterols, and plant volatiles, such as mono- and sesquiterpenes, etc.), phenolics (phenolic acids, flavonoids, lignin, tannins, lignans, coumarins, stilbenes, etc.), and nitrogen containing compounds (alkaloids and glucosinolates).

5.8 TERPENE AND ITS ROLE IN PLANT DEFENSE Terpenes are the most assorted group of secondary metabolites and include all plant volatiles, sterols, carotenoids, flavonoids, etc. Acetyl coenzyme A is the biogenic precursor of terpenes, which are synthesized by condensing isoprene (C5) as their basic unit. Isoprene is a five-carbon core structure whose numbers are used to classify the myriad different types of terpenes (Mahmoud and Croteau, 2002; Holopainen et al., 2018), such as monoterpenes (C10), sesquiterpenes (C15), diterpenes (C20), triterpenes (C30), tetraterpenes (C40), and so on. Mainly, mono- and sesquiterpenes are included in plant volatiles which are lipophilic compounds

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that have high vapor pressures; these chemicals play beneficial roles in protecting plants from different biotic and abiotic stresses. Biotic stresses include pests, pathogens, and herbivores, while abiotic stresses comprise of climatic variations, including fluctuations in temperature, sunshine, wind velocity, salinity, flooding, drought, etc. Major constituents of plant volatile oils are terpene derivatives that are responsible for the aroma and flavor of various plant species. These essential oils are used in the treatment of several diseases, like skin allergies and bacterial and fungal infections, as well as for perfumery, cosmetics, and in flavoring agents, etc. However, they also play a vital role in protecting plants against herbivory, pest, and pathogen attacks. It is mainly the antimicrobial activities of terpenes that have been highly explored. These essential oils are comprised of several compounds which are basically active compounds categorized into four groups on the basis of their structure: terpenes, terpenoids, phenylpropenes, and others. But not all terpenes show antimicrobial activity, for example, in an experiment, Dorman and Deans (2000) observed that terpenes, such as limonene, δ-3-carene, α-pinene, β-pinene, α-terpinene, and (1)-sabinene, showed no or very low antimicrobial activities against several bacterial strains. However, p-Cymene, a monoterpene, enhanced the antimicrobial activity of carvacrol and polymyxin B nonapeptide when used in combination (Mann et al., 2000; Aligiannis et al., 2001; Ultee et al., 2002; Bagamboula et al., 2004; Rattanachaikunsopon and Phumkhachorn, 2010; Hyldgaard et al., 2012). Compounds like p-Cymene have a high affinity for microbial membranes leading to membrane expansion (Ultee et al., 2002; Hyldgaard et al., 2012) and they disrupt proton motive force generation in the flagella of many microbes by disturbing the membrane potential (Gabel and Berg, 2003; Burt et al., 2007; Hyldgaard et al., 2012). These compounds are also known to affect the nutrient and ion transport and permeability of cell membranes, which was evaluated by the efflux of intracellular ions like K1 and H1 leading to cell death (Ultee et al., 1999; Lambert and Hammond, 1973; Bouhdid et al., 2010; Hyldgaard et al., 2012). Miron et al. (2000) confirmed the membrane as the site of action by monitoring the release of calcein encapsulated in membrane vesicles. Similarly, garlic contains allicin which is a diallyl thiosulfinate (i.e., sulfur containing compound) which exhibits antibacterial, antifungal, antiviral, and antiparasitic properties and also plays a vital role in plant defense (Ankri and Mirelman, 1999; Kyung, 2012). Likewise, cinnamaldehyde is a

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phenylpropene and is also reported to inhibit cell division by inhibiting cell wall synthesizing enzymes by acting as a noncompetitive inhibitor of β-(1,3)-glucan synthase in Saccharomyces cerevisiae (Bang et al., 2000).

5.9 PLANT PHENOLICS AND ITS ROLE IN PLANT DEFENSE Phenolics are extensively distributed throughout the entire plant kingdom and are the main compound that played a significant role in the evolution of plants from aquatic to terrestrial ecosystems. The screening of harmful radiations is majorly done by phenolic compounds which protect plants from intense light and high UV-B and the earliest of these components that were synthesized were mycosporine-like amino acids (Cockell and Knowland, 1999; Bourgaud et al., 2001). However, the diverse groups of plant phenolics are mainly known for their antioxidative defense systems, and they are synthesized by the phenylpropanoid pathway. Plant phenolics are comprised of several hundred different compounds basically on the basis of the biosynthetic pathways involved in the synthesis of particular compounds and the number of carbon atoms they contain (generally, 5dehydroquinic acid acts as a precursor to most of the phenols; but the incorporation of acetate units (anthraquinone), isoprenoid chains (gossypol, ubiquinone, etc.,), and/or amino groups also leads to the formation of highly complex phenols); moreover, the largest group is comprised of various categories of flavonoids (Daniel, 2006; Takshak and Agrawal, 2016). All compounds that are comprised of an aromatic ring and a hydroxyl group and/or its substituent are circumscribed as a phenolic compound. Compounds containing single aromatic rings include all aldehydes, alcohols, ketones, and related glucosides, for example, catechol and hydroquinone. Phenolic acids include cinnamic, salicylic, gallic, ellagic, and benzoic acids, for example, vanillic and syringic acid residues found in lignin; however, acetophenones are aromatic ketones, for example, acetovanillone. On the other hand, phenylpropanes are widely spread among phenolics, for example, coniferyl alcohol; while phenylpropenes are associated with the essential oils of spices. Benzophenones and xanthones have similar structures, but differ in that they have heterocyclic oxygen rings between two benzene rings. Additionally, coumarins are derived from the lactonization of o-hydroxy cinnamic acid, which is an aromatic constituent of many plants, for example, scopoletin. Furthermore, flavonoids are widely distributed low molecular weight phenolic compounds

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(Weston and Mathesius, 2013); they are comprised of a 2phenylchromone (flavone) skeleton and other related compounds, such as anthocyanins, isoflavones, and neoflavones. These compounds are classified on the basis of the oxidation pattern of their C3 fragments, the additional oxygen heterocyclic rings, and glycosylation (Daniel, 2006). Flavonoids stabilize and protect the thylakoid membrane (mainly lipid phase) and also act to quench the excited state of chlorophyll and singlet oxygen formed during oxidative stress (Agrawal and Rathore, 2007). Anthocyanins and flavones/ols act as accessory pigments and protect the leaves of plants from excess solar radiations. Anthocyanins are the glycosides of anthocyanidins-derivatives and provide various color shades to the flowers and fruits in plants and thus help in attracting pollinators; they also increase the antioxidant potential of plants (Lev-Yadun and Gould, 2008; Takshak and Agrawal, 2016). Similarly, flavones and flavonols are responsible for the color of flowers from yellow to white. Major phenolic compounds are also involved in the antioxidation of reactive oxygen species (ROS); the production and destruction of ROS in cells are an inherent and essential phenomenon that occurs in a highly controlled manner. Any oxidative stress imposes imbalances in this continuous phenomenon of production and destruction of ROS, thus, leading to excess ROS generation which may cause damage to lipids, proteins, and DNA; although, the inherent antioxidant defense system is regulated by enzymes like catalase and superoxide dismutase, phenolics are also known for their efficient defensive role in antioxidation (Agati and Tattini, 2010; Fini et al., 2011; Takshak and Agrawal, 2016). Antioxidants are generally classified as chain breaking (primary) and preventative (secondary) (Namiki, 1990; De Beer et al., 2017). Chain breaking antioxidants scavenge free radicals and donate hydrogen atoms; however, preventative antioxidants act as metal chelators. Antioxidants are also capable of peroxide decomposition and singlet oxygen quenching (Chiesi and Schwaller, 1995; De Beer et al., 2017). Phenolic compounds are most common water soluble antioxidants (Macheix et al., 1990; De Beer et al., 2017). In addition to antioxidation, phenolic compounds do have several other roles, such as signaling, UV light screening, (as phenolics usually accumulate in central guard cells, epidermal cells, and the sub epidermal cells of leaves and shoots), metal chelation, antinutritional properties (unpalatable-protection against herbivores); and a flavonoid-DNA complex provides mutual protection against oxidative damage (Beckman, 2000; Feucht et al., 2004; Onyilagha

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and Grotewold, 2004; Lattanzio et al., 2006). Plant phenolics also act as internal physiological or chemical messengers; some phenolic acids such as ferulic acid and hydroxycinnamic acids act as the source of phenylpropanoid units for lignin synthesis and phenolics are also associated with indole acetic acid oxidase which destroys indole acetic acid (IAA, growth hormone) (Lattanzio et al., 2006). Phenolics are also considered to be phytoanticipins, that is, preformed antifungal compounds which protect plants from fungal (pathogen) attack and also provide resistance against pests (Lattanzio et al., 2006). Lignin is a polyphenolic compound that provides mechanical support to plants and protects plants from pathogens, insects, and herbivores.

5.10 N-CONTAINING COMPOUNDS A discussion on secondary metabolites is incomplete without nitrogen containing compounds which are a major class of compounds that protect plants from grazers, that is, herbivores. N-containing compounds are broadly categorized into cyanogenic glucosides, glucosinolates, and alkaloids. Cyanogenic glycosides are widely distributed and found in more than 2000 plants. Due to grazing, vacuole-located glucosides get exposed to β-glucosidases and hydroxynitrile lyases resulting in the release of cyanide, which is a potent poison. However, glucosinolates are both sulfur and nitrogen containing compounds derived from amino acids with at least 100 different structures; these are usually located in vacuoles and the disruption of tissues by any physical damage releases an enzyme that hydrolyses glucosinolates, these breakdown into what are collectively described as “mustard oils” responsible for flavoring many plants, such as mustard, radish, and cabbage (Chew, 1988; Bennett and Wallsgrove, 1994). Moreover, alkaloids are also N-containing compounds that are widely distributed throughout the plant kingdom (Wink, 2003), and they are broadly classified into three major classes on the basis of their precursors and final structures: true alkaloids, pseudoalkaloids, and protoalkaloids. True alkaloids are derived from amino acids and contain nitrogen in a heterocyclic ring, for example, pyridines, pyrroles, indoles, and piperidines. While, pseudoalkaloids are basic but are not derived from amino acids, for example, caffeine and solanidine; however, protoalkaloids are basic and derived from amino acids, but their heterocycle is devoid of nitrogen, for example, mescaline. Alkaloids are effective against herbivory, toxic against fungi and bacteria, and provide resistance against insects (Harborne, 1988).

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5.11 SALICYLIC ACID AND JASMONIC ACID Salicylic acid (SA) is also a phenolic compound and is synthesized by the conversion of phenylalanine into trans-cinnamic acid in the presence of the enzyme phenylalanine ammonia lyase (PAL) (Kim and Hwang, 2014). Phenylalanine is a precursor to SA biosynthesis. SA acts as a systemic signal for various plant pathogen related protein syntheses and resistance against pathogens; it activates plant defense responses after pathogen attack (Klessig et al., 2000). It also regulates photosynthesis in plants by regulating leaf and stomatal closure, seed germination, respiration, growth, flowering, and senescence (Rivas-San Vicente and Plasencia, 2011). However, jasmonic acids (JA) are synthesized by linolenic acid (fatty acid) and are widely distributed among plants. JA and methyl jasmonates regulate various physiological processes, such as the induction of senescence, herbivore antifeedants, proteinase inhibition, meristematic growth, and signal transduction (Lattanzio et al., 2006; Taniguchi et al., 2014). JA is also involved in host immunity regulation in plants (Taniguchi et al., 2014).

5.12 PERIODICAL CHANGES: CLIMATE CHANGE VERSUS SECONDARY METABOLITES The appearance of land plants from their green algal ancestors happened about 500 million years ago (Kenrick and Crane, 1997). Initially land plants were under harsh environments and had to face key stresses, namely UV-B radiation and attack by microbial soil communities, thus, adaptations such as specialized secondary metabolic pathways were developed to cope (Kenrick and Crane, 1997; Waters, 2003). Under the influence of environmental factors, a respective group of secondary metabolites that acted as a chemical interface between plants and their surrounding environments developed. Although much effort has been made to understand the origination of the first living organism, it is still not clearly known when and how life emerged on Earth. However, there is a common notion that early organisms inhabiting an organically rich environment, spontaneously formed in the prebiotic world. The Oparin-Haldane theory is a widely accepted theory about the heterotrophic origin of life (Lazcano and Miller, 1996). If this is valid, life evolved from a primordial soup, having different organic molecules which probably formed spontaneously

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during the first billion years of Earth’s existence. Nutrient compounds in primordial soup caused minimum biosynthesis by the early heterotrophic organisms. The first living systems emerged straight from the primordial soup which evolved at a relatively fast pace up to a common ancestor, usually referred to as LUCA (last universal common ancestor). LUCA is an entity that represents the divergence startingpoint of all the extant life forms on Earth. Due to the sessile nature of plants, they cannot escape adverse conditions, therefore, they continuously optimize available resources strategically. Metabolic strategies are the easiest way, where a plant responds to adverse conditions by quickly producing metabolite(s) utilizing few members of enzyme(s). The same plant species growing under different environmental conditions exhibits significant differences in the production and accumulation of primary and secondary metabolites (Pavarini et al., 2012; Ramakrishna and Ravishankar, 2011; Gutbrodt et al., 2012; Edreva et al., 2008). Secondary metabolites intricately connect ecology, evolution, and human affairs in the plant kingdom. Chemical interactions between plants and their environment are mediated mainly by the biosynthesis of secondary metabolites, which impart plastic adaptation to changing environments. Such chemical interactions are often mediated by varieties of plant metabolites (Gutbrodt et al., 2012; Szakiel et al., 2010). Plants regulate their metabolic machinery for optimal growth and their physiology by evolving two pivotal metabolic pathways (Ober, 2005; Pichersky and Gang, 2000). The primary metabolism of plants forms essential molecules needed for plant survival, growth, and reproduction, such as carbohydrates, proteins, and nucleic acids. Despite the fact that these compounds are not pivotal for plant survival, their high structural diversity helps plants to interact with their ecosystem efficiently (Hartmann, 2007; Wink, 2003). Changes in the climate are expected to have a profound impact on plants. It is well known that many components of the climate, such as temperature, solar radiation, and CO2 concentration, are directly related to the optimal functioning of plant metabolic machinery. The extent of these effects are not well known due to inadequate investigation and the complexity of this issue (Ahuja et al., 2010). Environmental factors, either directly or indirectly (through the generation of ROS), damage DNA or induce mutations causing genomic perturbations (Britt, 1995; Balestrazzi et al., 2011). Neither H2O2 nor O22

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are capable of inducing DNA damage, but OH generated through Fenton or Haber Weiss reactions include DNA damage (Mello Filho and Meneghini, 1984). Oxidative damage to DNA results in altered bases as well as damaged sugar residues that are further subjected to fragmentation and, thus, generate strand breaks. In addition to direct oxidation, DNA bases may also be subjected to indirect damage through reaction with reactive products produced by ROS attack on other macromolecules. One of the major sources of such indirect oxidative damage is lipid peroxidation (LPO), caused by an attack of ROS on the polyunsaturated fatty acid residues of membrane phospholipids (Marnett et al., 2003). Major reactive products of LPO are malondialdehyde (MDA), acrolein, and crotonaldehyde. MDA reacts with guanine residues in DNA to form a pyrimidopurinone adduct called M1G (Fink et al., 1999). Acrolein and crotonaldehyde leads to the formation of DNA interchain crosslinks (Kozekov et al., 2003). To withstand the damage, plants evolved secondary metabolic pathways. Abiotic and genotoxic stresses induce specific genes in order to express proteins that are meant to perform different functions in DNA repair pathways. The sustenance, survival, and fitness chances of plants largely depend on their tolerance or adaptation to abiotic stress due to the impending climate change. Contemporary evolutionary concepts suggest that there is more to heredity than genes; that some acquired information is inherited; and that evolutionary change can result from instruction as well as selection. Stress tolerance in plants is a result of adaptive evolution where evolution is integrated with development as determined by the environment (Ioannidis, 2008). Adaptive evolution is usually directed by selective processes, whereas development is usually directed by instructive processes; evolution involves random genetic changes, while development involves induced epigenetic changes (Jablonka and Lamb, 1998).

5.13 EX ANTE AND EX POST INVESTMENTS IN SECONDARY METABOLISM: AN INVESTMENT WITH A RETURN Metabolic alterations in plants are known to be one of the basic reactions toward suboptimal conditions. Plants regulate their metabolic machinery for optimizing growth and functioning by evolving two distinct metabolic pathways (Ober, 2005; Pichersky and Gang, 2000). It is well known that many components of climate, such as temperature, solar radiation, and CO2 concentration can directly influence the functioning of plant

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metabolic machinery. Apart from plant defense, these changes in plant chemistry can have variable bottom-up effects on higher trophic levels (Kollberg et al., 2015). Studies have revealed both positive (Richards et al., 2015; Bukovinszky et al., 2009) and negative (Harvey et al., 2010a,b; Ode, 2006) effects of plant defense chemistry on parasitoids. Secondary metabolites provide cross-tolerance to plants against herbivory as well as to stressful abiotic conditions (Haugen et al., 2008). Siemens et al. (2009) suggest that the evolution of the capacity of plants to endure environmental stressors comprises of the ability of a plant to produce chemical defenses through the involvement of plant hormone signaling pathways. This has important implications on plant adaptations to climate change, as temperature stressed plants can be more vulnerable to herbivores (Siemens et al., 2012). The effect of elevated temperatures on the expression of plant secondary defense compounds seems to be specific to the type of compounds and the plant species involved. The growth-differential balance hypothesis states that plant defenses are an outcome of a tradeoff between growth and differentiation related processes (Herms and Mattson, 1992). Thus, a rise in temperature decreases levels of carbon-based secondary compounds due to increased allocation to growth rather than defense under resource limiting conditions (Jamieson et al., 2012). Carbon based compounds, including phenolics and condensed tannins, tend to decrease with increased temperature (Bidart-Bouzat and Imeh-Nathaniel, 2008). Contrary to this, hydrolysable tannins, terpenes, and VOCs, have been shown to increase in concentration with rising temperatures (Hansen et al., 2006; Loreto et al., 2006; Sallas et al., 2003). Similarly, nitrogen containing compounds display diverse responses to increased temperature. Higher temperatures may increase alkaloids (Salminen et al., 2005), whereas glucosinolates may either decrease or increase (Bidart-Bouzat and Imeh-Nathaniel, 2008). Plants produce a wide array of secondary metabolites and this diversity is thought to be the outcome of their coevolution with pathogens and herbivores (Ehrlich and Raven, 1964). Plants’ metabolisms respond to environmental stresses in various ways, which include the production of compatible solutes (e.g., proline, raffinose, and glycine betaine) which have the capacity to stabilize proteins and cellular structures or to balance cell turgor pressure through osmotic adjustment and the removal of excess ROS, thus, establishing cellular redox balance. The expression of various defense traits may incur allocation costs that negatively impact plant growth, particularly when resources such as light

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and nitrogen are limiting. The diversion of resources to defense at the expense of construction of new leaf tissue, which otherwise returns revenue in the form of photosynthate over the life time of the organ, also imposes so-called costs that are compounded as plants age (Coley et al., 1985; Herms and Mattson, 1992). These direct costs of defense may also impose ecological costs, including reduced ability to complete with neighboring plants or increased susceptibility to enemies that are not targeted by the induced defense (Futuyma and Agrawal, 2009; Stamp, 2003). The allocation of limited resources to plant growth and defense has, therefore, been described as a “dilemma” because growth rates must be sufficient to compete with neighboring plants for light while not neglecting the investment in defense against plant eating organisms (Herms and Mattson, 1992). Variations in plant metabolites against abiotic stresses are consolidated in Table 5.2. Diverse plant specialized metabolic pathways branch from core primary metabolism at different points. For instance, the enormously rich phenylpropanoid metabolism widely present in plants starts with the deamination of the aromatic amino acid phenylalanine through phenylalanine ammonia lyase (PAL; Vogt, 2010). The diverse family of secondary plant terpenes (isoprenoids) begin with primary metabolites, isopentenyl pyrophosphate, and dimethylallyl pyrophosphate (Chen et al., 2011). Caffeine and related purine alkaloids sparsely found in 13 orders of flowering plants begin with core purine nucleotides (Ashihara et al., 2008). This observation suggests that the initial birth of those major specialized pathways present in extant plants probably involves emergent catalytic activities toward certain primary metabolites, which yield new compounds that enhance host fitness in a particular environment. Second, in general the taxonomic distribution of plant specialized metabolic tracts is correlated with the gradual evolutionary development of specialized tissue types, organs, and/or lifestyles observed in land plants as they underwent extensive divergence over the past 500 million years. Several other pathways, such as the biosynthesis of core phenylpropanoid cuticles, sporopollenins, abscisic acid, and flavonoids, are absent in extant charophytic algae most closely related to land plants, but are ubiquitously present in all extant land plants (Weng and Chapple, 2010). The primary function of these compounds are for protection against UV radiation and desiccation, representing major abiotic stresses facing those early land plants when migrating from aquatic habitats to terrestrial environments.

Table 5.2 Changes in different secondary metabolites from different plants under various stresses Drought

Response

Plant

Plant part

Treatment

Reference

Glycosides

Increased

Scrophularia ningpoensis

Root

Wang et al. (2010)

Morphine Trigonelline Glucosinolates Chinolizidine

Increased Increased until pod setting

Papaver somniferum Glycine max Brassica napus Lupinus angustifolius

Leaves Leaves Seed Seed

Camellia sinensis

Leaves

Osmotic stress created by adding 100 and 170 g L21 PEG Limited water supply Nonirrigated Seed drying Irrigation withheld until availability of soil water Water withheld

NaCl, CaCl2, and Na2SO4 treatment given. Na:Ca and Cl2:SO422 ratios kept as 4. Five levels of treatments corresponded to EC 0, 5, 10, 15, 20 dS m21 0 200 mM NaCl

Khatkar and Kuhad (2000)

200 mM NaCl treatment 20, 40, 60, 80, 100, and 150 mM 100 mM

Lin et al. (2002) Kennedy and De Filippis (1999) Tari et al. (2010)

150 mM NaCl

Bor et al. (2009)

Treatment/metabolite

Glucosinolates

Increased during vegetative growth Increased

Hypericum brasiliense H. brasiliense

Betulinic acid Rutin

Szabo et al. (2003) Cho et al. (1999) Jensen et al. (1996) Christiansen et al. (1997) Hernández et al. (2006) Nacif et al. (2005) Nacif et al. (2005)

Salt

Proline and soluble sugar

Increased during crown root initiation stage

Triticum aestivum L. cvs HD2009 and KRL1-4

Leaves

Proline

Increased till 100 mM NaCl

Leaves

Proline Anthocyanin

Increased Increased

Vitis vinifera L. four genotypes, Car, PoC, PS, and Per Oryza sativa L. Grevillea ilicifolia; Grevillea arenaria

Sorbitol

Increased in leaf and no change in root Increased

Gamma-aminobutyric acid

Lycopersicum esculentum Sesamum indicum L.

Leaves Shoot cultures Leaves and roots Plant tissue

Singh et al. (2000)

(Continued)

Table 5.2 (Continued) Drought

Response

Plant

Plant part

Treatment

Reference

Jasmonic acid

• Increased in roots of Pera at 24 h and decreased in shoot at 6, 24, and 72 h • Increase in roots of HF at 72 h and in shoot increase at 24 h • Increase in Jerba at 100 mM • Reduced Tabarka at both levels of salinity Increased only in top leaves. In root, stem. and basal leaves, total alkaloid was decreased

L. esculentum cvs Pera and HF

Root and shoot

100 mM NaCl

Pedranzani et al. (2003)

Cakile maritime cvs Jerba and Tabarka

Leaves

100 and 400 mM NaCl

Ksouri et al. (2007)

Datura innoxia Mill.

153.8 mol m23 NaCl

Brachet and Cosson (1986)

Out of 17 cultivars; increased in 10 cultivars, decreased in 5 cvs, and remained unchanged in two cultivars

G. max

• Root • Stem • Basal leaves • Leaves • Top leaves Leaves

NaCl treatment in step-wise manner: 3 days 30 mM, 3 days 70 mM and 3 days 100 mM

Cho et al. (1999)

Six cultivars of wheat showed increase in measured metabolites at higher temperatures Increased up to 25°C in shoots; in flowers at 30°C

T. aestivum

Leaves

Three temperature treatments were given: 20°C, 25°C, and 30°C

Shamloo et al. (2017)

Hypericum perforatum

• Shoots • Flowers

Zobayed et al. (2005)

Under higher temperature increased in plants from higher temperature treatments

Polygonum minus

Leaves

Five temperature treatments were given: 15°C, 20°C, 25°C, 30°C, and 35°C 25°C day:18°C night 28°C day:20°C night 28°C day:20°C night 22°C day:15°C night

Polyphenol

Total alkaloid content

Trigonelline (nicotinic acid betaine)

Temperature

Phenolic acids Flavonoids Phytosterols Hypericin

Flavonoids

Gao et al. (2015)

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When vascular plants arose, the ancestral core phenylpropanoid pathway was further elaborated on to provide lignin, a phenolic polymer that provides physical rigidity to water-conducting xylem cells in vasculature and enables vascular plants to stand upright (Weng and Chapple, 2010; Weng et al., 2010). The evolution of trichomes in euphyllophytes coincides with the occurrence of a diverse array of metabolites enriched in these specialized surface structures, wherein most of these compounds are involved contributes in chemical defense against herbivores (Dai et al., 2010). The phenylpropanoid pathway provides vital compounds like lignin for vascularization, flavonoids for color, and phytoalexins to deter microbes (Dixon et al., 2002). Few copies of the PAL gene have been reported in gymnosperms, whereas numerous pal gene families have been reported in angiosperms (Bagal et al., 2012). During the evolution of gymnosperms, back in the Mesozoic era, they experienced large environmental and geographical changes and challenges (Eckert and Hall, 2006). During the colonization of terrestrial environments by pioneer land plant ancestors, it was crucial that they were associated with fungi and soil bacteria, for example, N2 fixing cyanobacteria started symbiosis with early fungal lineages and land plants. Fungi (Glomeromycota) started arbuscular-mycorrhizal (AM) symbiosis with the first land plants (Parniske, 2008). This is how PAL got into the higher plants and evolved to performing multiple functions. The phylogeny tree of PAL is given in Fig. 5.4. Other details about different organisms, their gene ontology and evolutionary lineage are given in Table 5.3. Some gene families are substantially larger in gymnosperms than in angiosperms, indicating the importance of gene duplication as an important mechanism for genome expansion in conifers. It seems that large multigene families are correlated with conifer genome size (Ahuja and Neale, 2005). The PAL gene family in plants is likely to be expanded by

Figure 5.4 The phylogeny tree of PAL through evolutionary lines.

Table 5.3 Details of numeric used in phylogeny tree (given in Fig. 5.5) S. no.

Gene

Organism

Genetic ontology (GO)

Taxonomic lineage

1

AC504_2635

Ammonia lyase

2

AC508_0700

3

AC499_5841

Pseudomonas syringae pv. maculicola Pseudomonas amygdali pv. Mellea P. amygdali pv. Lach

4

AC509_1838

5

FOXB_06709

P. amygdali pv. morsprunorum Fusarium oxysporum

Bacteria . Proteobacteria . Gammaproteobacteria . Pseudomonadales . Pseudomonadaceae . Pseudomonas Bacteria . Proteobacteria . Gammaproteobacteria . Pseudomonadales . Pseudomonadaceae . Pseudomonas Bacteria . Proteobacteria . Gammaproteobacteria . Pseudomonadales . Pseudomonadaceae . Pseudomonas Bacteria . Proteobacteria . Gammaproteobacteria . Pseudomonadales . Pseudomonadaceae . Pseudomonas NA

6

PAL

Arabidopsis thaliana

7

PAL-B0-1

Brassica oleraceae

Phenylalanine ammonia lyase

8

PAL-BR-1

Brassica campestris

Phenylalanine ammonia lyase

9

PAL-BN-1

B. napus

Phenylalanine ammonia lyase

10

N/A

Eutrema halophilum

Phenylalanine ammonia lyase

Ammonia lyase Ammonia lyase Ammonia lyase Phenylalanine ammonia lyase Phenylalanine ammonia lyase

Eukaryota . Viridiplantae . Streptophyta . Embryophyta . Tracheophyta . Spermatophyta . Magnoliophyta . Eudicotyledons . Gunneridae . Pentapetalae . Rosids . Malvids . Brassicales . Brassicaceae . Camelimeae . Arabidopsis Eukaryota . Viridiplantae . Streptophyta . Embryophyta . Tracheophyta . Spermatophyta . Magnoliophyta . Eudicotyledons . Gunneridae . Pentapetalae . Rosids . Malvids . Brassicales . Brassicaceae . Brassiceae . Brassica Eukaryota . Viridiplantae . Streptophyta . Embryophyta . Tracheophyta . Spermatophyta . Magnoliophyta . Eudicotyledons . Gunneridae . Pentapetalae . Rosids . Malvids . Brassicales . Brassicaceae . Brassiceae . Brassica Eukaryota . Viridiplantae . Streptophyta . Embryophyta . Tracheophyta . Spermatophyta . Magnoliophyta . Eudicotyledons . Gunneridae . Pentapetalae . Rosids . Malvids . Brassicales . Brassicaceae . Brassiceae . Brassica Viridiplantae . Streptophyta . Embryophyta . Tracheophyta . Spermatophyta . Magnoliophyta . Eudicotyledons . Gunneridae . Pentapetalae . Rosids . Malvids . Brassicales . Brassicaceae . Eutremeae . Eutrema

ammonia

Eukaryota . Viridiplantae . Streptophyta . Embryophyta . Tracheophyta . Spermatophyta . Magnoliophyta . Eudicotyledons . Gunneridae . Pentapetalae . Rosids . Malvids . Brassicales . Brassicaceae . Camelimeae . Arabidopsis Eukaryota . Viridiplantae . Streptophyta . Embryophyta . Tracheophyta . Spermatophyta . Magnoliophyta . Eudicotyledons . Gunneridae . Pentapetalae . Rosids . Malvids . Brassicales . Brassicaceae . Camelimeae . Arabidopsis Eukaryota . Viridiplantae . Streptophyta . Embryophyta . Tracheophyta . Spermatophyta . Magnoliophyta . Eudicotyledons . Gunneridae . Pentapetalae . Asterids . Campanulids . Asterales . Asteraceae . Asteroideae . Anthemisiinae . Artemisia Eukaryota . Viridiplantae . Streptophyta . Embryophyta . Tracheophyta . Spermatophyta . Magnoliophyta . Eudicotyledons . Gunneridae . Pentapetalae . Rosids . Malvids . Brassicales . Brassicaceae . Camelimeae . Arabidopsis Eukaryota . Viridiplantae . Streptophyta . Embryophyta . Tracheophyta . Spermatophyta . Magnoliophyta . Eudicotyledons . Gunneridae . Pentapetalae . Rosids . Malvids . Brassicales . Brassicaceae . Camelimeae . Arabidopsis Bacteria . Actinobacteria . Microeoceales . Micrococcaceae . Arthrobacter

ammonia

Bacteria . Actinobacteria . Microeoceales . Micrococcaceae . Arthrobacter

ammonia

Bacteria . Actinobacteria . Microeoceales . Micrococcaceae . Arthrobacter

ammonia

Bacteria . Proteobacteria . Alphaproteobacteria . Rhizobiales . Rhizobiaceae . Rhizobium/Agrabateria gp. . Rhizobium Bacteria . Firmicutes . Bacilli . Bacillales . Paenibacillaceae . Paenibacillus Bacteria . Firmicutes . Bacilli . Bacillales . Paenibacillaceae . Paenibacillus

11

PAL1

A. thaliana

Phenylalanine ammonia lyase

12

PAL1

A. thaliana

Phenylalanine ammonia lyase

13

N/A

Artemisia annua

Phenylalanine ammonia lyase

14

At2g37040

A. thaliana

Phenylalanine ammonia lyase

15

At2g37040

A. thaliana

Phenylalanine ammonia lyase

16

ASE96_11845

Arthrobacter sp. Leaf 69

17

ASF98_14405

Arthrobacter sp. Leaf 337

18

AS992_16390

Arthrobacter sp. soil 736

19

ASD85_20415

20

ASD40_12285

21

A0A0Q9K4T2

Rhizobium sp. Root 651 Paenibacillus sp. Root 444D2 Paenibacillus sp. soil 724D2

Phenylalanine lyase Phenylalanine lyase Phenylalanine lyase Phenylalanine lyase Phenylalanine lyase Phenylalanine lyase

ammonia ammonia

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gene duplication, including tandem and segmental duplication, and whole genome duplication (Cannon et al., 2004). Gene duplication is a ubiquitous process in the evolution of most genomes. Gene duplication arises as a byproducts of DNA recombination and DNA repair processes that sometimes duplicate stretches of an organism’s DNA. The duplicated stretches can be extremely short, comprising of only a few nucleotides, or they can be extremely long, comprising of long segments of either chromosome or even the entire genome. If any duplicated stretch of DNA includes at least one gene, then gene duplication has occurred. Most duplicated genes are eliminated from a genome shortly after the duplication (Lynch and Conery, 2000). However, a small fraction of duplicate is usually preserved, indicating that the duplication either did no harm or was favored by selection. Over time, duplication may preserve a similar function, or they may acquire specialized functions (Taylor and Raes, 2004; Conant and Wolfe, 2006). Accurate relation between genes in the gene family in evolutionary genomics is defined as orthologous and paralogous (Koonin, 2005). The genes that have been diverged as the result of speciation events are orthologs, while those that have been diverged following duplication events are paralogs (Fitch, 1970). As a result, ortholog genes, which originate from a single gene in the last common ancestor of a series of present species, have often retained identical biological functions. To assess the origin of PAL, it is reported that PAL is emerged in bacteria with an antimicrobial role then a member of a pioneer fungal lineage which was expected before the divergence of Ascomycota and Basidiomycota who obtained a PAL via HGT from a soil bacterium through an early symbiosis (Parniske, 2008). Fungi transferred PAL to an ancestor of land plants via an insect AM symbiosis. This was the starting point of the phenylpropanoid pathway development and the distribution of plants in terrestrial environments. It is reported that gymnosperm PAL genes have been clustered into three clades (Bagal et al., 2012). Duplication events are an important issue in the evolution of the PAL gene family. Different protein isoforms may be expressed from duplicate copies of genes or each duplicate copy may have a distinct expression pattern for response to different physiological conditions, such as tissue differentiation or resistance to environment stress (Su et al., 2006). Once the enzyme becomes a part of many metabolic pathways, there evolution is observed. The coexpression network for key enzymes (given in Fig. 5.5.) of phenylpropanoid pathway clearly their involvement enzymes in many pathways. Details about the different genes involved in this network are given in Table 5.4.

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Figure 5.5 The coexpression network for key enzymes of phenylpropanoid pathway showing their involvement in many other pathways.

Table 5.4 Description of the factors involved in the networks influenced by 4-Coumarate:CoA ligase (4CL2) Locus

Description

Annotation

• At3g21240

4-Coumarate:CoA ligase

• At2g37040

Phenylalanine ammonia lyase 1 (PAL1)

Encodes an isoform of 4-coumarate:CoA ligase (4CL), which is involved in the last step of the general phenylpropanoid pathway. The catalytic efficiency was in the (descending) order: p-coumaric acid, caffeic acid, ferulic acid, 5-OH-ferulic acid, and cinnamic acid Encodes phenylalanine ammonia lyase1 PAL1 is involved in these biological processes: • Salicylic acid catabolic process • Lignin catabolic process • Drought recovery • Polyamine catabolic process • Coumarin biosynthetic process • Cellular modified amino acid biosynthetic process (Continued)

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Table 5.4 (Continued) Locus

Description

At3g10340

Phenylalanine ammonia lyase 4

At3g53260

Phenylalanine ammonia lyase 2

At1g51680

4-coumarate-ligase 1

At5g48930

Hydroxycinnamoyl-CoAshikimate/quinate hydroxycinnamoyl transferase

At2g40890

Cytochrome P450, family 98, subfamily A, polypeptide 3 (CYP98A3)

At5g54160

O-methyltransferase 1 (OMT1)

Annotation

• Response to UV-B • Response to Karrikin • Phenylpropanoid metabolic process • Pollen development • Response to wounding • Response to oxidative stress • Defense response Encodes PAL4 and has role in these biological processes: • Glucuronoxylan metabolic process • Xylan biosynthetic process Encodes PAL2 and has role in these biological processes: • Polyamine catabolic process • Coumarin biosynthetic process • Cellular modified amino acid biosynthetic process • Positive regulation of flavonoid biosynthetic process • Response to Karrikin • Response to wounding • Phenylpropanoid metabolic process • Response to oxidative stress • Defense response Encodes 4CL1. It is involved in the last step of the general phenylpropanoid pathway. In addition to 4-coumarate, it also converts ferulate. The catalytic efficiency was in the (descending) order: p-coumarin acid, ferulic acid, caffeic acid, 5-OH ferulic acid. 4CL1 is unable to use sinapic acid as a substrate HCT synthesizes and catabolizes the hydroxycinnamoyl esters involved in the phenylpropanoid pathway. Also influences accumulation of flavonoids which in turn inhibits auxin transport and reduces plant growth Encodes coumarate-3-hydroxylase (C3H), a P-450 dependent monoxygenase. Involved in lignin and flavonoid biosynthesis. Also effects the biosynthesis of coumarins such as scopoletin and scopolin as a branching-out-pathway from phenylpropanoid acid level Encodes a flavonol-3-O-methyltransferase that is highly active toward quercetin and myricetin

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5.14 CONCLUSION As sessile organisms, plants had to evolve their metabolic systems to produce a diverse plethora of metabolites in response to challenging climatic conditions. Gradual change in climatic variables acted as a selective pressure for the evolution and emergence of metabolic activities and also led to functional shifts. This study on understanding climate change in relation to the evolution of secondary metabolism is crucial to predicting futuristic responses.

ACKNOWLEDGMENT The authors are thankful to the Head, Department of Botany; and Coordinator, Centre of Advanced Study in Botany, Banaras Hindu University, Varanasi, DST-SERB and CSIR (EMR).

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FURTHER READING Bond, T.C., Doherty, S.J., Fahey, D.W., Forster, P.M., Berntsen, T., Angelo, B.J., et al., 2013. Bounding the role of black carbon in the climate system: a scientific assessment. J. Geophys. Res. Atmos. 118, 5380 5552.

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Breitmaier, E., 2006. Terpenes: importance, general structure, and biosynthesis. Terpenes: Flavors, Fragrances, Pharmaca, Pheromones. John Wiley & Sons, Weinheim, pp. 1 9. Dev, R., Singh, S.K., Singh, A.K., Verma, M.K., 2016. Comparative in vitro multiplication of some grape (Vitis vinifera) genotypes. Indian J. Agric. Sci. 85, 1477 1483. Knutti, R., Rugenstein, M.A.A., 2015. Feedbacks, climate sensitivity and the limits of linear models. Philos. Trans. R. Soc. A Math. Phys. Eng. Sci. 373, 1 20. Lockwood, M., 2012. Solar influence on global and regional climates. Surv. Geophys. 33, 503 534. Pichersky, E., Lewinsohn, E., 2011. Convergent evolution in plant specialized metabolism. Annu. Rev. Plant Biol. 62, 549 566. Walther, G.R., Post, E., Convey, P., Menzel, A., Parmesan, C., Beebee, T.J., et al., 2002. Ecological responses to recent climate change. Nature 416 (6879), 389.

CHAPTER 6

Impact of Xenobiotics Under a Changing Climate Scenario Virendra Kumar Mishra1, Gurudutt Singh1 and Reetika Shukla2 1

Institute of Environment and Sustainable Development, Banaras Hindu University, Varanasi, India Department of Environmental Science, Indira Gandhi National Tribal University, Amarkantak, India

2

Contents 6.0 Introduction 6.1 Xenobiotics 6.2 Types of Xenobiotics 6.2.1 Exogenous 6.2.2 Endogenous 6.3 Physicochemical Nature of Xenobiotics 6.4 Sources of Xenobiotics 6.5 Distribution and Fate of Xenobiotics in the Environment 6.6 Impact of Xenobiotics on the Environment 6.6.1 Impact of Xenobiotics in Water 6.6.2 Impacts of Xenobiotics on Soil 6.6.3 Impact of Xenobiotics in Air 6.7 Impact of Xenobiotics on Climate Change 6.7.1 Loss of Species Diversity Among the Food Chain and Food Webs 6.7.2 Effect on Nutrient Cycling in Ecosystems 6.7.3 Effect on Pollinators 6.8 Impact and Interaction of Xenobiotics and Climate Change 6.9 Conclusion and Future Prospective References Further Reading

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6.0 INTRODUCTION 6.1 XENOBIOTICS Xenobiotics are defined as chemical substances that are found in a given organism but are not produced naturally by that organism (Schlegel, 1986). The term xenobiotics is a combination of two Greek words xeno Climate Change and Agricultural Ecosystems DOI: https://doi.org/10.1016/B978-0-12-816483-9.00006-2

Copyright © 2019 Elsevier Inc. All rights reserved.

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and biotic. The word xeno means strange or unnatural and the term biotic means life. Xenobiotics are found in both organic and inorganic forms and may imitate biologically produced compounds that are essential for life (Essumang, 2013). Some xenobiotic compounds are toxic to human life and the environment as they are not recognized by any biochemical processes in microorganisms and plants. The anthropogenic sources of xenobiotics include industrial, household, medicinal drug, agricultural, and transportation (Essumang, 2013). Xenobiotic compounds are categorized as food additives, environmental pollutants, carcinogens, and some pesticides, such as acephate, diazinon, DEET, and hydrocarbons. Xenobiotics can produce a variety of effects, including: drug toxicity, climate change, and immunological responses. Xenobiotics are necessary for human beings and society because many chemical and pharmaceutical industries use these compounds to produce medicines, plastics, detergents, gels, research laboratory chemicals and biochemical kits, perfumes, herbicides, pesticides, and various other products (Rieger et al., 2002). Xenobiotics are nonbiodegradable or partially biodegradable compounds, which may undergo slow biotransformation and may persist in the environment for a long duration. Organic and inorganic xenobiotics are a huge source of environmental pollution. They are released through dumping of waste and sewage, industrial discharge into aquatic environments, and in the air as air pollutants. The concentration of these persistent organic pollutants may increase as they pass through the food chain from one trophic level to another (Dubey et al., 2014). Large number of xenobiotic substances such as xylene, naphthalene,pyrene, acenaphthene are reaching to conventional sewage treatment plants but these plants are unable to treat these compounds and these compounds passed untreated into different matrices (Thakur, 2008).

6.2 TYPES OF XENOBIOTICS There are two types of xenobiotics, that is, exogenous and endogenous (Fig. 6.1)

6.2.1 Exogenous Xenobiotics that are not normally produced by an organism but enter into the organism through food stuffs or in the form of medicine or are inhaled from the environment are known as exogenous xenobiotics. For example: insecticides, chemicals, pollutants, drugs, and food additives.

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Figure 6.1 Different types of xenobiotic compounds in the environment.

6.2.2 Endogenous Endogenous xenobiotics are biologically synthesized products having some similarity to exogenous xenobiotic compounds. These are synthesized in the human body or are produced as metabolites of various processes in the body. For example: bile acid, bilirubin, eicosanoids, certain fatty acids, and steroids The word xenobiotic was coined in the mid-1960s by Dr. Howard Mason for any chemical to which an organism is exposed and is extrinsic to the normal metabolism of that organism. Xenobiotic substances are mainly produced by human activities and influence human beings through their ability to affect the living environment. Some organisms may also form them as a part of their defense system, for example, bacterial toxins, herbal toxins, and cytotoxins. This also includes pesticides, occupational chemicals, environmental contaminants, clinical medicine, drugs of abuse, polymer-related chemicals, and foreign chemicals created by other organisms. Xenobiotics become harmful when they enter into the food chain.

6.3 PHYSICOCHEMICAL NATURE OF XENOBIOTICS Xenobiotic compounds have complex chemical structures and some of these chemicals are joint by nonphysiological bonds and these compounds have some similarities in structure to the natural compounds from which they originate (Table 6.1). Xenobiotic compounds have high molecular masses, are insoluble or partially soluble in water, and they are condensed forming aromatic and polycyclic rings. These xenobiotics are polycyclic

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aromatic hydrocarbons (PAHs), halogen substituted aliphatic compounds, aromatic hydrocarbons, aromatic amines, organic and nitroaromatic compounds, azo compounds, and synthetic polymers (Southorn and Powis, 1988; Spain, 1995). Some xenobiotic compounds and their chemical formulas are given in Table 6.1. Table 6.1 Some xenobiotic compounds and their chemical formula Xenobiotic compounds Chemical formula Pesticides:

1. 2. 3. 4. 5.

Deltamethrin N-Hexane Paraquat Soman (nerve gas) Methyl isocyanate

C22H19Br2NO3 C6H14 C12H14Cl2N2 C7H16F C2H3NO

Pharmaceutical industry:

1. 2. 3. 4.

Paracetamol Ibuprofen Caffeine Acetaminophen

C8H9NO2 C13H14O2 C8H10N4O2 C8H9NO2

Food additives:

1. 2. 3. 4. 5.

Vinegar Citric acid Methyl cyclopropane Astaxanthin Lecithin

CH3COOH C6H8O7 C4H10 C40H52O4 C35H66NO7P

Paint industry:

1. Xylene 2. Toluene

C8H10 C7H8

Wood preservatives:

1. Creosate 2. Cupric chromate

(CH3)C6H4(OH) CrCuO4

Plastic industry:

1. Poly vinyl chloride (PVC)

(C2H3Cl)n

Textile industry:

1. Soda ash 2. Potassium carbamate

Na2CO3 K2CO3

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6.4 SOURCES OF XENOBIOTICS Different sources of xenobiotics include: • toothpaste (Capdevielle et al., 2008); • laundry detergents (Gordon et al., 2008); • volcanoes (Capaccioni et al., 1993; Pyle and Mather, 2003) and dolls (Biedermann-Brem et al., 2008); • mobile phones (Nnorom and Osibanjo, 2009; Monteiro et al., 2007); • railway sleepers (Mateus et al., 2008; Thierfelder and Sandström, 2008); • conventional wastewater treatment plants (Sabik et al., 2004); • as well as some other products and services and throughout the expanse of nature. Xenobiotics released from different sources may cause environmental pollution and pose potential challenges for the environment. Some of these sources are known and some are unknown, but anthropogenic activity is a major source. Different sources of xenobiotics are given in Table 6.2. The Stockholm convention was held in 2001 under the United Nations Environmental Program (UNEP) to discuss the impact of persistent organic chemicals on the environment. Xenobiotic compounds were considered as one of the most important pollutants of the environment (Thakur, 2008). Table 6.2 Sources of xenobiotic substances Sources Example

Agriculture Medicine Food industry Energy industry Transport Consumer industry Plastic industry Petrochemical industry Paper industry Pesticide industry Insecticides

Pesticides, herbicides Drugs Additives, food CO2, SO2 NOx, Lead, CO2 Coating, dyes Number of complex organic compound Antioxidant plasticizer, crosslinking agents Oil/gas industries, refineries, produced some chemicals, for example, benzene, vinyl chloride Paper and pulp effluent, chlorinated organic compound These are benzene and benzene derivatives, and chlorinated, heterocyclic DDT

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6.5 DISTRIBUTION AND FATE OF XENOBIOTICS IN THE ENVIRONMENT In conventional sewage treatment systems there are large number of xenobiotic substances, each having its own impact and removing these contaminants pose a serious environmental issue (Fig. 6.2). Xenobiotics, such as trichloroethylene (TCE) and polycyclic aromatic hydrocarbons (PAHs), are resistant to breakage and persistent in the environment. These xenobiotics are accumulated in the environment due to their specific chemical properties. Therefore, these xenobiotics show characteristics of toxicity and accumulation in the environment and affect the environment and human life. Xenobiotic contaminants are found mainly in water and wastewater sources, agricultural runoff, and biological systems and have the potential to affect the climate and human health (Fatta-Kassinos et al., 2011). The major xenobiotic compounds are introduced into the environment through industries such as paper and pulp, fossil fuel, pesticide, explosive, and pharmaceutical, etc. Examples of xenobiotic compounds Fate of xenobiotics

Degradable

Nonbiodegradable

CH3CI, CH2CI2, vinyl chloride

Chloroform

Hexachlorocyclohexane

DDT EtBr

Toluene, pentachlorophenol

Process of degradation

Photochemical Chemical

Biodegradation Biotransformation Cometabolism

Mineralized, transformed, and modified compounds mixed to soil, water, atmosphere, and participate in biogeochemical, cycles, and biomagnifications and also affects the climate change

Figure 6.2 Fate of xenobiotics in the environment.

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are synthetic organochloride and pesticides, and some are naturally occurring (Donner et al., 2010). Some microorganisms may be able to acclimatize to xenobiotics that are introduced into the environment through parallel gene transfer and to make use of such compounds as sources of energy. These processes may lead to supplementary changes in the metabolic pathways of microorganisms. Sometimes these microorganisms degrade harmful xenobiotic compounds under feasible conditions. Genetically modified microorganisms are capable of degrading xenobiotics and are effective agents in bioremediation processes. Some microorganisms are degradative in nature and increase the mobility of xenobiotic compounds as well as enhance the toxicity of these chemicals (Noyes and Lema, 2015) Bioremediation processes for xenobiotics have some limitations, that is, they require the best conditions for the metabolic activity of assured microorganisms, which may be hard to meet in practical environments. In some cases, a single microorganism is not able to perform the metabolic processes that are required for the degradation of xenobiotic compounds and these are known as syntrophic bacterial consortia. Different microbial consortia work together to degrade the xenobiotic compounds. These xenobiotic compounds are completely degrade into its constituents in different steps. One microorganism may inhibit the activity of another organism and this balance is maintained by these organisms. A variety of biological effects are caused by xenobiotic compounds which are used when xenobiotics are characterized in bioassay processes. Because of the biological effects of xenobiotics their registration is required before its marketing in several countries. The risk factor of pesticides must be evaluated, such as toxicity and persistence in the environment. A large number of bacteria and fungi have been recognized as having the capacity to degrade xenobiotic compounds (Lah et al., 2011). Biodegradation is a biological process which catalyzes the reduction process of xenobiotic compounds (Alexander, 1999). It is a combining of two processes and these are: (1) growth and (2) cometabolism 1. Growth: In this process, xenobiotics are a source of carbon, energy, and nitrogen when they are used by microorganisms and these microorganisms causes whole mineralization (biodegradation) of these pollutants. This type of biodegradation takes place in aerobic conditions (Riser-Roberts, 1998).

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2. Co-metabolism: The xenobiotic compounds are metabolized in the presence of growth substrates which are used as the primary source of carbon and energy.

6.6 IMPACT OF XENOBIOTICS ON THE ENVIRONMENT Xenobiotic contaminants are of universal occurrence and are released into the environment through human activities resulting from the increasing population and urbanization. The major sources of xenobiotics range from agriculture to pharmaceuticals, and while the current demands for pharmaceuticals are increasing due to the increase of population this also contributes toward xenobiotics. The different drugs consumed have led to the discharge of harmful contaminants into aquatic environments and this causes numerous short and long terms effects on natural ecosystems. Xenobiotics have direct effects on ecosystems, that is, change in community parameters, change in community structure, diversity, productivity, and energy transfer, succession and population density. Changes in population dynamics will lead to changes in productivity, reproduction, genetic, and compositional changes, thereby affecting the ecosystem (Gianfreda and Rao, 2008). It will also affect the food chain on all tropic levels (Bhat, 2013). Worldwide herbal medicine and some botanical plants species are gaining influence and these also might have some properties of xenobiotics. The environment is contaminated by some herbs that have the capacity to interfere with the biological systems of aquatic organisms (Guengerich, 1997a,b). Some plants are commonly used in the preparation of health supplements and remedies for varieties of health problems, including weight loss, menstrual cycles, and rheumatism (Grollman et al., 2007; Debelle et al., 2008).

6.6.1 Impact of Xenobiotics in Water In conventional sewage treatment plants there are common receptors of xenobiotics that have to be treated along with municipal wastewater before being discharged into aquatic systems. In water bodies, some trace metals, xenobiotic compounds, and synthetic organic chemicals, for example, PAHs, phthalates, and pesticides, can be found (Essumang and Ankrah, 2010). The primary degradation products of pharmaceuticals and xenobiotic compounds influence conventional sewage treatment plants and may inhibit biological processes, such as nitrification. The first step in

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the nitrification process is the oxidation of ammonium to nitrite and it is susceptible to the presence of xenobiotic compounds. The first steps of nitrification are inhibited by xenobiotics under uncontrolled conditions, it may totally inhibit the biological nitrogen process (Essumang and Adokoh, 2009). Xenobiotic compounds discharged into surface water may reach to groundwater through leaching process however, it is strongly discouraged now as discharge of these compounds may affect the ecological integrity of aquatic ecosystems. Some aquatic organisms are important biological indicators of xenobiotic pollution (Fent et al., 2006). Xenobiotic compounds are released into the environment either in their original forms or as metabolites. Xenobiotic compounds in human beings may follow the pathway of ingestion and excretion with disposal by wastewater (Singh et al., 2016a,b). Some xenobiotic compounds are nonbiodegradable in conventional sewage treatment plants and are released along with treated runoff which may result in the contamination of aquatic systems, such as rivers, lakes, and estuaries (Embrandiri et al., 2016). High production, persistence in the environment and biological effects are the most important and critical aspects of xenobiotics. Studies have observed the increasing amount of xenobiotic compounds found in aquatic systems (surface water) and these xenobiotics affect aquatic fauna and flora raising concerns worldwide (Embrandiri et al., 2016). Xenobiotics have an impact on animals in the food chain (insects and fish) (Rosi-Marshall, 2013).

6.6.2 Impacts of Xenobiotics on Soil Some xenobiotic compounds, like pesticides, affect the processes of soil and also the yield. Due to the toxicity of xenobiotics certain factors of the soil may be influenced. Some xenobiotics which are persistent in the soil may cause different problems. If these xenobiotics remain in the soil for long periods of time it may cause: 1. Xenobiotic compounds may be assimilated by plants and can accumulate in the edible portion of plants. 2. Some xenobiotic compounds are eroded with soil particles. 3. Xenobiotic compounds may accumulate in the food chain and change the nature of ecological balance (Alexander, 1965). 4. Incomplete mineralization may lead to the formation of harmful intermediate compounds.

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5. Xenobiotic compounds are metabolized by soil microbes; these metabolites may accumulate in the soil. It has been recognized that, some xenobiotics are toxic and their rates of degradation are extremely slow and that they cause adverse effects on human and on ecological health. Since xenobiotics and some of their metabolites are persistent and accumulative in nature these compounds tend to increase over time. Their enhanced concentration may cause harmful impacts on flora, fauna, and soil. Because of these concerns, it is important to understand xenobiotic degradation pathways in soil.

6.6.3 Impact of Xenobiotics in Air Human beings introduce xenobiotic pollutants into the environment in huge quantities every day and are hardly aware to these pollutants. Some xenobiotic compounds, such as PAHs, are exclusive environmental pollutants. These organic compounds are present everywhere and are released into the environment due to the incomplete combustion of materials and gas particles. There are different types of sources of xenobiotic compounds, such as PAHs, and these sources include domestic sources like the heating and combustion of fuel and coal, combustion of agricultural waste, and sometimes even the grilling of foods. Mobile sources, like trucks, ships, aircrafts, and cars (Essumang et al., 2006) also contribute toward the release of xenobiotics into the environment. Industrial sources, like power generation, aluminum production, cement kilns, and refineries are major sources of PAHs in the atmosphere (Essumang et al., 2006). These xenobiotic compounds reach the different trophic levels and disturb the distribution and cycles of food webs and chains. These compounds may affect the climate and environment and some xenobiotics are carcinogens which may cause a number of problems in human beings (Essumang et al., 2006).

6.7 IMPACT OF XENOBIOTICS ON CLIMATE CHANGE Xenobiotics are major stressors, in addition to eutrophication and climate change, which all affect the environment (Pace and Groffman, 1998). In contrast, there are only a few studies investigating the relationship between xenobiotics and climate change, it would be interesting to investigate the behavior of xenobiotics in modified climatic conditions. The use of xenobiotics will increase in the near future

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(Bloomfield et al., 2006; Navarro et al., 2000) which may be a great cause of environmental pollution. The occurrence of xenobiotic compounds in the environment and their impact on the environment depend on water solubility and vapor pressure as well as other environmental factors like humidity and temperature differences, the nature of the compounds, and the concentration of particulate matter. The emission of xenobiotic compounds caused during volatilization processes from plants and soil, wind erosion of soil particles, and the emission of pollutants present in the air have an impact on climate change as well as the environment (Mosleh et al., 2005; Coscollà et al., 2011). Differences in temperature caused several changes in xenobiotic compound and these compounds impact the dynamics of aquatic ecosystem (Gagne et al., 2007). Temperature increases lead to enhanced activity of organisms and increases the uptake rates of xenobiotic substances, including toxicants, in the environment. The characteristics and behavior of xenobiotics will change with changing environments and may influence climate change (Brubaker and Hites, 1998; Ma et al., 2004; MacDonald et al., 2002, Meyer and Wania, 2008; Sweetman et al., 2005; Sinkkonen and Paasivirta, 2000; Wania, 1999). A change in xenobiotic metabolism in water, soil, and some biota may be caused due to a change in temperature. It could increase the concentration of xenobiotic pollutants and promote their partitioning to aquatic ecosystems (MacDonald et al., 2003). Increases in the temperature of soil or water will increase the degradation rate of xenobiotics which may increase the concentration of xenobiotic pollutants from solvent depleting processes (Sinkkonen and Paasivirta, 2000; Sweetman et al., 2005). Increased volatilization of xenobiotics in water, air, and soil may result due to increasing temperatures. Under these circumstances they will be subject to photo degradation and transport (Beyer et al., 2003; Brubaker and Hites, 1998; Ma et al., 2004; Scheyer et al., 2005). Certain biotic factors can manipulate the behavior of xenobiotics and may change the pattern of migration of species which can impact on the climate. Some distributional processes of climate change will change the frequency and number of xenobiotic compounds which are used in agriculture and they thereby move to change the climate (Chen and McCarl, 2001; Reilly et al., 2003). Volatilization is an important environmental factor that increases global warming, while global warming could enhance the volatilization of xenobiotic compounds in water and soil (Van den Berg et al., 1999).

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6.7.1 Loss of Species Diversity Among the Food Chain and Food Webs Some microorganisms present in an ecosystem depend on associations between species but due to the presence of xenobiotics this association among species may be lost, which could lead to the loss of a keystone species. A keystone species is defined as a species that is disproportionately connected to more species in a food web. There are many more connections to keystone species, as it manages the organization and structure of a whole community. The loss of a keystone species may affect the trophic level, food web, and/or several chain connections and may thus cause the extinction of another species in the community (Mills et al., 1993; Stoytcheva, 2011). The major effect of xenobiotics on the diversity of animals and plants in an ecosystem is one of the major undesirable impact of xenobiotics in the environment (Zacharia, 2011).

6.7.2 Effect on Nutrient Cycling in Ecosystems A huge fraction of xenobiotic compounds used in the environment eventually reaches the soil where through soil building processes and nutrient cycling they are transported into plants. Soil microorganisms interact with some xenobiotics and this directly or indirectly affects the processes of nutrient cycling in an ecosystem. In the soil, xenobiotics hinder nitrogen fixation which is required for the growth of higher plants (Rockets, 2007).

6.7.3 Effect on Pollinators Honeybees and butterflies are natural pollinators and are highly susceptible to xenobiotic compounds. Xenobiotics may harm butterfly and bee species so that natural pollination decreases due to a loss of pollinator species and decreases in the number of pollinator species. It can result into a decrease in seed and fruit production. Therefore, xenobiotics may affect ecological behavior and the economy as well as affect climate change (Morse and Calderone, 2000).

6.8 IMPACT AND INTERACTION OF XENOBIOTICS AND CLIMATE CHANGE Currently, there is great focus on the study of xenobiotics in relation to climate change, for instance temperature, salinity, and acidification of aquatic systems in the environment. More studies are required to understand how

Impact of Xenobiotics Under a Changing Climate Scenario

Climate-induced

Xenobiotics-induced

Xenobiotics sensitivities

Climate change sensitivities

Restrict thermal tolerances

Enhanced bioavailability, bioaccumulation Temperature, salinity, UV light

145

Potentia

Xenobiotics exposure

Bioactivity

Disrupted Physiology

Induced climate change Reduced population fitness

Oxidative stress, xenobiotic Consumption (mental) Species phylogeny and life history Organism condition (e.g., nutritional status, life-stage)

Other stressors (e.g., disease, invasive, habitat quality, food availability

Figure 6.3 Interaction between xenobiotics and climate change.

the toxic effects of xenobiotics are influenced by climate change. Several man made xenobiotics, when coming into contact with human beings, flora, and fauna (Fig. 6.3), can act as pollutants and damage the nature, physiology, development, and behavior of animals in addition to affecting the ecological environment. Multiple chemical stressors contribute toward severe climate change due to the synergistic interaction of xenobiotics with climate change (Bozinovic and Pörtner, 2015; Helmuth, 2009). Different types of environmental parameters regulate the toxicity of xenobiotics and may influence the global climate change. Xenobiotic pollution has enhanced the vulnerability of species to climate change stressors (Hooper et al., 2013). The complex and nonlinear interactions between xenobiotics and the climate have been identified by several researchers (Bertin et al., 2013). The interaction between climate change and xenobiotics has some reciprocal effects on ecological systems, including: 1. Interactions of xenobiotics and climate change, how these interactionsmay affect the living beings. 2. The toxicity pathways of xenobiotic compounds are influence by climate change. 3. Some xenobiotic compounds, such as pesticides and endocrine disrupting compounds, could affect the physiological capacity of wild fauna and flora and may respond to climate change.

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The effects of xenobiotics can be predicted through different processes of climate change on human beings, animals, and plants. In some areas, an increase in temperature may lead to an increase in air pollutants, such as tropospheric ozone and particulate matter (IPCC, 2014). In areas where xenobiotic contaminants are found in maximum amounts they may enter into surface and ground water after the precipitation. Some xenobiotics are more accessible and may enter into food webs and the food chain (Braune et al., 2005; Dalla Valle et al., 2007a,b; Kong et al., 2014; Letcher et al., 2010; Pacyna et al., 2006). The interaction between xenobiotics and the climate and its effects will depend on the nature of xenobiotic compounds and the life cycle of organisms (DeLorenzo et al., 2009). This interaction influences will lead to the toxicity of xenobiotics in flora and fauna (Holmstrup et al., 2010; Noyes et al., 2009). Environmental parameters, such as pH, temperature, salinity, dissolved oxygen, etc., are affected due to the changing climate and this could affect xenobiotic speciation (Anawar, 2013; Weiss, 2014; Sokolova and Lannig, 2008).

6.9 CONCLUSION AND FUTURE PROSPECTIVE Currently, most researchers are focused on the impact of xenobiotics on climate change because xenobiotics may affect the environment, including air and aquatic systems, as well as human health. Xenobiotics will affect climate change due to increasing temperatures and salinity. Increasing temperatures will enhance the metabolism and degradation of xenobiotics. These xenobiotic contaminants may get into surface and groundwater and could thereby affect aquatic fauna and flora. Xenobiotic contaminants are transferred into food webs and the food chain and will thereby affect all tropic levels as well as the nutrient cycle. To mitigate the combined effect of xenobiotics and climate change on human health and the environment, there needs to be an enhanced awareness in people about ecosystems and the use of xenobiotic compounds. Increasing temperatures and salinity, which are linked to climate change, could affect the distribution of toxicity of xenobiotics, such as persistent organic compounds and pesticides in aquatic ecosystems, and this will also affect climate change. Currently, there are huge number of researchers focused on determining ways to minimize the effect of xenobiotics on climate change and ecosystems as well as on making sustainable use of these xenobiotics for sustainable living with the environment.

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Lah, L., Podobnik, B., Novak, M., Koroˇsec, B., Berne, S., Vogelsang, M., et al., 2011. The versatility of the fungal cytochrome P450 monooxygenase system is instrumental in xenobiotic detoxification. Mol. Microbiol. 81 (5), 1374 1389. Letcher, R.J., Bustnes, J.O., Dietz, R., Jenssen, B.M., Jørgensen, E.H., Sonne, C., et al., 2010. Exposure and effects assessment of persistent organohalogen contaminants in arctic wildlife and fish. Sci. Total Environ. 408 (15), 2995 3043. Ma, J., Hung, H., Blanchard, P., 2004. How do climate fluctuations affect persistent organic pollutant distribution in North America? Evidence from a decade of air monitoring. Environ. Sci. Technol. 38 (9), 2538 2543. MacDonald, R.W., Mackay, D., Hickie, B., 2002. Contaminant amplification in the environment: revealing the fundamental mechanisms. Environ Sci Technlol 36, 457 462. MacDonald, R.W., Mackay, D., Li, Y.-F., Hickie, B., 2003. How will global climate change affect risks from long-range transport of persistent organic pollutants? Human and Ecological Risk Assessment 9, 643 660. Mateus, E.P., da Silva, M.D.G., Ribeiro, A.B., Marriott, P.J., 2008. Qualitative mass spectrometric analysis of the volatile fraction of creosote-treated railway wood sleepers by using comprehensive two-dimensional gas chromatography. J. Chromatogr. A 1178 (1 2), 215 222. Meyer, T., Wania, F., 2008. Organic contaminant amplification during snowmelt. Water Res. 42 (8 9), 1847 1865. Mills, L.S., Soulé, M.E., Doak, D.F., 1993. The keystone-species concept in ecology and conservation. BioScience 43 (4), 219 224. Monteiro, M.R., Moreira, D.G., Chinelatto, M.A., Nascente, P.A., Alcântara, N.G., 2007. Characterization and recycling of polymeric components present in cell phones. J. Polym. Environ. 15 (3), 195 199. Morse, R.A., Calderone, N.W., 2000. The value of honey bees as pollinators of US crops in 2000. Bee Cult. 128 (3), 1 15. Mosleh, Y.Y., Paris-Palacios, S., Couderchet, M., Biagianti-Risbourg, S., Vernet, G., 2005. Metallothionein induction, antioxidative responses, glycogen and growth changes in Tubifex tubifex (Oligochaete) exposed to the fungicide, fenhexamid. Environ. Pollut. 135 (1), 73 82. Navarro, S., Barba, A., Navarro, G., Vela, N., Oliva, J., 2000. Multiresidue method for the rapid determination in grape, must and wine—of fungicides frequently used on vineyards. J. Chromatogr. A 882 (1 2), 221 229. Nnorom, I.C., Osibanjo, O., 2009. Toxicity characterization of waste mobile phone plastics. J. Hazard. Mater. 161 (1), 183 188. Noyes, P.D., Lema, S.C., 2015. Forecasting the impacts of chemical pollution and climate change interactions on the health of wildlife. Curr. Zool. 61 (4), 669 689. Noyes, P.D., McElwee, M.K., Miller, H.D., Clark, B.W., Van Tiem, L.A., Walcott, K. C., et al., 2009. The toxicology of climate change: environmental contaminants in a warming world. Environ. Int. 35 (6), 971 986. Pace, M.L., Groffman, P.M., 1998. Successes, limitations, and frontiers in ecosystem science: reflections on the seventh Cary Conference. Ecosystems 1 (2), 137 142. Pacyna, E.G., Pacyna, J.M., Fudala, J., Strzelecka-Jastrzab, E., Hlawiczka, S., Panasiuk, D., 2006. Mercury emissions to the atmosphere from anthropogenic sources in Europe in 2000 and their scenarios until 2020. Sci. Total Environ. 370 (1), 147 156. Pyle, D.M., Mather, T.A., 2003. The importance of volcanic emissions for the global atmospheric mercury cycle. Atmos. Environ. 37 (36), 5115 5124. Reilly, J., Tubiello, F., McCarl, B., Abler, D., Darwin, R., Fuglie, K., et al., 2003. US agriculture and climate change: new results. Clim. Change 57 (1 2), 43 67.

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Rieger, P.G., Meier, H.M., Gerle, M., Vogt, U., Groth, T., Knackmuss, H.J., 2002. Xenobiotics in the environment: present and future strategies to obviate the problem of biological persistence. J. Biotechnol. 94 (1), 101 123. Riser-Roberts, E., 1998. Remediation of Petroleum Contaminated Soils: Biological, Physical, and Chemical Processes. Lewis. Rockets, R., 2007. Down on the farm? Yields, nutrients and soil quality. Scienceagogo.com. Rosi-Marshall, E., 2013. Streams stressed by pharmaceutical pollution. www.environmentalchange.nd.edu/events/2 (last visited 10.08.2013.). Sabik, H., Gagnon, C., Houde, F., Deblois, C., 2004. Distribution, fate, and behaviour of nonylphenol ethoxylates and degradation products in the dispersion plume of a major municipal wastewater effluent. Environ. Forensics 5 (2), 61 70. Scheyer, A., Graeff, C., Morville, S., Mirabel, P., Millet, M., 2005. Analysis of some organochlorine pesticides in an urban atmosphere (Strasbourg, east of France). Chemosphere 58 (11), 1517 1524. Schlegel, H.G., 1986. Xenobiotics, General Microbiology, sixth ed. Cambridge University Press, New York, p. 433. Singh, A., Prasad, S.M., Singh, R.P. (Eds.), 2016a. Plant Responses to Xenobiotics. Springer, Singapore. Singh, S., Bashri, G., Singh, A., Prasad, S.M., 2016b. Regulation of xenobiotics in higher plants: signalling and detoxification. Plant Responses to Xenobiotics. Springer, Singapore, pp. 39 56. Sinkkonen, S., Paasivirta, J., 2000. Degradation half-life times of PCDDs, PCDFs and PCBs for environmental fate modeling. Chemosphere 40 (9 11), 943 949. Sokolova, I.M., Lannig, G., 2008. Interactive effects of metal pollution and temperature on metabolism in aquatic ectotherms: implications of global climate change. Clim. Res. 37 (2 3), 181 201. Southern, P.A., Powis, G., 1988. Free radicals in medicine. I. Chemical nature and biologic reactions. Mayo Clin. Proc. 63 (4), 381 389. Elsevier. Spain, J.C., 1995. Biodegradation of nitroaromatic compounds. Annu. Rev. Microbiol. 49 (1), 523 555. Stoytcheva, M., 2011. Pesticides in the Modern World—Risks and Benefits. InTech, Rijeka, Croatia. Sweetman, A.J., Dalla Valle, M., Prevedouros, K., Jones, K.C., 2005. The role of soil organic carbon in the global cycling of persistent organic pollutants (POPs): interpreting and modelling field data. Chemosphere 60 (7), 959 972. Thakur, I.S., 2008. Xenobiotics: pollutants and their degradation-methane, benzene, pesticides, bioabsorption of metals. Environmental microbio, 1-26. Thierfelder, T., Sandström, E., 2008. The creosote content of used railway crossties as compared with European stipulations for hazardous waste. Sci. Total Environ. 402 (1), 106 112. Van den Berg, F., Kubiak, R., Benjey, W.G., Majewski, M.S., Yates, S.R., Reeves, G.L., et al., 1999. Emission of pesticides into the air. Fate of Pesticides in the Atmosphere: Implications for Environmental Risk Assessment. Springer, Dordrecht, pp. 195 218. Wania, F., 1999. On the origin of elevated levels of persistent chemicals in the environment. Environ. Sci. Pollut. Res. 6 (1), 11 19. Weiss, J., 2014. Chapter 4: Osmoregulation and excretion. Physiological, Developmental and Behavioural Effects of Marine Pollution. Springer Netherlands, Dordrecht, pp. 97 124. Zacharia, J.T., 2011. Ecological effects of pesticides. In: Stoytcheva, M. (Ed.), Pesticides in the Modern World—Risks and Benefits, ISBN: 978-953-307-458-0, InTech. Tech. Available from: http://www.Intechopen.Com/books/pesticides-in-the-modern-worldrisks-and-benefits/ecological-effects-of-pesticides.

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FURTHER READING Balbus, J.M., Boxall, A., Fenske, R.A., McKone, T.E., Zeise, L., 2013. Implications of global climate change for the assessment and management of human health risks of chemicals in the natural environment. Environ. Toxicol. Chem. 32 (1), 62 78. Bolliger, C., Schönholzer, F., Schroth, M.H., Hahn, D., Bernasconi, S.M., Zeyer, J., 2000. Characterizing intrinsic bioremediation in a petroleum hydrocarboncontaminated aquifer by combined chemical, isotopic, and biological analyses. Biorem. J. 4 (4), 359 371. Dalla Valle, M., Codato, E., Marcomini, A., 2007. Climate change influence on POPs distribution and fate: A case study. Chemosphere 67 (7), 1287 1295. Dalla Valle, M., Codato, E., Marcomini, A., 2007b. Climate change influence on POPs distribution and fate: a case study. Chemosphere 67 (7), 1287 1295. Delorenzo, M.E., 2015. Impacts of climate change on the ecotoxicology of chemical contaminants in estuarine organisms. Curr. Zool. 61 (4), 641 652. Essumang, D.K., Kowalski, K., Sogaard, E.G., 2011. Levels, distribution and source characterization of polycyclic aromatic hydrocarbons (PAHs) in topsoils and roadside soils in Esbjerg, Denmark. Bull. Environ. Contam. Toxicol. 86 (4), 438 443. Gordon, C.J., 2003. Role of environmental stress in the physiological response to chemical toxicants. Environ. Res. 92 (1), 1 7. Gordon, C.J., 2005. Temperature and Toxicology: An Integrative, Comparative, and Environmental Approach. CRC Press, Boca Raton. Ibrahim, H.Z., 2014. Climate change impacts on pests and pesticide use. Alexandria Research Center for Adaptation to Climate Change (ARCA) 1 33. IPCC (United Nations Intergovernmental Panel on Climate Change), 2007a. Climate Change 2007: Synthesis report. Cambridge University Press, Cambridge, UK. Available at: http://www.ipcc.ch/ipccreports/assessments-reports.htm. IPCC (United Nations Intergovernmental Panel on Climate Change), 2007. Climate Change 2014: The physical science basis. Cambridge University Press, Cambridge, UK. Available at: http://www.ipcc.ch/ipccreports/assessments-reports.htm. Lapertot, M.E., Pulgarin, C., 2006. Biodegradability assessment of several priority hazardous substances: choice, application and relevance regarding toxicity and bacterial activity. Chemosphere 65 (4), 682 690.

CHAPTER 7

Impact of Climate Change on Plant Microbe Interactions under Agroecosystems Vipin Kumar Singh1, , Awadhesh Kumar Shukla2, and Amit Kishore Singh3 1

Department of Botany, Center of Advanced Study in Botany, Institute of Science, Banaras Hindu University, Varanasi, India Department of Botany, K.S. Saket P.G. College, Ayodhya, Faizabad, India 3 Botany Department, Kamla Nehru P.G. College, Raebareli, India 2

Contents Introduction Effects of Elevated CO2 on Beneficial Microbes Effects of Elevated CO2 on Microbial Processes Effects of Elevated Temperature and Drought on Microbial Structure and Functions 7.4.1 Effect of Elevated Temperature 7.4.2 Effect of Drought 7.5 Effect of Climate Change on Host Pathogen Interactions 7.6 Effect of Climate Change on Biocontrol Agents 7.7 Climate Change and Xenobiotics 7.8 Future Perspective References Further Reading 7.1 7.2 7.3 7.4

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7.1 INTRODUCTION It is understood that planet Earth is continuously experiencing climate change, influenced by atmospheric CO2, a major GHG, which has increased by nearly 30% due to anthropogenic sources (fossil fuels, agroecosystems; IPCC, 2013). Anticipated increases in temperature, O3, nitrogen deposition, and changes in precipitation patterns may substantially affect plant taxa and interactive soil/rhizosphere microbes that constitute 

First two authors share equal contribution

Climate Change and Agricultural Ecosystems DOI: https://doi.org/10.1016/B978-0-12-816483-9.00007-4

Copyright © 2019 Elsevier Inc. All rights reserved.

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terrestrial ecosystems. Biological components, such as plants and microbes, are essential interacting elements of terrestrial ecosystems. Typically, there are several types of plant microbe interactions, including competition, commensalism, mutualism, and parasitism, essential for performing ecological services, like decomposition, mineralization, and nutrient cycling. Generally, all land plant taxa studied to date have well-established symbiosis with diversified beneficial microorganisms (Brundrett, 2009). Many of these are plant growth-promoting microbes, such as plant growthpromoting rhizobacteria (PGPR) and plant growth-promoting fungi while others prefer to enter the root systems of their hosts and impart beneficial effects with their endophytic lifestyle (Kloepper and Schroth, 1978; Stone et al., 2000; Lugtenberg and Kamilova, 2009; Bashan and Holguin, 1998). This is the case for arbuscular mycorrhizae, ectomycorrhizae, and other endophytic fungi (Das and Varma, 2009). Mostly, beneficial microbes are key elements of biocontrol processes, constitute ecofriendly strategies for curbing plant disease, and are an alternate to hazardous chemical pesticides. However, the changes associated with global warming (i.e., elevated CO2 (eCO2), increased temperatures, ozone levels, and drought, etc.,) may affect the structure and composition of relative microbial communities and the incidence and severity of plant diseases, which directly or indirectly influence the further coevolution of plants and their pathogens (Garrett et al., 2006; Eastburn et al., 2011). As a matter of fact, in many developing countries, climate change has emerged as a major constraint causing huge economic loss in the agricultural sector. Under the current scenario, in many developing countries, including in India, initiatives are being taken to develop resilient crops for climate change and climate vulnerability through strategic research and technological demonstration. Moreover, numerous microbes have been explored that are well known for providing resistance to crops against abiotic and biotic stress. With the ever-changing phenomena of climatic conditions, outbreaks of new diseases have been reported which could affect microbial assisted biocontrol processes and therefore may weaken their disease controlling capacity. Therefore, it is imperative to deeply explore the impact of climate change on the numerous aspects of plant microbe interactions. In this regard, the present chapter endeavors to address, extensively, the effect of climate change on various plant microbe issues and also to highlight future research perspectives for enhancing crop productivity under changing climatic conditions using throughput technologies. In addition, sustainable means of controlling climate change will be outlined.

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7.2 EFFECTS OF ELEVATED CO2 ON BENEFICIAL MICROBES Soil is as an excellent natural medium that supports the growth and development of numerous plants as well as microbes (Melillo et al., 2002). Plant microbe interactions in soil are either beneficial or harmful depending upon the prevailing environmental conditions (Glick, 2012). Beneficial plant microbe interactions are caused by symbiotic or nonsymbiotic bacteria and a highly specialized group of fungi (mycorrhizal fungi). Beneficial plant-associated bacteria are known to stimulate plant growth and enhance their resistance to degenerative diseases and abiotic stresses. Bacterial genera such as Azospirillum, Bacillus, Pseudomonas, Rhizobium, Serratia, Stenotrophomonas, and Streptomyces fall under this category. These are popularly known as PGPR. As evidenced by numerous reports, growth promoting substances (phytohormones, siderophores, and antimicrobials) released in copious amounts by PGPR indirectly influence the overall morphology of plants. Under eCO2 environments, plants tend to stimulate microbes through enhanced carbon flow or rhizodeposition into rhizospheric soil, which are important for critical ecosystem processes. Generally, the number and diversity of rhizosphere inhabiting bacteria depend on root exudates (Rangel-Castro et al., 2005) or plant metabolites and are substantially influenced by environmental parameters due to plant physiological changes (Rasche et al., 2006a,b). Hence, it can be hypothesized that environmental factors associated with climate change will likely affect these soil microbial communities. For instance, Ma et al. (2017) noticed that a CO2 concentration .10,000 is lethal for some soil bacterial species. A possible reason for the reduction in bacterial population could be attributed to their low adaptability under eCO2. In another experiment, eCO2 was found to decrease the relative abundance of Methylocystaceae, the major methane-oxidizing bacteria in rice roots (Okubo et al., 2015). Furthermore, several studies have evidenced that endophytic bacterial populations that colonize the internal tissues of plants, such as roots, stems, shoots, leaves as well as flowers, fruits, and seeds (Hallmann, 2001; Compant et al., 2008, 2010), may be affected in a similar manner and grow under extreme environmental conditions, such as in plants containing high levels of heavy metals (Lodewyckx et al., 2002; Idris et al., 2004). Many of them have the potential to induce plant growth-promoting effects that may help in mitigating biotic and abiotic stresses by priming specific plant genes under adverse conditions (Hallmann and Berg, 2007).

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Mycorrhizal fungi are well known for their symbiotic associations with the vast majority of terrestrial plants, from garden vegetables to trees of old forests (Smith and Read, 1997). It has been estimated that apart from 6000 species of Glomeromycotina and Ascomycotina, there also exists unique mycorrhizal helper bacteria that usually participate in mycorrhizal establishment and functioning (Klett et al., 2007). Changes in climatic conditions can alter environmental conditions drastically and as a result of which mycorrhizal structure and function may be significantly reduced (Staddon et al., 2002). In addition, eCO2 concentrations have also been found to reduce hyphal growth in mycorrhizae. Moreover, eCO2 concentrations are likely to increase the carbon allocation to plant roots and thereby significantly hamper the normal physiological and growth promoting activities of plant root associated microbes (Groenigen et al., 2011). Reports have revealed increased the ectomycorrhizal colonization of some species (Garcia et al., 2008) and enhanced microbial nitrogen fixation in some leguminous crops (Rogers et al., 2009) under changing environmental conditions. A myriad of available reports have demonstrated the impacts of climate change on belowground microbial dynamics, however, little effort has been made so far in elucidating the effect of eCO2 on aboveground inhabitants, such as symbiotic endophytes. It is a well-established concept that endophytes confer environmental stress tolerance. For instance, fungal endophytes can improve plant performance at during water stress (Kannadan and Rudgers, 2008), heat stress (Rodriguez et al., 2008), low nutrient availability (Lyons et al., 1990), and intense grazing/herbivory pressure (Hartley and Gange, 2009). Thus, endophytic associations may be of considerable importance in determining how plant hosts respond to climate change. Newman et al. (2003) demonstrated strong interactions between CO2 and endophyte status in the growth and chemical composition of closely related Festuca arundinacea Schreb. The findings of Brosi et al. (2011) advocated that eCO2 has more pronounced impacts than changes in temperature or precipitation, which may encourage grass fungal (Lolium arundinaceum Neotyphodium coenophialum) symbiosis, leading to higher endophyte infection frequency in tall fescue in old-field communities.

7.3 EFFECTS OF ELEVATED CO2 ON MICROBIAL PROCESSES Soil is an important elemental unit of terrestrial ecosystems and home to a vast array of heterogeneous microflora that regulate nutrient cycling and

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other ecological services. Alterations in CO2 concentrations (i.e., eCO2) may alter soil microbial processes, such as microbial biomass, soil respiration, soil carbon dynamics, and microbial community structure. The main cause of these effects is a change in organic carbon content in soil. Earlier studies have demonstrated reductions in pasture microbial decomposition rates after exposure to high concentrations of CO2 (van Ginkel et al., 2000). Furthermore, Hu et al. (2001) also suggested that a high level of CO2 would result in decreased amounts of available N for microorganisms due to the consequent improvement of plant growth, thus, reducing the degradation ability of several microorganisms. These effects of eCO2 concentrations on the dynamics of soil organic matter may lead to indirect effects on soil structure. In addition, microbial-mediated processes that play an important role in the nitrogen cycle may also be influenced as a result of climate change. Drissner et al. (2007) reported a 48.1% and 23.1% increment in soil microbial biomass under 60 Pa pCO2, during spring and autumn, respectively. These trends of enhanced microbial biomass have been supported by many researchers (Hu et al., 2006; Sillen and Dieleman, 2012). In contrast, Kandeler et al. (2006) found no significant changes in microbial biomass. However, this does not mean that bacterial communities are not affected by eCO2, since changes, that is, gene expression or enzyme activity, can occur without alterations in biomass size. Studies have indicated that profound global warming by 1°C could directly stimulate microbial activity, therefore, climate change will probably influence nearly all belowground processes to some unknown extent with unpredictable long-term consequences. In a free-air carbon dioxide enrichment (FACE) metaanalysis conducted by Tarnawski et al. (2006), the community function of Lolium perenne- and Molinia coerulea (purple moor grass) associated Pseudomonas populations were altered following 8 years of increased pCO2 (60 Pa). Moreover, this study, revealed that increased CO2 concentrations (60 Pa) did not affect the population density of Pseudomonas sp. Bardgett et al. (2013) documented the effect of eCO2 on root exudates which was found to modulate the function of the rhizospheric soil microbial community. Likewise, the presence and expression of functional genes in soil microbial communities have also been shown to change in response to climatic disturbances. For instance, spore forming bacteria are expected to prevail and circumvent adverse conditions such as a drought better than others; this mechanism allows numerous bacteria to survive under long periods of stress.

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Additionally, the impacts of eCO2 on microbial populations and microbial enzyme activities have also been elucidated using metaanalysis by several authors. In an extensive metaanalysis of 47 studies, Zak et al. (2000) found that eCO2 generally increases microbial respiration. Similarly, in a metaanalysis of 30 studies, De Graaff et al. (2006) concluded that CO2 enrichment enhanced microbial respiration by approximately 17%. Enzyme activities reflect the status of microbial nutrient availability and specific soil microbial functions (Allison and Vitousek, 2005; Manzoni et al., 2012). Furthermore, Kelley et al. (2011), while investigating the response of soil enzymatic activities to eCO2, observed that the activity of N-acetylglucosaminidase (chitin-degrading enzyme) was increased substantially by 12.6% under eCO2 conditions. This suggests a progression of microbes that can produce enzymes responsible for degrading more recalcitrant forms of N with increasing CO2. Additionally, they showed that other enzymes, including those that degrade starch, cellulose, lignin, xylan/hemicellulose, and organic P and S, respond variably to eCO2 due to the wide variety of ecosystems and experimental designs (Kelley et al., 2011). Similarly, Drissner et al. (2007) examined temporal changes in enzymatic activities. They observed that during spring, there was increased activity in invertase (36.2%), xylanase (22.9%), urease (23.8%), protease (40.2%), and alkaline phosphomonoesterase (54.1%), while in the autumn, however, the activity of these enzymes was reduced by 3% 12%.

7.4 EFFECTS OF ELEVATED TEMPERATURE AND DROUGHT ON MICROBIAL STRUCTURE AND FUNCTIONS 7.4.1 Effect of Elevated Temperature It has been experienced by many researchers that global warming would decrease the soil moisture content, may check the competence of microbes to disperse, survive, and colonize soil spaces (Carson et al., 2010). Also, an increase in ambient temperature would result in the heating of soil, which can modify the structure of rhizosphere microbiomes established after interactions with plants. Okubo et al. (2014) reported significant increases in the relative abundances of methanotrophs, Methylosinus and Methylocystis, due to rising temperature. Nevertheless, researches have proposed that elevated temperatures under climate change scenario has been shown to increase plant growth as well as the allocation of nutrients to belowground ecosystems. This allocation may result in

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alterations in plant microbe interactions, specifically PGPR populations that are dependent on rhizodeposition. Kachhap et al. (2015), while investigating the impact of elevated temperature on PGPR in the rhizosphere of groundnut variety B-95, observed tremendous changes in the PGPR populations. Their findings revealed a greater number of PGPR populations under elevated temperature as compared to ambient temperature, which could be attributed to the increase in organic C-containing root exudates. Another study implicated PGPR as an effective, ecofriendly, and sustainable approach for the amelioration of heat stress in wheat (Triticum aestivum) by Pseudomonas aeruginosa strain 2CpS1 (Meena et al., 2015). Mosier et al. (2015) assessed the impact of high temperatures on the expression of proteins involved in carbon use and found repression in two genotypes of Leptospirillum. The influence of high temperatures on symbiosis in early host rhizobia molecular signal exchange has been witnessed when the nodules were already established (Hungria and Vargas, 2000). From various sources, it is now established that rhizobia rely on the activation of heat shock proteins (HSPs) that provide thermotolerance (Alexandre and Oliveira, 2011). More or less, HSPs strategies under high temperatures are almost identical to arbuscular mycorrhizal fungi (AMF) (Rhizophagus irregularis), which generally prevent the misfolding or aggregation of proteins under heat and other abiotic stresses (Estrada et al., 2013). These fungi are widely known to improve plant growth and health by intensifying mineral nutrition and by increasing tolerance to abiotic and biotic stresses (Turnau and Haselwandter, 2002). Another strategy for thermotolerance in beneficial microbes, including AMF, is the accumulation of trehalose, that is, reserved carbohydrate (Lenoir et al., 2016). Interestingly, it was shown that under heat and other stresses, enzymes not only from the trehalose synthase complex but also neutral trehalase responsible for the breakdown of internal trehalose are induced. This phenomenon of trehalose cycling with the activation of anabolic and catabolic enzymes during heat stress was first observed in the yeast Saccharomyces cerevisiae (Hottiger et al., 1987).

7.4.2 Effect of Drought Drought stress is one of the most important consequences of global warming. Increases in temperature leading to increased soil moisture evaporation is known to reduce the formation of extra mycorrhizal mycelium in

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plant roots. Different PGPRs and mycorrhizal taxa (ecto- or endo-), however, respond differently to droughts in terms of their patterns of abundance. For instance, citrus plant inoculated with mycorrhiza (Glomus versiforme) is known to improve the osmotic adjustment under drought stress through elevated levels of carbohydrates and essential minerals (K1, Ca1, and Mg21) (Wu and Xia, 2006). Drought is also known to bring about losses in the photosynthates that accumulate in plants during photosynthesis. In agriculture, rhizobacterial inoculants have been successfully used as an effective tool to confer resistance against drought stress (Figueiredo et al., 2008; Sandhya et al., 2009a,b). Like eCO2, drought is also known to influence the belowground quality and quantity of carbon inputs into soil, thereby altering the C:N ratios of leaf and root litter (Walter et al., 2012; Sanaullah et al., 2014; García-Palacios et al., 2015) and reducing belowground C allocation (Ruehr et al., 2009; Sanaullah et al., 2012; Fuchslueger et al., 2014; Hasibeder et al., 2015), which in turn can affect microbe-mediated carbon turnover (García-Palacios et al., 2015). Any change in the soil carbon content can be assessed using important soil parameters, including microbial biomass carbon (MBC) and soil extracellular enzymes. Numerous studies have revealed significant changes in MBC under drought conditions indicating that plants increase rhizodeposition to facilitate soil water as well as nutrient transport (Sanaullah et al., 2011, 2012; Xue et al., 2017) and rhizodeposition as the result of root exudation and root cell sloughing. Moreover, enhanced β-cellobiosidase (Salamon et al., 2004; Kreyling et al., 2008; Sanaullah et al., 2011), chitinase (Parham and Deng, 2000; Kreyling et al., 2008; Sanaullah et al., 2011), and changing leucineaminopeptidase (Sardans and Penuelas, 2005; Weintraub et al., 2007; Sanaullah et al., 2011) were the commonly reported patterns. Beneficial microbes respond to drought either by adopting biochemical strategies or going into a dormant state. The semipermeable membranes of microbes help to maintain their intracellular water potential which can lead to desiccation during drought through a reduction in extracellular water potential. To cope with dehydration under drought conditions, microbes tend to accumulate compatible solutes like proline (Or et al., 2007). Therefore, switching from growth to survival state is the first line of defensive in the strategy adopted by microbes, which involves the production of antioxidants (SOD, CAT, APOX) and protective molecules like osmolytes to reduce water potential and maintain hydration and synthesis of chaperones to stabilize proteins. However, all these processes

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are energetically expensive and greatly decrease the microbial growth rate (Schimel et al., 2007; Or et al., 2007). In addition, some beneficial microbes, such as Piriformospora indica (an endophyte), confer drought tolerance through priming the expression of quite a diverse set of stress related genes in plant parts such as leaves, etc. (Sheramati et al., 2008). Alternatively, microorganisms prefer to go into a dormant state in order to escape unfavorable conditions or even death (Lennon and Jones, 2011).

7.5 EFFECT OF CLIMATE CHANGE ON HOST PATHOGEN INTERACTIONS The impact of climate change on plant pathogens and their role in disease development have been examined in several pathosystems (host plant). Predicted climatic changes are expected to affect pathogen development, survival rates, and host modulation, resulting in changes in disease development in crops (Fig. 7.1). Earlier reports generalized that the impacts of climate change differ by pathosystem and geographical region. These changes may affect not only the optimal conditions required for infection

Figure 7.1 An overview of causes of climate change and their consequences on terrestrial ecosystems.

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but also the host specificity and mechanisms of plant infection (Elad and Pertot, 2014). Past studies based on FACE concluded that eCO2 and/or ozone (O3) consistently affect the leaf surface wax composition; such changes would likely alter host pathogen communication/interaction at a molecular scale prior to infection. Moreover, local environmental setup, such as increases in leaf number, leaf area, canopy size, and density, under eCO2 can alter canopy temperature and microclimate humidity which could promote plant diseases of different severity levels. The impacts of eCO2 on plant disease have been deeply reviewed. There was an increased level of disease severity for 6 out of 10 pathosystems involving biotrophic fungi. Among the 15 necrotrophic fungi under observation, an increase in disease severity was noticed for 9, decreased disease severity for 4 species, and no significant changes for the remaining 2 pathogens (Chakraborty et al., 2000). Overall, the effect of increased eCO2 concentrations on a particular pathogen depends on the interaction between the effects of eCO2 on the pathogen and the effects of this change on the specific plant under specific environmental conditions. Similarly, FACE experiments have realistically evaluated the impact of eCO2 on agricultural fields or natural systems such as forests. The findings revealed that eCO2 may favor disease through denser, more humid plant canopies and increased pathogen reproduction, but may reduce disease risk by enhancing resistance (Chakraborty, 2005). Moreover, some studies have also predicted that elevated O3 concentrations would also increase the likelihood of plant diseases (Chakraborty et al., 2008).

7.6 EFFECT OF CLIMATE CHANGE ON BIOCONTROL AGENTS The introduction of coevolved natural enemies from regions of origin to regions of invasion (classical biological control agents [BCAs]) has been one of the key methods of suppressing exotic species (McFadyen, 1998; Moran et al., 2005; Messing and Wright, 2006). A survey of existing literature predicted that the current rate of GHGs may influence alien species invasions, causing destruction to natural ecosystems, agriculture, and human society (Chown et al., 2012; Sandel and Dangremond, 2012). Thus, it is imperative to address the impact of climate change on the performance and diversity of biocontrol agents essential for current and future biodiversity conservation. Under climate change, the effectiveness of some BCAs may change. Stireman et al. (2005) predicted that the frequency and intensity of pest outbreaks will increase as climatic conditions become

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more variable, leading to disruptions in the stability of existing biological control systems. Earlier studies have pointed out that the direct impacts of climate change lead to ineffective biocontrol processes, resulting in ecosystem imbalances. The potential host shifts and/or host range expansion of insects (Louda et al., 2003; Messing and Wright, 2006) is a major cause which would probably affect native plants in a negative manner. For instance, a Eurasian weevil, Rhinocyllus conicus, deliberately introduced to control invasive Carduus spp. was found to heavily attack native thistles in North America (Louda et al., 1997). However, purposefully introducing invasive species may latterly lead to direct or indirect cascading effects on natural food webs and ecosystems (Henneman and Memmott, 2001; Carvalheiro et al., 2008) and may even affect human health (Pearson and Callaway, 2006). Moreover, like with insects, warming is expected to change the life history or growth strategy of native plants, from annual to perennial, potentially leading to more exposure to biocontrol insects. For instance, Alternanthera sessilis reproduces entirely via seeds in areas .26°N in China; however, it is perennial at lower latitudes with both seed-based and vegetative reproduction. The warming experiment showed that plant A. sessilis nodes bearing dormant buds could surpass winter and develop new shoots and plants in the coming spring season, indicating that global warming will change its growth strategy as well as reproduction mode. These responses may connect the insect plant geographical gap and potentially increase exposure to biocontrol insects. Therefore, studies on the risks to nontarget species from BCAs under climate change are critical for the future management of invasive species and the conservation of native species (Simberloff, 2012). Furthermore, such an “invasive plant-introduced insect native plant” system provides a valuable model to predict how climate change and novel biotic exchange from species range shifts and/or expansions will impact insect host use and plant interactions.

7.7 CLIMATE CHANGE AND XENOBIOTICS From Section 7.6, it is now clear that changing climate conditions and climate change consequences, like drought, salinity, and emergence of new pests, have enforced growers to go for frequent use of chemical pesticides for greater crop yield at any cost (González-Alcaraz and van Gestel, 2016). The indiscriminant use of pesticides and other anthropogenic

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activities leads to the release of xenobiotic compounds (pesticides, fuels, solvents, alkanes, polycyclic aromatic hydrocarbons (PAHs), and nitrogen and phosphorus compounds) into the environment including soil. Reports suggest that soil can be considered as a natural and preferred sink for contamination. According to published reports, the overuse of xenobiotics leads to nontarget effects, such as animal death, environmental problems, and ecological imbalances (Li and Jennings, 2018; Sanchez-Bayo, 2014). In the context of soil biological activities, pesticides once entering into soil systems may influence the structure and functions of microbial communities. The adverse impacts of pesticides on soil microbial diversity and activities have been described by many researchers (Niewiadomska, 2004; Ingram et al., 2005; Littlefield-Wyer et al., 2008). For instance, chlorothalonil, chlorimuron-ethyl, and metham could reduce the abundance of genes involved in N cycling (Li et al., 2017a; Tan et al., 2013; Yang et al., 2014; Zhang et al., 2016). Similarly, chloropicrin may lead to a decline in soil bacterial community and decreases in ammonia-oxidizing archaea (AOA). Wu et al. (2014) found the same impact on soil microbial community by analyzing phospholipid fatty acid composition. Zhang et al. (2017) while investigating the impact of fungicide iprodione and nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) found that repeated application of iprodione decreased soil enzyme activities, bacterial biomass, and community diversity. DMPP application increased soil bacterial biomass and relative to iprodione applications alone, extra DMPP application alleviated the toxic effects of iprodione applications on soil bacterial biomass and community diversity. Moreover, the bacterial community structure was changed by repeated iprodione application, alone or together with DMPP. Some more negative impacts of pesticides have been listed in Table 7.1. Another major cause of environmental deterioration is PAHs (Acenaphthene, Anthracene, Fluorene, Phenanthrene, etc.). Some environmental problems originate from natural sources such as open burning, natural losses or seepage of petroleum or coal deposits, and volcanic activities. Typically, these are produced from pyrogenic, petrogenic (crude oil maturation processes), or biological (by certain plants and bacteria or formed during the degradation of vegetative matter) sources. In soil, atmospheric deposition is a major source of PAHs (Hussein et al., 2016). Exposure to PAHs from industrial areas of the world is suspected to be hazardous to health and linked to genotoxic, reproductive, and mutagenic effects in humans. The majority of PAHs are hydrophobic in nature and resistant to

Table 7.1 Effects of heavy metals on microbial community structure and functions Sites

Xenobiotic compound (pesticides)

Parameters evaluated

Results

References

China

Tetraconazole

Microbial biomass carbon (MBC), basal respiration, substrate-induced respiration, structure diversity and functional community profiling

Zhang et al. (2014a)

China

Fomesafen

CLPP, PLFA, nif gene abundance by q-PCR

China

Fluopyram

Basal respiration, substrate-induced respiration, MBC, microbial community function and structure

MBC, basal respiration, and substrate-induced respiration were suppressed. The ratios of gram negative to gram positive (GN to GP) bacteria decreased, and the fungi to bacteria ratio increased after a temporal decrease on the seventh day. PCA of PLFAs revealed that tetraconazole application significantly altered the microbial community structure on day 7. Different functional community profiles were observed, depending on the tetraconazole application rates. The addition of fomesafen can alter the microbial community structure and functional diversity of the soil, and these parameters did not recover even after a 90day incubation period. Fluopyram treatment decreased the MBC C but accelerated basal respiration, substrate-induced respiration, and ecophysiological indices (qCO2); PLFA demonstrated that supplementation of fluopyram decreased the total amount of PLFAs, bacterial biomass (both GP bacteria and GN), fungal biomass, and the ratios of GN/GP and fungi/bacteria at all incubation times. PCA suggested that the addition of fluopyram shifted the soil microbial community structure and function. Overall, fluopyram had a harmful effect on overall soil microbial activity and changed the soil microbial community structure and function.

Wu et al. (2014)

Zhang et al. (2014b)

(Continued)

Table 7.1 (Continued) Sites

Xenobiotic compound (pesticides)

Parameters evaluated

Results

References

China

Chlorothalonil (CTN)

nif H, chiA, aprA, AOA, and AOB gene abundances related to N cycling

Zhang et al. (2016)

Cuttack, India

Imidacloprid

Growth pattern and activities of microbes in tropical rice soil ecosystem, MBC, soil enzymes

Beijing, China

Chloropicrin (CP)

Bacterial abundance and community structure

Repeated CTN applications led to CTN residue accumulation in soil at concentrations of 5.59 and 78.79 mg kg21, respectively, by the end of incubation causing negative effects on the chiA and aprA gene abundances. Imidacloprid application disturbed the bacteria, actinomycetes, fungi, and phosphate solubilizing bacteria populations. Total soil MBC was reduced on imidacloprid application. Except dehydrogenase and alkaline phosphatase activities, all other soil enzymes namely, β-glycosidase, fluorescein diacetate hydrolase, acid phosphatase, and urease responded negatively to imidacloprid application. The extent of the negative effects of imidacloprid depends on dose and exposure time. CP had a significant impact on the abundance of the bacterial microbiome. There was a shift in the predominant populations. Staphylococcus, Actinomadura, Acinetobacter, and Streptomyces were significantly reduced in number or disappeared; and Bacteroides, Lachnoclostridium, Pseudoalteromonas, Colwellia, Idiomarina, and Cobetia became the new predominant populations.

Mahapatra et al. (2017)

Li et al. (2017b)

Vegetable farmland of Shandong Province, China

Iprodione and 3,4-dimethylpyrazole phosphate (DMPP)

Soil enzymes; 16S rRNA gene abundance; soil bacterial community structure

Experimental site of Faculty of Agriculture of the University of Concepción, Chile Experimental site of Chinese Academy of Agricultural Science,; China

Chlorpyrifos

Soil enzymes

Clomazone

Microbial communities structure (16S rDNA, 18S rDNA), N-cycling

Laboratory study at north Italy and agricultural field of Piacenza, Italy

Chlorpyrifos (CHL), Isoproturon (IPU), Tebuconazole (TBZ)

Abundance of 11 microbial taxa and 8 functional microbial groups; enzymatic activities involved in biogeochemical cycles

Regular intervals of iprodione application decreased the activities of all enzymes tested, and DMPP application inhibited soil urease activity; repeated iprodione applications decreased the bacterial 16S rRNA gene abundance and community structure Both application rates (4.8 and 24 kg a.i. ha21) caused a strong inhibition of carboxylesterase (62% 78% of controls), acid phosphatase (56% 60%) and glucosidase (43% 58%) activities. Higher doses of clomazone exerted negative impacts on bacterial community structure. Fungal abundance was increased after 60 days of application in loam and clay. N2fixing bacteria were significantly inhibited after day 60, and the AOB were stimulated during the first 30 days in T100-treated soils. In lab conditions, CHL and TBZ significantly reduced the relative abundance of ammonia oxidizing bacteria (AOB) and archaea (AOA). In field conditions, all the pesticides negatively affected the relative abundance of AOA.

Zhang et al. (2017)

Hernandeza et al. (2017)

Du et al. (2018)

Karas et al. (2018)

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biodegradation and are, thus, persist as environmental contaminants for extremely long periods of time. A pertinent literature survey and ongoing research has considered bioremediation as the most ecofriendly cleanup technology, utilizing either single potent microbes or a group of microbes with novel enzyme coding genes (Sawulski et al., 2014). Therefore, investigating the responses of indigenous microbes and their ecological, biochemical, and molecular strategies for PAHs degradation becomes inevitable. Several published reports have indicated that the presence of PAHs significantly affect the community structure and abundances of dioxygenase degrading genes. For instance, Sawulski et al. (2014) noticed that phenanthrene (3-ring PAH) and fluoranthene (4-ring PAH) were rapidly degraded and responded well to bioremediation using bacterial communities as compared to benzo(a)pyrene-affected bacterial communities. These differences in degradation rates and responses could be due to variations in relative molecular weight which reduced the competence of microbial populations to degrade high molecular weight PAHs, such as benzo(a)pyrene. Similar patterns of degradation rates and microbial community responses were corroborated by several researchers (Muckian et al., 2009; Zhang et al., 2011; Thavamani et al., 2012). However, a few articles have revealed that the introduction of PAHs is not the only factor that influences microbial community structure, but sometimes it may be determined by other substantial factors like pH rather than xenobiotics alone (Wu et al., 2017). In a nutshell, available reports suggest that the impact of xenobiotics on the biological attributes of soil, that is, structure and functions of microbes, in most of cases, were found to be dependent on xenobiotics physicochemical characteristics, their interaction with soil matrices, and most importantly their persistence in soil environments (Table 7.2).

7.8 FUTURE PERSPECTIVE Climate change is a consequence of anthropogenic activity, leading to drought and salinity which limit crop yields. Crop improvement through conventional plant breeding or biotechnological tools like genome editing or transgenic technology are key steps in obtaining desired characteristics associated with future needs. In addition, soil amendment using biochar (a carbon rich product formed by pyrolysis) can be an effective tool for mitigating climate change impacts. Published reports revealed that the addition of biochar not only improves the physiological properties of soil, like

Table 7.2 Impacts of PAH on microbial community structure and functions Sites Xenobiotic Parameters studied compounds

New South Wales, Australia

PAHs and heavy metals

Soil enzymes; bacterial 16S rRNA-DGGE

Experimental site; Ireland

Phenanthrene, fluoranthene, and benzo(a) pyrene

Soil enzymes, archaeal, bacterial, and fungal community profiles using T-RFLP, ARISA dioxygenase gene abundance

Contaminated land of Upper Silesia, Poland

PAHs and heavy metals

Bacterial community structure using CLPPs, soil enzymes

Results

References

Inhibition of soil enzymes in PAHs and heavy metal contaminated soil; reduced microbial diversity in contaminated soil and presence of few distinctive species by exerting selective pressure. Bacterial community responded rapidly to phenanthrene and fluoranthene compared to benzo(a)pyrene; phenanthrene and fluoranthene were degraded relatively rapidly, bringing about a change in the bacterial community structure in same time period; benzo(a)pyrene was degraded much more slowly and to a lower level, reflected in a more slowly changing bacterial community structure. High soil enzyme activities indicated bacterial adaptation to long-term heavy metal and PAHs contaminated soil; CLPP showed reduced bacterial community in contaminated soil revealing that soil enzymes alone did not reflect the effect of contaminants on bacterial diversity.

Thavamani et al. (2012)

Sawulski et al. (2014)

Markowicz et al. (2016)

(Continued)

Table 7.2 (Continued) Sites Xenobiotic compounds

Parameters studied

Results

References

Las Cabezas de San Juan, Spain

Naphthalene

Bacterial 16S rRNA profiling

Martirani-Von Abercron et al. (2016)

Agricultural plots in southwest Nanjing, Jiangsu Province, China

PAHs

Power plant site,; China

PAH (Acenaphthene)

16S rRNA and PAHRHDα gene abundances using q-PCR; bacterial community using Illumina MiSeq PAH analysis using HPLC, 16S rRNA profiling using PCR DGGE

PAH has the ability to alter the bacterial community; observed significant reduction in Bacteroids and Actinobactera. Soil pH was the primary determinant of bacterial community in these arable soils compared to PAH contamination.

Spatial variability in bacterial community was due to changes in total organic matter rather than PAH

Ma et al. (2017)

Wu et al. (2017)

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171

bulk density, water holding capacity, nutrient retention, soil pH, and cation exchange capacity, but also acts as a carbon source for bulk or rhizosphere soil that could be beneficial for plant growth too. New data vividly reported the role of biochar in provoking plant defense system responses against phytopathogens in several agronomic crops and fruit plants. Thus, biochar application in soil is not only a potential strategy for climate change mitigation but also an effective means of coping with biotic and abiotic stresses. Microorganisms possess an inherent potential to adapt themselves under changing environmental conditions. These microbes can be successfully used as an effective agent for the bioremediation of several xenobiotic compounds present in soil with the use of modern high throughput molecular tools and techniques through genetic manipulation. Future research related to the biodegradation of xenobiotic compounds should be focused on both the basic and applied areas. Bioremediation is a promising strategy for the detoxification of environmental contaminants. There is an urgent need to understand the genetics and biochemistry of microbes for their successful application in lab as well as field conditions. Attempts are required to bridge the gap between success at laboratory and field scales. Environmental factors, such as the physiological conditions of microbes, temperature, pH, nutrient level, moisture, concentration of targeted xenobiotic compounds, etc., should be considered while proceeding toward successful bioremediation of contaminated sites. Focus should be given to the selection of compatible microbes with high potential catabolic activities, plasmid, and catabolic enzymes for bioaugmentation at the field scale. The potential of efficient microbes could be exploited in bioreactor systems at large scale so that the effective mitigation measures of pollutants could be possible. More concerted efforts are needed to detect specific genes/enzymes responsible for degrading specific xenobiotic compounds using genomics and proteomics-based studies. Additionally, studies on the interaction between different microorganisms, such as bacteria, fungi, actinomycetes, etc., are likely to be useful in the bioremediation of xenobiotics as synergistic interactions could be responsible for enhanced remediation. It is a well-known fact that native microbial communities are the key component for maintaining plant health. Therefore, it is crucial to promote beneficial communities. New “omics” technologies, such as metagenomic, metaproteomics, and metatranscriptomic analyses, will certainly reveal the hidden knowledge of microbial dynamics in soil and plants or

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other environments, and vis a vis furnish the establishment of plant pathogen-suppressive microbial populations. As the effects of climate change on microbial communities are better understood through new experiments and new high throughput sequencing approaches, this additional form of environmental interaction can be included in models of climate and disease risk evaluation. Indeed, climate change is a reversible phenomenon yet GHGs will continuously accelerate global warming which may affect beneficial and harmful pathogens. Therefore, researchers should surely make an effort to address the effects of climate change on pathogens. To date, the longterm dynamics of plant diseases, with reference to climate change models, have been poorly studied, thus, climate models are required in order to better understand the areas with high levels of uncertainty, especially with respect to regional-scale changes. Plant biotechnologists as well as plant pathologists should adopt an interdisciplinary approach for defining key processes and factors influencing vital plant physiological processes for controlling different plant diseases to improve food security in the context of climate change.

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McFadyen, R.E.C., 1998. Biological control of weeds. Annu. Rev. Entomol. 43, 369 393. Meena, H., Ahmed, M.A., Pravin, P., 2015. Amelioration of heat stress in wheat, Triticum aestivum by PGPR (Pseudomonas aeruginosa strain 2CpS1). Biosci. Biotechnol. Res. Commun. 8 (2), 171 174. Melillo, J.M., Steudler, P.A., Aber, J.D., 2002. Soil warming and carbon-cycle feedbacks to the climate system. Science 298, 2173 2176. Messing, R.H., Wright, M.G., 2006. Biological control of invasive species, solution or pollution? Front. Ecol. Environ. 4, 132 140. Moran, V.C., Hoffmann, J.H., Zimmermann, H.G., 2005. Biological control of invasive alien plants in South Africa, necessity, circumspection, and success. Front. Ecol. Environ. 3, 71 77. Mosier, A.C., Li, Z., Thomas, B.C., Hettich, R.L., Pan, C., Banfield, J.F., 2015. Elevated temperature alters proteomic responses of individual organisms within a biofilm community. ISME J. 9, 180 194. Muckian, L.M., Grant, R.J., Clipson, N.J.W., Doyle, E.M., 2009. Bacterial community dynamics during bioremediation of phenanthrene- and fluoranthene-amended soil. Int. Biodeterior. Biodegrad. 63, 52 56. Newman, J.A., Anber, M.L., Dado, R.G., Gibson, D.J., Brookings, A., Parsons, A.J., 2003. Effects of elevated CO2, nitrogen and fungal endophyte-infection on tall fescue, growth, photosynthesis, chemical composition and digestibility. Glob. Change Biol. 9, 425 437. Niewiadomska, A., 2004. Effect of carbenzadim, imazetapir, and thiramon nitrogenous activity, the number of microorganuisms in the soil and yield of red clover (Trifolium protense L.). Pollut. J. Environ. Stud. 13, 403 410. Okubo, T., Tokida, T., Ikeda, S., Bao, Z., Tago, K., Hayatsu, M., et al., 2014. Effects of elevated carbon dioxide, elevated temperature, and rice growth stage on the community structure of rice root associated bacteria. Microbes Environ. 29, 184 190. Okubo, T., Liu, D., Tsurumaru, H., Ikeda, S., Asakawa, S., Tokido, T., et al., 2015. Elevated atmospheric CO2 levels affect community structure of rice root-associated bacteria. Front. Microbiol. 6, 136. Or, D., Smets, B.F., Wraith, J.M., Dechesne, A., Friedman, S.P., 2007. Physical constraints affecting bacterial habitats and activity in unsaturated porous media—a review. Adv. Water Res. 30, 1505 1527. Parham, J.A., Deng, S.P., 2000. Detection, quantification and characterization of betaglucosaminidase activity in soil. Soil Biol. Biochem. 32, 1183 1190. Pearson, D.E., Callaway, R.M., 2006. Biological control agents elevate hantavirus by subsidizing deer mouse populations. Ecol. Lett. 9, 443 450. Rangel-Castro, J.I., Killham, K., Ostle, N., Nicol, G.W., Anderson, I.C., Scrimgeour, C. M., et al., 2005. Stable isotope probing analysis of the influence of liming on root exudate utilization by soil microorganisms. Environ. Microbiol. 7 (6), 828 838. Rasche, F., Hodl, V., Poll, C., Kandeler, E., Gerzabek, M.H., van Elsas, J.D., et al., 2006a. Rhizosphere bacteria affected by transgenic potatoes with antibacterial activities compared with the effects of soil, wild-type potatoes, vegetation stage and pathogen exposure. FEMS Microbiol. Ecol. 56, 219 235. Rasche, F., Velvis, H., Zachow, C., Berg, G., van Elsas, J.D., Sessitsch, A., 2006b. Impact of transgenic potatoes expressing antibacterial agents on bacterial endophytes is comparable to effects of soil, wild-type potatoes, vegetation stage and pathogen infection. J. Appl. Ecol. 43, 555 566. Rodriguez, R.J., Henson, J., Van Volkenburgh, E., Hoy, M., Wright, L., Beckwith, F., et al., 2008. Stress tolerance in plants via habitat-adapted symbiosis. ISME J. 2, 404 416.

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Zhang, M., Xu, Z., Ying, T., Christie, P., Wang, J., Ren, W., 2016. Non-target effects of repeated chlorothalonil application on soil nitrogen cycling, the key functional gene study. Sci. Total Environ. 543, 636 643. Zhang, M., Wang, W., Zhang, Y., Teng, Y., Xu, Z., 2017. Effects of fungicide iprodione and nitrification inhibitor 3,4-dimethylpyrazole phosphate on soil enzyme and bacterial properties. Sci. Total Environ. 599 600, 254 263.

FURTHER READING Abdel-Shafy, H.I., Mansour, M.S.M., 2016. A review on polycyclic aromatic hydrocarbons, Source, environmental impact, effect on human health and remediation. Egypt. J. Petrol. 25, 107 123. Fu, D., Teng, Y., Shen, Y., Sun, M., Tu, C., Luo, Y., et al., 2012. Dissipation of polycyclic aromatic hydrocarbons and microbial activity in a field soil planted with perennial ryegrass. Front. Environ. Sci. Eng 6, 330 335. Ma, J., Zhang, W., Chen, Y., Zhang, S., Feng, Q., Hou, H., et al., 2016. Spatial variability of PAHs and microbial community structure in surrounding surficial soil of coalfired power plants in Xuzhou, China. Int. J. Environ. Res. Public Health 13, 878. Roger, A., Colard, A., Angelard, C., Sanders, I.R., 2013. Relatedness among arbuscular mycorrhizal fungi drives plant growth and intraspecific fungal coexistence. ISME J. 7, 2137 2146.

CHAPTER 8

Medicinal Plants Under Climate Change: Impacts on Pharmaceutical Properties of Plants Akanksha Gupta1, Prem Pratap Singh1, Pardeep Singh2, Kalpna Singh3, Anand Vikram Singh1, Sandeep Kumar Singh1 and Ajay Kumar1 1

Department of Botany, Center of Advanced Study in Botany, Institute of Science, Banaras Hindu University, Varanasi, India 2 Department of Environmental Science, PGDAV College, University of Delhi, New Delhi, India 3 Department of Obstetrics Gynaecology, Institute of Medical Science, Banaras Hindu University, Varanasi, India

Contents 8.1 Introduction 8.2 Importance of Medicinal Plants 8.2.1 Why Secondary Metabolites of Plants Are Important 8.3 Impacts of Climate Change on Secondary Metabolites of Medicinal Plants 8.3.1 Impact of CO2 on Secondary Metabolites of Plants 8.3.2 Impact of Drought on Secondary Metabolites of Plants 8.3.3 Impact of Cold on Secondary Metabolites of Plants 8.3.4 Impact of Ozone on Secondary Metabolites of Plants 8.4 Concluding Remarks References Further Reading

181 182 183 184 189 197 198 200 202 202 209

8.1 INTRODUCTION Currently and over the past two decades, climate change has been one of the most severe global concerns for researchers and governments. According to the CO2, CH4, and N2O, etc., are the most common and severe greenhouse gases causing global warming and ultimately changes in

Climate Change and Agricultural Ecosystems DOI: https://doi.org/10.1016/B978-0-12-816483-9.00008-6

Copyright © 2019 Elsevier Inc. All rights reserved.

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climatic conditions. Changing climatic conditions adversely affect normal human life, agriculture, forestry, biodiversity as well as ecosystem function (Lepetz et al., 2009). Changing climatic conditions have a wide range of adverse effects on different sectors, including the health of humans, water, air, soil, microbial communities, and plants and their pharmaceutical components as well as on secondary metabolites and food security. The rising global population, rapid industrialization, and utilization of huge amounts of chemical fertilizers/pesticides in agricultural fields are some important factors responsible for climate change. Changing climatic conditions include rising temperatures, cold, drought, and changes in rainfall patterns. All these factors affect the normal functioning of humans, plants, and microbial communities, etc. In the present chapter the impacts of changing climatic conditions on the pharmaceutical components or secondary metabolites of plants are discussed.

8.2 IMPORTANCE OF MEDICINAL PLANTS Since human civilization started developing, the value of different plant groups has been recognized for their nutritional, healing, curative as well as sustenance providing properties for daily life needs. Among these, the group of medicinal plants is very important because of its secondary metabolites and pharmaceutical properties, which are broadly used in various pharmaceutical, medical, cosmetic, and nutritional industries (Hassan et al., 2012). In Indian culture, there are many cues related to the uses of different medicinal plants as medicine against various diseases which are mentioned in the “Vedas” (Petrovska, 2012). According to a report by the World Health Organization (WHO) (Gurjar et al., 2016; Wannes and Marzouk, 2016), approximately 45,000 different plants in more than 21,000 species of plants are being used for medicinal purposes that can be verified by the Ayurveda (Taur and Patil, 2011). According to the WHO, almost 80% of the world’s population and 65% of the Indian population are using natural and traditional methods of healing and curing with the use of medicinal plants (Bannerman, 1980; Prashantkumar and Vidyasagar, 2013). In Indian society, there are special persons called vaidhya who have knowledge of medicinal plants and who know how to use these plants and their products for healing. These vaidhya use local herbal plants as raw material for making drugs for the treatment of diseases (Chopra, 1986).

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Currently in India, the sees herbal medicines as an alternative way of treating inveterate diseases. Various kinds of secondary metabolites are produced by these herbal plants that have shown many characteristics, including the improvement of the immune system, which help in the treatment of various diseases (Anand et al., 2017). Plant extracts have been shown to have more beneficial effects than negative impacts in terms of toxicity or side effects (Van Huyssteen, 2007). A plethora of chemicals are extracted from herbal plants including substances like terpenoids, phenols, steroids, flavonoids, tannins, and aromatic compounds. These are commonly known as secondary metabolites and are used for immunity in plants against pathogens and herbivores; around 12,000 secondary metabolites have been isolated and many more are in the process of identification. The cost effectiveness of herbal medicines makes them an attractive choice for people living in developing countries but now a days developed countries also use these herbal products. During consumption,the frequency of different parts of plants is more common than their extracted oil. Currently, a limited number of medicinal plants have been discovered; just over half a million plants so there is a hopeful future in studying medicinal plants.

8.2.1 Why Secondary Metabolites of Plants Are Important Since ancient times medicinal plants have been used to cure diseases (Mongalo et al., 2016). Herbal plant products and medicines are thought to be safe for use and are affordable as well and that is why around 80% of the world’s population and 65% of the Indian population are directly dependent on traditional medicine for meeting their primary healthcare needs (Prashantkumar and Vidyasagar, 2013). Secondary metabolites of the plant presently use in various aspect including developments of drugs, due to enormous chemical diversity, less side effect and economic values. Plant metabolites represent a highly efficient raw commodity for drug development (Verpoorte and Memelink, 2002). The vast chemical variety of secondary metabolites and their inherent capacities provide opportunities for the discovery of new drugs. Isolated plant metabolites, like phenols, terpenes, and alkaloids, have been used through many modifications which take place in the main basic skeleton of products and drugs. In plant systems, the isoprenoid pathway, polyketide pathway, and shikimate pathway are the main pathways for the synthesis of secondary metabolites (Verpoorte and Memelink, 2002).

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Now a days, many researchers studied the interaction between plant secondary metabolites and virulent factors of many human diseases for the long term benefits of secondary metabolites (Davies and Espley, 2013). The diversity of plant secondary metabolites is vast and this chemical variety is successfully manipulated to the advantage of humans (Balandrin et al., 1985; Edreva et al., 2008; Veberic et al., 2010; Singh and Sharma, 2015). Many studies have shown that secondary metabolites reduce the chances of many serious diseases or syndromes even diabetes, tuberculosis, ulcers, asthma, cancer, Alzheimer’s disease, and cardiovascular diseases, etc. (Basu and Imrhan, 2007; Holst and Williamson, 2008; Crozier et al., 2009; Fang et al., 2011; Ramakrishna and Ravishankar, 2011; Miller and Snyder, 2012) (Table 8.1). In the current scenario, plant derived pharmaceutical components show fewer side effects in comparison to allopathic drugs, thereby having reduced incidences of multidrug resistance pathogens. According to research, around 60 plant extracts and 110 purified compounds were obtained from 112 medicinal plants within 10 years (2005 15) and they show efficiency in the treatment of multidrug resistant pathogenic diseases.

8.3 IMPACTS OF CLIMATE CHANGE ON SECONDARY METABOLITES OF MEDICINAL PLANTS Changing climate conditions confer changes in normal environmental conditions, like drought, salinity, flood, and extremely low as well as high temperature variations (Wani et al., 2008, 2016; Gosal et al., 2009; Wani and Kumar Sah, 2014). Abiotic factors are important for the growth and development of plants and each plant species needs special environmental conditions for growth. Currently, extreme changes in the environment are frequent and predictable, like an increase in normal temperature (Bhatla and Tripathi, 2014). Increases in temperature of up to 5°C have been seen of late and these fluctuations can affect different plant species and their yields (Cleland et al., 2012). Plant secondary metabolites and plant growth are affected by spikes in temperature due to alterations in the metabolic pathways that regulate signaling, physiology, and defense mechanisms (Fig. 8.1). Not only secondary metabolites but also primary metabolites, that is, amino acids, sugars, and intermediate products of the Krebs cycle, are also affected by climatic change. Generally, it is found that an increase in secondary metabolite production provides protection

Table 8.1 Secondary metabolites synthesized from different plants and their uses in disease management Sr. no. Phytochemical Family Plants Disease management

1

Glycosides

Scrophulariaceae Apocynaceae

Convallariaceae Rosaceae

2

Flavonoids

Acanthaceae

Amaranthaceae

Anacardiaceae

Digitalis purpura Digitalis lanata Nerium oleander

Convallaria majalis Prunus persica Prunus serotine Prunus armeniaca Adhatoda vasica Nees. Ruellia tuberosa Linn. Andrographis paniculate Nees. Achyranthes aspera Linn. Amaranthus spinosus Linn. Mangifera indica Linn.

References

Heart failure and cardiac arrhythmia Asthma, epilepsy, cancer, painful menstrual periods, leprosy, malaria, ringworm, indigestion, and venereal disease Used for congestive heart failure Hypothyroidism, cardiac pain

Oerther (2011) and Bernhoft et al. (2010)

Asthma, chronic bronchitis, and other respiratory conditions. Anti-hypertensive, kidney disorder, bronchitis, syphilis.

Mahajan and Chaudhari (2012)

Vetter (2000)

Treatment of cough, bronchitis and rheumatism, malarial fever, dysentery, asthma, hypertension and diabetes Useful for patients suffering from scabies, diabetes, skin irritation, diphtheria, rheumatism, diarrhea and hemorrhoids (Continued)

Table 8.1 (Continued) Sr. no. Phytochemical

Family

Plants

Disease management

Annonaceae

Annona squamosa Linn

Zingiberaceae

Curcuma longa Linn.

Solanaceae

Solanum nigrum Linn

Meliaceae

Azadirachta indica A Juss.

Treatment of epilepsy, dysentery, cardiac problems, worm infestation, constipation, hemorrhage, antibacterial infection, dysuria, fever Treatment of digestive disorders, liver diseases, cancer, atherosclerosis, osteoarthritis, menstrual problems, bacterial infection, wounds, eye disorder Good for cooling hot inflammation, ringworm, ulcers, testicular swelling, gout and ear pain Leprosy, eye disorders, bloody nose, intestinal worms, stomach upset, loss of appetite, skin ulcers, diseases of the heart and blood vessels (cardiovascular disease), fever, diabetes, gum disease (gingivitis), and liver problems

References

3

4

Tannin

Anacardiaceae

Anacardium occidentale L. Schinopsis brasiliensis

Fabaceae

Anadenanthera colubrine Tephrosia purpurea (L.) Caesalpinia ferrea

Myrtaceae

Psidium guajava L.

Taxus buccata Artemisia annua

Terpenoids

Solanaceae

Thapsia laciniata Arabidopsis thaliana Nicotiana plumbaginifolia Nicotiana tabacum

Araceae

Spirodela polyrrhiza

Apiaceae Brassicaceae Solanaceae

Anthelmintic, aphrodisiac, ascites, dysentery, fever, inappetence, leukoderma, piles, tumors, and obstinate ulcers Anthelmintic, aphrodisiac, ascites, dysentery, fever, inappetence, leukoderma, piles, tumors, and obstinate ulcers Diabetes, rheumatism, and cancer and also are said to litigate diarrhea, inflammation, and pain Improves the skin, treats cough and cold, constipation, dysentery, and scurvy Antineoplastic and antimalarial agents Scabies Cure sores in the mouth Piles Joint pain, dental caries, gingivitis, strychnos poisoning and is also used as nerve stimulant Treatment of colds, measles, edema and difficulty in urination

de Sousa Araújo et al. (2008)

Croteau et al. (2006) and Pollier et al. (2011) Drew et al. (2013) Tholl and Lee (2011) ´ et al. (2001) Jasinski Sasabe et al. (2002) and Schenke et al. (2003) Van Den Brûle et al. (2002) (Continued)

Table 8.1 (Continued) Sr. no. Phytochemical

5

6

Alkaloids

Phenolic Compounds

Family

Plants

Disease management

References

Apocynaceae

Catharanthus roseus

Yu and De Luca (2013)

Solanaceae Ranunculaceae

N. tabacum Coptis japonica

Brassicaceae

A. thaliana

Diabetes, blood pressure, asthma, constipation, and cancer and menstrual problems Joint pain, dental caries, gingivitis Intestinal catarrh, dysentery, enteritis, high fevers, inflamed mouth and tongue, conjunctivitis Cure mouth sores

Solanaceae

Solanum lycopersicum

Vitaceae

Vitis vinifera

Poaceae

Zea mays

Diabetes, diseases of the heart and blood vessels (cardiovascular disease), cataracts, and asthma. Improves heart health, prevents cancer, fights thrombosis, prevents intestinal disorders, fights excess uric acid and gout, reduces the risk for Alzheimer’s disease Diarrhea, dysentery, urinary tract disorders, prostatitis, lithiasis, angina, hypertension and tumor

Shitan et al. (2009) Yazaki et al. (2009)

Alejandro et al. (2012) Mathews et al. (2003)

Gomez et al. (2009, 2011)

Goodman et al. (2004)

Medicinal Plants Under Climate Change: Impacts on Pharmaceutical Properties of Plants

189

Figure 8.1 Overview of medicinal plants under climate change conditions.

against biotic stress to plants, this can be described as a connecting link between biotic and abiotic stress (Arbona et al., 2013). Some genotypic changes or modifications may help in the mitigation of or adaptation to changing environmental conditions (Springate and Kover, 2014). In plants, cells can rejuvenate chemical imbalances which are life-sustaining for their survival by activation of early metabolic responses. With respect to environmental changes, plants that can change their morphology and physiology can survive in extreme conditions (Millar et al., 2007) (Table 8.2).

8.3.1 Impact of CO2 on Secondary Metabolites of Plants Since the era of industrialization, elevations in the concentration of CO2 has become a severe problem to normal human life and plant physiology. Due to anthropogenic activities the level of CO2 emission has increased drastically in the past 40 years since 1750 (Field et al., 2014). Medicinal plants have the ability to adapt according to the changing environment. The elasticity in their metabolic pathway is due to secondary metabolites, but to some extent this elasticity may affect metabolite production which is the basis of their medicinal activity (Mishra, 2016). Digitalis lanata has pharmaceutical properties and is used for the treatment of heart failure (Rahimtoola, 2004), but when treated with high levels of CO2, a 3.5-fold digoxin (a cardenolide glycoside) increase was observed, while the concentrations of other glycosides, like digoxin-mono-digitoxoside, digitoxin, and digitoxigenin, were significantly decreased (Stuhlfauth et al., 1987; Stuhlfauth and Fock, 1990). The concentration of CO2 is not the only important factor in the concentration of secondary metabolites in plants but the duration of exposure is also a significant factor. For example, the bulbs of Hymenocallis littoralis are known for their antiviral and antineoplastic activity; Idso et al. (2000) reported increases in the concentration of

Table 8.2 Impact of climate change factors on the secondary metabolites of plants S. no.

Plant

Family

Used in

Changes

Impact of changes

References

1

D. lanata

Plantaginaceae

Heart failures

Elevated CO2

3.5-fold increase in digoxin, a cardenolide glycoside

2

Hymenocallis littoralis

Amaryllidaceae

Antineoplastic and antiviral properties

Elevated CO2

3

Ginkgo biloba

Ginkgoaceae

Alzheimer’s disease, vascular or mixed Dementia

Elevated CO2

4

Papaver setigerum

Papaveraceae

Elevated CO2

5

Hypericum perforatum

Hypericaceae

Treat variety of ailments, including eye and lung inflammation Relieve depression and Anxiety

Increase in three types of alkaloids (pancratistatin, 7deoxynarciclasine and 7deoxy-trans dihydronarciclasin) Altered terpenoid content, 15% increase in quercetin aglycon and 10% decrease in kaempferol aglycon, 15% in isorhamnetin and bilobalide to some extent Enhancement of four alkaloids viz. morphine, codeine, papaverine and noscapine

Stuhlfauth et al. (1987) and Stuhlfauth and Fock (1990) Idso et al. (2000)

6

Brassica oleracea

Brassicaceae

Treatment of glaucoma and Pneumonia

Elevated CO2 Elevated CO2

Enhancement of hypericin, pseudohypericin and hyperforin belonging to the class of phenolics Increase in methylsulfinylalkyl glucosinolates, glucoraphanin and glucoiberin derived from Glucosinolates

Weinmann et al. (2010) and Huang et al. (2010a,b)

Ziska et al. (2008)

Zobayed and Saxena (2004)

Schonhof et al. (2007)

7

C. roseus

Apocynaceae

Anticancerous, antiviral and diuretic

Elevated CO2

8

Zingiber officinale

Zingiberaceae

9

Quercus ilicifolia

Fagaceae

Motion sickness, Nausea and Vomiting Gynecological problems

10

Pseudotsuga menziesii

Pinaceae

11

Melissa officinalis

Lamiaceae

12

Capsicum baccatum

Solanaceae

13

Salvia officinalis

Lamiaceae

Ezuruike and Prieto (2014) and Saravanan and Karthi (2014)

Elevated CO2

Increase in almost all of the Plant Secondary Metabolites viz. alkaloids, flavonoids, phenolic and tannins Increase in Flavonoid and Phenolic content

Elevated CO2

Increase in tannins and phenolic content

Kidney, bladder problems, Venereal disease Dementia, Anxiety and Central nervous system (CNS) related disorders Asthma and digestive problem

Elevated CO2

Digestive problems, stomach pain (gastritis), diarrhea, bloating, and heartburn

Ozone stress

Level of terpenes specifically monoterpenes decreased significantly Increased in total anthocyanins to a substantial extent along with phenolics and tannins 50% decrease in capsaicin and dihydrocapsaicin, seeds showed significant reduction in capsaicin but no change in dihydrocapsaicin Increase in phenolic content, notably in Gallic acid, Catechinic acid, Caffeic acid and Rosmarinic acid

Stiling and Cornelissen (2007) and Ibrahim and Jaafar (2012) Snow et al. (2003)

Elevated ozone

Elevated ozone

Ghasemzadeh et al. (2010a,b)

Pellegrini et al. (2011) and Shakeri et al. (2016) Bortolin et al. (2016)

Pellegrini et al. (2015)

(Continued)

Table 8.2 (Continued) S. no.

Plant

Family

Used in

Changes

Impact of changes

References

14

Betula pendula

Betulaceae

treatment of high blood pressure, high cholesterol, obesity, gout, kidney stones, nephritis

Elevated ozone

Lavola et al. (1994)

15

Pinus taeda

Pinaceae

Treatment of kidney and bladder complaints

Elevated ozone

16

Pueraria thomsonii

Fabaceae

Elevated ozone

17

S. officinalis

Lamiaceae

Petroselinum crispum Hypericum brasiliense

Apiaceae

Labisia pumila

Primulaceae

Increase in the concentration of monoterpenes Enhancement of monoterpenes Concentration and the total amount of the phenolic compounds, are drastically enhanced Concentration and also overall production of total phenolics and flavonoids increased

Nowak et al. (2010)

18

Treatment of fever, acute dysentery, diarrhea, diabetes, and cardiovascular diseases Digestive problems, stomach pain Plague and malaria

Increase in hyperoside a flavonoid, decreased papyriferic acid a triterpenoid and dehydrosalidroside hyperoside, betuloside belonging to phenolics Increasing in condensed tannins without any rise in total concentration of phenols Increase in puerarin production

19

20

Hypericaceae

Antiseptic, diuretic, digestive, expectorant, antidepressive cardiovascular protection and osteoporosis

Drought stress Drought stress Drought stress

Drought stress

Jordan et al. (1991)

Sun et al. (2012)

Petropoulos et al. (2008) Nacif de Abreu and Mazzafera (2005)

Jaafar et al. (2012)

21

Salvia sclarea

Lamiaceae

Stomach and digestive problems, kidney complaints and for insomnia

Cold stress

22

Teucrium polium

Lamiaceae

Gastrointestinal disorders, inflammations, diabetes and rheumatism

Cold stress

23

Thymus sibthorpii

Lamiaceae

Ulcer and digestive problem

Cold stress

24

Withania somnifera

Solanaceae

Cold stress

25

A. thaliana

Brassicaceae

Bronchitis, asthma, ulcers, hypertension and intestinal worms Cure sores in the mouth

26

Mentha piperita

Lamiaceae

Treatment of indigestion, pain in joints, diarrhea, cough, dysmenorrhea and fever

Salt stress

Cold stress

Reduction in individual leaf area but an increased length and number of spikes and a longer inflorescence with higher content of essential oils under cold conditions Leaves are smaller and thicker, have more stomata and glandular hairs, higher photosynthetic rate and stomatal conductance was measured Leaves become smaller and thicker, higher photosynthetic rate and stomatal conductance was measured Increase in with anolide (steroidal lactones, key bioactive compound) Formation of sterol glycosides as well as a higher enzymatic activity Lower number of leaves, leaf area and leaf biomass

Kaur et al. (2015a,b)

Lianopoulou et al. (2014a,b)

Lianopoulou et al. (2014a,b)

Mir et al. (2015) and Kumar et al. (2012) Mishra et al. (2013)

Tabatabaie and Nazari (2007)

(Continued)

Table 8.2 (Continued) S. no.

Plant

Family

Used in

Changes

Impact of changes

References

27 28

C. roseus Aloysia citrodora

Apocynaceae Verbenaceae

Salt stress Salt stress

Decrease in protein content Leaf number become less, reduce in leaf area and biomass

Osman et al. (2007) Tabatabaie and Nazari (2007)

29

Satureja hortensis

Lamiaceae

Salt stress

Increase in carbohydrates

Hendawy and Khalid (2005) and Najafi and Khavari-Nejad (2010)

30

Matricaria chamomilla

Asteraceae

Used to treat cancer Digestive disorders such as flatulence, indigestion and acidity Antirheumatic, Antiseptic, Aromatic, Carminative, Digestive, Expectorant, Stings, Stomachic Skin disease and Rheumatism

Salt stress

Kováˇcik et al. (2009)

31

Panax quinquefolius

Araliaceae

Boost energy, lower blood sugar and cholesterol levels, reduce stress, promote relaxation, treat diabetes, and manage sexual dysfunction in men

Heat stress

Increase of Phenolic compounds like protocatechuic, chlorogenic and caffeic acids significantly Enhancement of ginsenoside and reduction in photosynthesis

Jochum et al. (2007)

32

Cucumis sativus

Cucurbitaceae

33

Triticum aestivum

Poaceae

34

Perilla frutescens

Lamiaceae

35

Beta vulgaris

Amaranthaceae

36

Ocimum basilicum

Lamiaceae

37

Aquilaria sinensis

Thymelaeaceae

Help to stay hydrated, support heart health, protect brain from neurological diseases, reduce risk of cancer immune-modulator, antioxidant, astringent, laxative, diuretic, antibacterial and used in the acidity, colitis, kidney malfunction, swelling wounds Used for cure of asthma and cough Improve blood flow, lower blood pressure Lowering blood pressure, an antispasmodic as well as cleansing the blood Used against cancer, abdominal pains, asthma, colic, and diarrhea

Heat stress

Decrease in 5-aminolevulinate dehydratase, the first enzyme of pyrrole biosynthetic pathway

Mohanty et al. (2006)

Heat stress

Decrease in 5-aminolevulinate dehydratase, the first enzyme of pyrrole biosynthetic pathway

Mohanty et al. (2006)

Heat stress

Reduction in productivity of anthocyanins Release of the anthocyanin pigment from hairy root

Zhong and Yoshida (1993) Thimmaraju et al. (2003)

Heat stress

Plant tolerance improves by Salicylic acid

Clarke et al. (2004)

Heat stress

Upregulation in the expression of Jasmonic Acid

Xu et al. (2016)

Heat stress

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three alkaloids: 7-deoxy-trans dihydronarciclasin, pancratistatin, and 7-deoxynarciclasine in the first year of experiment, however, they found significant reductions in concentration with the passing of time. Weinmann et al. (2010) reported the use Ginkgo biloba for the treatment of Alzheimer’s disease and vascular or mixed dementia. When G. biloba is exposed to high levels of CO2 and O3 together, the terpenoid content gets altered along with an elevation in quercetin aglycon of 15%, and a decline in the concentrations of kaempferol aglycon, isorhamnetin, and bilobalide of 10%, 15%, and to some extent, respectively (Huang et al., 2010a,b). Ziska et al. (2008) carried out an experiment on Papaver setigerum with an elevation in the level of CO2 from 300 to 600 lmol mol21. This elevated level showed comparable concentrations and a significant increase in the concentrations of four alkaloids: codeine, noscapine, morphine, and papaverine. In addition, this study also augurs that high plant carbon:nutrient ratios may produce excess amounts of nonstructural carbohydrates due to an increase in the level of CO2. According to Heyworth et al. (1998) these nonstructural carbohydrates may be approachable for internalization in C-based secondary metabolites. Relevant to this forecast, Zobayed and Saxena (2004) also carried a test on Hypericum perforatum, which showed that pseudohypericin, hypericin, and hyperforin belong to the class of phenolics that are increased due to a rise in the level of CO2. Mahn and Reyes (2012) reported that the secondary metabolites of broccoli (Brassica oleracea var. italica Plenck) have possible effects on cardiovascular diseases and cancer. A rise in the level of CO2 resulted in the enhancement of glucoiberin and methylsulfinylalkyl glucosinolates glucoraphanin which are derived from Glucosinolates (Schonhof et al., 2007). Catharanthus roseus is known worldwide for its antiviral, anticancer, and diuretic properties (Ezuruike and Prieto, 2014). Secondary metabolites from this plant, that is, phenols, alkaloids, tannins, and flavonoids, increase with the increasing CO2 levels (Saravanan and Karthi, 2014). Ghasemzadeh et al. (2010a,b) reported an increase in the concentrations of phenolic and flavonoid in Zingiber officinale with increases in CO2 level. Stiling and Cornelissen (2007) observed elevation in the concentrations of phenols and tannins in Quercus ilicifolia related to increases in CO2 level. The same kind of study was done on Elaeis guineensis (oil palm). In this experiment the level of CO2 was elevated from 400 to 1200 lmol mol21. An increase in phenols and flavonoids was observed, that was assigned as the cause of an increase in the primary metabolite phenylalanine, which is

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a precursor of various secondary metabolites (Ibrahim and Jaafar, 2012). Similar work was also done by Ibrahim et al. (2014) on the plant Labisia pumila, in which the results showed elevated levels of phenols and flavonoids after exposure to elevated levels of CO2. However, Snow et al. (2003) reported a decline in the level of terpenes especially monoterpenes of Pseudotsuga menziesii after exposure to elevated CO2 levels. Panax ginseng is a traditional Chinese medicinal plant used in China, Korea, and Japan for the treatment of cancer and other stress related disorders (Wang et al., 2007; Chang et al., 2003). When its root suspension was treated with increased level of CO2 an increase in flavonoids and phenols contents were observed (Ali et al., 2005).

8.3.2 Impact of Drought on Secondary Metabolites of Plants Many experiments have been carried out to study the effects of drought on the content of secondary metabolites in plants, which show that plants cumulate more secondary metabolites, like terpenoids, phenolics, and N-containing compounds (alkaloids, glucosinolates, and cyanogenic glucosides) in water stressed conditions. In water stressed conditions plant growth is reduced in contrast to secondary metabolite content. The concentration of secondary metabolites is increased due to a reduction in biomass production, however, there is no increase in the rate of synthesis of metabolites. So, the overall concentration of metabolites increases based on fresh or dry weight (Kleinwächter and Selmar, 2014). Nowak et al. (2010) reported an extremely high increase in the monoterpene concentration of Salvia officinalis due to water stress conditions. The increase in monoterpene concentration was much higher than the reduction in biomass as compared to controls, which were grown in water sufficient conditions. An experiment was conducted on Petroselinum crispum commonly known as parsley, which showed that increases in the concentration of monoterpenes were much greater than the reductions in the biomass of the leaves (Petropoulos et al., 2008). In Origanum vulgare (Greek oregano), the content of essential oil per plant remained constant, while the concentration of metabolites increased in drought conditions (Ninou et al., 2017). It seems that the rate of monoterpene synthesis remained constant. But, the same amount of monoterpenes was acquired using much less biomass. Paulsen and Selmar (2016) used this explanation to explain the increase in monoterpene concentration in thyme plants. There were no any changes in the total content of monoterpenes.

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Based on dry weight, the rate of synthesis was found to be different in drought stressed plants compared to the control plants grown in water sufficient conditions. In the first part of the experiment, based on dry weight, the synthesis rate was massively high in stressed plants compared to the control set, while this statement become just the opposite when the drought become extended (Paulsen and Selmar, 2016). Phenolic compounds also follow the same trend. According to Nacif de Abreu and Mazzafera (2005), the total amount of phenols as well as its concentration showed massive increase in Hypericum brasiliense under drought conditions. It seems that plants grown in stressed conditions were generally smaller than the control plants because of the massive increase in phenolic content. According to Nogués et al. (1998), anthocyanin was increased by 25% in stressed plants of Pisum sativum compared to plants grown in normal conditions. In L. pumila, the concentration as well as the overall production of total phenols and flavonoids per plant were found to be elevated in plants grown in water deficient conditions (Jaafar et al., 2012). Many studies have been carried out to date which show a trend in the increase of secondary metabolite concentration when exposed to water stress conditions. But variations were also found during some experiments regarding the increase in total content of natural compounds per plant.

8.3.3 Impact of Cold on Secondary Metabolites of Plants The effect of low temperature on plant systems is one of the most severe stress factors; it affects developmental processes, productivity, diversity, and distribution in a broad spectrum of ways (Chinnusamy et al., 2007; Rahman, 2013). Low temperatures directly affect the physiology of plants (Ruelland et al., 2009). In fact to tolerance cold stress, plants change their survival strategies by redistributing resources and lowering growth (Eremina et al., 2016). Low temperatures also affect the fluidity of cell membranes by changing its concentration (Sevillano et al., 2009; Upchurch, 2008). Low temperatures also generate free oxygen radicles, which increases the stress inside cells and then plant cells have to get rid of them by activating antioxidants (Ruelland et al., 2009; Sevillano et al., 2009). Besides of all this, plant cells have to increase their levels of amino acids, soluble solids, and cryoprotective proteins in order to maintain integrity. For survival, plant cells have to activate many enzymatic and metabolic pathways (Ruelland et al., 2009; Eremina et al., 2016).

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Many medicinal plants, such as Satureja thymbra, Thymus sibthorpii, Phlomis fruticosa, Teucrium polium, and Cistus incanus, show seasonal dimorphism through the development of many defense systems (Lianopoulou and Bosabalidis, 2014). The dimorphism in these plants is adapted by many defensive mechanisms, which are activated by different hormones. Origanum dictamnus induces many structural (shape, size, and distribution of leaves) and anatomical changes (thicker cuticle, formation of a wax layer on the epidermis of leaves, dense layer of nonglandular trichomes) to fight against low temperature. A large intracellular space for accumulating air at high temperatures was found in mesophyll cells (Lianopoulou and Bosabalidis, 2014). The composition of essential oils seems to change with changing temperatures; for instance, a 60% p-Cymene content was found during winter, while a 42% carvacrol content was found during summer time (Lianopoulou and Bosabalidis, 2014). Under low temperature, Salvia sclarea reduces its leaf area individually, while increases in length and spike number were noticed. Increases in inflorescence and essential oil were also noticed (Kaur et al., 2015a,b). In T. polium and T. sibthorpii some changes, on exposed to cold, such as increased number of glandular hairs and stomata, smaller leaves, and thick cuticles, were reported. Crystals of calcium oxalate and dark phenolic compounds are also found in their epidermal and mesophyll cells (Lianopoulou et al., 2014a,b). According to Fahn and Cutler (1992), plants which grow in cold atmospheres possess some special features, like dense glandular and nonglandular hairs, thick, curled and small leaves, increased number of stomata, high content of phenol in epidermis and mesophyllic cells, numerous sclerenchymatic fibers, and compact mesophyll with developed palisade parenchyma. Abiotic stresses not only modify plants structurally and anatomically, but also lead to elevation in their antioxidant quantities (Lianopoulou and Bosabalidis, 2014; Khan et al., 2015; Saema et al., 2016), which also increases their medicinal and nutritional importance (Khan et al., 2015; Nourimand et al., 2012). According to Khan et al. (2015), how low temperature stress affects plants and plant physiology is of great importance. Knowledge of the role of phytohormones in the response of plants to stress is also important for future perspective. Foeniculum vulgare, also known as fennel, under low temperature stress, showed altered content of chlorophyll, biomass production, and β-carotene, when seedlings of this plant were exposed to 2°C for 2, 3, and 4 hours, however, this stress significantly elevated antioxidant activity

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(Nourimand et al., 2012). Xiao et al. (2011) showed membrane damage among 11 varieties of fennel species, and explained that adaptation to different environmental stresses is dependent on plant species. When Withania somnifera (ashwagandha), also known as Indian ginseng, is exposed to low temperatures it shows increased accumulation of withanolides in its leaves (Khan et al., 2015; Kumar et al., 2012). Saema et al. (2016) showed that elevation in the metabolite withanolide due to exposure to low temperature is also found in transgenic plants of this species. The cooccurrence of elevation in the enzymatic activities of antioxidants SOD, GR, CAT, and APX was found in ashwagandha (Khan et al., 2015) and in the leaves of T. sibthorpii due to low temperature stress (Lianopoulou et al., 2014b). Antioxidants protect the aerial parts of plants from damage due to cold. In contrast to leaves, the concentration of withanolides was found to be lower in root tissues due to low temperature stress (Khan et al., 2015), which advises that low temperatures affect different plant organs and tissues differently, thus, leading to the development of special mechanisms in each kind of tissue. The dispersion of metabolites in different organs is also affected by cold stress and is important for the internal balance and survival of plants. According to Kumar et al. (2012), seasonal incidence of low temperatures leads to an elevation in withanolides. The glycosylation of sterols is catalyzed by the enzyme sterol glycosyl transferases (SGT); this enzyme plays an important role in plant adaption to cold stress. For the glycosylation of sterols and withanolides, the WsSGTL1 gene is responsible, which is specifically responsible for the position of 3b-hydroxy.

8.3.4 Impact of Ozone on Secondary Metabolites of Plants Planet Earth has a natural umbrella which surrounds it, protecting it from harmful U-VB radiations. Ozone is found in the stratosphere, but its occurrence in the lower layer of the atmosphere (troposphere) acts as a pollutant. Ozone pollution in the lower atmosphere affects crop yields globally. It decreases the crop yield of maize by 2.2% 5.5%, wheat by 3.9% 15%, and soybean by 8.5% 14%. Harmful effects are seen on medicinal plants as well. The effects of O3 on medicinal plants were never highlighted, so it has become necessary to study its effects in future. Melissa officinalis is used for the treatment of central nervous system related problems, dementia, and anxiety, but when exposed to high concentration of O3, the concentrations of phenols, tannins, and anthocyanins

Medicinal Plants Under Climate Change: Impacts on Pharmaceutical Properties of Plants

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were little elevated (Pellegrini et al., 2011; Shakeri et al., 2016). The effect of prolonged O3 exposure on Capsicum baccatum was studied by some Brazilian scientists who found that on exposure to ozone plants had 50% less dihydrocapsaicin and capsaicin in the pericarp of their fruit. The seeds of fruit from exposed plants showed less capsaicin, but there was no change in the concentration of dihydrocapsaicin compared to the control fruit. Besides these, the concentration of phenols gets elevated by 17% and carotenoids by 52.8% (Bortolin et al., 2016). A similar kind of experiment was also done on S. officinalis to analyze their impact on antioxidant and ecophysiological traits. Plants were exposed to O3 (120 6 13 ppb) for 90 consecutive days; and an elevation in the concentration of phenols, a two-fold increase in gallic acid, an eight-fold increase in caffeic acid, and a 122% elevation in rosmarinic acid (60th day of treatment) was found (Pellegrini et al., 2015). Lavola et al. (1994) reported an increase in flavonoids specifically hyperoside, as well as a decline in the level of dehydrosalidroside hyperoside, betuloside (a kind of phenol), and papyriferic acid (triterpenoid) after ozone treatment in Bitola pendula. Jordan et al. (1991) reported an elevation in tannin concentration in Pinus taeda L. when exposed to O3, however, there was no sign of any elevation in phenol content. This suggests that there was a change in the distribution pattern of phenols, which leads to an increase in tannin in the foliar region. Although the effect of O3 is well known on edible crops, its effects on medicinal plants still need to be widely studied and a strategy needs to be developed to cope with the problem. In the recent past, the effect of ozone pollution on “in vitro” condition have been studied.In a study a suspension culture of Pueraria thomsonii was exposed to O3, and it showed no elevation in the production of puerarin after 20 hours of exposure time (Sun et al., 2012). An almost 2.6 fold increase in puerarin was reported after 35 hours, which was the highest amount. This experiment highlighted the idea of increasing the production of puerarin from P. thomsonii with the help of ozone exposure. An elevation in abscisic acid was also noticed in experimental sets when exposed to ozone, and this elevation was much higher as compared to controls. After 15 hours of O3 exposure, the highest amount of abscisic acid was recorded. This concentration was 11 fold that of the control plants (Sun et al., 2012). Xu et al. (2011) performed a similar type of experiment on the suspension culture of H. perforatum, where 6 day-old cultures were treated

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with between 30 and 180 nL O3 L21 for 0 6 hours. The 5 day-old culture set, which was treated with 90 nL O3 L21, showed the highest amount of hypericin production when harvested on the 21st day of the experiment. The highest amount of hypericin production was also obtained when the 15 day-old culture suspension was exposed to O3 and harvested on the 21st day. According to Xu et al. (2011) the time of O3 exposure should be standardized to 3 hours when other parameters are constant.

8.4 CONCLUDING REMARKS Plant have high diversity among species and produce many secondary metabolites; many of which are biologically active and have been highly useful to the human race over the centuries, being used for daily life needs as well as for pharmaceutical purposes. Over the past two decades, medicinal plants have been used for the discovery of new drugs in the allopathic treatment of many severe diseases even cancer, HIV, tuberculosis, ulcers, etc. Changing climatic conditions or abiotic stress factors influence the normal behavior of plants as well as their physiology, which ultimately affects the important secondary metabolites of pharmaceuticals. Changes in climatic conditions influence the availability of water, normal pH, and salinity, of plants, which will have direct effects on the yields of metabolites. The secondary metabolites of plants provide a unique source of flavors and pharmaceutical properties, etc., and also protect plants from stress conditions. In various studies, it had been found that some of environmental factors, like temperature, elevated CO2, and temperature exposure, enhances the secondary metabolites of plants, whereas low temperature, drought conditions, and high salinity adversely affect the metabolites, growth, and productivity of plants. Furthermore, there is a need to understand on a molecular level the stress response of plants toward the improvement of the growth and productivity of plants.

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FURTHER READING Ullmann, P., Ury, A., Rimmele, P., Benveniste, P., Bouvier-Navé, P., 1993. UDPglucose sterol ß-d-glucosyltransferase, a plasma membrane-bound enzyme of plants: Enzymatic properties and lipid dependence. Biochimie 75 (8), 713 723.

CHAPTER 9

Air Pollution: Role in Climate Change and Its Impact on Crop Plants Bhanu Pandey1 and Krishna Kumar Choudhary2 1

Natural Resources and Environmental Management, CSIR-Central Institute of Mining & Fuel Research, Dhanbad, India 2 Department of Plant Sciences, School of Basic and Applied Sciences, Central University of Punjab, Bathinda, India

Contents 9.1 Introduction 9.2 Factors Driving Climate Change 9.2.1 Natural Drivers 9.2.2 Anthropogenic Drivers 9.3 Impacts of Climate Change on Crop Plants 9.3.1 Average Temperature Increase 9.3.2 Alteration of Precipitation Pattern 9.4 Conclusion Acknowledgments References Further Reading

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9.1 INTRODUCTION Global warming relating to climate change has been one of the most debated topics of the 21st century. A report by the Intergovernmental Panel on Climate Change (IPCC, 2014) has strongly highlighted the dominance of anthropogenic influences in inducing extensive changes in climatic systems and the concomitant rise in average temperature along with changed precipitation patterns globally. As per the assessment details provided by the IPCC (2014), each successive decade since 1850 has been experiencing higher temperatures in comparison to the preceding decades. Global agricultural production is facing challenges due to such fluctuations in climatic variability (Oseni and Masarirambi, 2011), which are increasing Climate Change and Agricultural Ecosystems DOI: https://doi.org/10.1016/B978-0-12-816483-9.00009-8

Copyright © 2019 Elsevier Inc. All rights reserved.

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at an alarming rate. According to estimations, Asseng et al. (2015) reported a 6% decline in global wheat production for each 1°C of further temperature increase which predicts a direct risk to global food security. Therefore, world food production is currently facing the risk of unprecedented shifts in the spatial and temporal patterns of climatic variables, such as the rising of sea levels, prevalence of much higher temperature than previously, higher incidences of extreme weather events, enhancement of atmospheric CO2 concentrations, and unpredictable rainfall patterns (Chen et al., 2015). Scientific evidences have displayed that many anthropogenic activities have significantly contributed to the surface temperature increment since 1951. Global concentrations of atmospheric CO2 have enhanced since the preindustrial era from 280 ppm (IPCC, 2014) to the present level of 410 ppm (www.esrl.noaa.gov/gmd/ccgg/trends/) and is further predicted to increase to 448 ppm by 2050 (IPCC, 2014). Such a substantial rise of global atmospheric CO2 concentrations and alterations in rainfall distribution patterns are expected to have far reaching consequences on crop growth and production, thereby threatening global food security in future. Of late, this subject has received stupendous attention with an enormous amount of research being conducted in order to better understand crop responses to changes in climatic variability. With growing concern for agriculture and alterations in the Earth’s system dynamics, the direct and indirect impacts of climate change have been critically analyzed here along with the need for plausible mitigation strategies that can be adapted by farmers through different agronomic practices to cope with the threat of food insecurity.

9.2 FACTORS DRIVING CLIMATE CHANGE The Earth’s climatic system is of a dynamic nature and is inherently subject to continuous processes of change since preindustrial times, mainly associated with anthropogenic activities and to a lesser extent to sources of natural origin. Humanity’s understanding of the global climate system has been challenged by the complex interconnected nature of its different components, such as atmosphere, biosphere, cryosphere, and hydrosphere. However, the major physical drivers of climate change are initially associated with atmospheric composition, the cloud effect, and radiative forcing. The drivers of climate change can be categorized into those of natural or those of anthropogenic origin (Fig. 9.1).

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Figure 9.1 Various natural and anthropogenic factors influencing climate change.

9.2.1 Natural Drivers 9.2.1.1 Milankovitch Cycle or Changes in the Earth’s Orbit The emerging consensus regarding the rise of global mean temperature has been a topic of discussion lately and researchers mostly believe that anthropogenic emissions are a major cause driving such changes. However, such variation in the Earth’s mean temperature can also be associated with both long-term and short-term natural cycles occurring over considerable time periods. Such natural cycles are termed Milankovitch cycles, which are also believed to have contributed to a variation in the global mean temperature by 5°C during the glacial and interglacial periods (Rehman et al., 2015). Although, such cycles take centuries to cause noticeable variations in the global mean temperature. Solar irradiance is mainly governed by three important factors: eccentricity in the Earth’ orbit, the tilt and position of the Earth’s axis, and the wobbling of Earth’s axis of rotation, which are responsible for variations in the timing of perihelion. However, such changes are generally insignificant and do not contribute much to the global mean temperature in the long term (U.S. EPA, 2016). It is estimated that about 0.12 W m22 has been contributed to global atmospheric radiative forcing due to the increment in solar irradiance since 1750 (Forster et al., 2007; Rehman et al., 2015), while the approximately 2.64 W m22 increase in radiative forcing is due to anthropogenic activities (Rehman et al., 2015). 9.2.1.2 Volcanoes Volcanic activity represents a natural process contributing to the cause of global warming through the emission of different tracer constituents directly into the atmosphere leading to alterations in atmospheric chemical composition. Although volcanoes are found in specific regions across the

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globe, their effects can be spread widely across great distances through gases, dust, and ash that easily make their way into the atmosphere. Owing to atmospheric circulation patterns, eruptions at mid-latitude or higher latitudes generally cause a huge impact on the whole hemisphere, while volcanisms in the tropics can have a much greater impact on the climatic system in both hemispheres. Dust particles and volcanic ash reduce the amount of solar radiation reaching the surface of the Earth, thus, causing reductions in temperature in the troposphere and changing atmospheric circulation patterns (Myhre et al., 2013). However, the extent to which this occurs is still unclear. Although, the effects of large-scale volcanisms may last for a few days or weeks, climate patterns get disturbed for years due to the substantial outpouring of harmful ash and gaseous substances. Emissions of sulfuric gases from volcanic eruptions get converted into sulfate enriched aerosol particles consisting of about 75% H2SO4, which can linger in the stratosphere for 3 4 years after a volcanic eruption. Such major eruptions disrupt the Earth’s radiative balance as the materials that enter into the atmosphere have tremendous potential to absorb and scatter a major portion of Earth’s solar radiation, which is well-known as “radiative forcing” and can have long lasting effects on the global climate (Stenchikov et al., 1998). Volcanic activity also releases enormous amounts of various greenhouse gases (GHGs), such as CO2 and water vapor. However, such eruption does not cause much change in the global atmospheric concentration of these gases. Although, there have been times in history when intense volcanic activities have significantly enhanced the atmospheric concentration of CO2 and caused global warming.

9.2.2 Anthropogenic Drivers 9.2.2.1 Greenhouse Gas Emissions Demographic growth has been the primary reason behind the aggravation of atmospheric concentrations of GHGs. Global average economic growth has outpaced the process of GHG-intensity enhancement. Human induced GHG emissions have enhanced from 27 to 49 GtCO2eq y21 between 1970 and 2010 (IPCC, 2014). GHG emissions rose on an average of 1 GtCO2eq y 2 1 between 2000 and 2010 as compared to 0.4 GtCO2eq y 2 1 between 1970 and 2000, (IPCC, 2014). In the industrial age, atmospheric CO2 concentrations have displayed an exponential growth rate, with the major sources of such emissions being industrial processes and fossil fuel combustion, which are responsible

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for increases of about 78% (2000 10) in total GHG emissions (IPCC, 2014). Over the past 40 years, CO2 emission due to fossil fuel combustion has enhanced consistently and contributed to 69% of global GHG emissions in 2010. It was further increased by about 3% (2010 11) and by between 1% and 2% (2011 12). Other anthropogenic activities, such as agricultural practices, deforestation, and other alterations in land use patterns constitute the second-largest contributor to GHG emission in this industrial era. Since 1970, CO2, methane (CH4), nitrous oxide (N2O), and fluorinated gas emissions have increased by about 90%, 47%, 43%, and less than 3%, respectively. In 2010, CO2 remained the main human induced GHG, contributing about 76% of the total anthropogenic emissions (IPCC, 2014). Although the past four decades have experienced an enhancement of GHG emissions in every region across the globe, the increment patterns in different regions continue to be nonuniform. GHG emission has displayed an increment of 330% in the Middle East Asia and of 70% in Africa. GHG emissions from international transportation have also been growing rapidly. 9.2.2.2 Other Anthropogenic Factors Ozone Ozone is found in both the troposphere and stratosphere and its formation and destruction are driven by anthropogenic and natural activities. Abundances of ozone vary spatially and temporally owing to its unstable nature and mechanisms of production, destruction, and transport processes controlling its abundances, which contribute to the complexity of ozone radiative forcing calculations. Destruction of stratospheric ozone through photochemical reactions occurs in the presence of halogen species, such as chlorine and bromine. Stratospheric ozone depletion, which mainly contributes to a net negative radiative forcing, is most notable in the Polar regions. In the tropospheric region, ozone is known as surface or ground level ozone, which is generally produced photochemically due to the emission of methane, oxides of nitrogen (NOx), carbon monoxide (CO), and nonmethane volatile organic compounds (VOCs) both near and far downwind of these precursor sources, resulting in a contribution to worldwide radiative forcing due to ozone abundances (Dentener et al., 2005). Tropospheric ozone (O3) is the most important secondary air pollutant causing significant phytotoxicity in crops, thus, leading to global yield

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losses (Wilkinson et al., 2012). Presently, O3 concentrations already display deleterious impacts on crop yields (Wahid, 2006), which are predicted to rise further at a rate of 0.3 ppb per annum worldwide (Wilkinson et al., 2012). Asian countries, such as India and China, are the most affected by increasing O3 pollution owing to rapid economic growth, industrialization, and unfavorable meteorological conditions, which lead to high emissions of O3 precursors (Zhang and Oanh, 2002). Aerosol Increased aerosol particles in the atmosphere due to anthropogenic activities have influenced global radiative forcing in mainly three ways: (1) through radiation aerosol interactions, which is known as a direct effect, mainly involving the scattering and absorption of shortwave and longwave radiation; (2) through cloud aerosol interactions, known as an indirect effect, resulting from alterations in the size and number of cloud droplets due to changes in aerosol; (3) through albedo pattern alteration as a result of absorbing-aerosol deposition onto snow and ice (IPCC, 2014). However, not all aerosols produce a negative radiative forcing. Aerosols, such as black carbon, absorb solar radiation and are responsible for producing a positive radiative forcing, leading to the warming of the atmosphere. Land Use Changes Alterations in land use patterns due to anthropogenic activities in the industrial era have also altered the albedo properties of land surfaces, primarily through afforestation and deforestation. There are strong scientific evidences that such changes have caused an increment in global surface albedo, resulting in a net negative (cooling) radiative forcing of about 20.15 W m22 (IPCC, 2014). However, such changes in land use patterns have also lowered surface albedo causing a net positive radiative forcing due to afforestation and pasture abandonment. In addition to direct radiative forcing, such activities have led to indirect forcing effects on the global climatic system, viz., changes in carbon cycles and alterations in the emission of dust particles through effects on the hydrologic cycle. Areas with significant irrigation generally govern surface temperatures and precipitation patterns through changes in the partitioning of energy from sensible to latent heating. Irrigation induced direct radiative forcing can be both negative and positive, depending on the balance of surface cooling and increased cloudiness effects (Cook et al., 2015).

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Contrails Contrails are visible evidence of humans’ contribution to climate change. It is a special type of cloud/cirrus cloud that is produced due to the movement of a jet-engine aircraft in the mid to upper troposphere in the presence of high humidity. Contrails can be persistent in nature and can last for many hours, spreading and drifting with the local direction of the winds. During their movement, they lose their linear features and create additional cirrus cloudiness that interact with solar and thermal radiation to contribute to a global net positive radiative forcing and are, thus, harmful to the global climate system.

9.3 IMPACTS OF CLIMATE CHANGE ON CROP PLANTS The climate changes over the past few decades have been quite rapid in various agricultural regions throughout the world, and increases in the levels of atmospheric CO2 and O3 have also been ubiquitous. The accelerating increase in atmospheric CO2 as well as other GHGs has been well documented to occur since the industrial revolution. The inference is that the natural and life sustaining greenhouse effect is heightened, the Earth is warmed up, and the result is in the form of climate change. The virtual certainty that the phenomenon of climate change will continue in the future gives rise to numerous questions related to food security (Fig. 9.2). Climate change can alter the productivity of plants either by influencing variables that directly affect plant growth, that is, temperature and

Figure 9.2 Various consequences of climate change affecting crop productivity.

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precipitation, or indirectly by affecting equally crucial factors arising from altered agroecosystem conditions, such as soil properties, pests, weeds, and insects. Important consequences of climate change that affect crop productivity include: • average mean temperature increase, • alteration of precipitation patterns, • various air pollutants, • rising atmospheric CO2 concentrations, • change in sea level, • herbivory, • weed species, and • insects and pests.

9.3.1 Average Temperature Increase It is estimated that the average global temperature will increase by between 1.4°C and 5°C by the year 2100 (IPCC, 2014), while Rao et al. (2006) estimated an approximately 17% decrease in crop yields for every 1°C increase during the average growing season. Plants have learnt to adapt to fluctuations in temperature through the development of different stress tolerance strategies during the course of their evolution. Heat stress affects the growth and morphology of various plants negatively Table. 9.1. Though, plant responses toward heat stress are dependent on many factors, such as plant species, age, and growth stage of plant. The responses of plant species also depend on the extent and length of the stress. Elevated temperatures expedite different aspects of plant metabolism (Pastore et al., 2017) and can affect crop yields by causing rapid crop development (Badeck et al., 2004). Due to rapid crop development, the duration of various aspects of plant metabolism are shorter (Larcher, 2003). Changes in the timing and duration of different plant developmental processes result in lower crop plant yields (Allen et al., 2018). Temperature is a key factor in seed germination due to its affecting the rate of absorption of water and other substrates, which are crucial for the proper growth and development of seeds. Heat stress hinders seed germination and plant emergence and promotes poor seedling vigor, abnormal seedlings, reduced radicle and plumule growth, and causes an overall reduction in plant growth and development, modifications in photosynthesis, dry matter partitioning, and a decline in net assimilation rate (Kumar et al., 2011). High temperature for long durations, particularly

Table 9.1 Effects of various stress factors on wheat, rice, maize and soybean crops Stress

Crops

Treatment

Effects

References

Temperature

Wheat

30°C/25°C (day/night)

Rice

32°C (night temperature)

Reductions in leaf size, grain size, yield, number of grains per spike, shortened period for booting, heading, anthesis, and maturity Reductions in yield, grain size, width and weight, and increased spikelet sterility

Maize

35°C/27°C (day/night), 14 days 38°C/28°C (day/night), 14 days

Rahman et al. (2009) Mohammed and Tarpley (2010) Suwa et al. (2010) Djanaguiraman et al. (2011)

Soybean

Flooding

Wheat Rice

Maize

Water logged pots (for 28 days) Stagnant flooding (50 cm above soil surface) Water logged (2 cm above soil surface)

Soybean

Drought

Wheat

90%, 60%, and 30% of field capacity

Rice

Mild (aerobic soil near field capacity) to severe Moderate to severe (100 and 150 mm evaporation from class A pan) 30% of field capacity

Maize

Soybean

Reduced ear expansion and cob size by impairing hemicelluloses and cellulose synthesis due to a reduced photosynthate supply Decrease in photosynthetic rate and stomatal conductance, increased thicknesses of the palisade layers, membrane damage in chloroplast, thylakoid, mitochondria, and damage to cristae and matrix Retarded root growth, reduction in biomass, low tiller number, shorter leaves and accelerated senescence, and decreased leaf nitrogen content Reduction in number of tillers and panicles; significant decline in yield and harvest index; increased aerenchyma gas space, and declined root oxidase activity Reduced biomass, lowered shoot growth, increased adventitious roots, and aerenchyma formed in roots Various physical injuries, anaerobic stress conditions, poor vegetative growth, reduced photosynthetic activities, reduced nodulation, and significant yield losses Decreased biomass, leaf relative water content, chlorophyll, carotenoids, biomass, inorganic solutes (Ca, K, Mg), and increased organic solutes (soluble sugars and proline) Significant decline in yield along with spikelet fertility, reduced plant height and number of tillers Reduced test weight, harvest index, yield, and biomass

Decreased photosynthetic rates, leaf water potentials, flowers, pods, leaf sucrose, and starch content

Malik et al. (2002) Kuanar et al. (2017) Abiko et al. (2012) Tewari and Arora (2016) Loutfy et al. (2012) Lafitte et al. (2006) Khalili et al. (2013) Liu and Stützel (2004) (Continued)

Table 9.1 (Continued) Stress

Crops

Treatment

Effects

References

Ozone

Wheat

Ambient 1 10 ppb and ambient 1 20 ppb

Rice

Ambient 1 10 ppb and ambient 1 20 ppb

Sarkar and Agrawal (2010) Sarkar et al. (2015)

Maize

Ambient 1 15 ppb and ambient 1 30 ppb

Soybean

82.5 and 61.3 ppb

Wheat

0.06 ppm

Damage to vegetative parts (shoot and root height, leaf number, leaf area), reproductive parts (pollen viability and viable pollen floret), and reduced yield Induction of superoxide dismutase, catalase, peroxidase, ascorbate peroxidase, glutathione reductase, ascorbic acid, thiols, and phenolics. Reductions in RuBisCO, yield, and changes in the quality of grains Induction of ROS (superoxide radical and hydrogen peroxide), secondary metabolites (total phenol, flavonoids, and anthocyanin), antioxidative enzymes (superoxide dismutase, catalase, peroxidase, ascorbate peroxidase, and guaiacol peroxidase). Reduced yield Significant reduction in yield, leaf area index, chlorophyll content, increased antioxidative defense and phenolic content Significantly reduced plant height, leaf area, relative growth rate, biomass, and yield

Rice

0.001 0.008 ppm

Maize

45, 70, and 110 nL SO2 L21

Soybean Wheat

1.2, 97, and 490 ppb 700 ppm

Rice

Ambient 1 200 μmol mol21

Maize

550 ppm

Soybean

580 ppm

Sulfur dioxide

Carbon dioxide

Reduced plant height, number of tillers and leaves, leaf area, chlorophyll content, biomass, and yield Decrease in leaf soluble protein, aspartic acid, glycine, glutamine, and arginine concentration, while glutamic acid, asparagine, and alanine concentrations were increased Reduced biomass, stunted plant growth, and lower grain yields Increased plant height, leaf area, number of leaves and tillers, total biomass, harvest index, test weight, total soluble sugars, and starch content in seeds, while protein and total free amino acids were decreased Declined content of protein, minerals (iron, zinc), vitamins (B1, B2, B5, and B9), and increase in vitamin E Increased grain yield and harvest index, cob length, cob diameter, grain weight cob21, number of grains cob21, and test weight. Decreased N and P concentrations in grain but increased K content Increased seed yield, nodes, and number of branches, leaf area, chlorophyll, and leaf N content

Singh et al. (2014)

Betzelberger et al. (2010) Deepak and Agrawal (2001) Singh et al. (2009) Ranieri et al. (1990) Li et al. (2011) Mishra et al. (2013) Zhu et al. (2018) Abebe et al. (2016) Jin et al. (2017)

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during reproductive periods, not only reduces total biomass, but also the reproductive efforts of crops (Polowick and Sawhney, 1988) mostly during flowering and fertilization (Morison and Lawlor, 1999), which can cause sterility, reduced pollen germination and viability, fruit abortion, and can finally result in complete crop failure (Teixeira et al., 2013). Incidences of agricultural disease are often linked with an increase in temperature (Ziska and Bunce, 2007). Heat stress negatively affects various physiological processes in plants, such as reduced photosystem II (PSII) activity, photosynthetic pigments, enhanced reactive oxygen species (ROS) production, altered starch and sucrose synthesis, reduced ADP-glucose pyrophosphorylase, and invertase activities (Rodríguez et al., 2005). An exponential rise in air saturation vapor pressure because of increased air temperature leads to increased vapor pressure deficit between air and plant leaves which reduces wateruse efficiency and ultimately plants lose more water per unit carbon gained (Ahad and Reshi, 2015). Physiological changes in plants are due to biochemical alterations in response to environmental stress. The plasma membrane acts as a primary target to heat stress by increasing the fluidity of lipid membranes responsible for changes in calcium influx along with cytoskeletal rearrangement, finally leading to the upregulation of mitogen-activated protein kinase and calcium-dependent protein kinase. These proteins, in turn, mediate the activation of various tolerance responses, including the production of antioxidant enzymes in defense against ROS or the production of osmolytes (Hasanuzzaman et al., 2013). Heat acclimation triggers the accumulation of heat shock proteins (HSPs) (Hua, 2009; Bray, 2000), and the activation of phytohormones, such as Abscisic acid (ABA) and other protective molecules, such as proline, sugars, sugar alcohols, tertiary sulfonium compounds, and tertiary and quaternary ammonium (Hasanuzzaman et al., 2013).

9.3.2 Alteration of Precipitation Pattern Different studies have reported that GHG-induced atmospheric warming may lead to enhanced surface aridity and more drought events in the present century due to reductions in precipitation in tropical and subtropical regions (Dai et al., 2018), whereas the rising of extreme and sudden rainfall events are more likely to affect the rainfed low-lying areas of Asian countries, such as India, China, Bangladesh, and Nepal (Ismail et al., 2013). Different climate models have projected enormous increases in

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precipitation intensity with reductions in frequency (Dai et al., 2018), which could potentially result in more dry periods and flooding events. As per available reports, about 10% of global agricultural areas are affected by flooding events and crop yield losses range between 15% and 80%, depending on other factors, such as species, duration of stress exposure, and different soil types (Patel et al., 2014). 9.3.2.1 Flooding Stress Soil water logging generally induces hypoxia in the organs and plant parts that are submerged fully or partially, causing deleterious effects to the productivity of different crop plants, such as soybean (Tewari and Arora, 2016), rice (Kuanar et al., 2017), maize (Abiko et al., 2012), and wheat (Malik et al., 2002) (Table 9.1). Flooding stress caused a disruption in physiological mechanisms of the plants leading to yield losses and a reduction in productivity in different crop species. It is evident that flooding induced a reduction in stomatal conductance and assimilation rate and enhanced transpirational processes, conditional to the species-specific tolerance capacity to water logging stress (Promkhambut et al., 2010). Promkhambut et al. (2010) also reported that an increase in intercellular CO2 in flooded plants caused a detrimental impact on the assimilation process in sweet sorghum cultivars as a consequence of ineffective diffusion of internal CO2 from substomatal cavities to the specific site of carboxylation. Hypoxia is a common consequence of flooding stress which generally induces root and shoot injuries and disrupts the nutrient allocation mechanism of plants (Tewari and Arora, 2016). Several adaptive mechanisms developed under flooding stress are: 1. As an effective strategy to develop proper anchorage and transportation of water and nutrients to different aboveground plant parts, plants produce adventitious roots under flooding stress (Promkhambut et al., 2010; Zaidi et al., 2003). 2. To maintain proper oxygen balance under stressful conditions, the development of lysigenous aerenchyma in the rooting system is a common adaptive mechanism of plants which reduces the number of oxygen consuming cells (Sauter, 2013). Such aerenchymal spaces also help in rapid oxygen diffusion over long distances within plants (Kuanar et al., 2017) and are essential for survival under hypoxic or anoxic conditions. 3. Under flooding conditions, the development of a barrier to manage radial oxygen loss in the outermost cellular layer of the roots has been

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reported in maize species (Zea nicaraguensis) to prevent the diffusion of oxygen from the tip of the roots to the circumforaneous rhizospheric zone (Abiko et al., 2012). 4. Stomatal closure generally corresponds to an adaptive reaction of plants under flooding stress, inducing a reduction of root water permeability and a restriction in water loss through transpiration, thereby helping the plants survive under such conditions (Soldatini et al., 1990). Different literatures have reported an enhancement in the level of antioxidants (ascorbic acid and glutathione) and antioxidative enzymes, such as ascorbate peroxidase (APX), superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), and glutathione reductase (GR) to cope with the oxidative stress generated due to flooding stress (Hossain et al., 2009; Yan et al., 1996). It is evident that abiotic stress can cause a substantial increase in the total endogenous antioxidative enzyme concentration under flooding conditions, which further accelerates photooxidative injury due to an oxygen deficiency (Yordanova et al., 2004). Furthermore, Kuanar et al. (2017) has reported a declination in root oxidase activity at a faster rate under stagnant soil flooding conditions in rice plants, signifying a reduction in the oxygen releasing efficacy of roots, further generating greater oxidative damage due to hypoxic condition. 9.3.2.2 Drought Stress According to a report by the IPCC, the emission pattern of GHGs will increase global warming further and will likely enhance extreme climatic events in the future (IPCC, 2014). Drought is usually a recurring climatic event resulting from a natural reduction in the amount of precipitation over an extended period of time (Wilhite, 2000). The growing impacts of climate change on natural water resources has led to changes in the pattern of precipitation globally (Dai, 2013); and the intensification of agricultural activities is affecting the dynamics of soil moisture (Zhang et al., 2017), which has led to the enhanced frequency and harshness of droughts worldwide (Table 9.1). Stomatal closure and reduced carbon uptake are common strategies utilized by crop plants to overcome drought stress, however, they lead to a rise in canopy temperature by lowering the latent heat (Bernacchi et al., 2007) and they further increase heat-related impacts causing negative effects on crop production. Climate change models predict that increased emissions of GHGs will create significant disturbances to hydrological systems, influencing freshwater systems negatively (Strzepek and McCluskey, 2007) and making agriculture vulnerable.

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In the early developmental growth stage of plants, water deficiency is a limiting factor particularly affecting the phases of cell division, differentiation, and cell elongation and lead to a reduction in cell turgor pressure and other physiological events (Taiz and Zeiger, 2010). Harb et al. (2010), suggested acclamatory responses through cell wall expansion in Arabidopsis thaliana as an early avoidance strategy under water limiting conditions. Liu and Stützel (2004) reported a decrease in specific leaf area in drought treated plants determining a reduction in cell expansion and hence resulting in thinner leaves. Nonami (1998) suggested that cell elongation is primarily correlated with the water absorption capacity of the elongating cells under water deficient conditions. Crop yield and productivity are basically determined by the allocation of biomass to the reproductive sink. A common observation reported from drought studies is that under water deficient conditions there is an enhancement of root-shoot biomass ratio, with a greater reduction in the shoot biomass as compared to root biomass (Erice et al., 2010). Water deficient conditions cause the accumulation of solutes in the root tips resulting in potential differences between the surrounding soil and the root hairs, which in turn attracts water to these tip and consequently plants are able to maintain root turgor pressure and growth (Liu and Stützel, 2004). A reduction in leaf biomass was also observed in Jatropa curcas L. seedlings by 28% of the total produced leaf biomass under drought stress (Achten et al., 2010). The growth and development of several crops are affected under drought conditions, such as rice (Lafitte et al. 2006; Manickavelu et al., 2006), maize (Monneveux et al., 2006), soybean (Samarah et al., 2006), barley (Samarah, 2005), cow pea (Turk et al. 1980), Amaranthus spp. (Liu and Stützel, 2004), and wheat (Loutfy et al., 2012). Furthermore, the impacts of drought lead to an alter physiology of plants. Cell contraction is resulted due to reduction of cellular volume which in turn induce enhancement of cell viscosity (Farooq et al., 2009). Such an increase in cell viscosity due to high concentrations of solute accumulation may prove to be detrimental for normal plant functioning and photosynthetic machineries (Hoekstra et al., 2001). Stomatal limitation has been documented under drought stress in different crop species, such as maize (Cochard, 2002), wheat (Khan and Soja, 2003), soybean (Ohashi et al., 2006), kidney bean (Miyashita et al., 2005), and rice (Praba et al., 2009). Besides stomatal closure, a reduction of stomatal size was also

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reported under moderate drought conditions (Farooq et al., 2012). Klamkowski and Treder (2008) reported the CO2 deficiency and stomatal closure contributed to a reduction in photosynthesis in strawberry cultivars under moderate drought conditions. Miyashita et al. (2005) also observed reductions in the photosynthesis and transpiration rates of Phaseolus vulgaris L. due to stomatal limitation. Besides a reduction in stomatal conductance, nonstomatal limitations are also taken into account as a major factor in determining the detrimental impacts on carbon assimilation processes in plants under drought stress. Flexas and Medrano (2002) reported the downregulation of different metabolic activities due stomatal limitation under water stress conditions, which impaired ribulose-bisphosphate (RuBP) regeneration and adenosine triphosphate (ATP) synthesis ultimately inducing events of photoinhibition and the disruption of normal photochemistry. Drought induced increases in photorespiration in plants has been reported by Massacci et al. (2008), which could be an acclimation strategy to counter balance the over excitation in the PSII. Drought induced photorespiration, can also offset carbon fixation and assimilation processes leading to the generation of ROS in the photosynthetic tissues of plants (Farooq et al., 2012). Plants often encounter ROS toxicity owing to a reduction in CO2/O2 ratio in photosynthetic tissues and the enhancement of photorespiration under water deficit conditions. Uncontrolled generation of ROS may result in membrane leakiness and lipid peroxidation, ultimately leading to malondialdehyde (MDA) production as well as the impairment of functional macromolecules, such as DNA, proteins, lipids, nucleic acid, and chlorophyll pigments (Moussa and Abdel-Aziz, 2008). Drought induced free radical bursts inside the cellular and subcellular components of plants induce the production of enzymatic antioxidants, such as SOD, CAT, GR, APX, POD, dehydroascorbate reductase (DHAR); and nonenzymatic antioxidants, like ascorbic acid, flavanoids, anthocyanins, carotenoids, and α-tocopherol, thereby imparting resistance against abiotic stress at different growth stages of plants (Reddy et al., 2004). Accumulation of proline is one of the most important adaptive responses to drought stress in plants. Bandurska et al. (2017) reported an increase in proline concentration in the leaves and roots of barley. The accumulation of osmolytes, like amino acid, protein, and sugar is a common phenomenon directly correlated to the improvement of drought tolerance mechanisms owing to their capacity to cope with osmotic stress

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and their role in the maintenance of nutrient homeostasis (Iqbal et al., 2014). An increased accumulation have been observed for free proline content accompanied by other free amino acids and soluble protein under preflowering drought stage in the leaves of peanut cultivars (Zhang et al., 2017), contributing to osmotic regulation and improving drought tolerance mechanism in the cultivars. Similarly, the enhancement of glycine betaine and free proline in maize plants highlights the protective role of these nonenzymatic antioxidant molecules against oxidative injury under drought stress (Moussa and Abdel-Aziz, 2008). Drought has been a serious concern for agricultural crop yield losses, such as wheat (Zhao et al., 2017) maize, (Kamara et al., 2003), barley (Samarah, 2005), rice (Lafitte et al., 2006; Pantuwan et al., 2002), and chickpea (Mafakheri et al., 2010). Also the impact of drought on crop plants depends on the severity, which is directly correlated with the stringency and duration of the stress period. In barley, at post-anthesis, drought has been proven to be detrimental to grain yield irrespective of stress severity as reported by Samarah (2005). The study also documented a shortening of the duration of grain filling processes in barley under drought stress as compared to well-watered plants. Moreover, drought induced maturity acceleration associated with a faster rate of grain filling has been reported in common beans (P. vulgaris L.), which displayed a positive correlation to seed yield, determining the drought adaptation strategy in resistant cultivars (Rosales-Serna et al., 2004). Various Air Pollutants Ozone Pollution Ozone is a phytotoxic gas not emitted directly into the atmosphere; the formation of tropospheric O3 in the atmosphere includes several photochemical reactions between primary air pollutants. Therefore, it is a secondary pollutant formed in the atmosphere through solar radiation-driven chemical reactions between O3 precursor gases, for instance, CO, NOx, CH4, and non-methane VOCs (Von Schneidemesser et al., 2015). Ozone has toxic effects on vegetation (Ainsworth et al., 2012); it enters via the stomata and forms ROS. Although plants have the capacity to detoxify O3 and ROS increased damage have been observed when the detoxification capacity reduced and finally leading to O3 induced symptoms on the leaves (Burkey et al., 2007). The impact of O3 on plants, however, varies between crops, cultivars, agricultural practices, and environmental factors (Table 9.1). There are

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various approaches to study the effects of O3 on plants, such as exposureresponse studies using open-top chambers, free air concentration enrichment (FACE) studies, and the biomonitoring of sensitive plants. Symptoms of O3 toxicity usually occur between the veins on the adaxial portion of the older and middle-aged leaves, but may also involve both abaxial and adaxial leaf surfaces for some species (Cho et al., 2011). O3 induced visible foliar injuries, such as chlorotic stippling or interveinal yellowing, have been reported by Sarkar and Agrawal (2010) in the leaves of mature rice plants, under both ambient and elevated O3 concentrations in open-top chambers (OTCs), and it was observed that the magnitude of the injury depends on the cumulative effect of both duration and concentration of O3 exposure. Ahmad et al. (2013) observed the development of O3 induced visible foliar injuries on onion, potato, and cotton plants when mean monthly O3 concentrations exceeded 45 ppb in north-west Pakistan. Differential magnitudes of symptoms of ozone induced injury were also recorded for the different cultivars of clover (Chaudhary and Agrawal, 2013). Responses to increased O3 concentrations have been well pronounced on the growth and biomass of plants from several studies reported to date and such responses exhibited variability owing to species or cultivar differences and the different developmental stages of plants (Guidi et al., 2009; Morgan et al., 2003). Sarkar and Agrawal (2010) reported adverse impacts of ambient and elevated O3 (ambient 1 10 ppb and ambient 1 20 ppb) doses on two wheat cultivars (HUW 510 and Sonalika) and observed that cultivar HUW 510 displayed a higher degree of O3 damage in its vegetative parts than cultivar Sonalika, whereas damage to reproductive structures (viable pollen floret per plant and pollen viability) was greater in Sonalika as compared to HUW 510. Likewise, this variability in responses was used to discriminate O3 sensitivity in 10 different wheat cultivars exposed to eight O3 regimes by Saitanis et al. (2014). O3 induced reductions in plant height, number of tillers, number of leaves, and total leaf area have been reported in different crop species, like wheat (Pleijel et al., 2018), maize (Singh et al., 2014), mung bean (Chaudhary and Agrawal, 2015), and soybeans (Singh et al., 2010). O3 induced biomass reduction in belowground plant parts is generally associated with the reduction in root:shoot ratio (Andersen, 2003). Furthermore, some studies have reported an increased carbon allocation in leaves due to O3 exposure mainly attributed to a reduction in phloem loading and transportation to meet the higher carbon demand to repair

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the O3 induced foliar tissue damage (Cho et al., 2011). A meta-analysis study performed by Morgan et al. (2003) toward investigating the response of soybean (Glycine max (L.) Merr.) to an O3 exposure of 60 ppb exhibited an approximately 21% reduction in shoot and root dry biomass. Moreover, Saitanis et al. (2014) also observed a reduction in total biomass in different wheat cultivars due to O3 fumigation with a maximum reduction of about 24.9% in cultivar Akbar followed by 20.2% in cultivar Sufi. Meta-analysis data revealed that an increase in relative grain yield losses is associated linearly with O3 concentration, showing maximum losses within a 20% 30% range for various sites in India, China, and the United States (Pleijel et al., 2018). Several controlled environment and field studies have observed that current background O3 concentrations are adversely affecting the yields of different crops species, such as wheat (Mishra et al., 2013; Sarkar and Agrawal, 2010; Rai et al., 2007), rice (Sarkar et al., 2015), soybean (McGrath et al., 2015; Singh et al., 2010, Jaoudé et al., 2008), maize (Singh et al., 2014), barley (Wahid, 2006), mustard (Singh et al., 2009), and mung bean (Chaudhary et al., 2013). A regression analysis study carried out by McGrath et al. (2015) showed that ambient O3 concentrations in the United States resulted in approximately 5% and 10% yield losses in soybean and maize respectively during the period of 1980 2011 in rainfed field conditions. Feng and Kobayashi (2009) reported yield losses of more than 10% for wheat, soybean, and rice; and more than 20% for beans under projected O3 concentrations of 51 75 ppb, thereby indicating that future increasing O3 concentrations pose a serious threat to global food security. Furthermore, previous studies have observed higher O3 sensitivity in leguminous crops followed by crops of the Poaceae family, such as wheat, rice, and barley (Sarkar and Agrawal, 2010; Feng and Kobayashi, 2009). Effects of ambient O3 on wheat crops were investigated by comparing the yields of OTCs with ambient air to filtered air, depicting an average yield loss of 8.4% at 35.6 to 13.7 ppb, with reductions in starch (10.9%) and protein (6.2%), as revealed by 33 experiments (from 3 continents and 9 countries using 17 cultivars along with 1 set of 4 cultivars) (Mills et al., 2018). Physiological damage can occur early and even at low O3 concentrations prior to the appearance of visible injury. Different experimental studies have been conducted to evaluate the effects of ambient and elevated O3 exposure on physiological processes that have exhibited detrimental impacts on the assimilation rate of plants.

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Five modern wheat cultivars were exposed to elevated levels of O3 in a fully open air field experiment and significant impacts of elevated O3 concentrations were depicted, such as increased lipid oxidation, and faster declines in pigments and photosynthetic rates. A lowered carboxylation rate causes a reduction in photosynthetic activity and electron transport rate (Feng et al., 2016). Lowering of gs, and a decline in net photosynthetic capacity and carboxylation efficiency are some common O3 induced phytotoxic impacts on the physiological processes of plants (Rai et al., 2011; Cho et al., 2011; Morgan et al., 2003). The impaired activity of mesophyll cells and structural damage to the cellular membrane, as evidenced by enhanced intercellular CO2 concentrations and lipid peroxidation, are associated with O3 induced photosynthetic loss, as reported by Rai et al. (2011). An O3 induced alteration in photosynthetic electron transport rate in plants via a decrease in the efficiency of excitation capture, indicate a lowering of Fv/Fm ratio (Guidi et al., 2001). Reduced Fv/Fm ratio indicates photoinhibition to the PSII complexes, causing increased sensitivity of plants to light exposure (Rai et al., 2011). The impacts of O3 on vegetation are best correlated with accumulative stomatal O3 flux, calculated over a species-specific time period, using a threshold for the stomatal O3 flux (Mills et al., 2011). Harmens et al. (2018) reported that wheat yield and 1000-grain weight declined linearly with increasing O3 flux, therefore, wheat yield is determined by the accumulated O3 stomatal flux above a threshold value, irrespective of O3 concentration. Similar response was observed by Osborne et al. (2016) on soybean cultivars. O3 flux in the apoplastic region is determined by uptake largely via stomatal aperture present on leaf surfaces. Although the mechanism through which O3 influences stomatal conductance is still not clear, researches have postulated the activation of O3 induced abscisic acid signaling pathway and outbursts of ROS directly in guard cells (Kangasjärvi et al., 2005). Due to the short residence time of O3 in the apoplastic region, it gets rapidly degraded to form ROS and/or reacts with cellular biomolecules, such as protein, lipid, DNA, or apoplastic fluid present there (Mishra et al., 2013). ROS induces cellular membrane damage and causes detrimental effects to the normal functioning of cells. Plants have developed various mechanisms to make use of nonenzymatic and enzymatic antioxidants present in different cellular compartments to cope with oxidative injury caused by O3 stress (Singh et al., 2015).

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Elevated CO2 Concentration Atmospheric concentrations of CO2 have increased from 280 ppm, at the beginning of the industrial revolution to 411 ppm currently and are expected to rise to 750 ppm by 2100 (IPCC, 2014). This trend of unprecedented increasing CO2 concentrations is of concern to agricultural production and food quality because elevated atmospheric carbon dioxide levels have revealed strong diversified effects on crops (Table 9.1). Although the effects depend on plant species and nutrient availability (Zhang et al., 2017) as well as on the specific processes that plants use to fix carbon during the process of photosynthesis (Wang et al., 2008), it is well accepted that elevated atmospheric CO2 markedly affects numerous plant processes, like growth (O’leary et al., 2015), biomass allocation (Wang and Taub, 2010), biochemical processes (Arndal et al., 2014), photosynthesis (Kimball, 2016), and respiration (Xu et al., 2015). Some of the most common responses of crop plants toward elevated CO2 concentrations include growth rate changes (Xu et al., 2014), biomass allocation (White et al., 2012), rate of nutrient uptake (Prior et al., 2008), and water-use efficiency (Varga et al., 2015). Plants have been categorized primarily into three categories (C3, C4, and CAM plants) depending on their operational photosynthetic pathway and variations in the physiological response of plants to carbon dioxide enrichment, mainly CO2 fixing enzymes. C3 plant species, such as wheat, rice, oilseeds, and pulses respond favorably to elevated CO2 when compared to C4 and CAM species, as C3 plants are competitively inhibited by O2. C4 plants, such as sorghum, maize, and sugarcane display little or no photosynthetic response to elevated CO2 because the C4 pathway is completely CO2 saturated (Cousins et al., 2003). An increase in atmospheric CO2 usually increases plant development and growth by improving photosynthesis and water-use efficiency. Increasing atmospheric CO2 might be beneficial for crops, particularly C3 plants. However, the advantages may or may not be realized in long-term growth due to interactions of various environmental factors (Poorter et al., 2013). Fernando et al. (2015) and Broberg et al. (2017) observed declines in several nutrients along with protein concentrations in food crops, while Högy and Fangmeier (2008) and Myers et al. (2014) showed decreases in vitamins, some macro and micro elements of food crops, under elevated CO2. It has been well identified that elevated CO2 accelerates photosynthesis but the degree of stimulation depends on the nature of the species as well as on environmental conditions. Experiments carried out under ideal conditions reflected that doubling the CO2 concentration increased

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photosynthesis by 25% 45% (Ainsworth and Long, 2005) and yield by 10% 20% in C3 crops, while in C4 crops under natural atmospheric conditions, doubling of CO2 leads to increased photosynthesis by 10% 25% and yield by 0% 10% (Ainsworth and Long, 2005). Sage et al. (1989), and Chen et al. (2005) reported the acclimation or downregulation of photosynthesis, that is, after long-term exposure to elevated CO2 the effect on photosynthetic rates were either positive or negative and were variable among and within species. Decreased leaf nitrogen and disturbed source-sink balance due to an increase in sugar production may be probable reasons of photosynthetic acclimation (Seneweera et al., 2011). However, Radin et al. (1987) showed acclimation does not occur in Gossypium hirsutum grown at elevated CO2 concentration. High atmospheric CO2 in general causes reductions in stomatal density (Teng et al., 2009), stomatal conductance (Gao et al., 2015), leaf transpiration (Katul et al., 2009), and evapotranspiration/water use (Bernacchi and VanLoocke, 2015). Contradictory responses are also reported as reverse responses might occur during interactions between CO2 and other climatic factors. Elevated CO2 cut down the amount of water required to produce an equivalent amount of biomass. This enhancement in wateruse efficiency is due to a closing of the stomata to adjust to the CO2 flux and these partially closed stomata monitor the amounts of H2O that are transpired by the plant (Lambers et al., 1998). The elevated CO2 environment plays a significant role in various stages of growth, as well as economic yield of agricultural crops. Plant growth responses comprise of increases in leaf size (Ghannoum et al., 2000), specific leaf weight, leaf thickness, leaf area index, branches, nodes (Allen, 1990) stem length, and extensively long roots (Allen, 1990; Bowes, 1993; Lee-Ho et al., 2007). The effects of increased CO2 also include changes in allocation of biomass to roots from leaves (Stulen and Den Hertog, 1993). Elevated CO2 enhances the flower, fruit, and seed number (Jablonski et al., 2002), which results in higher total seed mass (Jablonski et al., 2002), while on the other hand, it reduces protein content in flowers and seeds (Ziska et al., 2004). Plant chemical composition is altered due to elevated CO2 which affects growth (Poorter et al., 1997). CO2 enhancement causes the accumulation of nonstructural carbohydrates along with soluble phenolic compounds, while declines have been reported for minerals, nitrogen, and phosphorus (Rogers et al., 1999). Plants under increased CO2 have higher photosynthetic nitrogen use efficiency and higher nitrogen use efficiency (Tuba et al., 2003). Growth,

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biomass, and yield for C3 species are increased under CO2 enriched environments (Deepak and Agrawal, 2001a,b). It was estimated that a doubling of CO2 from 340 to 680 ppm would increase the growth and yield of major crops by 10% 50% (Warrick, 1988). Atmospheric CO2 enrichment, however, provides plants with counteracting tendencies against various environmental stress factors to avoid harmful effects on their growth and development. An increase in CO2 ameliorates the effects of increasing temperatures (Idso and Idso, 1994), drought (Karl et al., 2009), O3 pollution (Karl et al., 2009), herbivory, and pest stress (Bidart-Bouzat et al., 2005). Sulfur Dioxide Sulfur dioxide (SO2) is a gaseous pollutant present in the atmosphere, which is acidic in nature, with a pungent and irritating odor. SO2 is released from volcanic eruptions, rock weathering processes, sea spray, microbial activities, and hot springs, while anthropogenic activities also generate SO2 from the combustion of biomass and fossil fuels, vehicular exhaust emissions, and industrial processes have supplemented extra SO2 emissions into the atmosphere. The atmospheric concentration of SO2 increased to a severe level after the industrial revolution which was documented in London in 1952 as London smog which drew the attention of the scientific community of that time. Lu et al. (2011) reported that between 1996 and 2000, China displayed a relative reduction in SO2 concentration of 13% as a consequence of the reduction in economic growth and the implications of air pollution control legislation. But such economic growth again took place dramatically during 2000 06 and decreased again between 2006 and 2010. In nature, pollutants do not occur singly but in combination with other gaseous pollutants and their combined effects are much more damaging to plants. SO2 pollution is not restricted only to urban areas or the site of emission, but also extends to rural areas where it induces phytotoxicity to vegetation and agriculture (Agrawal et al., 1987). The degree of phytotoxicity induced due to SO2 pollution is impelled by the concentration of SO2 (Table 9.1), present meteorological conditions, the duration of exposure, and also the genetic composition of the plants (DeKok, 1990). The entry of air pollutants is mainly driven through stomatal openings present on the leaf surface. Agrawal et al. (1987) reported that after entry through stomatal openings, SO2 reacts with oxygen molecules (O2) to produce sulfite and bisulfate ions, which further get photooxidized into sulfate ions, generating more free radicals concomitant with the formation

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of superoxide radical (_O22) which causes adverse impacts on the health of the plant. Marginal and bifacial chlorosis, followed by necrosis, are some of the most common visible symptoms of SO2 toxicity in plants. Upon entry through the stomatal opening, SO2 reacts with water and the formation of acid takes place, leading to the development of injuries at the margins and degradation in leaf surface area, which in turn affect the assimilative efficacy of plants. Rai and Agrawal (2008) observed significant declines in the photosynthetic rate and stomatal conductance of rice when the plants were exposed to ambient SO2 concentration. Various studies have displayed that SO2 toxicity may cause chlorophyll pheophytinization and swelling of thylakoids under SO2 exposure (Wellburn et al., 1972). Agrawal et al. (1987) suggested that the detoxification of free radicals takes place upon exposure to high SO2 concentrations in tolerant cultivar/ species. Oxides of Nitrogen Oxides of nitrogen have adversely affected forest ecosystems, and it has been observed that humid temperate regions of the world are facing the threat of increased nitrogen deposition (Dise and Wright, 1995; Adams et al., 2004) leading to a situation popularly known as nitrogen saturation. Högberg et al. (2006) has reported that nitrogen saturation is caused due to an excess in the availability of ammonium and nitrate of the total combined plant and microbial nutritional demands. Oxides of nitrogen play a significant role in the O3 formation process. It is evident that biomass burning contributes to total NOx emissions (Sahai et al., 2011) maximally. Studies have reported that the combustion of agricultural residues from different staple crops, such as wheat, rice, and sugarcane have enhanced NOx emissions from 58.9 to 117.4 Gg from 1980 to 2010 (Oksanen et al., 2013). The entry of NOx into plants is governed by the cuticular region and stomatal opening and once inside the mesophyll layer, NOx reacts with water resulting in the formation of chlorotic and necrotic patches on leaf surfaces. Coniferous trees are affected due to chronic nitrogen wet and dry deposition resulting in frost damage and disturbances in the normal physiological functioning of plants which in turn reduces their productivity. Redling et al. (2013) reported that human induced acidic precipitations are gradually altering the pH of soil, which further changes its cation exchange capacities, leading to the increased uptake of mobile metal by plants which ultimately cause disturbances to crop productivity.

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According to Rai et al. (2007), wheat crop (Triticum aestivum L.) grown in nonfiltered chambers displayed phytotoxic effects of NOx pollution, which induced the production of secondary metabolites and hence crop yield was compromised. Volatile Organic Compounds VOCs are organic compounds produced from anthropogenic emissions. They are also released naturally from plants, having sufficiently high vapor pressure to get vaporized into the atmosphere under normal conditions (Yuan et al., 2009). The most common VOCs are alkanes, carnonyls, alkenes, esters, alcohols, and acids (Peñuelas and Llusià, 2003). Laothawornkitkul et al. (2009) has estimated that total VOC emissions from vegetation are approximately 700 1000 3 1012 g C y21 per annum globally and studies have projected that a rise of 2°C 3°C in the mean global temperature could further enhance such biogenic emissions by 30% 45%. Anthropogenic sources of VOCs emissions mostly include the incomplete combustion of fuel, evaporation of fuel, biomass burning, and several industrial processes (Holopainen, 2004). Hallquist et al. (2009) estimated that VOCs emissions from biogenic sources have exceeded that of anthropogenic sources in the atmosphere. Plants have the potential to emit a considerable amount VOCs, such as isoprene and mono- and sesquiterpenes. Such VOCs take part in photochemical reactions to produce secondary phytotoxic pollutants, such as surface level ozone. Some common responses in plants to VOCs toxicity include chlorosis, plant biomass reduction, decreased number of flowering plants21, and reductions in leaf area (Cape, 2003). Change in Sea Level Alterations in sea level will affect plant growth and productivity in some regions of the globe. Allison et al. (2009) reported that there has been a 12 22 cm rise in sea level globally during the 20th century, and new scientific observations confirm an increase of about 3.4 mm per annum in the past two decades. Such rises in sea level induce vulnerability in low-lying agricultural lands to inundation, which can result in the loss of agricultural land and poses a threat to crop production in these areas. Warming of air temperatures has led to an increase in sea level as a consequence of the melting of glaciers and polar ice as well as the thermal expansion of the sea, in the past part of the 20th century (Douglas, 1997), exposing arable land to wastewater contamination to a severe extent (ESCAP, 2009). All these factors are responsible for the reduction of crop production in the immediate future.

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Weed Species The presence of weeds in crop fields can cause reductions in yield as a consequence of competition for moisture, light, space, and mineral nutrients (Xu et al., 2009). Temperature increases due to global climate change will positively affect the survival of weed species (Woodward and Williams, 1987) and consequently their growth and distribution. Mostly such weeds species avoid the higher latitudinal regions owing of the prevalence of low temperatures, making their survival almost impossible. Therefore, such species are restricted to tropical and subtropical regions (Holm et al., 1997). Rahman and Wardle (1990) reported that the warming of air temperatures will facilitate and stimulate the northward expansion of these weeds as climate change influences the distribution of such invasive species. Several studies have reported that plant growth and development are generally affected by the interaction of various biotic and abiotic factors simultaneously. It is evident that the presence of weeds can amplify the deleterious impacts of air pollutants, such as O3, and can cause an increased loss in crop productivity as compared to, for example, under O3 stress alone (Li et al., 2016). Li et al. (2013) elucidated the performance of weed species and crop plants under O3 stress and reported significant resistance in weed species owing to the presence of exceptionally developed antioxidant defense systems. Therefore, the invasion of weed species in crop fields concomitant with other abiotic and biotic factors may result in competition and the related potential damage to crops, and hence, the adaptation of weed management practices is essential to monitor such types of climate change induced crop yield losses (Ghosh et al., 2018). Insects and Pests Generally, the warming of air temperatures promotes and accelerates the life cycle and growth of different insects and pests. GHG-induced global warming promotes successful colonization by insects and pests due to their early maturation and migration (Bale and Hayward, 2010). There will be an overall positive feedback on the amplification of different ranges of insects due to such temperature increases (Parmesan, 2006; Walther, 2010). Dixon (2012) has reported that about 20% 25% of harvested crops are lost to pre- and postharvest diseases worldwide and climatic change is expected to amplify such losses. Such a situation of the projected increase of extreme climate events will give rise to outbreaks of pest populations (Hawkins and Holyoak, 1998; Srygley et al., 2010), which will further hamper the growth, productivity, and yield of plants.

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Presently, it has been depicted that crop plants are more at risk of getting infected by pests and insects. Climate change causes alteration of temperature scale, precipitation pattern, wind velocity, and periodicity. These changes further alter the vigor and activities of different types of pathogens. Some pathogens causing harm to crops and harvested products have evolved as more active and damaging owing to the expansion of their geographical ranges as a result of climate change (Dixon, 2012). The enrichment of atmospheric CO2 levels can further increase the production of simple sugars in the leaves of plants and can in turn reduce the leaf nitrogen content. Such a situation can promote the consumption of more leaves by pest species in order to meet their metabolic requirements for nitrogen, inducing a more severe pest attack. Moreover, higher temperatures reduce the effectiveness of some pesticides and further favor the growth of many disease carrying pathogens.

9.4 CONCLUSION Global climate change has become univocal, and the probable impacts of which are projected to aggravate further if the emission of GHGs persist in an unmitigated way. Since 1950, about 20% more carbon dioxide has been added into the Earth’s atmosphere, increasing the chances of potentially disastrous outcomes. The consequences of global climate changes are difficult to predict owing to their complexity and the incomplete insight into several atmospheric processes and interactive relationships among different environmental variables, such as temperature, radiation, water availability, soil salinity, and soil nutrition. Presently, humanity has been challenged with improving agricultural productivity in order to feed the global population adequately and to achieve future sustainability under such disastrous consequences of global climate change. Hence, a cumulative effort is necessary to exploit various adaptive strategies and plausible mitigation measures including more research and the development activities using different crops/or cultivars to gain an enhanced understanding of ways to counteract such negative impacts of climate change.

ACKNOWLEDGMENTS BP is grateful to Director, CSIR-Central Institute of Mining and Fuel Research, Dhanbad and KKC is thankful to RSM grant, Central University of Punjab, Bathinda and to UGC startup grant, NewDelhi.

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Yan, B., Dai, Q., Liu, X., Huang, S., Wang, Z., 1996. Flooding-induced membrane damage, lipid oxidation and activated oxygen generation in corn leaves. Plant Soil 179, 261 268. Yordanova, R.Y., Christov, K.N., Popova, L.P., 2004. Antioxidative enzymes in barley plants subjected to soil flooding. Environ. Exp. Bot. 51, 93 101. Yuan, J.S., Himanen, S.J., Holopainen, J.K., Chen, F., Stewart Jr, C.N., 2009. Smelling global climate change: mitigation of function for plant volatile organic compounds. Trends Ecol. Evol. 24, 323 331. Zaidi, P.H., Rafique, S., Singh, N.N., 2003. Response of maize (Zea mays L.) genotypes to excess soil moisture stress: morpho-physiological effects and basis of tolerance. Eur. J. Agron. 19, 383 399. Zhang, B.N., Oanh, N.K., 2002. Photochemical smog pollution in the Bangkok Metropolitan Region of Thailand in relation to O3 precursor concentrations and meteorological conditions. Atmos. Environ. 36 (26), 4211 4222. Zhang, M., Wang, L.F., Zhang, K., Liu, F.Z., Wan, Y.S., 2017. Drought-induced responses of organic osmolytes and proline metabolism during pre-flowering stage in leaves of peanut (Arachis hypogaea L.). J. Integr. Agric. 16, 2197 2205. Zhao, G., Xu, H., Zhang, P., Su, X., Zhao, H., 2017. Effects of 2, 4-epibrassinolide on photosynthesis and Rubisco activase gene expression in Triticum aestivum L. seedlings under a combination of drought and heat stress. Plant Growth Regul. 81, 377 384. Zhu, C., Kobayashi, K., Loladze, I., Zhu, J., Jiang, Q., Xu, X., et al., 2018. Carbon dioxide (CO2) levels this century will alter the protein, micronutrients, and vitamin content of rice grains with potential health consequences for the poorest rice-dependent countries. Sci. Adv. 4, 1 8. Ziska, L.H., Bunce, J.A., 2007. Predicting the impact of changing CO2 on crop yields: some thoughts on food. New Phytol. 175 (4), 607 618. Ziska, L.H., Morris, C.F., Goins, E.W., 2004. Quantitative and qualitative evaluation of selected wheat varieties released since 1903 to increasing atmospheric carbon dioxide: can yield sensitivity to carbon dioxide be a factor in wheat performance? Glob. Change Biol. 10, 1810 1819.

FURTHER READING IPCC, 2007. Climate Change 2007: the physical sciences basis. Retrieved from ,http:// ipcc-wg1.ucar.edu/wg1/wg1-report.html.. Liu, F., Jensen, C.R., Andersen, M.N., 2003. Hydraulic and chemical signals in the control of leaf expansion and stomatal conductance in soybean exposed to drought stress. Funct. Plant Biol. 30, 65 73. Specht, J.E., Chase, K., Macrander, M., Graef, G.L., Chung, J., Markwell, J.P., et al., 2001. Soybean response to water. Crop Sci. 41, 493 509. Tripathy, J.N., Zhang, J., Robin, S., Nguyen, T.T., Nguyen, H.T., 2000. QTLs for cellmembrane stability mapped in rice (Oryza sativa L.) under drought stress. Theor. Appl. Genet. 100, 1197 1202.

CHAPTER 10

Cyanobacteria and Their Role Under Elevated CO2 Conditions Savita Singh

Department of Botany, Babu Shivnath Agrawal College, Mathura, India

Contents Introduction Global Warming and the Climate Change Scenario Elevated Carbon Dioxide Levels and Cyanobacteria Ecophysiological Adaptations Exclusive to Cyanobacteria 10.4.1 Photosynthesis 10.4.2 Temperature 10.4.3 Buoyancy 10.4.4 Heterocyst and Nitrogen Fixation 10.4.5 Akinete Formation 10.4.6 Phosphorous Uptake and Storage 10.5 Future Prospects Acknowledgments References 10.1 10.2 10.3 10.4

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10.1 INTRODUCTION Cyanobacteria, usually referred to as blue green algae, comprise of an interesting group of gram negative photosynthetic prokaryotes that can fix carbon dioxide and release oxygen (Castenholz, 2001; Rippka, 1988). Cyanobacteria have a history dating back 3.5 billion years from the era of the anoxygenic environment of Earth to the present oxygenic environment (Castenholz, 2001; Kump, 2008). They are of diverse natures, distributed in a variety of habitats, and are important ecologically (Abed and Garcia-Pichel, 2001; Tamura et al., 2011). Cyanobacteria are recognized as major players in regulating the carbon and nitrogen metabolisms of soil and marine ecosystems. They serve as nitrogenous biofertilizer and also produce amino acids, biological compounds, and products of pharmaceutical importance. Climate Change and Agricultural Ecosystems DOI: https://doi.org/10.1016/B978-0-12-816483-9.00010-4

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The presence of pigments, such as chlorophyll a besides phycocyanin and phycobilins, which make up the phycobilisome, storing starch as reserve food material, and the absence of motile reproductive cells distinguishes cyanobacteria from other algae and bacteria. Cyanobacteria vary in shape and size ranging from unicellular to filamentous forms. They also exhibit diversity in terms of mode of reproduction, such as binary fission, budding, fragmentation, hormogonia formation (seen in Oscillatoria and Nostoc), and akinete (Nostoc). Many species of cyanobacteria possess gas vesicles, such as Microcystis, Aphanizomenon, Nodularia, etc., enabling them to regulate buoyancy and to maintain position in the water column in response to physical and chemical factors (Reynolds et al., 1987; Walsby, 1994). Another unique ability that define cyanobacteria is in their ability to fix atmospheric nitrogen by means of specialized cells known as heterocysts (Kumar et al., 2010; Merrick and Edwards, 1995; Adams, 2000). In the heterocysts, the synthesis of the nitrogenase enzyme occurs, which is required for nitrogen fixation. Nitrogenase is oxygen sensitive so the structure and physiology of heterocysts enable or ensure anaerobic conditions thus providing a suitable environment for nitrogen fixation. Besides heterocystous cyanobacteria some nonheterocystous cyanobacteria have developed the ability to fix nitrogen under anoxic conditions in microbial mats (Trichodesmium). Among heterocystous cyanobacteria, strains of section IV (Nostocales) and section V (Stigonematales) are considered to be the most important components of the N2 fixing community. In short, cyanobacteria exhibit a number of unique and highly-adaptable ecophysiological traits (Litchman et al., 2010), namely: (1) the ability to grow in warm temperatures; (2) buoyancy due to the presence of gas vesicles; (3) the uptake and storage of phosphorus; (4) nitrogen fixation; (5) akinete production and the associated life history characteristics; and (6) the ability to capture light at low intensities and a variety of wavelengths. They form blooms in water reservoirs that are the primary source of several toxins (cyanotoxins) and the have deleterious effects on human as well as animal health (Ferrao-Filho and Kozlowsky-Suzuki, 2011; Deore and Bansal, 2013). The issue of climate change is a serious matter of global concern (IPCC, 2007). The rise in human activities since the arrival of industrialization in the mid-18th century has resulted in a serious rise in greenhouse gases (GHGs) in the atmosphere of Earth. Since the industrial revolution

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in the mid-18th century, large scale burning of fossil fuels, land use change, and forestry activities have considerably enhanced the concentration of GHGs in the atmosphere; for example, the concentration of carbon dioxide has increased from 275 to 285 ppm in the preindustrial era (AD 1000 1750) to 398 ppm in 2015. Additionally, synthetic GHGs like chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), and sulfur hexafluoride (SF6) are also steadily accumulating in the atmosphere. Enhanced accumulation of GHGs and the resulting global warming of the entire atmosphere has led to changes in the patterns of rainfall, the disruption of hydrological cycles, melting of ice caps and glaciers, a rise in sea levels, and increases in the frequency and intensity of extreme events, such as heavy precipitation and cyclonic activities (IPCC, 2007; Allen and Ingram, 2002). All these effects have great impact on the sustainability of water resources, agriculture, forests, and ecosystems. Thus, the climate change scenarios predict that rivers, lakes, and reservoirs will experience increased temperatures, more intense and longer periods of thermal stratification, modified hydrology, and altered nutrient loading (Wilhelm and Adrian, 2008; Winder and Schindler, 2004; Søndergaard et al., 2003). These environmental drivers will have substantial effects on freshwater phytoplankton species composition and biomass, thereby increasing the occurrence of blooms (Beardall et al., 2009; Paerl and Huisman, 2009; Carey et al., 2012) potentially favoring cyanobacteria over other phytoplankton due to its unique and highly-adaptable ecophysiological traits (Mooij et al., 2005). This chapter, thus, aims to discuss the role of cyanobacteria under elevated carbon dioxide (CO2) conditions as a consequence of global warming.

10.2 GLOBAL WARMING AND THE CLIMATE CHANGE SCENARIO Increasing human activities that are transforming the biosphere include land use changes, industrial development, energy production from fossil fuels, and urbanization. Whenever a forest is lost, either due to deforestation or grazing, there occurs a loss of stored carbon which indirectly affects the carbon cycle. Agricultural practices like jhoom cultivation also release CO2 into the atmosphere. Due to domestic and industrial coal burning, CO2 is being pumped into the atmosphere heavily. Moreover, the concentrations of gases, like methane (CH4), nitrous oxide (N2O),

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and CFCs, are increasing in the troposphere mainly due to different human activities. Certain gases, such as CO2, CH4, N2O, and CFCs, are radiatively active gases (GHGs) as they possess the ability to absorb long wave infrared radiation. The increasing abundance of these GHGs in the atmosphere is affecting the global climate and this is known as global climatic change. The continuous increase of these GHGs in the atmosphere is anticipated to have three broad effects: 1. CO2 fertilization 2. Global warming 3. Depletion of the ozone layer in the stratosphere. It is estimated that with a twofold increase in CO2 concentration the growth of C3 plants may increase by 30% on average in a brief period of time under favorable water, nutrient, light, and temperature conditions. This particular response of plants to elevated concentrations of CO2 is referred to as the carbon dioxide fertilization effect. It is also anticipated that increased CO2 concentrations will cause the rate of photosynthesis to increase and under such circumstances stomatal conductance in plants will decrease (due to partial closure of stomata). Because of partial stomatal closure, a reduction in transpiration rate is expected, which will result in increased water use efficiency allowing newer species of plants to grow successfully in areas with less water. Under increased atmospheric CO2 conditions, plants should allocate a larger proportion of photosynthate to their roots (Thompson et al., 2017). Profuse root production is expected to boost mycorrhizal development and the fixation of N2 in root nodules, thus, enabling plants to grow successfully even in soils with less of nutrients. However, it has been proposed that under a natural scenario, the beneficial effects of increased CO2 may not actually be realized mainly due to the intense negative effects of global warming which would shadow any beneficial effects. The predicted global warming scenario in the future has the potential to affect the weather and climate, sea level, and the distribution and phenology of organisms, food production, and fishery resources in the oceans (Easterling et al., 2000; Ullah et al., 2018). The global mean temperature has increased by approximately 0.6°C in the 20th century. The average temperature of the Earth may increase by 1.4°C 5.8°C by the year 2100 from that of the mean temperature of 1990 (Trenberth et al., 2007). Temperature changes are expected to be extreme in areas of middle and higher latitudes (Solomon et al., 2007). Of particular relevance for cyanobacterial blooms is the prediction that heat

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waves will become more frequent, more intense, and will last longer (Meehl et al., 2007). Climate warming is expected to have a severe effect on the onset (earlier), strength (stronger), and duration (longer) of the stratification of lakes (Peeters et al., 2002), even to the extent that some polymictic lakes may become dimictic, dimictic lakes may become warm monomictic, and monomictic lakes may become oligomictic (Gerten and Adrian, 2002). The warming of the atmosphere will considerably increase its moisture carrying capacity. While the troposphere warms up, the stratosphere will cool down causing widespread changes in precipitation patterns due to changed patterns of air-mass movements (Jeppesen et al., 2007; Parry et al., 2007). Precipitation will increase at higher latitudes in both summer and winter and in southern and eastern Asia in summer. Precipitation in winter months may decrease at lower latitudes. Moreover, the frequency of extreme events like droughts and floods may increase severely. The sea level has constantly been on the rise by almost 1 2 mm each year during the 20th century. Predictions say that by the year 2100, the global mean sea level may rise by up to 0.88 m compared to what it was in 1990. Global warming will result in an increase in sea level rise due to the thermal expansion of oceans and the melting of glaciers on the Earth’s surface. The rising sea levels are going to affect human populations inhabiting sea shores all over world. Innumerable coastal cities will be confronted with the threat of flood. The inundation of coastal salt marshes and estuaries may result in a deprivation of breeding grounds for many important bird and fish species, which may result in the extinction of such species. Thus, sea level rise is anticipated to have negative impacts on human settlements, tourism, drinking water, fisheries, infrastructure, agricultural lands, and wetlands. Both altitudinal and latitudinal distribution patterns of organisms may shift due to the change in temperature ranges caused by global warming. With increasing global warming many species are expected to shift slowly poleward or toward high elevations in mountainous areas. Slow migrating species may not be able to migrate fast enough and so may totally disappear. Increased temperatures will cause an eruption of plant diseases and pests, explosive growth of weeds, and an increased basal rate of respiration of plants. A combination of all these factors will decrease crop production. Small temperature increases may slightly enhance crop productivity in temperate regions, but larger temperature changes will reduce crop productivity.

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10.3 ELEVATED CARBON DIOXIDE LEVELS AND CYANOBACTERIA Due to urbanization and development in agriculture and industry eutrophication is on the rise. This eutrophication (nutrient overloading especially of nitrogen and phosphorus) has favored the growth and dominance of harmful cyanobacterial blooms (Fig. 10.1). In response to these climatic changes, the physical and chemical characteristics of many lakes will change, potentially synergistically, thereby affecting phytoplankton communities (Carey et al., 2012). It is being hypothesized that cyanobacteria may continue to increase in response to global climate change (Wagner and Adrian, 2009; Paerl et al., 2011; Paerl and Huisman, 2009). In freshwater systems, enriched with nutrients like N and P, cyanobacterial blooms exhibit high photosynthetic demands for CO2; to a level that ambient water have no free CO2, scaling the pH up to 10 or even higher. Under such conditions, the rate of CO2 supply becomes the controlling factor for algal biomass production (Huisman et al., 2005;

Figure 10.1 Schematic representation of changes in phytoplankton community structure and dominance of cyanobacteria as a consequence of elevated CO2 levels and global warming.

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Badger and Price, 2003). It is suggested that an increase in atmospheric CO2 levels may provide a sufficient influx of C, thereby enabling a substantial increase in the productivity of surface dwelling cyanobacterial blooms (Visser et al., 2016). On the other hand, models and laboratory experiments have shown that rising CO2 concentrations may indeed exacerbate cyanobacterial blooms (Schippers et al., 2004; Verspagen et al., 2014; Visser et al., 2016). Cyanobacteria and many phytoplankton have evolved a CO2-concentrating mechanism (CCM) in order to overcome the problem of low affinity of Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) for CO2 and produce an environment of high CO2 concentration around RuBisCO (Giordano et al., 2005; Badger et al., 2006; Price et al., 2008; Price, 2011). In cyanobacteria, five different Ci uptake systems are known: BCT1, SbtA, BicA, NDH-I3, and NDH-I4. BicA and NDH-I4 have a low affinity for bicarbonate and high flux rates, while others exhibit high affinity and low flux rates (Shibata et al., 2002; Omata et al., 2002; Maeda et al., 2002). Thus, cyanobacteria have the advantage of diverse uptake systems that enable them to respond effectively under changes in Ci availability. It has also been speculated by experimental findings that rising pCO2 may shift strain dominance in Microcystis, which was attributed to the presence of different Ci uptake systems (Sandrini et al., 2014; Van de Waal et al., 2011). Whole genome sequencing of harmful cyanobacteria, like Anabaena (Thiel et al., 2014; Cao et al., 2014; Christiansen et al., 2014), Aphanizomenon, and Planktothrix strains, suggest that they exhibit genetic as well as phenotypic variation in Ci uptake systems, which may provide a selective advantage in high CO2 environments (Visser et al., 2016).

10.4 ECOPHYSIOLOGICAL ADAPTATIONS EXCLUSIVE TO CYANOBACTERIA Cyanobacteria, blue green algae, were responsible for the initial rise of atmospheric O2 around 2.3 billion years ago (Holland, 1994; Farquhar et al., 2000). All eukaryotes, including algae and higher plants, derived their photosynthetic capabilities from cyanobacteria by way of endosymbiosis (Margulis, 1993). It is believed that oxygenic photosynthesis—an extremely complex biochemical process—was “invented” only once, and a primitive cyanobacterium was the organism responsible for it (Kasting and Siefert, 2002).

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10.4.1 Photosynthesis Photosynthesis is the process where light energy is converted into chemical-bond energy through an ion gradient generating source of energy in the form of adenosine triphosphate (ATP). Cyanobacteria show photolithotrophic autotrophy where light serves as the source of energy, inorganic hydrogen as an electron donor, and CO2 acts as a carbon source. Cyanobacteria are reputed to be strong competitors for light due to their accessory pigmentation and the structural organization of their light-harvesting antenna complex. Lab experiments have found that Synechocystis sp. outcompete other phytoplankton species under limiting light conditions (Passarge et al., 2006). On the other hand, Huisman et al. (1999) and Reynolds (2006) found that Microcystis sp. were unable to compete for light and showed poor light efficiency. Thus, increased temperature due to elevated CO2 conditions may affect the growth of various cyanobacterial taxa which not only depend on photosynthetic response but also change in dynamics of fluidity of plasma membrane or fatty acid composition (Dmitry and Murata, 2004; Nanjo et al., 2010).

10.4.2 Temperature Temperature is one of the most important factors that affects the growth and survival of organisms. With the increase in water temperatures to values approaching physiological optima for a wide range of phytoplankton species, more phytoplankton will grow and replicate faster, at least until water temperatures rise beyond the optimal temperature for growth (Carey et al., 2012). Global climate change is expected to increase the global mean temperature, and under such circumstances the growth of microbial populations, especially of cyanobacteria, is expected to enhance (Luring et al., 2013). The temperature at which maximum replication rates may occur for cyanobacteria varies from 20°C for Aphanizomenon flos-aquae and Planktothrix agardhii, to 28°C for Microcystis aeruginosa, and even to 41°C for Synechococcus sp. (Reynolds, 2006).

10.4.3 Buoyancy Many species of planktonic cyanobacteria produce gas vesicles, which provide buoyancy and help access surfaces where light is optimum (Walsby, 1994). Changes in climatic conditions resulting in increased temperatures are predicted to increase stratification in lakes to support the growth of buoyant cyanobacteria (Jones et al., 2005). M. aeruginosa and Anabaena

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spiroides were able to migrate 12 m to access light and nutrients despite substantial density barriers (Ganf and Oliver, 1982). So it is evident that fast-migrating genera (e.g., Microcystis, Anabaena) may benefit from the climate-induced strengthening of stratification by gaining a competitive advantage over other nonmigrating or slow migrating phytoplankton. While increased stratification is predicted to favor buoyancy-regulating cyanobacteria in comparison to non-buoyant algae in most environmental conditions (Huisman et al., 2004).

10.4.4 Heterocyst and Nitrogen Fixation Some cyanobacterial genera belonging to Nostocales and Stigonematales orders have the ability to form specialized structures known as heterocysts which are sites of nitrogen fixation (Theil, 2004). Heterocysts are thick walled cells with a nitrogen fixing enzyme called nitrogenase (Elhai and Wolk, 1990). Nitrogenase complex has the ability to convert NRN to N 2 N under anoxic conditions, which is created inside heterocysts by laying thick wall external to plasma membrane (Ernst et al., 1992). This nitrogen fixation (N-fixation) ability of some cyanobacterial species provides them with a competitive advantage in the water column when available sources of N are strongly depleted. Besides heterocystous cyanobacteria, some nonheterocystous cyanobacteria have developed the ability to fix nitrogen under anoxic conditions in microbial mats (Trichodesmium). In the context to global warming and the resulting warmer climatic conditions, enzymatically controlled processes, such as N fixation, might be expected to increase at the approximate rate of cyanobacterial growth in responses to temperature (i.e., Q10 of $ 1.8). Observed that Q10 values for N fixation of heterocystous strains of Nodularia spumigena and Anabaena sp., determined via nitrogenase activity rates, were indeed .1.8 and commonly close to 2.0 in the light. It was found that in 21 different heterocystous species, the rates of N fixation in the light were 2.5 4 times higher than in dark conditions, furthermore, there was substantially greater temperature dependence of N fixation in the light.

10.4.5 Akinete Formation Cyanobacteria enjoy the division of labor exhibited by their ability to transform vegetative cells into spore forming structures known as “akinetes.” Akinete differentiation is confined to the heterocyst-forming cyanobacteria and these structures are found to be more resistant toward cold

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and desiccation (Adams and Duggan, 1999). Akinetes are generally recognized by their larger size and conspicuous granulation. This occurs due to the accumulation of glycogen and cyanophycin, a nitrogen storage polymer consisting of equimolar amounts of arginine and aspartate (Wolk et al., 1994; Flores and Herrero, 2010). The position of akinetes in relation to heterocysts is mainly strain specific (Kantz and Bold, 1969; Meeks and Elhai, 2002). It has been speculated that with increased temperature and drought, cyanobacterial populations with the ability to form akinete will surely have an added advantage over other non akinete forming genera. Hence, some taxa within the Nostocaceae, Rivulariaceae, and Stigonemataceae families of cyanobacteria, like genera of Anabaena, Cylindrospermopsis, Gloeotrichia, and Nodularia, have an added advantage.

10.4.6 Phosphorous Uptake and Storage Cyanobacteria produce extracellular or membrane associated alkaline phosphatase (Ray et al., 1991; Hirani et al., 2001; Tiwari et al., 2015) that mineralizes organic phosphate available in the environment (soil or water); besides this they also have the ability to store excess phosphorous intracellularly (Healey, 1982; Whitton, 1987) providing a competitive advantage in P depleted environmental conditions. In low nutrient conditions, the high affinity of cyanobacteria for P due to the induced synthesis of high affinity uptake systems for Pi and its further storage as polyphosphate inside cells help with survival in both low and high P conditions.

10.5 FUTURE PROSPECTS The problem of global warming is surely not at all a myth. It is sure to confront human civilization looking at the greed of comfort in terms of modernization and urbanization. Humans are over exploiting nature and its resources. Only caring for nature, evolving new methods that cause minimal damage to nature, and an inevitable promise to care for nature and natural resources may protect human civilization from the consequences of global warming. Organic farming without the use of chemical fertilizers and the use of age old biofertilizers, like Anabaena, Aulosira, Scytonema, and many more such ecofriendly microbial species, must be emphasized and encouraged in future. The emergence of cyanobacteria on Earth dates back billions of years ago and their ecophysiological traits benefit them immensely and, therefore, their candidature for dominance compared to other microbial species cannot be denied.

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Cyanobacteria are reputed to be strong competitors compared to other microbial species due to their ability to adapt to changing environments. Their ecophysiological traits, like adaptation to varying temperatures, the production of gas vesicles, nitrogen fixing ability, akinete formation, and photosynthetic adaptability helps in their survival and dominance under elevated CO2 levels and the global warming scenario.

ACKNOWLEDGMENTS The author is thankful to Dr. Ajay Kumar and Dr. Krishna Kumar Choudhary for helpful discussions on the topic.

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CHAPTER 11

Rising Atmospheric Carbon Dioxide and Plant Responses: Current and Future Consequences Amit Kumar Mishra1, Shashi Bhushan Agrawal2 and Madhoolika Agrawal2 1

Texas A&M AgriLife Research and Extension Center, Texas A&M University, Uvalde, TX, United States Laboratory of Air Pollution and Global Climate Change, Department of Botany, Institute of Science, Banaras Hindu University, Varanasi, India

2

Contents 11.1 Introduction 11.2 Current Status and Trends of Atmospheric CO2 Levels 11.3 Plant Responses to Atmospheric CO2 11.3.1 CO2 Fertilization Effect 11.3.2 Growth Responses 11.3.3 Physiological Responses 11.3.4 Biochemical Responses 11.3.5 Molecular Changes in Plants Under CO2 Enrichment 11.3.6 Yield 11.4 Interaction With Air Pollutants 11.5 Summary Acknowledgments References Web references

265 267 268 268 269 271 281 283 284 286 294 295 295 306

11.1 INTRODUCTION The world’s population is predicted to increase by 2.3 billion people between 2009 and 2050, thus creating the need for a substantial rise in global food production with the aim of meeting the imminent food demand (Alexandratos and Bruinsma, 2012). The present and expected future changes in the climate will inhibit the attainment of food production goals Climate Change and Agricultural Ecosystems DOI: https://doi.org/10.1016/B978-0-12-816483-9.00011-6

Copyright © 2019 Elsevier Inc. All rights reserved.

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even further. Environmental variations are associated with increasing abiotic and biotic stresses, such as drought and heat stress, insect attack, new disease outbreaks, and rising greenhouse gas emissions. Amongst these, the levels of atmospheric CO2 are increasing continuously over time; the present atmospheric CO2 concentration has increased from 280 to more than 400 µmol mol21 (https://www.esrl.noaa.gov/gmd/ccgg/trends/global. html; Tans and Keeling, 2018) since the 1800s and is predicted to double by the end of the 21st century (IPCC Climate Change, 2013). Long-term exposure of plants to high CO2 levels, elevated temperature, and drought will considerably affect the equilibrium of ecosystem processes equally at local and global levels. Global food security will be contingent on key physiological processes of agricultural crop species and will be affected by the combined effects of the factors of climate change together with increasing levels of atmospheric CO2 (Tausz et al., 2013). Carbon dioxide is the key substrate for photosynthesis, and therefore, can be anticipated as the main contributor toward global food production. Approximately 90% of the existing plant species of the world possess C3 type processes to fix carbon during photosynthesis though saturation is not observed at current ambient CO2 levels, and thus, photosynthesis and growth are predicted to increase under high CO2 environments (Makino and Mae, 1999; Kimball, 2016). In contrast, plants having C4 type pathways develop a special mechanism of carbon fixation that principally prevents photorespiration (a respiratory process in which plants utilize oxygen in the presence of light and release some CO2 thereby significantly wasting energy generated by photosynthesis). In C3 plants, Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) catalyze the process of photosynthesis by reacting with both CO2 and oxygen (O2), instigating a photosynthetic carbon reduction (PCR) and photorespiratory carbon oxidation (PCO) cycles, respectively (Drake et al., 1997; Makino and Mae, 1999). Photosynthesis, respiration, and water relations are the three main physiological processes of plants swayed by high CO2 levels (Gamage et al., 2018). Thus, understanding the mechanisms of photosynthesis, respiration, and water use, and their impact on plant growth under high CO2 levels offers an exclusive opportunity to improve crop productivity under future climate change. The increase in global atmospheric CO2 levels will have a major influence on agricultural crop production. Plants could acclimatize to these changes by utilizing excess CO2 during the process of photosynthesis to produce photoassimilates leading to increased growth and productivity. Still, mechanisms to assimilate higher CO2 concentrations and their consequences are not yet completely

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clear. In this chapter, the effects of increasing CO2 levels on overall plant growth and development are highlighted and potential knowledge gaps in the understanding of plant responses to CO2 enrichment are outlined.

11.2 CURRENT STATUS AND TRENDS OF ATMOSPHERIC CO2 LEVELS The burning of fossil fuels, the continued growth of the population, and other anthropogenic activities have resulted in an increase in carbon dioxide (CO2) input into the atmosphere, from less than 300 ppm before the industrial revolution to the current concentration of above 400 ppm (https://www.esrl.noaa.gov/gmd/ccgg/trends/global.html) (Fig. 11.1), which is a major contributor to global greenhouse gas emissions. The increase in atmospheric greenhouse gases, such as CO2, methane, nitrous oxide, and halocarbons, is likely to result in an increased radiative forcing of 9% between 1998 and 2007, leading to a warming of the atmosphere (Forster et al., 2007). The concentrations of CO2 in the atmosphere are increasing linearly decade to decade. Data from the past 10 years suggests that the average yearly rate of increase is 2.24 ppm (https://www.esrl. noaa.gov/gmd/ccgg/trends/gl_gr.html). This rate of increase is more than

Figure 11.1 Annual mean data of global atmospheric CO2 from 1980 to 2017. Data adopted from https://www.esrl.noaa.gov/gmd/ccgg/trends/global.html.

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double that observed during the 1960s. Nevertheless, CO2 is an important substrate for the growth of plants other than being a potent greenhouse gas and a major contributor to global warming. The A1B emissions scenario indicates that the CO2 concentration will reach 700 1000 ppm by the end of 21st century (IPCC Climate Change, 2013), but this will sturdily depend on the future consequences of anthropogenic emissions (Woodward, 2002). This rapid surge in atmospheric CO2 concentrations encouraged researchers to study plant responses in order to better understand crop management in a future world under high CO2 levels.

11.3 PLANT RESPONSES TO ATMOSPHERIC CO2 11.3.1 CO2 Fertilization Effect The principal response of plants to elevated CO2 (EC) is an upsurge in net photosynthetic rate (Ps) and a diminution in stomatal conductance (gs) (Long et al., 2004; Gifford, 2004; Ainsworth and Rogers, 2007) (Fig. 11.2). According to Drake et al. (1997), in C3 plants, an increase in Ps arises because RuBisCO is not inundated at ambient levels of CO2. Ainsworth and Long (2005) noticed that treatment with EC resulted in a 31% rise in light-saturated leaf Ps and an upsurge of 28% in 24-hour (diurnal) photosynthetic carbon (C) assimilation during the analysis of 12 large scale free air CO2 enrichment (FACE) experiments. Under EC, contingent upon the type of plant species and C assimilation pathway, enhanced photosynthetic efficiency resulted in the modification of growth and yield responses, known as “CO2-fertilization” effect. According to Drennan and Nobel (2000), most vascular plants utilize the C3-C assimilation pathway, while approximately 3% use C4 carbon fixation, such as sorghum (Sorghum bicolor), sugarcane (Saccharum sp.), and maize (Zea mays), however, 6% 7% of species are reported to use Crassulacean acid metabolism (CAM) and these three mechanisms for C-assimilation show differential response to enhanced CO2. The current levels of atmospheric CO2 set a higher edge of Ps in C3 plants and apparently, in the past, the lower concentration of CO2 was even more curbing (Drake et al., 1997; Ainsworth and Rogers, 2007). According to Ainsworth et al. (2008), the kinetic properties of RuBisCO suggest that it functions optimally at a CO2 concentration of 200 ppm. An increase in atmospheric CO2 will certainly enhance the Ps in C3 plants (Drake et al., 1997; Makino and Mae, 1999). In comparison to C3 plants, C4 plants are not very responsive to EC as they possess a CO2 concentration mechanism

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Figure 11.2 Schematic representation of the effects of CO2 enrichment on the regulation of plant growth and metabolism.

in the mesophyll cells of their leaves (Ziska and Bunce, 1997; Ghannoum, 2009). However, the preliminary stimulation of C3 photosynthesis is not constantly sustained when plants are treated with EC for a long time and this modification is termed “photosynthetic acclimation” (Bowes, 1991; Moore et al., 1998; Seneweera et al., 2002), which conveys morphological and biochemical changes from cellular to whole plant level (Drake et al., 1997; Makino and Mae, 1999; Stitt and Krapp, 1999; Seneweera et al., 2002; Seneweera and Norton, 2011) (Fig. 11.2).

11.3.2 Growth Responses Morphological changes, growth, and development of plants in response to EC are well reviewed in both C3 and C4 species (Ghannoum et al., 2000; Ainsworth and Long, 2005), while there is a pronounced amount of interspecific variation. Commonly, EC increases the efficiency of leaf photosynthesis, causing tall plants with thick stems possessing additional

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branches and leaves (increased number of sinks) (Ainsworth and Long, 2005). Pritchard et al. (1999) observed increments in root length/diameter and branching patterns under EC, however, variations in nature and distribution could be an adaptation to root/shoot limitations, functioning rather than an inherent allometric association between shoot and root growth (Hunt and Nicholls, 1986). An increase in root growth could be an adaptation to the need to attain more nutrients to meet the increased C supplied to the roots (Rogers et al., 1997). An increase in root/shoot ratio (RSR) under EC may be attributed to higher photoassimilate partitioning to the roots due to greater Ps. This might be an adaptation to sequester more carbon for the initiation of the growth of more roots (Uprety et al., 1996). Under EC, taller and more branched plants are observed when changes arises in the shoot apices and vascular cambium. Such variations, particularly in the number of apical meristems, have a great effect on the establishment of prospective sink strength. The response of different plant species may be diverse but intraspecific variations also exist, with more determinate types showing less response than less determinate types, such as in wheat (Ziska, 2008) and soybean (Ziska and Bunce, 2000; Ainsworth et al., 2002). An explanation for higher shoot or pod numbers under EC could be the greater supply of photoassimilates (Nakano et al., 1997), while Seneweera et al. (2003) suggested that CO2 may modulate morphology and overall plant development via its impact on the variations in hormonal balance in plants (ethylene biosynthesis), such as in Oryza sativa L. (rice). Regardless of any mechanism, a yield response against EC needs an associated increase in sink capacity to compete for action of the source. Previous literature suggest substantial plasticity and several structural modifications/adaptations in leaves in response to varying environment conditions (Pritchard et al., 1999), including light and nitrogen (N) supply (Gutiérrez et al., 2009). Under elevated CO2, Ainsworth and Long (2005) established that the number of leaves increases, but leaf area index did not show a significant change in C3 grasses, while leaf expansion rate in the early growth stage may be higher (Pritchard et al., 1999; Seneweera and Conroy, 2005). Under high CO2, leaf thickness (i.e., leaf mass per unit area) frequently increases due to variations in the number and size of mesophyll cells per unit leaf area (Gutiérrez et al., 2009). Previous evidence on crop responses to EC, suggest that stomatal density reduces as the concentration of CO2 increases (Tricker et al., 2005),

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while there are some studies where density is higher, lower, or has not changed (Ainsworth and Rogers, 2007). Ainsworth and Rogers (2007) observed an average reduction in stomatal density of 5% in FACE experiments, but the findings were not statistically significant. They also suggested that variations in stomatal density are normally small ( 6 10%) and there are few evidences for a significant decline in stomatal density, so any variation in leaf conductance may be the outcome of changes in aperture and not density. The consequences of high CO2 levels on plant development and structure are many and diverse, working together with both C assimilation in plants and water relations (Seneweera and Norton, 2011). Pritchard et al. (1999) established that the most important effects of elevated CO2 are an upregulation of carbohydrate availability and a diminution in water use efficiency (WUE) and these cumulative events would lead to a rise in cell proliferation, phenological development, and availability of nutrients. In the past few years, the growth and productivity of agricultural crop species have been shown an upsurge under EC and this has enticed a substantial interest due to the variations in their response. In general, high CO2 levels pose potential positive effects on agricultural crop plants. The effects of CO2 enrichment on various plants studied so far are listed in Table 11.1.

11.3.3 Physiological Responses Elevated CO2 enhances the rate of photosynthesis and thereby the growth and productivity of plants. The key reason for this enriched photosynthesis is the increased carboxylation efficiency of RuBisCO, which is comparatively low at ambient atmospheric CO2. However, at EC, the rise in CO2 concentration at the place of CO2 fixation will unbalance the CO2/ O2 ratio; thus, the carboxylation efficiency of RuBisCO will be supported by a decreasing rate of photorespiration. C3, C4 and, CAM plants respond differently to increased CO2, which is further discussed. 11.3.3.1 Responses of C3 Plants In C3 plants, elevated CO2 stimulates photosynthesis due to the increased gradient of CO2 from ambient air to the place of CO2 fixation. According to Bowes (1991), RuBisCO is the main enzyme of the PCR cycle and also participates in the PCO cycle or photorespiration. When Ribulose-1,5-bisphosphate (RuBP) is carboxylated by RuBisCO, it generates two molecules of 3-phosphoglyceric acid (PGA) (Seneweera and Norton, 2011). Alternatively, when RuBP is oxygenated by RuBisCO, it

Table 11.1 Effects of elevated CO2 on various plants Plant CO2 dose Experimental setup

Parameter(s)

Reference

Triticum aestivum L.

Rao et al. (1995) Mulholland et al. (1997)

T. aestivum L., Triticum durum L., Triticum monococcum L.

800 ppm

CSTR

Total chl k (14.8%), carotenoids k, rbcL k, rbcS k

550 and 660 ppm

OTC

Double that of ambient 600 ppm

OTC OTC

680 ppm

OTC

680 ppm

OTC

600 ppm

OTC

702 ppm

Controlled chambers

Dry matter accumulation m (7% 23%), stem dry weight m (174%), ear dry weight m (5%), total grain dry weight m (10% 33%) Yield m (13.4% 33.8%), above ground biomass m (12.2% 32.2%) Shoot length m (14.7% 19.7%), total plant length m (13% 18%), number of tillers m (61.9% 65.8%), number of leaves m (32.8% 36.5%), leaf area m (33.6% 38.15%), leaf biomass m (46% 52.9%), total biomass m (21.6% 28.3%), grain yield m (30.4%) Grain yield m (21%), grain protein m (B12%), straw yield m (29%) Flag leaf photosynthesis m (B50%), gs k (B60%), WUE m (B40%) WUE m (81.3 91.2), foliar protein k (10.7% 11.1%), foliar nitrogen k (11.6% 14%), phenol m (17.3%), TSS m (7.5%), starch m (16% 16.4%) Total dry weight m (65.8%), RGR m (6.6%)

550 ppm

FACE

gs k (9.1% 33%), leaf area m (7.5% 21.8%), dry weight of whole plant m (21% 46%), grain yield m (13% 22%), harvest index m (7.6% 31%)

Bender et al. (1999) Deepak and Agrawal (1999)

Pleijel et al. (2000) Donnelly et al. (2000) Agrawal and Deepak (2003) CardosoVilhena et al. (2004) Uprety et al. (2009)

T. monococcum L., Triticum dicoccoides L., T. aestivum L. T. aestivum L.

550 ppm

FACE

700 ppm

OTC

OTC

Solanum tuberosum L.

280 ppm

OTC

570 ppm

OTC

Average size of starch grain m (54% 93%), starch content m (1.6 5 times), grain protein (11% 47%) Plant height m (27.5% 30.3%), number of leaves m (16.2% 32%), total biomass m (16.6 19.8), NAR m (24% 30%), LAR k (16.8% 18.6%), RSR m (26.3% 57%), grain yield m (46% 54.6%), grain protein k (5.4% 8.9%), TFAA k (6.9% 10.4%), TSS m (18.6% 28.9%), SC m (8.3% 19.7%) Total chl m (7.6% 11.2%), carotenoids k (9.6% 12.7%), Ps m (29.7% 30.8%), gs k (18.9% 35.4%), Fv/Fm m (1.7% 3.4%), H2O2 k (16% 41.6%), 2O2 k (23% 33%), LPO k (13% 32.6%), SL k (6.6% 22.5%), AA k (9.2% 15%), SOD k (12% 14%), APX k (44% 54%), GR k (33.3 38.8), foliar protein k (5.7% 7.3%), PAL m (23.9% 26.7%), total phenolics m (17.6% 24.6%) Haulm dry weight m (15%), haulm/tuber ratio m (13%), average size of tubers m (18%) Plant height m (29.3%), number of leaves m (21.4%), number of tubers m (82.5%), fresh tuber weight m (92.4%), tuber dry weight m (135%), total biomass m (17.4 107.4%), RGR m (9.3%), NAR m (12.5%), SLA k (28.7%), starch content m (130.6%), reducing sugars k (32%), soluble sugars m (63.8%), total nitrogen k (15.2%), organic carbon m (20%), C/N ratio m (41.6%)

Sinha et al. (2009) Mishra et al. (2013a)

Mishra et al. (2013b)

Persson et al. (2003) Kumari and Agrawal (2014)

(Continued)

Table 11.1 (Continued) Plant

Glycine max L.

Zea mays L.

Vigna radiata L.

CO2 dose

Experimental setup

Parameter(s)

Reference

OTC

Ps m (57.9% 65.3%), gs k (27.9%), Fv/Fm m (8.6%), total chl m (37.3%), carotenoids m (28.1% 31.6%), protein content k (13.9%), ascorbic acid k (8.1%), APX k (17.5%), CAT k (16.2%), SOD k (15.4%), GR k (24.7%) A m (56%), photorespiration k (36%), glycolate oxidase k, CAT k, hydroxy pyruvate reductase k A m (60%), relative RuBP regeneration limitation m (44%), relative stomatal limitation m (5%)

Kumari et al. (2015)

726 ppm

OTC

727 ppm

OTC

600 ppm

OTC

575 630 ppm

OTC

700 ppm

OTC

1180 ppm 575 630 ppm

OTC OTC

Plant height m (8% 14%), number of branches m (29% 48%), leaf area m (26% 47%), total biomass m (26% 35%), Ps m (30% 33%), foliar protein k, yield m (31% 38%) Ps m (ns-10%), dark respiration k (ns), R/P ratio k (ns-29%) gs k (35.6%), E k (32%), leaf area m (7%), leaf dry weight m (29%), SLW m (9%), shoot length m (11%), root dry weight m (19%), dry stem weight m (44%) gs k (38%), Am (ns) Ps m (23% 55%), dark respiration k (15% 37%), R/P ratio k (50% 59%), dry leaf weight m, weight of roots m (30% 57%), root/shoot weight m (28% 42%), seed yield m (19% 23%)

Booker et al. (1997) Reid and Fiscus (1998) Deepak and Agrawal (2001) Uprety et al. (1996) Vanaja et al. (2011)

Bunce (2014) Uprety et al. (1996)

Vigna mungo L.

600 ppm

OTC

600 ppm

OTC

600 ppm

OTC

700 ppm

OTC

600 ppm

OTC

Ps m (73.9% 124.9%), leaf dry weight m (12% 133%), leaf area m (30% 71%), root length m (12% 28%), root weight m (106% 236%), pod number m (32%), seed number m (39.6%) Dry weight m (36% 47%), leaf area m (3% 9%), leaf number (16% 29%), plant height m (3% 35%), chl a k (10% 18%), Ps m (13% 96%), rate of respiration k (54% 62%) Total soluble protein k (27.4%), total nitrogenase activity m 2 O2 k (11.8% 24.8%), H2O2 k (23.5% 27.8%), LPO k (17.6% 22.9%), SOD k (11.8% 14.7%), CAT k (64.3%), total chl m (30.6% 38.9%), carotenoids k (8.1% 14.7%), Ps m (25.4% 29.2%), gs k (18.6% 33.9%), Ci m (9% 14%), WUE m (47.5% 61.5%), plant height m (19% 26%), number of leaves m (21.3% 24.6%), leaf area m (20.5% 22.3%), number of nodules m (43% 50%), total biomass m (19.8% 33.6%), seed yield m (25.3% 40%), seed protein k (9.9%), TSS m (9.3% 15.1%), SC m (15.5%) % germination m (5.2% 22%), vigor index m (20% 31%), shoot length m (9.8% 12%), leaf area m (17% 27%), dry leaf weight m (28.43%), dry stem weight m (36% 44%), dry root weight m (20% 40%), total dry weight m (31% 41%)

Srivastava et al. (2001)

Das et al. (2002)

Srivastava et al. (2002) Mishra and Agrawal (2014)

Vanaja et al. (2006)

(Continued)

Table 11.1 (Continued) Plant

CO2 dose

Experimental setup

Parameter(s)

Reference

550 and 700 ppm

OTC

Vanaja et al. (2007)

Brassica juncea L.

600 ppm

OTC

Helianthus annus L.

700 ppm

OTC

550 ppm

OTC

550 ppm

OTC

550 ppm

OTC

Shoot length m (2% 34%), stem dry weight m (4% 80%), root length m (2% 38%), leaf area m (18% 90%), total biomass m (2% 70%), pod weight m (18.4% 51.3%), seed yield m (2.3% 47.6%), harvest index m (38.7% 39.5%) Carbon content in plant parts m (29% 40%), nitrogen content in plant parts k (29% 30%), C/N ratiom (50% 95%), nonreducing sugars m (44%), reducing sugars m (39%), starch content m (64%) gs k (46.8%), E k (18%), leaf area m (8%), leaf dry weight m (7%), SLW m (17%), shoot length m (11%), root dry weight m (45%), dry stem weight m (24%) Ps m (25.5% 52.1%), plant height m (14.6% 68%), 100-seed weight m (50% 64.3%), seed yield m (35% 46%), percentage of seed oil m (5.3% 15.4%), total seed protein k (10.9% 12.7%), Ca k (8.2%), Na k (43.6%), K k (35%), Fe k (34.6%), Mn k (26.2), Cu k (23.3%), Zn k (22.6%), percentage of oleic acid m (9%) Percentage of N (ns), leaf area (ns), shoot dry mass m, seed yield m TDM m (22.2% 54%), root dry weight m (65.3%), flower number m, pod yield m (76.7% 119.7%)

Phaseolus vulgaris L.

Uprety and Rabha (1999)

Vanaja et al. (2011)

Pal et al. (2014)

Bunce (2008) Rao et al. (2015)

Oryza sativa L.

760, 1140 and 1520 ppm 1200 ppm

Growth chamber FACE

Ps, gs, and E m up to 1140 ppm but k at 1520 ppm

1200 ppm

FACE

N content k, Ci/Ca ratio (ns), RuBisCO content k

gs k (0% 64%), decrease in leaf water potential k

Bokhari et al. (2007) Shimono et al. (2010) Zhu et al. (2012)

CSTR, Continuous stirred tank reactor; FACE, free air CO2 enrichment; OTC, open top chamber; m, increase; k, decrease; A, net assimilation rate; R/P ratio, respiration/photosynthesis ratio; TDM, above ground total dry matter; FBPase, fructose-1, 6-bisphosphate phosphatase; Vc,max, in vivo apparent RuBisCO activity; Amax: assimilation rate per unit leaf area under light and CO2 saturation; Ps, net photosynthetic rate; gs, stomatal conductance; E, transpiration rate; WUE, water use efficiency; Ci, internal CO2 concentration; Ca, atmospheric CO2 concentration; Fv/Fm, chlorophyll fluorescence; ANPP, cumulative above-ground net primary production; chl, chlorophyll; 2O2: superoxide radical; H2O2, hydrogen peroxide content; LPO, lipid peroxidation; SOD, superoxide dismutase activity; POD, peroxidase activity; CAT, catalase activity; APX, ascorbate peroxidase activity; GR, glutathione reductase activity; PAL, phenylalanine ammonia lyase activity; rbcL, RuBisCO large subunit; rbcS, RuBisCO small subunit; N, nitrogen; ns, not significant.

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forms one molecule each of PGA and 2-phosphoglycolate (PG). PGA is further reduced into carbohydrates and is also utilized in regenerating RuBP. Oxygenation of RuBP generates PG, which is considered a waste product that utilizes a considerable amount of light energy originating from the light reaction of photosynthesis. Sharkey (1985) suggested that at the present atmospheric concentration of O2 of 21 kPa and 380 µmol CO2 mol21, the generation of PG will lead to a decrease in potential photosynthetic capacity by 20% 50% depending on temperature. Bokhari et al. (2007) subjected 10 day-old rice seedlings to 760, 1140, and 1520 ppm CO2 concentrations, respectively for 24 hours each and noticed that Ps, gs, and transpiration rate (E) increased maximally at 1140 ppm CO2, but further treatment to 1520 ppm for 24 hours caused the downregulation of these. Doubling the present atmospheric CO2 will totally prevent C3 photorespiration, which will lead to a rise in the photosynthetic efficiency of these plants (Bowes, 1991; Sage and Kubien, 2007). Seneweera and Norton (2011) suggested three major limitations in C3 photosynthesis, namely: 1. Limitation of photosynthesis urged by RuBisCO due to the supply and utilization of CO2. 2. Supply and utilization of light, which confines the electron transport rate for the regeneration of RuBP. 3. Utilization of triose phosphate, which limits the ease of use of inorganic phosphorus (Pi) in the chloroplast for synthesis of ATP to regenerate RuBP (Farquhar and Sharkey, 1982; Sharkey, 1985). Under elevated CO2 conditions, the second and third limitations are generally observed (Sharkey, 1985; Makino and Mae, 1999). The second limitation may result due to low photosynthetic photon flux densities or the inability to transform light energy into chemical energy. The third constraint, triose phosphate limitation, arises when there is a disproportion in carbohydrate synthesis and its utilization (Sharkey, 1985; Paul and Foyer, 2001). 11.3.3.2 Responses of C4 Plants Plants possessing a C4 photosynthesis mechanism for C-assimilation are able to concentrate CO2 in the mesophyll tissue up to 2000 µmol mol21, which totally suppresses the oxygenation reaction leading to the saturation of carboxylation process (Hatch and Slack, 1968; Poorter and Navas, 2003). Because of this reason, photosynthesis in C4 plants is not supposed

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to rise under EC. However, previous reports suggest that C4 growth is stimulated under EC (Samarakoon and Gifford, 1996; Seneweera et al., 1998; Ziska et al., 1999; Leakey et al., 2006b; Ghannoum, 2009). Increased C4 growth under EC is partially interceded by changes in plant water relations (Seneweera et al., 1998, 2001; Ziska et al., 1999; Ghannoum et al., 2001; Leakey et al., 2004). The progressive responses of C4 plants to high CO2 may be the outcome of several other factors, such as leakage of CO2 into the bundle sheath cells, direct fixation of CO2 in the bundle sheath, and the occurrence of C3-plant-like photosynthesis during the expansion of leaves (Wand et al., 1999). From a meta-analysis, a decrease of 30% in gs of C4 plants under EC was reported, which is comparable to the response of C3 plants (Ghannoum et al., 2000; Ainsworth and Rogers, 2007). In general, EC decreases gs, which in turn lessens the transpiration rate of plants leading to better soil water availability at later growth stages (Seneweera et al., 1998; Leakey et al., 2004, 2006a); however, the mechanisms controlling stomatal movement under EC have not been clearly described. In a report, photosynthesis in maize plant did not increased under EC where there was no soil water scarcity during its growing season, but photosynthesis was enhanced in that year when episodic water stress took place (Leakey et al., 2006b). They concluded that EC indirectly enriches C gain during drought conditions. Therefore, enhanced growth in C4 plants under EC is not a straight photosynthetic response, but might have resulted due to decreased drought stress as water use is lesser, which reserves water in order to increase the duration of photosynthesis (Seneweera et al., 2002; Leakey et al., 2009). 11.3.3.3 Responses of CAM Plants CAM is an adaptation observed in some vascular plants, such as prickly pear (Opuntia stricta), agave (Agave salmania), and pineapple (Ananas comosus). Fixation of CO2 and CO2 metabolism are progressively divided in CAM plants. Fixation of CO2 occurs at night, the early morning, and/or the late afternoon catalyzed by the cytosolic enzyme phosphoenolpyruvate carboxylase (PEPC) to produce malate or aspartate, which is finally stored in the vacuoles (Seneweera and Norton, 2011). Decarboxylation takes place during the daytime and results in the release of CO2 from the malic or aspartic acid, which is finally converted into carbohydrates (Winter and Smith, 1996). Usually, CAM plants possess three to five-fold greater transpiration efficiencies than C3 or C4 plants

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(Nobel, 1996) and frequently these species exist in environments where water shortages prevail (Drennan and Nobel, 2000). Drennan and Nobel (2000) noticed an increase in both day and nighttime CO2 uptake in 10 species with an average biomass rise of about 35% when CO2 was doubled. They suggested that CO2 fixation by RuBisCO is enhanced in the late afternoon along with nocturnal CO2 fixation, however carboxylation activities of both RuBisCO and PEPC are reduced in response to EC. Under EC, nocturnal malate levels increase with increments in carbohydrate contents (Drennan and Nobel, 2000). Reductions in RuBisCO content in CAM species under EC are compensated by the upregulation of enzyme activities in order to maintain photosynthesis. With diminutive evidence of photosynthetic acclimation against EC, some CAM plants display greater CO2 assimilation (source capacity), higher transport of sucrose in the phloem, and sturdy sink strength (Drennan and Nobel, 2000; Osmond et al., 2008). Due to these adaptations, a better understanding of the mechanisms directing C gain in CAM plants may pave the way for new insights into physiological mechanisms that could help in the genetic manipulation of C3 species for C rich environments. 11.3.3.4 Effect of CO2 on RuBisCO In C3 plants, RuBisCO is a rate-limiting enzyme used in the process of photosynthesis and is comprised of about 56% of all soluble protein and 26% of total leaf nitrogen (N) (Makino and Osmond, 1991). According to Mae et al. (1983), the amount of RuBisCO in the leaves is the outcome of the equilibrium between its production and degradation. In plant cells, RuBisCO synthesis is regulated by transcriptional, posttranscriptional, and translational processes (Moore et al., 1999; Stitt and Krapp, 1999). RuBisCO is promptly generated during the development of leaves followed by a progressive degradation (Suzuki et al., 2001). The synthesis of RuBisCO and its degradation are influenced by environmental factors, such as light intensity, soil nitrogen, CO2, and O3. Under EC, changes in leaf N status are strongly associated to a diminution in RuBisCO content and photosynthetic acclimation. Under elevated CO2, Makino et al. (2000) reported a loss of 30% in RuBisCO before it begins to confine photosynthesis. The suppression of RuBisCO synthesis takes place when an imbalance occurs between supply and utilization of carbohydrates under EC (Moore et al., 1998, 1999). Approximately 80% 90% of RuBisCO is formed just before the full expansion of leaf blades in

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cereal crops (Suzuki et al., 2001; Feller et al., 2008; Seneweera and Norton, 2011). Similarly, upregulation in rbcS and rbcL mRNA occurs during leaf expansion and reaches a maximum a few days before full expansion, while little RuBisCO is synthesized after full expansion. According to Ludewig and Sonnewald (2000), the downregulation of photosynthetic genes under EC is evident only in senescing leaves and no association was noticed between gene transcripts and soluble sugars. In monocots, the degradation of RuBisCO is constantly preponderant after complete expansion of the leaf blade leading to a prompt decrease in RuBisCO content. This might be a modifying recovery mechanism in relation to nutrient remobilization for the development of sinks as RuBisCO characterizes a significant store of N as well as a role in its metabolism. However, the mechanism by which EC speeds up degradation of RuBisCO is not well understood. Under elevated CO2, the activities of antioxidative enzymes like superoxide dismutase (SOD), peroxidase (POX), catalase (CAT), ascorbate peroxidase (APX), and glutathione peroxidase (GPX) are lower (Pritchard et al., 2000; Vurro et al., 2009). These enzymes are well-known to combat highly reactive oxygen species (ROS). Under elevated CO2, a decrease in the activities of antioxidative enzymes may lead to an upregulation in ROS levels in the chloroplast, which could possibly contribute to the degradation of RuBisCO.

11.3.4 Biochemical Responses The primary effects of elevated CO2 include the stimulation of photosynthesis and growth in C3 species, declined RuBisCO content, and decreased stomatal conductance (Bowes, 1991; Drake et al., 1997; Ainsworth and Long, 2005; Ainsworth and Rogers, 2007). These physiological effects altogether have led to the general hypothesis that the antioxidant metabolism will be downregulated in plants grown under EC. This hypothesis has led to many assumptions, including: (1) that ROS formation will be conquered by lower rates of RuBisCO oxygenase reaction and subsequent photorespiration at EC (Polle et al., 1993; Mishra and Agrawal, 2014); (2) EC will reduce the electron leakage from photosystem I to oxygen and decrease the chloroplastic oxidative stress (Polle, 1996); and (3) less susceptibility to drought as a result of decreased stomatal conductance will reduce ABA-mediated upregulation of the antioxidant metabolism (Jiang and Zhang, 2002). However, direct evidence to

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support any of these predictions is not regular in the literature and the available studies show mixed up/down-regulation of specific biochemical constituents of the antioxidant metabolism in plants treated with EC (Rao et al., 1995; Polle et al., 1997; Pritchard et al., 2000; di Toppi et al., 2002; Mishra and Agrawal, 2014). Increments in the ascorbic acid and total phenolic contents were recorded in Beta vulgaris (Kumari et al., 2013) and stimulations in the ascorbate (ASC), glutathione (GSH), and ASC/GSH levels, along with their redox status were noticed in Lolium perenne and Medicago lupulina (Farfan-Vignolo and Asard, 2012), when the plants were subjected to elevated CO2. AbdElgawad et al. (2015) noticed that elevated CO2 can decrease the hydrogen peroxide (H2O2) level, lipid peroxidation (LPO), and lipoxygenase (LOX) activity, while the activities of SOD, CAT, GPX, and glutathione reductase (GR) levels were reduced and the ascorbate-glutathione cycle was unaffected in C3 grasses (L. perenne, Poa pratensis) and legumes (M. lupulina, Lotus corniculatus). Therefore, the principal form of the enzymatic antioxidant defense mechanism may sturdily depend on species and applied abiotic stress (Duarte et al., 2013; Singh and Agrawal, 2015). Havir and McHale (1987) found decreased CAT activity but no effect on the activity of SOD in tobacco treated with elevated CO2. Antioxidative enzyme activities were reduced in plants grown under EC compared to plants grown at ambient CO2 levels (Wustman et al., 2001). Based on a study performed by Badiani et al. (1998) with plants grown in naturally occurring EC springs, a mix of up/downregulation of antioxidant enzyme activities was revealed, with SOD and GR upregulated; and CAT, APX, dehydroascorbate reductase (DHAR), and monodehydroascorbate reductase (MDHAR) downregulated. Pritchard et al. (2000) noticed that antioxidant enzymes are commonly downregulated in soybean at elevated CO2. Mishra and Agrawal (2014) reported a marked downregulation of ROS levels, membrane disruption, and the activities of SOD and CAT in mung bean cultivars under elevated CO2. In contrast, a different hypothesis has been formulated which suggests that endogenous ROS production will increase in plants grown under EC leading to greater oxidative stress. Plants grown under EC possess increased rates of respiration (Leakey et al., 2009), a greater number of mitochondria (Griffin et al., 2001), and greater protein carboxylation (Qiu et al., 2008). Therefore, the question of, how the plant antioxidant system will be affected by elevated CO2 levels, is yet to be clearly investigated.

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11.3.5 Molecular Changes in Plants Under CO2 Enrichment Carbon dioxide enrichment can induce a noticeable decrease in photorespiration, advising that there may be an involvement of the expression of the genes in the photorespiration pathway (Sharkey, 1988; Novitskaya et al., 2002; Foyer et al., 2009; Florian et al., 2014; Wang et al., 2014). Differential expression in a number of genes and proteins associated with the process of photosynthesis take place when plants are subjected to elevated CO2 (Gamage et al., 2018). A study by Eisenhut et al. (2013) in Arabidopsis suggested that A BOUT DE SOUFFLE (BOU), a gene encoding a mitochondrial carrier, possibly participates in photorespiration since the knockout mutant bou-2 can check growth at ambient atmospheric CO2, but not at elevated CO2. In another study by Timm and Bauwe (2013), defective plants (glyk1 mutants) containing a gene which encodes glycerate kinase (GLYK), cannot develop at ambient CO2 levels but completely recuperate at elevated CO2, however, the exact mechanism that facilitates the requirement of high CO2 by the mutants is unknown. Florian et al. (2014) noticed that the transcript levels of photorespiratory genes were almost unaffected at elevated CO2 except for reductions in the transcript levels of GDCH1 (glycine decarboxylase H-protein) which is known to be involved in photorespiratory carbon recovery in Arabidopsis. Therefore, the role of photorespiratory gene expression in response to variations in atmospheric CO2 are typically unknown and need further examination (Foyer et al., 2009; Timm and Bauwe, 2013; Florian et al., 2014). Markelz et al. (2014) described the expression of the respiratory genes in Arabidopsis thaliana plants treated with elevated CO2 as having sufficient and limited N availability. The analysis showed that 4439 transcripts were significantly different under ambient and elevated levels of CO2, in particular, the genes related to protein synthesis (constituents of glycolysis, the TCA cycle, and the mitochondrial electron transport chain (ETC, and mitochondrial protein import complexes)) were higher during the day due to CO2 enrichment. Fukayama et al. (2011) noticed the upregulation of the transcription of genes related to respiration in rice under elevated CO2. The grain proteome is altered under CO2 enrichment in wheat, mainly the gluten proteins leading to a poor quality of bread (Wieser et al., 2008; Hogy et al., 2009a; Fernando et al., 2015). Hogy et al. (2009b) observed variations in metabolic proteins which are involved in different physiological processes under elevated CO2. Wieser et al. (2008)

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found that treatment with elevated CO2 decreased the gliadin to glutenin ratio, which led to a damaging effect on dough rheology physiognomies. A study by Panozzo et al. (2014) on wheat reported increments in metabolic proteins under CO2 enrichment. Arachchige et al. (2017) found altered protein composition during proteome-wide analysis in wheat under CO2 enrichment. Additionally, large genetic variations in grain protein concentrations (GPC) have been noticed (Fernando et al., 2014), but grain proteome response mechanisms in different genetic contexts are still unknown under elevated CO2. Hogy et al. (2009b) suggested that responsive proteins under EC may serve as genetic/molecular markers for the selection of associated traits for quality and could, thus, play an important role in prospective breeding programs for adaptation to global climate change. However, analysis of the grain proteome has been always a challenging task due to its broad range of proteins, but the main benefit of this approach is the estimation of post-transcriptional modifications in gene products that are not identified via analysis of transcriptomes. Thus, understanding the response of the grain protein and/or proteome under elevated CO2 conditions will become progressively significant as atmospheric CO2 levels have been predicted to rise in the near future.

11.3.6 Yield The forthright effects of elevated CO2 on photosynthesis and gs lead to variations in crop growth, the allocation of carbon, biomass accumulation, and finally seed yield. In general, crop responses under CO2 enrichment show higher growth and yield, although there are important interactions with N, water, and temperature. It is well-known that increases in seed yield due to EC are lesser in magnitude than the stimulation of photosynthesis and aboveground biomass, signifying that feedbacks constrain the prospective benefits of EC (Long et al., 2004). Foliar respiration during the nighttime in soybean is stimulated under EC (Leakey et al., 2006a, 2009), which decreases plant carbon balance but might be essential for the generation of energy for distribution of extra carbohydrates from the leaves to reproductive sinks. Meta-analysis data from 40 species across 12 FACE sites revealed that growth and aboveground biomass generally increase under EC, with an average crop yield increment of 17% (Ainsworth and Long, 2005). Among different plant groups, C3 species are the most responsive, although there are reports that N-fixing dicots respond well under low nutrient levels (Poorter and Navas, 2003).

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Increased economic yield under EC may involve larger seed or grain size, greater number of seeds/grains per pod/plant or ear/panicle, and/or additional reproductive structures per plant. Under CO2 enrichment, the yield benefit for most C3 crops, like wheat, rice, soybean, and mung bean, is due to increased aboveground dry matter production contributing toward a greater amount of reproductive structures (Pinter et al., 1996; Deepak and Agrawal, 1999; Kim et al., 2001, 2003; Morgan et al., 2005; Mishra and Agrawal, 2014). Under EC, the response of a number of C3 crops other than principal cereals have been studied in FACE experiments, including sugar beet (B. vulgaris), potato (Solanum tuberosum), barley (Hordeum vulgare), and oilseed rape (Brassica napus), which were reported to show greater yields. Tuber production was significantly higher under CO2 enrichment, whereas the production of aboveground dry matter was not changed in potato (S. tuberosum) (Bindi et al., 2006). Miglietta et al. (1998) suggested that the number of tubers, rather than the size of the tubers contributed to an increase in yield. Also, the proportion of deformed tubers was not affected by EC (Bindi et al., 2006). The response of two C4 crops, viz., sorghum (S. bicolor) and maize (Z. mays), under elevated CO2 has been evaluated in FACE experiments (Ottman et al., 2001; Leakey et al., 2004, 2006b). The results suggested that EC had no effect on seed yield when averaged across varying growth conditions and two growing seasons (Ottman et al., 2001; Leakey et al., 2004, 2006b). Under elevated CO2 treatment, the final yield, grain weight, and harvest index of C4 crops were not affected. Ottman et al. (2001) observed an inclination toward higher yield and aboveground biomass when sorghum was grown under high CO2 concentrations and water stress. According to Leakey et al. (2009), this observation supports the fact that in future, C4 plants will benefit from elevated CO2 in times and areas affected by drought, but more studies are needed to reduce the uncertainty of this prediction. A study conducted on sugarcane in opentop chambers (OTCs) showed that elevated CO2 (720 ppm) enhanced photosynthesis, plant height, biomass, and sucrose content by 30%, 17%, 40%, and 29%, respectively (de Souza et al., 2008). The data collected in the study also suggested that sugarcane productivity might increase in the near future; however, the OTCs may have overestimated the effects of elevated CO2 and caused transitory water stress (de Souza et al., 2008). Under CO2 enrichment, the two most studied characteristics of yield quality; protein and nitrogen, are important issues (Uddling et al., 2018). Taub et al. (2008) found a reduction in the protein content of grains

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during a meta-analysis of crops grown under EC. Significant reductions ranging between 10% and 14% were observed in nonleguminous crops, like barley, wheat, rice, and potato, however, for soybean, an important leguminous crop worldwide, the reduction was much smaller, only 1.5% (Taub et al., 2008). The reason may be accredited to the fact that legumes possess the ability to fix N, which would prevent N dilution. A decrease in N concentration has been observed in the grains of wheat (Manderscheid et al., 1995; Pleijel and Uddling, 2012), barley (Manderscheid et al., 1995), and rice (Kobayashi et al., 2006) under CO2 enrichment. Under elevated CO2, GPC was reduced by between 3.9% and 14.1% depending on the treatment system and volume of rooting (Kimball et al., 2002). The largest decrease in GPC was noticed in OTC experiments with restricted rooting volumes, which can be ascribed to a feedback inhibition of the photosynthetic CO2 response and the accrual of nonstructural carbohydrates (Weigel and Manderscheid, 2005). In a meta-analytic study by Taub et al. (2008), a 10% mean reduction in total GPC under elevated CO2 across a range of different environmental conditions was reported. Arachchige et al. (2017) reported a significant decrease in GPC in wheat and the responses varied between different genotypes at an elevated CO2 concentration of 550 6 20 µmol mol21. The findings of Arachchige et al. (2017) also suggested that it was mainly the storage proteins that was reduced at elevated CO2. A significant decline in the protein content of rice grains have also been noticed under CO2 enrichment (Conroy et al., 1994; Seneweera and Conroy, 1997; Uprety and Reddy, 2008). Mishra and Agrawal (2014) found a significant decrease in the soluble protein contents in the leaves and seeds of mung bean cultivars.

11.4 INTERACTION WITH AIR POLLUTANTS Key air pollutants that cause damage to flora include sulfur dioxide (SO2), fluorides, nitrogen oxides (NOx, principally NO and NO2), peroxy-acyl nitrates (PAN), and tropospheric ozone (O3). The different processes of fossil fuel combustion that release CO2 into the atmosphere also will introduce several other air pollutants or their precursors. NOx is produced by high-temperature incineration and participates in photochemical reactions with hydrocarbons, which generate phytotoxic oxidants, O3, and PANs. Other phytotoxic pollutants comprise of carbon monoxide, ambient oxidant complexes (other than O3, PAN, or NOx), chlorine,

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ammonia, hydrogen chloride, mercury vapors, fly ashes, dust, sulfuric acid mist, hydrogen sulfide, and ethylene. Presently, three major phytotoxic pollutants that cause damage to vegetation are O3, SO2, and NO2 due to their prevalent distribution across the commercial world. According to Rozema (1993), high atmospheric CO2 levels are accompanied by gaseous air pollutants, like SO2, NOx, and O3, in industrialized and populated areas. This has provoked widespread concern to estimate the effects of high CO2 and air pollutants (Barnes and Pfirrmann, 1992; Mulchi et al., 1992). The effects of elevated levels of CO2 and SO2 on plant development and productivity have been comprehensively studied individually for a large number of plant species, but in spite of widespread recognition that the levels of several trace gases will rise concurrently with atmospheric CO2 in ambient air, relatively few studies are available on plant responses to increasing concentrations of combined CO2 and SO2 (Carlson and Bazzaz, 1982, 1985; Carlson, 1983; Miszalski and Mydlarz, 1990; Rao and DeKok, 1994; Deepak and Agrawal, 1999, 2001; Agrawal and Deepak, 2003). Reductions in the growth and yield of wheat (Triticum aestivum L. cv. Malviya 234) have been observed, when plants were treated with SO2 alone at a concentration of 0.06 ppm, however, elevated CO2 (600 6 25 ppm) alone and in combination with SO2 stimulated growth and yield (Deepak and Agrawal, 1999). Another study by Agrawal and Deepak (2003) on wheat cultivars (T. aestivum L. cv. Malviya 234 and HP1209) suggested that elevated CO2 changed the plants’ response to elevated SO2. Similar observations were recorded by Deepak and Agrawal (2001) in a study conducted on two cultivars of soybean (Glycine max cv. Bragg and PK 472), where the adverse effects of SO2 was mitigated by elevated CO2 treatment. Saxe (1986) found an increase in photosynthesis of an average of 41% across a range of species under CO2 enrichment of 1000 µmol mol21 as compared to ambient air. Increased photosynthesis was also observed in a combined treatment of CO2 and nitrogen monoxide (NO), but the response was less than that found under CO2 enrichment alone (Saxe, 1986). The responses of the plants to elevated CO2 alone and in combination with NO were closely related day-by-day. Transpiration rate was also decreased under elevated CO2, which was further reduced after the addition of NO. According to Hand (1986), such gaseous conditions occur in commercial greenhouses when hydrocarbon fuels are burned for direct CO2 enrichment, so the effects of the mixture of these gases may be interesting.

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Ozone (O3) present in the troposphere is toxic, and adversely affects crop productivity, thus, posing a threat to global food security (Ainsworth et al., 2012; Mishra and Agrawal, 2015). Both CO2 and O3 have substantial effects on plant physiology and crop production, thus, understanding crop responses to a combination of elevated concentrations of both gases is one of the most important issues in view of future global climate change. Elevated CO2 generally has a growth stimulating effect as it causes a rise in photosynthesis; however, O3 tends to have the opposite effect (Fig. 11.3). According to Barnes and Davison (1988), both factors can directly affect physiological and biochemical processes, such as plant senescence, that might affect plant responses to other biotic/abiotic stresses. The nature of the interaction may be influenced by the features of the O3 exposure pattern (timing in relation to phenological development, chronic/acute exposure), plant species, water availability, and other climatic parameters, but it will also depend upon the kind of effect that is considered, that is, visible injury, photosynthesis, total biomass, or economic yield, etc. In general, elevated CO2 reduces O3-induced leaf

Figure 11.3 A diagrammatic representation of the effects of elevated CO2 and O3, singly and in combination.

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damage and yield losses, primarily through O3 exclusion via a decrease in gs, but also to a certain extent due to an increased detoxification capacity. To date, studies that have observed the effects of CO2 and O3, singly and in combination have shown a variety of plant responses (Table 11.2). Foliar O3 injury (interveinal chlorosis/chlorotic spots) was decreased substantially by CO2 enrichment in a number of agricultural crop plants, for example, tobacco (Heck and Dunning, 1967), wheat (Mortensen, 1990; McKee et al., 1995; Rao et al., 1995; Fangmeier et al., 1996; Mulholland et al., 1997; Cardoso-Vilhena et al., 1998; Mishra et al., 2013a), radish (Barnes and Pfirrmann, 1992), barley (Fangmeier et al., 1996), snap bean (Cardoso-Vilhena et al., 1998), and potato (De Temmerman et al., 2002). For wheat, CO2 treatment swayed the severity of visible foliar damage and provided protection against O3-induced early senescence during vegetative plant growth (Mulholland et al., 1997; Mishra et al., 2013a). However, CO2 enrichment had only a partial protective effect (decreased foliar injury) for a sensitive clone of white clover (Heagle et al., 1993) and had no effect on O3 injury in the leaves of Phaseolus vulgaris (Heck and Dunning, 1967). According to Volin et al. (1998), the exposure of C3 and C4 grass species to O3 increased leaf dark respiration and reduced photosynthesis, which was not observed in an elevated CO2 environment. As repair processes on a cellular level depend primarily on dark respiration, the cost of repair is lower in elevated CO2 conditions. The duration of the dark period is also an important factor for a plant to recover from O3 exposure during the day. De Temmerman et al. (2002) suggested that crops, such as potato, show visible injury symptoms at much lower O3 concentrations during the long days in summer when the dark period becomes too short for repair processes. Previous studies on the impact of high CO2 and air pollutant concentrations revealed that the elimination of toxicity caused by air pollutants at elevated CO2 was largely due to the stimulation of internal detoxification mechanisms rather than reduced pollutant uptake (Barnes and Pfirrmann, 1992; Mulchi et al., 1992). The relative stimulation by elevated CO2 tends to be larger in an atmosphere with increased levels of O3, or vice versa; in a CO2-enriched atmosphere, the negative effects of O3 are less than at ambient CO2. In determinate crops (such as cereals), yield not only depends on photosynthesis but also on the extent of the active period of leaf photosynthesis and the sink capacity of the grains. Booker et al. (2007) demonstrated that elevated CO2 alleviated the inhibitory effects of O3 on the photosynthesis and biomass of peanuts.

Table 11.2 Response of crop plants against combined exposure of CO2 and O3 Plant

CO2 dose

O3 dose

Experimental setup

Plant characteristics

Reference

G. max L.

700 ppm

80 ppb

OTC

Reid et al. (1998)

718 ppm

72 ppb

OTC

700 ppm

1.5 3 Ambient

OTC

550 ppm

1.23 3 ambient

SoyFACE

RuBisCO activity k (ns), RuBisCO content k (ns) Whole plant water loss k (22%), leaf area m (9%), WUE (ns), seed yield (ns) A m (20% 26%), Ci m (2.1 times), total soluble protein k (22% 29%) A m (19%), JPSII m (3%), gs k (16%)

550 ppm

Twice the ambient

SoyFACE

gs k (26%)

Raphanus sativus L.

765 ppm

73 ppb

Controlled chambers

Asat (ns), gs k (62%), WUE m, SLA k, plant growth (ns)

T. aestivum L.

800 ppm

120 ppb

CSTR

1150 ppm

40 ppb

OTC

550 and 660 ppm

140 ppb

OTC

Double than ambient 680 ppm

Low and high

OTC

150 ppb

OTC

680 ppm

1.5 3 ambient and 2 3 ambient

OTC

Shoot biomass m, total chl (ns), carotenoids (ns), rbcL (ns), rbcS (ns) Dry biomass (ns), straw (ns), grain yield (ns), number of seeds (ns), harvest index (ns), 1000 seed weight (ns) Dry matter accumulation (ns), stem dry weight (ns), ear dry weight (ns), total grain dry weight (ns) Grain yield m (18.7% 30%), above ground biomass m (16.9% 30.6%) Flag leaf photosynthesis m (B10% 20%), gs k (B20% 75%), WUE m (B10% 80%) Grain yield (ns), grain protein (ns), straw yield (ns), harvest index (ns)

Booker et al. (2004) Booker and Fiscus (2005) Bernacchi et al. (2006) Gillespie et al. (2012) Barnes and Pfirrmann (1992) Rao et al. (1995) Rudorff et al. (1996) Mulholland et al. (1997) Bender et al. (1999) Donnelly et al. (2000) Pleijel et al. (2000)

S. tuberosum L.

700 ppm

75 ppb

700 ppm

Ambient 1 10 ppb

Controlled chamber OTC

700 ppm

Ambient 1 10 ppb

OTC

714 ppm

72 ppb

Greenhouse

550 and 680 ppm 550 and 680 ppm 680 ppm

60 ppb

OTC

60 ppb

OTC and FACE OTC

50 and 70 ppb

Total dry weight m (95.5%), RGR m (9.06%), K m (3.5%), Fv/Fm k (3.47%) Plant height m (8.7% 9.3%), number of leaves m (11.6% 13.6%), total biomass m (7% 11%), LAR (ns), LWR m (27.6%), RSR m (18.3% 21.2%), number of grains m (12.8% 19%), weight of grains m (34.8% 37.5%), grain protein (ns), TSS m (9.7% 13.9%), starch m (8.3% 10.2%) Total chl m (4% 5.6%), carotenoids k (5.7% 21.6%), Ps m (8.4% 16.4%), gs k (49.5% 50.6%), Fv/Fm m, H2O2 m (8.1% 8.8%), 2O2 m (6% 14.4%), LPO m (8.6% 27%), solute leakage m (12.4% 13%), ascorbic acid m (2% 13.2%), SOD m (5%), APX m (16% 23%), GR m (44.6% 54%), PAL m (34.3% 39.2%), total phenolics m (11.7% 49.3%), protein (ns) Fv/Fm k (2.4%), Ci k (5.1% 8.3%), Vcmax m, Jmax m, Jmax/Vcmax m, K k (6%), RGRs m Tuber yield m (19% 29.7%), fructose content k, glycoalkaloids (ns) Tuber number k, above ground biomass k, green leaf k, senesced leaf dry weights k Dry weight of above ground biomass (ns), tuber number k, fresh weight of tubers m

Cardoso-Vilhena et al. (2004) Mishra et al. (2013a)

Mishra et al. (2013b)

Biswas et al. (2013) Donnelly et al. (2001) Craigon et al. (2002) Finnan et al. (2002) (Continued)

Table 11.2 (Continued) Plant

Beta vulgaris L.

CO2 dose

O3 dose

Experimental setup

Plant characteristics

Reference

Ambient 1 280 ppm 550 ppm

Ambient 1 20 ppb

OTC

Ambient 1 20 ppb

OTC

Persson et al. (2003) Kumari and Agrawal (2014)

550 ppm

Ambient 1 20 ppb

OTC

550 ppm

Ambient 1 20 ppb

OTC

Haulm dry weight k, haulm/tuber ratio k, average size of tubers k, number of tubers m Plant height m (15.5%), number of leaves m (13.3%), leaf area m (58.3%), total biomass m (10.9% 56.2%), starch content m (60.9%), soluble sugar m (26.6%), total nitrogen k (2.4%), organic carbon m (14.4%), C/N ratio m (17.2%), protein k, amino acid k Ps m (21.1% 47.1%), gs k (37.7%), Fv/Fm m (6.4%), total chl m (19.2%), carotenoids m (31.6%), solute leakage k, protein content k (9.6%), ascorbic acid m (5%), POD k (10.3%), SOD k (10.4%), CAT k (28.7%), GR k (76.7%), APX m (13.7%) Root length k (%), shoot length k (11.4%), total plant biomass m (12.8%), total chl k (18%), MDA m (52.2%), CAT m (53.3%), GR m (44%), POD k (18.7%), ascorbic acid m (56.9%), foliar protein k (25.4%), starch m (17.8%), soluble sugar k (29.4%)

Kumari et al. (2015)

Kumari et al. (2013)

OTC, Open top chamber; FACE, free air CO2 enrichment; m, increase; k, decrease; A, net assimilation rate; Asat, light saturated rate of CO2 assimilation; JPSII, whole chain electron transport through photosystem II; K, allometric root: shoot growth; LAR, leaf area ratio; LWR, leaf weight ratio; RSR, root shoot ratio; TSS, total soluble sugars; Vc,max, maximum in vivo rate of Rubisco carboxylation; Jmax, maximum electron transport rate for RUBP regeneration; RGRs, relative growth rate of shoot; MDA, malondialdehyde; Ps, net photosynthetic rate; gs, stomatal conductance; E, transpiration rate; WUE, water use efficiency; Ci, internal CO2 concentration; F0, initial fluorescence; Fm, maximum fluorescence; Fv, variable fluorescence; Fv/Fm, chlorophyll fluorescence; ANPP, Cumulative above-ground net primary production; chl, chlorophyll; 2O2, superoxide radical; H2O2, hydrogen peroxide content; LPO, lipid peroxidation, MDA, malondialdehyde; SOD, superoxide dismutase activity, POD, peroxidase activity, CAT, catalase activity; APX, ascorbate peroxidase activity; GR, glutathione reductase activity.

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The ameliorative effects provided by CO2 enrichment are mainly due to the exclusion of O3 from the interior of the leaves, which is caused by a decline in the gs of the plants upon CO2 enrichment (Cardoso-Vilhena et al., 2004) (Fig. 11.3). Elevated CO2 has been found to escalate the ability of plants to tolerate O3 toxicity (Rao et al., 1995; Gillespie et al., 2012). The amelioration of O3 induced damage has also been reported by Barnes and Pfirrmann (1992) and Mulchi et al. (1992) under CO2 enrichment. In wheat, elevated CO2 fully protects the detrimental effects of O3 on biomass, but not yield (McKee et al., 1997). Similar results have been observed with soybean (Fiscus et al., 1997), cotton (Heagle, 1989), and tomato (Reinert et al., 1997). On the other hand, Pleijel et al. (2000) noticed that wheat grain yield was negatively affected by O3 at ambient CO2 levels but unaffected by O3 at elevated CO2 levels. According to Bender et al. (1999), the response of wheat to elevated O3 and CO2 appears to be cultivar-dependent, as some cultivars do not respond significantly to elevated O3 levels and for those cultivars, no significant interactions between O3 and CO2 were observed. In potato, although CO2 enrichment did not prevent O3 induced yield losses, the increase in yield in response to high CO2 far exceeded the O3-induced losses (Craigon et al., 2002). Vorne et al. (2002) noticed significant interactions between CO2 and O3 regarding the glucose and reducing sugar content in potato tubers. Although a favorable impact of CO2 enrichment on the growth and yield of C3 cereal crops is observed, reductions in flour quality due to declined N content are likely in a CO2-enriched environment (Fangmeier et al., 1999), thereby counteracting the effect of O3 on flour quality (Vandermeiren et al., 1992; Pleijel et al., 1999). Rudorff et al. (1996) indicated that the maximum benefits for wheat production in response to elevated CO2 will not be accomplished under a simultaneous increase in O3 concentration. This observation suggests that predictive models based simply on the impacts of elevated CO2 will result in an overestimation of the possible effects of atmospheric changes on plant productivity (Barnes and Wellburn, 1998). A study conducted by Long et al. (2005) suggests that chamber studies, which have been the main mechanistic base for crop yield models, overestimate the increase in yield by elevated CO2 compared to what was observed under FACE systems (fully open-air conditions) in the field. Based on chamber experiments, the average yield stimulation for C3 crops with CO2 doubling was estimated at 30%, whilst estimates based on results from field-scale

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experiments under realistic conditions (including varying water availability) were lower. According to a review based on the responses of crop plants in FACE systems (Kimball et al., 2002), CO2 enrichment increases biomass in C3 grasses by an average of 12%, grain yield in wheat and rice (O. sativa L.) by 10% 15%, and tuber yield in potato by 28%. As compared to C3 crops, yield stimulation in C4 crops is much lesser. Morgan et al. (2003) suggested that some environmental differences between chamber and open-air microclimates also have an impact on plant interactions with O3 uptake and detoxification. Morgan et al. (2006) found that in an open-air study, the effects of season-long elevation in O3 induced significantly greater grain losses in soybean as compared to chamber experiments. According to Long et al. (2005), if season-long O3 elevation is representative of other major crops growing areas, then yield losses due to rising O3 will even compensate any gains due to rising CO2. Although leaf-level responses to elevated CO2 and O3 are well reviewed, a few studies have focused on canopy level responses to rising levels of these pollutants. In SoyFACE (Soybean-FACE), the results showed reductions in the evapotranspiration rate of soybean for all three treatments (elevated CO2, elevated O3, and elevated CO2 and O3), with the largest decrease observed for growth in elevated O3 (Bernacchi et al., 2006). When combined over a season, plants grown in elevated CO2 and O3 utilized 10% and 18% less water, respectively. While the direct response of soybean exposed to increases in CO2 and in O3 were similar, the mechanisms for these responses differ. Growth under elevated O3 resulted in reduced leaf area as compared with the control. It was expected that the O3-induced damage to the plant canopy, responsible for the lower biomass and leaf area, resulted in lower evapotranspiration in soybean. On the other hand, soybean grown under elevated CO2 revealed higher leaf area while showing a reduction in evapotranspiration, suggesting that a decrease in gs was sufficient enough to offset an increase in leaf area (Bernacchi et al., 2007). These results show that future changes in the atmosphere may influence soybean response to drought conditions and may have feedback effects on atmospheric moisture, potentially altering regional patterns of precipitation.

11.5 SUMMARY The effects of CO2 enrichment have been described with focus on phenotypical, physiological, biochemical, and molecular responses in relation

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to whole plant growth and development. In general, CO2 enrichment imposes its positive effects on plant growth and productivity. The responses of plant species to CO2 are variable and include photosynthetic acclimation at high levels of CO2. Generally, prolonged treatment with elevated CO2 decreases the primary stimulation of photosynthesis in many plant species and often suppresses photosynthesis. Excess accumulation of carbohydrates in leaf tissues may lead to the downregulation of photosynthetic gene expression and increased starch in order to impede the diffusion of CO2. Thus, plants possessing high sink strength for carbohydrate accumulation do not show a suppression of photosynthesis. Suppression of photosynthesis is always associated with reductions in leaf N and RuBisCO contents under high CO2 conditions. Rising global atmospheric CO2 contents counteract the negative effects of atmospheric pollutants (especially SO2, NOx, and O3) on vegetation. Prospective future research on the direct effects of CO2 and air pollutants on agricultural crop species should include interactions with environmental variables (e.g., rainfall, temperature, soil moisture, availability of nutrients, and vapor pressure deficit) that may be involved with predicted future global climate change. Global climate change poses a threat to agricultural productivity, and therefore, to global food and nutrient security. As CO2 is the key substrate for photosynthesis and plant development, exploring mechanisms of atmospheric CO2 utilization strategies in plants will pave the way to enhancing the productivity of agricultural crops to feed the growing population.

ACKNOWLEDGMENTS Authors are thankful to the Head, Department of Botany, Coordinator CAS, Botany, Institute of Science for necessary research facilities and to DST-Purse, ISLS (DBT), Banaras Hindu University and DST-FIST for providing financial support for the work.

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Zhu, C.W., Ziska, L., Zhu, J.G., Zeng, Q., Xie, Z.B., Tang, H.Y., et al., 2012. The temporal and species dynamics of photosynthetic acclimation in flag leaves of rice (Oryza sativa) and wheat (Triticum aestivum) under elevated carbon dioxide. Physiol. Plant. 145, 395 405. Ziska, L.H., 2008. Three-year field evaluation of early and late 20th century spring wheat cultivars to projected increases in atmospheric carbon dioxide. Field Crops Res. 108, 54 59. Ziska, L.H., Bunce, J.A., 1997. Influence of increasing carbon dioxide concentration on the photosynthetic and growth stimulation of selected C4 crops and weeds. Photosynth. Res. 54, 199 208. Ziska, L.H., Bunce, J.A., 2000. Sensitivity of field-grown soybean to future atmospheric CO2: selection for improved productivity in the 21st century. Aust. J. Plant Physiol. 27, 979 984. Ziska, L.H., Sicher, R.C., Bunce, J.A., 1999. The impact of elevated carbon dioxide on the growth and gas exchange of three C4 species differing in CO2 leak rates. Physiol. Plant. 105, 74 80.

WEB REFERENCES https://www.esrl.noaa.gov/gmd/ccgg/trends/global.html (accessed 25.04.2018.). https://www.esrl.noaa.gov/gmd/ccgg/trends/gl_gr.html (accessed 25.04.2018.).

CHAPTER 12

Climatic Resilient Agriculture for Root, Tuber, and Banana Crops using Plant Growth-Promoting Microbes Manoj Kaushal

International Institute of Tropical Agriculture, Ibadan, Nigeria

Contents 12.1 Introduction 12.2 Agriculture in Sub-Saharan Africa 12.3 Impacts on Major Crops of Sub-Saharan Africa 12.3.1 Cassava 12.3.2 Sweet Potato 12.3.3 Potato 12.3.4 Banana 12.3.5 Maize 12.4 Plant Growth Promoting Microbes in Sub-Saharan Africa Agriculture 12.5 Mechanisms of Plant Growth-Promoting Rhizobacteria 12.5.1 Direct Mechanisms 12.5.2 Indirect Mechanisms 12.6 Plant Growth-Promoting Rhizobacteria as Biofertilizers and Biocontrol Agents 12.7 Role of Microbes in Resilience of Root, Tuber, and Banana Crops Impacted by Climate Change 12.8 Microbial Adaptation in Sub-Saharan Africa 12.9 Conclusion References Further Reading

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12.1 INTRODUCTION Sub-Saharan Africa (SSA) is amongst the poorest regions of the world, where around 386 million people earn less than US$1.25 per day

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(Ravallion, 2012). Agriculture is a major source of income here, employing 65% of the labor force and contributing up to 32% to the national gross domestic product (Chauvin et al., 2012). Root, tuber, and banana (RTB) crops are major crops and they represent the backbone of food security and nutrition across the tropics of SSA. They are the most important source of nutrition and income for an average of 300 million poor people in different developing countries. RTB crops constitute the major staple foods of SSA, with banana, cassava, yam, cocoyam, Irish and sweet potatoes as the major ones. Besides the low production costs, RTB crops (such as yam, cassava, potato, and sweet potato) are also rich in nutrients. They contribute to the energy and an important source of income in various areas of SSA mostly populated by smallholder farmers. Along with food security, they are regular food crops, cash crops, livestock feed and serve as raw material for many industrial products. However, the production of RTB crops is greatly influenced by nutrient exchange, energy, soil environment, and atmosphere (Lehmann and Kleber, 2015). Also, the indiscriminate use of chemical fertilizers has led to a reduction in soil pH and exchangeable ions causing an unavailability of nutrients for crops, thus, leading to a decrease of productivity. RTB crops also suffer from biotic stress (insects, disease) and abiotic stress (light, temperature, etc.,) that the environment imposes (Gabriela et al., 2015). In SSA agricultural productions are mostly rainfed, and thus, their success is totally dependent on climate variability. The global temperature has been rising since the late 19th century. The impacts of climate change will lead to a decrease in the crop productivity of RTB crops in various regions of SSA where 95% of agriculture is still rainfed. This is because high temperatures can lead to reduced yields due to elevated development rates and enhanced respiration. Although, climate change could also result in increased incidence of diseases, leading to economic losses and vulnerability of various crops. Plant microbe interactions are vital to responding to these intense biotic and abiotic situations, resulting in better economic viability and environmental intensification (Compant et al., 2016; Khan et al., 2016). There are different approaches to achieving this, which include biofertilization, phytoremediation, and plant stress control (Goswami et al., 2016). Microbial populations colonize, interact, and associate with their hosts in various activities through different means. These include absorption of water, nutrient uptake from a limited soil nutrient pool, and stabilizing plant stress. However, environmental conditions are drastically altered due to climate change, which impacts on beneficial plant microbe

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associations. For instance, elevated CO2 concentrations diminished the growth of mycorrhizal hyphae and thus, alter the whole structural network of mycorrhizal systems. Higher CO2 concentrations increase C allocation to plant roots, which impacts on the normal physiological and growth promoting attributes of rhizospheric microbes. Drought is expected to be responsible for severe impediments to the growth of crops on more than 50% of the Earth’s arable land by 2050 (Vinocur and Altman, 2005). In drylands, drought stress is a major consequence of climate change and is responsible for significantly decreasing the microbial colonization process (Kaushal and Wani, 2016). Drought usually affects root activity, general morphology, and the functioning of host plants and their interactions, which are parallely related to impacts on potential crops as well modifications of pest and pathogen activities. In addition, drought causes losses in photosynthates acquired in plants during photosynthesis as well as decreasing the formation of extra mycorrhizal mycelium in plant roots. Sometimes late-maturing cultivars can face drought, however, crops with shorter life spans can escape harm due to early maturity before the drought arrives. In regards to the climate change issue, exploration of beneficial microbes in integrated nutrient management systems is necessary to combat the agriculture against drought and disease stress situations. There is sufficient data published on the enhancement of plant growth through plant growth-promoting microbes but only a paucity of information is available on the potential of these microbes under drought stress conditions in crops. Very little effort has also been made to introduce beneficial rhizospheric microbes as a mitigative tool in climate resilient agriculture. This chapter highlights the impacts of climate change on RTB crops and describes approaches involving plant growth-promoting microbes used to mitigate climate change impacts and enhance their productivity under drought stress in SSA. The chapter also identifies some of the challenges that climate change might pose to crop improvement and describes the efficacy of rhizobacteria to overcome these challenges for dryland agriculture. The potential impacts of climate change on the performance of plant growth-promoting microbes are also reviewed. Special focus is given to countries designated by the United States Agency for International Development (USAID) for the Feed the Future (FTF) program. These countries are specified because of their high poverty and hunger rates, greater opportunities for agricultural-led growth, host country leadership and governance, and resources availability (Ho and Hanrahan, 2011).

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12.2 AGRICULTURE IN SUB-SAHARAN AFRICA Agriculture is a major contributor to the economies of various countries in the world. In SSA, agriculture is dominated by smallholder farmers who contribute up to 90% of the agricultural production (Wiggins and Keats, 2013). Major crops of RTB are used as food as well as cash crops and include cassava, yam, potato, maize, sweet potato, and banana. Despite the major contribution of RTB crops to the economies of the countries of SSA, yields remain beneath the global average. In general, availability of water is considered as the primary limiting factor with only 3.5% of the total cultivated area under irrigation (Foster and Briceno-Garmendia, 2010). Temperature in SSA is projected to rise more rapidly than in the rest of the world, which may outpace a 4°C increase by the end of 21st century. However, in areas with enough water and heat (due to climate change) it has been predetermined that pathogen and insect prevalence will further damage agricultural crops (Ziska et al., 2011). Also, if the temperature in a particular region goes higher than normal, climate change and precipitation variability could become the limiting factors for RTB crops. Precipitation variability also adds to the magnitude and recurrence of drought, decreases water availability of crops, and reduces the productivity of rainfed agriculture in SSA. Among the various physiological changes brought about because of climate change, drought stress is the most important and widely studied. Drought stress leads to an overall decrease in the yields of RTB throughout SSA by reducing the length of the growing season, amplifying water stress, and increasing the incidences of disease and pest outbreaks. During developmental phases drought alters carbon-assimilation processes, including transpiration, photosynthesis, and respiration, resulting in low plant growth and productivity (Bita and Gerats, 2013). Even short duration heat shock when coinciding with the reproductive stage substantially lowers the crop yield (Teixeira et al., 2013), with a reduction in leaf area and the closing of stomata to minimize water loss. Drought stress is also projected to reduce the length of the growing season while spatially shrinking the suitable areas for crop production (Kaushal and Wani, 2015).

12.3 IMPACTS ON MAJOR CROPS OF SUB-SAHARAN AFRICA 12.3.1 Cassava Cassava is one of the most vital crops in SSA in relation to caloric intake (Rosenthal and Ort, 2012). In terms of its total production and as an

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important staple food, cassava is traditionally cultivated mainly in the Great Lake regions of Africa (Kenya and Tanzania) and in some parts of northern Zambia and Mozambique (Fig. 12.1). Cassava is more resilient to drought due to its tolerance to high temperatures as compared to other RTB crops. However, a prolonged drought period during the root thickening initiation stage leads to root yield declines of up to 60% (Jarvis et al., 2012). It is also well-studied that cassava has superior yield gains than that of other crops at high CO2 concentrations and can even recover from severe drought conditions (Rosenthal and Ort, 2012). Studies on cassava reported positive or minimum impacts and better performance of cassava crop in the near future under the raised CO2, elevated temperature, and uncertain rainfall patterns that have been projected. Using 16 models under the A1B storyline, 8% yield reduction is projected for cassava compared to the 17% 22% reductions for other crops, such as maize, sorghum, millet, and groundnut, by the mid-21st century in SSA (Schlenker and Lobell, 2010). Also, a slight enhancement of cassava production is projected in east Africa by 2030 compared to 2000 (Lobell et al., 2008). Another study, utilizing the Improved Global Agro-Ecological Zones method under the A1B storyline, projected a 10% enhancement in cassava yields in Africa by the 2090s compared to the 1990s (Tatsumi et al., 2011). However, studies conducted by the International Food Policy Research Institute (IFPRI) using the IMPACT model, showed an elevation in cassava production of between 40% and 100%

Figure 12.1 Production of (A) cassava and (B) sweet potato in SSA. FAO

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in Malawi, Rwanda, and Uganda with no significant change in Mozambique and Tanzania by 2050 (Maure et al., 2012). Employing the same model, a raise in cassava productivity by 0.42% and 0.75% in eastern Africa and southern Africa respectively is projected for 2050 (Ringler et al., 2010). Overall, cassava yield will be the least impacted under different climate change scenarios compared to maize, beans, potato, and banana, thus, making it a potential candidate to ensure future food security in SSA.

12.3.2 Sweet Potato Sweet potato is the most widely grown crop in SSA, mostly in Uganda, Rwanda, and parts of Tanzania, Kenya, and Ethiopia. Mainly cultivated by smallholder farmers, sweet potato is a major staple food in SSA and is also the most important source of carbohydrates. Sweet potato is mostly grown at altitudes of 800 1900 m a.s.l with temperatures between 20°C and 25°C, sometimes ranging from 15°C to 33°C. For tuber formation, low night temperatures are required, but higher temperatures during the day support vegetative growth. The susceptibility of sweet potato to drought stress and the low temperatures needed during the night for tuber formation make the crop vulnerable to climate change (Agili, 2012). The impacts of climate change on the production of sweet potato are sparsely known, but in comparison to other SSA crops it was the second most impacted after wheat. Hikes of 1.06% in eastern Africa and 1.14% in southern Africa are projected in sweet potato yield by 2050 through the utilization of the IMPACT model (Ringler et al., 2010). Also, a 15% decline in production is projected in eastern Africa by the 2090s compared to the 1990s if five general circulation models (GCM) are employed under the A1B storyline and the Improved Global Agro-Ecological Zones model (Tatsumi et al., 2011).

12.3.3 Potato Mostly grown by smallholders, potato is grown in all the FTF nations, mainly focused in the highland areas. Malawi and Kenya are the biggest potato producers in SSA (Fig. 12.2). Studies have also indicated that Kenya has exceeded Malawi with over 5 million tons of potato production in 2012 (FAO, 2013). In Kenya and Rwanda, potato is the second most important crop following maize and banana, respectively, mostly grown in the highlands of the

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Figure 12.2 Production of potato in SSA. FAO

southwest and northern regions (Muhinyuza et al., 2012). Potato is cultivated by about 500,000 smallholder farmers, making it one of the most important sources of income and employment in rural areas. Also, Rwanda (2.3 million tons) and Tanzania (1.8 million tons) rank third and fourth among the FTF nations, respectively, while Zambia is the lowest producer with only 30,000 tons in total annual production (FAO, 2013). Optimally grown at 17°C, potato production has been observed to decrease above this temperature due to the reduced development and productivity of the plant caused by stress or decreased assimilative partitioning to the tubers. Stress due to moisture also reduces crop yields by contracting the growing and dormancy periods and by lowering the number and size of potato tubers. Like the C3 crop sweet potato, elevated atmospheric CO2 concentrations enhance potato yields by multiplying the number of tubers, but actual yield profits may be insignificant under limited fertilization and water stressed conditions (Ainsworth and McGrath, 2010). Also, like sweet potato, the impacts of climate change on potato production in SSA has been scarcely studied.

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The impacts of increased CO2 concentrations on water use efficiency and yield were observed using six coupled GCMs under the A2 storyline in four potato-growing agroecosystems (Haverkort et al., 2013). Employing the IMPACT model, the IFPRI predicted an up to 100% enhancement in potato yields and a 50% increase in cultivation area in Rwanda in 2050 compared to 2010 (Tenge et al., 2012), leading to the prediction of double or triple the potato production by 2050. However, using 20 GCMs under the A1B storyline, a 15% reduction in potato yield is projected in Africa by 2030 (Jarvis et al., 2012). Similarly, utilizing five GCM models under the A1B storyline, a 17% decline in potato yield is projected in Africa in the 2090s compared to the 1990s (Tatsumi et al., 2011).

12.3.4 Banana In Uganda and Rwanda banana has an annual per capita consumption of .135 kg (FAO, 2013), but also being largely produced and consumed in other regions of SSA. The crop is mostly grown in East Africa, including southwestern and central Uganda, some parts of Rwanda, the northern, southern, and eastern highlands in Tanzania, and the central and Kisii regions in Kenya (Fig. 12.3). Studies have revealed that drought stress is the most critical constraint to banana production in the region (Van Asten et al., 2011). Another study demonstrated that banana plants can survive water stress for long periods of time as well as minimal soil moisture, but extended exposure to intense temperatures (above 35°C) can lower banana production (Thornton and Cramer, 2012). In parts of SSA that receive annual rainfall of below 1100 mm, droughts reduce yields by up to 65% (Van Asten et al., 2011). By 2020, in parts of Eastern and Southern Africa, banana production is projected to experience an increase in suitable areas ranging from 1% to 11% (Ramirez et al., 2011). On the other hand, highland bananas are projected to observe a significant loss in overall yield due to the inflation of pests and diseases if the temperature were to rise by 2°C (Thornton and Cramer, 2012). Similar to other RTB crops, quantitative measures under climate change impacts for banana yield are limited. In general, slight increase in environmental temperature would bring positive impacts for banana yield in highland areas, however, it could bring negative impacts for banana yield in lowland areas in the near future.

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Figure 12.3 Production of banana in SSA. FAO

12.3.5 Maize Maize, primarily grown by smallholders, is the most widely cultivated (on about 25 million hectares) staple crop in SSA, 77% of which is consumed as food (Fig. 12.4). In addition to being a source of dietary protein, maize is also the second most significant source of calories in eastern and southern Africa (Broughton et al., 2003). Compared to other crops of SSA, the impacts of climate change on maize are well studied. An increased maize yield was observed with an elevation in temperature up to 29°C followed by a sharp decline with further increases (Schlenker and Roberts, 2009). The optimum temperature for growing maize is 25°C, however, each degree increase above 30°C has been found to lower the final maize yield by 1% (Lobell et al., 2011). Some studies suggest that a 1°C increase above the norm lowers maize yield by 10%. Temperatures between 32.2°C and 37.8°C are good for corn yield if available with adequate moisture. A 9% decrease in maize yield with every 1-inch cutback in rainfall was observed with high

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Figure 12.4 Production of maize in SSA. FAO

temperatures, which suggests that maize is susceptible to heat as well as to moisture stress. Also, between 3% and 19% decrements in maize yields are projected in the FTF countries by 2055 compared to 2000 utilizing the CERES-Maize model with Mozambique, the most affected country ( Jones and Thornton, 2003). Yield profits in Kenya and Rwanda of 5% and 11% by 2030 and 18% and 15% by 2050, respectively, are attributed to elevated temperatures (Thornton et al., 2010) that may bring growing season temperatures close to optimum. A 7% 10% attrition in maize yields were observed by 2050 in SSA under the A2 storyline (Nielson, 2009). A 1°C rise in temperature is projected to lower maize yield by 65% even under good rainfed conditions (Lobell et al., 2011). Despite the large variations in projections observed, it has been widely accepted that climate change will adversely disturb maize yield in SSA and could increase losses by up to 40% of its production by the end of the 21st century.

12.4 PLANT GROWTH PROMOTING MICROBES IN SUBSAHARAN AFRICA AGRICULTURE Rhizosphere is the ecological niche surrounding plant roots with high microbial populations that are greatly influenced by root exudates.

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In general, the ratio of microbial population in non-rhizospheric to rhizospheric soil is 1:10. The higher microbial population in the rhizosphere region is because of root exudates secreted by plant roots which microbes utilize efficiently. Plant roots also secrete photosynthetic products (about 5% 30%) in the form of different sugars, which in turn are used by microbial populations (Glick, 2014). The group of bacteria that reside in rhizospheric soil regions and that improve plant growth and yield are known as plant growth-promoting rhizobacteria (PGPR). The majority of PGPR belong to genera Acinetobacter, Agrobacterium, Azotobacter, Azospirillum, Bacillus, Burkholderia, Rhizobium, Frankia, Serratia, Pseudomonads, and Bacillus (Vessey, 2003). PGPR boost plant growth and yield through various direct and indirect mechanisms. PGPR also help plants cope and increase yield during stress conditions through various mechanisms called RIDER (rhizobacteria induced drought endurance and resistance), a term coined by Kaushal and Wani (2015).

12.5 MECHANISMS OF PLANT GROWTH-PROMOTING RHIZOBACTERIA In order to maximize growth and yield, complete knowledge of PGPR mechanisms is required to manipulate microbial flora in the rhizosphere region. In general, PGPR aids the direct and indirect mechanisms of plants. Direct mechanisms activate plant metabolisms toward enhancing their adaptive capacity (Govindasamy et al., 2011), whereas indirect mechanisms involve plant defensive processes (induced systemic resistance and systemic acquired resistance).

12.5.1 Direct Mechanisms Rhizosphere bacteria have high potential to produce various classes of well-known phytohormones, including auxins, gibberellins, cytokinins, ethylene, and abscisic acid. Plants respond well to these phytohormones in the rhizosphere which can mediate various processes, including plant cell enlargement, division, and extension in roots (Glick, 2014). Indole-3-acetic acid (IAA), also known as auxin, produced by various PGPR are primarily involved in plant growth and development processes, such as cell elongation, cell division, and tissue differentiation. Continuous treatment of IAA in plants with highly developed roots, allows plants to uptake more nutrients ultimately improving overall plant growth (Aeron et al., 2011). IAA produced by rhizobacteria also elevate the size and surface area of root systems in contact with soil, which leads to an increased

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ability for nutrient and water uptake ultimately improving plant growth and yield (Kaushal et al., 2017). Cytokinins is another class of phytohormones produced by PGPR. Cytokinin application in plants results in enhanced cell division and shoot initiation (Jha and Saraf, 2015) by influencing their physiological and developmental mechanisms. Some other processes in plants, like nutritional signaling, expansion of leaf, promotion of seed germination, and delay of senescence are also greatly influenced by cytokinins (Wong et al., 2015). Gibberellins (GAs), another group of phytohormones, influence many developmental processes in higher plants and can travel from roots to the aerial parts of plants. GAs are responsible for seed germination, flowering, and fruit setting (Hedden and Phillips, 2000). The enhancement of plant growth and yield by GA producing PGPR has been widely reported (Gutierrez-Manero et al., 2001). Impacts on the aerial parts of plants are increased when PGPR also secrete auxins that stimulate their root system architecture (RSA) through elevated nutrient supply, which supports growth of the aerial parts (Wong et al., 2015). Nitrogen and phosphorous are the most limiting nutrients to plants. Despite the abundance of phosphorous present in soil, it is not in an available form that is suitable for plant uptake. Plants can only absorb monoand dibasic phosphate in soluble form (Jha and Saraf, 2015). PGPR that behave as phosphate-solubilizing bacteria (PSB) are responsible for the solubilization of complex structured phosphates, such as tricalcium phosphate and rock phosphate. Phosphate solubilizing microbes (PSB) converts organic phosphorus into inorganic form through the secretion of acids because of sugar metabolism and ultimately available to the plants. Microbes inhabiting the rhizosphere utilize sugars from root exudates and metabolize them to produce organic acids (Goswami et al., 2014). These organic acids further act as good chelators of divalent Ca21 cations and release phosphates from insoluble phosphate compounds. PSB also lowers the pH of the medium through the secretion of organic acids, such as acetic, malic, oxalic, and citric acids. PSB isolated from the rhizosphere regions are metabolically more active than those isolated from non-rhizosphere/bulk soil. Among the soil bacterial communities, Pseudomonas and Bacillus spp., have been identified as excellent phosphate solubilizers (Goswami et al., 2014). The most common organic acids produced by PSB are oxalic acid, citric acid malonic acid, succinic acid, and glycolic acid (Jha and Saraf, 2015).

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Nitrogen fixation by PGPR is an important mechanism that has been widely studied. Traits of nitrogen fixation by PGPR are either of root/legume-associated symbiotic bacteria (Rhizobium spp.) or free living bacteria (Azotobacter spp.). Symbiotic bacteria possess the specificity and infect the roots to produce nodule or free-living nitrogen fixers (Oberson et al., 2013). Free-living nitrogen fixers include Azospirillum, Azotobacter, Bacillus, Burkholderia, and Herbaspirillum. In general, these nitrogen fixers fix between 20 and 30 kg of nitrogen per hectare per year. Azotobacter chroococcum and Azospirillum brasilense have gained importance especially in cereals as they possess the nif gene cluster which codes nitrogenase, a key enzyme required for nitrogen fixation.

12.5.2 Indirect Mechanisms Iron, an essential nutrient for plants, also acts as a cofactor for many enzymes required for physiological processes such as nitrogen fixation. Despite the fact that iron is quite abundant in soils, like phosphorus, iron is frequently unavailable for plants and soil microbes. Plants either release certain organic compounds that can chelate iron and convert it into soluble form that can then be absorbed by the enzymatic system, or they directly absorb the complex formed by the organic compound and Fe31 and the iron is then reduced inside the plant and thus readily absorbed. Some PGPR release low molecular weight iron-chelating compounds (siderophores) in the rhizosphere, which serves to attract iron toward the rhizosphere and thus be readily absorbed by plants. Mostly the genus Pseudomonas produce siderophores to increase their competitiveness, and thus, improve plant health. Siderophores also improve iron nutrition, inhibit the growth of other microbes by releasing antibiotics, and suppress the growth of pathogens by limiting the iron availability to them. Another indirect mechanism of PGPR is to control soil borne pathogens through the secretion of cell wall breaking enzymes, such as β-1,3glucanase, chitinase, and cellulose, which exert a direct inhibitory effect on the cell wall of phytopathogens. These cell wall-degrading enzymes secreted by rhizobacteria negatively impact on the structural integrity of the walls of the targeted phytopathogen. For instance, chitinase degrades chitin, the major component of fungal cell walls. Various strains of Paenibacillus sp. and Streptomyces spp. can synthesize β-1,3-glucanase, which degrades the cell walls of Fusarium oxysporum. Similarly, Bacillus cepacia

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synthesizes β-1,3-glucanase, which inhibits the cell walls of Rhizoctonia solani, Phythium ultimum, and Sclerotium rolfsii (Compant et al., 2005). The most common PGPR belong to Bacillus and Pseudomonas species and also play an important role in the suppression of pathogens through the production of antibiotics. These antagonists suppress phytopathogens by secreting extracellular metabolites, which are inhibitory even at very low concentration. These include a wide variety of antibacterial and antifungal antibiotics, such as subtilin, subtilosin A, mycobacillin, rhizocticins, surfactin, iturin, and fengycin. An effective strategy of PGPR for plant protection is the induced systemic resistance. In this PGPR elicits host defense responses and thus reduce the incidence of diseases caused by phytopathogens that are spatially differentiated from the inducing agent. In this process the inducing rhizobacteria in plant roots produce a signal which disseminates systemically in different parts of the plants and enhances the defensive capacity of far-flung tissues against subsequent infection by phytopathogens (Thakker et al., 2012).

12.6 PLANT GROWTH-PROMOTING RHIZOBACTERIA AS BIOFERTILIZERS AND BIOCONTROL AGENTS Several PGPR strains are used as biofertilizers and biocontrol agents and are commercially obtained as formulated products (Jha and Saraf, 2015). In general, alginate gel is used to prepare bacterial and fungal formulations (Desai et al., 2002). Bacterial biofertilizers are formulated in many ways depending upon the nature of isolated strains. Sporulating Gram-positive bacteria that possess heat-resistant spores are exploited to formulate stable and dry powder products. The suspension of microbes in oil is an alternative to solid-powdered formulation, where oxygen is excluded to prevent respiration (Kamilova et al., 2015). Sometimes silica gel is added to oil formulations in order to enhance shelf life. Currently, commercialization of microbial based products are receiving huge attentions especially in SSA and thousands of companies are commercializing this as biofertilizers/biocontrol agents, of which the most exploited genera are Pseudomonas, Bacillus, F. oxysporum, Pythium aphanidermatum, Streptomyces griseoviridis strain K61, Bacillus licheniformis strain SB3086, Coniothyrium minitans, and many others. Many Gram-negative bacterial strains are also highly efficient as biocontrol agents, but they are difficult to formulate due to nonproduction of spores, short shelf life, and being easily destroyed

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when the formulations are desiccated. The main challenge faced by commercial developers is varying agroclimatic and environmental conditions, such as temperature, rainfall, soil type, cultivar, which change from one field/location to another, which cause variations in the potentiality of biofertilizer strains (Kamilova et al., 2015). However, researchers are still trying to develop better biofertilizers/biocontrol agents with improved shelf life that possess better adaptability in various agroclimatic zones and compatibility with different hosts. These improved bio-fertilizers could bring huge scope for enhancing agricultural productivity.

12.7 ROLE OF MICROBES IN RESILIENCE OF ROOT, TUBER, AND BANANA CROPS IMPACTED BY CLIMATE CHANGE Statistical shifts in the circulation of weather patterns over a period of time is known as climate change (Compant et al., 2010). Plant microbe interactions are greatly affected as climate change alters environmental conditions drastically because all microbial processes are dependent environment (Classen et al., 2015). Rising CO2 concentrations increase carbon allocation to plant roots and thus, impact on the normal physiological and growth promoting exercises of root linked microbes. The impact of elevated temperatures on plant microbe interactions are variable and may affect the performance of plant-beneficial bacteria (Egamberdiyeva and Hoflich, 2003) with varying soil types. However, there are certain PGPR that perform better at high temperatures and are, thus, of greater interest for application in climate smart agriculture. Global temperatures are continuously rising due to climate change and are predicted to elevate between 1.8°C and 3.6°C by the year 2100. Another major consequence of climate change is drought stress which effects many RTB crops in SSA and also leads to microbial community shifts under low soil water availability. Drought is also known to bring about many physiological, biochemical, and molecular changes in plants leading to reductions in crop productivity. In such stress conditions, rhizobacterial inoculants can, thus, be used either as a biofertilizer or phytostimulator depending on their mode of action and efficacy (Sharma et al., 2014). The ability of plants to sustain growth and survive during long periods of drought stress is known as drought resistance (Chaves et al., 2003). Plants have developed mechanisms to fight drought stress, including morphological adaptations, osmotic adjustment, antioxidant systems, reactive oxygen species, and a variety of stress-responsive genes and proteins (Kaushal and Wani, 2016).

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Drought stress also affects rhizospheric microbial populations through osmotic stress and resource competition (Chodak et al., 2015), which can result in the damage of nucleic acid via alkylation or oxidation, crosslinking, or base removal. It also results in the accumulation of free radicals because of conformational protein changes, restricted enzyme efficiency, and changes in electron transport chains (Berard et al., 2015) leading to the denaturation of proteins, lipid peroxidation, and ultimately cell lysis. Rhizobacteria employ a variety of physiological mechanisms to protect cell structures and other organelles from drought stress. These include the accumulation of compatible solutes and heat shock proteins and the production of exopolysaccharide and spores. Compatible solutes, such as proline, glycine betaine, and trehalose, enhance thermotolerance of enzymes, suppress protein denaturation, and preserve membrane integrity (Kaushal and Wani, 2016). Exopolysaccharides (EPS) protects the cell membrane integrity of plants. Some bacteria store large quantities of ribosomes and respond with rapid protein synthesis during stress conditions (Placella et al., 2012). Many drought-tolerant varieties developed through conventional plant breeding techniques have been used to diminish the negative impacts of drought stress on crop growth and yields (Eisenstein, 2013). Currently, plant microbe interactions have received a lot of attention for increasing crop productivity by providing resistance against various types of stress (Yang et al., 2009). Some well-studied examples of plant microbe interactions include mycorrhizal fungi (Azcon et al., 2013), nitrogen-fixing bacteria (Lugtenberg and Kamilova, 2009), and plant growth-promoting rhizobacteria (Glick, 2012). PGPR have the ability to colonize roots and produce various enzymes and metabolites that benefit plants in biotic and abiotic stress tolerence (Chauhan et al., 2015). Studies have been conducted on harnessing beneficial soil microbes to boost crop production under changing climatic conditions (Nadeem et al., 2014). PGPR assist all RTB crops with their efficacy to confer drought tolerance. In SSA, plants possess adaptive traits to endure drought stress and improved RSA with PGPR. RSA integrates root system topology, primary and lateral roots spatial distribution, and the number and length of various diameters of roots (Vacheron et al., 2013). A correlation between prolific root systems and drought resistance has been observed in several crops, such as soybean (Sadok and Sinclair, 2011), maize (Hund et al., 2011), and wheat (Wasson et al., 2012). Plants inoculated with PGPR under drought stress are able to maintain normal shoot growth, leading to an enhancement in crop productivity. Relative water content

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(RWC) in plants measures water status and maintains metabolic activity in tissues. A decline in RWC results in loss of turgor pressure which limits cell expansion and reduces growth in plants (Castillo et al., 2013). Maize plants, when treated with PGPR strains under drought stress, displayed elevated RWCs compared to nontreated plants (Naveed et al., 2014). Osmotic adjustment protects enzymes, proteins, cellular organelles, and membranes from oxidative damage (Huang et al., 2014). Proline is another important osmolyte that accumulates in plants due to drought stress and contributes to stabilizing cellular structures and redox potential. Increased proline levels have been demonstrated in maize (Naseem and Bano, 2014) which confers drought tolerance by protecting plants from dehydration. Antioxidant enzymes (such as catalase, superoxide dismutase, glutathione reductase, ascorbate peroxidase) represent another approach used to assess and cope with drought stress, which serves to minimize oxidative injury. PGPR also promote plant growth during drought stress by modifying the phytohormone content (Bresson et al., 2014), such as decreasing ethylene production and balancing Abscisic acid (ABA) and Indole acetic acid (IAA) signaling. It is also evident, that plant-associated bacteria elicit a plant response by inducing systemic tolerance. Climate change/stress may impact on all types of beneficial plant microbe interactions. In general, drought stress decreases the colonization of plantbeneficial microbes, but inoculation with PGPR diminishes drought stress and enhances plant performance. Also, the compositions of microbial communities directly relate to plant physiology and are driven by root exudates. Sometimes plant growth-promoting rhizobacteria support plants to adapt to drought stress/new environmental conditions.

12.8 MICROBIAL ADAPTATION IN SUB-SAHARAN AFRICA Microbial adaptation aimed at improving the growth and yield of RTB crops in SSA involves several other critical components, such as soil health, water conservation, and capacity building. It is evident from this chapter that RTB crops will be primarily affected by drought stress in the future in whole of SSA being most severe in the eastern regions. For example, with maize being more affected by drought stress, it would be useful to switch drought-sensitive with drought-tolerant crops, such as cassava, which may mitigate temperature stress-related crop failure. As an adaptive precaution to climate change, growers in SSA have already begun mixed cropping selections, such as cassava with coffee crops, based

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on the prevailing climate. Climate impacts in future could bring more water runoff in northern parts of SSA than that of southern regions (Adhikari et al., 2015). Also, wide resource and social constraints may impact on the capacity of smallholder farmers to adopt irrigation as an adaptation in SSA. Thus, currently, farmers are also more aware and enthusiastic for the use of commercialized technologies of PGPR/endophytes available on the market to mitigate various types of stress. The development and commercialization of drought-tolerant microbial strains also helps to reduce climate change impacts on RTB crops in droughtprone areas. Growth and yield of microbially inoculated plants raised up to 40% advising the potentiality of PGPR in drought stressed agriculture. Thus, the role of plant-associated microbiomes against drought in plant adaptation is highly emerging. Microbiome configuration may differ significantly in fluctuating environmental circumstances depending on the taxonomic vulnerability of the linked plant species.

12.9 CONCLUSION Climate change negatively influences the growth and yield of RTB crops in SSA due to changes in plant physiology and root exudation. It is projected that the yields of different RTB crops will decrease by up to 65% by 2030. Drought stress induced by climate change also has a significant impact on beneficial microbial populations in soil and thus on plant microbe interactions. In general, drought decreases the colonization of plant-microbes, but it is also true that plant performance is improved by reducing the impacts of drought when inoculated with PGPR. Growing drought-tolerant varieties is one way to cope with the climate change scenario. Sometimes drought-tolerant cultivars are not fully adapted to new environmental conditions, which could also be supported by inoculating with potential plant growth-promoting microbes. Microbial strains collected from local drought-affected locations performed better in enhancing tolerance of plants to drought stress than those that were isolated and brought from other regions that do not experience drought. However, integrating the testing of microbial strains from drylands into plant breeding techniques for drought resistant may help SSA agriculture to adapt to the continually changing climate. Future research is required to quantify the impacts of climate change on regionally imperative staple food crops as well as cash crops in order to better formulate potential coping techniques against drought.

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Metabolites in Sustainable Agroecosystem. Springer International, Cham, pp. 105 158. Yang, J., Kloepper, J.W., Ryu, C., 2009. Rhizosphere bacteria help plants tolerate abiotic stress. Trends Plant Sci. 14, 1 4. Ziska, L.H., Blumenthal, D.M., Runion Jr, G.B., Hunt, E.R., Diaz-Soltero, H., 2011. Invasive species & climate change: an agronomic perspective. Clim. Change 105, 13 42.

FURTHER READING You, L.S., Crespo, Z., Guo, J., Koo, K., Sebastian, M.T., Tenorio, S., et al., 2000. Spatial Production Allocation Model (SPAM). Version 3 Release 6.

CHAPTER 13

Understanding Soil Aggregate Dynamics and Its Relation With Land Use and Climate Change Pratap Srivastava1, , Rishikesh Singh2, , Rahul Bhadouria3,4, Sachchidanand Tripathi5, Hema Singh3 and Akhilesh Singh Raghubanshi2 1

Department of Botany, Shyama Prasad Mukherjee Post-graduate College, University of Allahabad, Allahabad, India 2 Institute of Environment and Sustainable Development (IESD), Banaras Hindu University, Varanasi, India 3 Ecosystems Analysis Laboratory, Department of Botany, Institute of Science, Banaras Hindu University, Varanasi, India 4 Natural Resource Management Laboratory, Department of Botany, University of Delhi, New Delhi, India 5 Department of Botany, Deen Dayal Upadhyaya College, University of Delhi, New Delhi, India

Contents 13.1 Introduction 13.2 Soil Aggregates: The State of the Art 13.2.1 Biophysical Interaction in Soil Aggregate Development 13.2.2 Aggregate Characteristics 13.3 Importance of Soil Aggregates: Relative Proportion 13.4 Aggregate Stability 13.5 Practices Influencing Soil Aggregate Dynamics 13.6 Management of Belowground Interaction Using Soil Aggregate Dynamics as Surrogate 13.7 Conclusion Acknowledgment References Further Reading

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13.1 INTRODUCTION Soil is the crucial component of the Earth’s biosphere (Ellert et al., 1997; Coleman et al., 2004). It is at the core of sustainable human development



Authors contributed equally

Climate Change and Agricultural Ecosystems DOI: https://doi.org/10.1016/B978-0-12-816483-9.00021-9

Copyright © 2019 Elsevier Inc. All rights reserved.

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Figure 13.1 Soil as the core of sustainability development.

(see Fig. 13.1) and is responsible for the rise and downfall of civilizations. Soil is the product of variable spatiotemporal interactions between climate, organisms (i.e., plant roots, animals, and microorganisms), parent materials, and topography (Ellert et al., 1997; Coleman et al., 2004). Moreover, the biotic interaction in soil is invariably affected by the soil’s physical and chemical properties (Milleret et al., 2009). These intricate biotic abiotic interactions affect the ecosystem’s properties due to their impact on principal ecosystem processes such as nutrient cycling, primary production (Ellert et al., 1997), and plant community dynamics (Frank et al., 2015). In particular, these interactions are crucial for the functioning of global biogeochemical processes as they regulate the accessibility and decomposition of soil organic matter (Siddiky, 2011). It is consistent with the current findings that not only the chemical recalcitrance but also the biophysical interactions of soil (as observed in aggregate development, determining its occluded C characteristics), are the major determinants of soil organic carbon (SOC) turnover and sequestration (Schmidt et al., 2011). Tisdall and Oades (1982) demonstrated that the SOC level closely relates to the soil aggregate formation and stability. These factors together represent the integrative effects of soil type, environment, plant species, and soil management (Martens and Frankenberger, 1992; Nyamangara et al., 1999; Martens, 2000). It has been established that the loss of SOC and aggregate stability (or turnover) represents the unsustainable nature of soil management (Amezketa, 1999; Carter, 2002). Therefore, in order to understand the soil carbon dynamics of an agro-ecosystem and to restore soil

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Figure 13.2 Illustration showing integrative nature of soil aggregate dynamics

multifunctionality, soil aggregate dynamics and its response to management needs to be assessed closely (Fig. 13.2). In this chapter, we collate studies showing the development in soil aggregate and its importance in the present ecological context. Therefore we discuss the recent advances in soil aggregates (role of biophysical factors and its characteristics) followed by aggregate relative proportion in soil nutrient dynamics and its stability under different management practices, and conclude by proposing a model showing soil aggregate as a surrogate to manage the belowground interactions in the soil system. Moreover, the emerging management practices which have a significant role in soil aggregate and related nutrient dynamics (in relation to nutrient stoichiometry) have also been reviewed to derive a holistic management perspective. The recent advances in this field suggest that although soil aggregates and related dynamics is an old topic, it has considerable impact under the present ecological perspective in terms of soil health management and carbon sequestration.

13.2 SOIL AGGREGATES: THE STATE OF THE ART Recently, focus has been given on identifying the key indicators of soil functions and processes. In this context, soil structure (aggregate structure

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and pore spaces) has been regarded as a key indicator for determining soil functioning and processes (Rabot et al., 2018). Aggregates are the fundamental, although virtual, unit of soil structure. It represents a threedimensional matrix of soil particles, organic matter, air, and water with distributed biological components (Six et al., 2000, 2002). It is important to understand that soil aggregate develops and is visible only when the soil is dried and mechanically stressed to a degree that it ruptures through a plane of weakness. It is a function of microbial behavior which helps in aggregation due to its particle orientation property. Therefore it is primarily affected by soil management practices which have a considerable effect on microbial activity. Like an organism, community, and ecosystem, aggregate develops hierarchically (see Box 13.1) in a management-specific manner. The qualitative (size distribution and stability) and quantitative (carbon pools and its characteristics) attributes of soil aggregate can be the functional traits which could be widely used to monitor the effect of management practices on agricultural sustainability (Fig. 13.2). Moreover, it could be used as surrogate to understand the fundamental principle of

BOX 13.1 Aggregate Development Concepts Fundamental assumptions of soil aggregate development that are used for understanding soil organic carbon dynamics include: (1) aggregate hierarchy concept, which describes a spatial scale dependence of mechanisms involved in micro- and macroaggregate formation (Tisdall and Oades, 1982); and (2) the formation of microaggregates occurring within macroaggregates (Oades, 1984). Tisdall and Oades (1982) proposed the aggregate hierarchy concept based on the infuence of soil organic matter as a binding agent. It was considered that binding agents act through three major mechanisms: temporary (mainly polysaccharides), transient (roots and fungal hyphae), and persistent (humic substances, polyvalent metal cation complexes, and oxides). In this model, fine particles (,20 µm) bind together by persistent binding agents forming microaggregates (53 250 µm). These microaggregates, in turn, bind together into macroaggregates by means of roots, hyphae, or fresh organic residues ( . 250 µm). However, Six et al. (2000) reported a contrasting theory that microaggregates are formed within macroaggregates by the mineral encapsulation of particulate organic matter (POM) and the microbial byproducts produced due to its decomposition. In addition, inorganic binding agents (such as oxides and carbonates) have also been reported to play an important role in aggregate formation (Six et al., 2004).

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Figure 13.3 Factors affecting soil aggregate development.

how physical and biological forces interact to define the efficiency of soil ecosystem in the utilization and conservation of energy and resources. Soil aggregate stability depends on pedoclimatic characteristics such as temperature and precipitation, and on several soil properties including texture, clay mineralogy, cation content, aluminum, and iron oxides as well as soil organic matter content (Bronick and Lal, 2005; Abiven et al., 2009). The factors affecting soil aggregate development are shown in Fig. 13.3. However, considering soil aggregate as a measure of soil function and processes is still a debatable issue as its quantitative estimate is dependent on the methods used for its estimation (Rabot et al., 2018). Therefore most of the studies suggest soil pore space measurement as the potential indicator for soil functioning (Rabot et al., 2018). SOC is a primary factor influencing soil structure and aggregate stability (Kay, 1998), and is itself influenced strongly by the dynamics of soil structure (i.e., aggregation stability and distribution) (Elliott and Cambardella, 1991). SOC is an intrinsic component of soil aggregate, therefore, considering soil aggregates (qualitative and quantitative characteristics) could be of vital importance to understand its management activities. An illustration of how the bidirectional interaction between SOC and aggregate development plays a crucial role in agricultural sustainability is provided in Fig. 13.4. Based on the studies in the literature, aggregates have been classified into three major size classes (see Box 13.2). Studies suggest that the availability of the organic matter source and management regulates the soil aggregate formation and soil carbon sequestration potential mediated by the soil aggregate development (Six and Paustian, 2014; Castellano et al., 2015; Toosi et al., 2017a,b). For example, the presence of fresh organic matter acts as a binding agent for

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Persistent SOC (Humic substance)

Temporary SOC (Polysaccharide) Soil chemical attributes

Soil biological attributes

Soil physical attributes

Soil physical occlusion

Soil organic Carbon

Soil chemical recalcitrance

Soil aggregate characteristics

Soil quality

Agricultural sustainability

Figure 13.4 Complicated aggregate development and its relation with global environmental crisis.

BOX 13.2 Aggregate Size Classes Soil crumbs are divided in aggregate size fractions using different sieve sizes (Tisdall and Oades, 1982; Six et al., 2002). The choice of sieve size and number depends on the experiment and idea to be tested. Generally, sieves of 2 mm, 250 µm, and 53 µm are used in tandem to get fractions of the respective size classes because the major variation under different land use and management practices across spatiotemporal scales have mostly been reported on these size fractions. These size fractions so obtained are, respectively, termed as macroaggregate ( . 2 mm), mesoaggregate (2 mm 250 µm), microaggregate (250 53 µm), and silt and clay (mineral) fractions (,53 µm). However, due to their similar behavior irrespective of management regime, macro- ( . 2 mm) and mesoaggregates (2 mm 250 µm) are sometimes lumped together as macroaggregates ( . 250 µm).

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the formation of macroaggregates (Tisdall and Oades, 1982; Six et al., 2000) whereas decomposition of soil organic matter within macroaggregates resulted in the formation of stable microaggregates (Gale et al., 2000). However, qualitative (aggregate associated carbon content) and quantitative (percent distribution) properties of the aggregates are of higher relevance for understanding soil nutrient dynamics and carbon sequestration potential. Garland et al. (2018) suggested that the soil aggregate-associated nutrients (especially soil P) showed more stability and closed-loop cycling under tropical soils. Moreover, macroaggregates showed fast response to changes in management practices related with organic matter input (Toosi et al., 2017b). Furthermore, organic matter decomposition is regulated by the pore spaces; Toosi et al. (2017a,b) observed that the pore space ,32 and .136 µm showed less organic matter decomposition within the macroaggregate under natural or cover-cropping systems. Moreover, change in the microbial community structure by the addition of soil ameliorant led to the differential behavior of soil organic matter decomposition and soil aggregate stability (Quilliam et al., 2013; Whitman et al., 2016). Zheng et al. (2018) observed an increase in microbial biomass carbon and a decrease in metabolic quotient under the biochar applied soils showing the increased stability of soil aggregates and microbial soil carbon sequestration within aggregates. Therefore we will critically review the role of various factors (physical, chemical, and biological) influencing aggregate development and their nutrient dynamics.

13.2.1 Biophysical Interaction in Soil Aggregate Development Soil microorganisms and plant root secretions have been found to play a significant role in soil carbon management affecting soil structural dynamics. Microbial secretions serve various purposes like attachment, nutrient capture, and desiccation resistance (Rillig, 2004; Rillig and Mummey, 2006). Bacteria and fungi either act synergistically or antagonistically in soil aggregate development through their polymeric secretions. Fungusderived polysaccharides have been found to contribute significantly in soil aggregation (Chenu, 1989). Fungal metabolic products are either secreted outside of, or contained in, the hyphal wall, which has been implicated as an important mechanism in soil aggregation (Tisdall and Oades, 1982). For example, biopolymers like hydrophobins and glomalins help to enhance the aggregation in addition to hyphal enmeshment (Miller and Jastrow, 2000). Root exudates stimulate the formation of aggregates by

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providing carbon into the soil which constitute about a third of the photosynthetic production. For example, root mucilage can cause short-term stabilization of aggregates by sticking the particles together (Morel et al., 1991). The marked impact of differential microbial dominance is found on soil carbon sequestration. Fungi seem relatively more important than bacteria in carbon sequestration. Comparatively, fungi possess unique carbon metabolic pathways and secrete more recalcitrant organic compounds than bacteria in the soil. Also, they are more carbon-efficient, and, therefore, sequester more carbon in biomass than they respire (Malik et al., 2016). Despite seemingly higher fungal contribution in soil structural dynamics and carbon sequestration, an extensive study on this aspect has not yet been possible due to some technical limitations in experimentation. Fungi can modify the physical environment to their advantage. Fungi have been underestimated in the regulation of aggregate turnover compared to bacteria. Fungi possess particle orientation capability, also termed as physical ecosystem engineering (Jones et al., 1997) which is the alteration in pore architecture and microbial habitation leading to modification in soil structural dynamics. Further, soil structural dynamics controls the compositional and functional attributes of the bacterial community through biotic and abiotic means such as predator prey relations and the availability of substrate, nutrient, oxygen, and water. Fungi (particularly arbuscular mycorrhizal fungi) which have been mostly studied in soil structural aspects, exert a strong influence at the scale of macroaggregates. However, bacteria and archaea modify the formation and stability of microaggregates. Moreover, arbuscular mycorrhizal fungi-facilitated alteration in prokaryotic communities (bacteria and archaea) may have considerable impact on soil carbon sequestration via a top down effect on soil structural dynamics (Rillig and Mummey, 2006). Fungi bacteria interaction is also important in soil structural dynamics. Fungi alter the physiological states of hyphae-associated microbes through microenvironmental changes (Budi et al., 1999; Bezzate et al., 2000; Mansfeld-Giese et al., 2002). Arbuscular mycorrhizal fungi can directly affect bacterial community characteristics via fungal metabolic secretions containing bacterio- or fungi-static agents (Rillig, 2004; Rillig and Mummey, 2006) which can modify growth and activity of a specific microbial group/species (Rillig, 2005; Rillig and Mummey, 2006). Fungal hyphae and roots help in aggregate development via various physical means in addition to its metabolic secretions and entanglement.

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Fungal hyphae are considered as tunneling “machines” (Wessels, 1999) which create a considerable amount of pressure on adjacent soil particles while invading the soil matrix (Money, 1994). This pressure leads to the formation of the microaggregate, enabling organic matter and clay particles to be more coherently attached in a similar way that roots do. Enmeshment by fungal hypha stabilizes the aggregate not only because of its primary involvement as it also brings various other factors into play through this mechanism. It has the ability to align the primary particles (especially clay particles) along its hyphae (Tisdall, 1991; Chenu et al., 2002). Roots induce aggregate formation by the temporal dry wet cycle in the rhizosphere. It contributes to increased binding between clay and root exudates under the pressure built up due to the localized dry wet cycle which helps in microaggregate development (Reid and Goss, 1982). A similar mechanism operates during fungi-mediated soil aggregate development, though at a smaller scale. The production of bacterial polysaccharide changes under localized drying wetting, compaction, and nutrient availability in the soil. It affects the soil structural development and dynamics considerably. Extreme drying wetting cycles have a strong impact for soil aggregation due to the differential response of soil microorganisms (Six et al., 2004). Decreased soil moisture typically enhances the contact points between primary particles and soil organic matter, which increases cohesion and strength in the soil aggregates (Horn and Dexter, 1989; Horn et al., 1994). Soil moisture affects bacterial growth and activity more than osmotolerant fungi. However, it affects fungal growth dynamics and metabolism affecting hyphal turgidity which is needed to penetrate through the soil matrix, especially during the dry season. It has been found that the sensitivities to water and nitrogen addition vary among microbial groups within soil aggregates (Wang et al., 2017a,b). Fungal growth dynamics and invasion is crucial for soil carbon sequestration due to its physical nature. It helps to disperse the carbon in the soil from distinct islands (like plant roots and surface layers), as it possesses the unique ability to link spatiotemporal dimensions. It has been found that the management in agroecosystems, which favors a fungal-dominated community, shows quantitative and qualitative improvements of SOC (Six et al., 2006).

13.2.2 Aggregate Characteristics Assessments of the soil organic matter’s chemical characteristics are commonly used to infer its potential reactivity (Kögel-Knabner et al., 2008)

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which is an important aspect in soil carbon sequestration studies. Several studies (Saroa and Lal, 2003; Mikha and Rice, 2004; Kong et al., 2005; Lagomarsino et al., 2009) showed higher concentration of C and N in macroaggregates than microaggregates because microaggregates are bound together by soil organic matter to form the macroaggregate. However, microaggregates hold stabilized SOC formed within stable macroaggregates (Six and Paustian, 2014). Moreover, soil aggregates withhold soil nutrients on its surface rather than allowing their availability in soil mineral fractions (Garland et al., 2018). Puget and Chenu (1995) suggested that greater C content in macroaggregates could be due to the less-decomposable soil organic matter associated with this fraction. However, in some studies, the SOC mineralization rate and protection has been found similar in macroaggregates and microaggregates (Rabbi et al., 2014). Furthermore, the stability of organic carbon in aggregate fractions is highly regulated by the pore structure (Toosi et al., 2017a). The distribution of microbial biomass and community (composition and function) are found to differ considerably in different aggregate size classes (Gupta and Germida, 1988; Mummey et al., 2006) under varied management systems (Singh and Singh, 1996). It was found that microbial communities inhabiting aggregates dynamically adjust their attributes in response to changes in soil moisture and other external conditions (Ebrahimi and Or, 2016), which determines the nature of soil biogeochemical activity and gas fluxes emitted from soil profiles. Therefore it is imperative to understand the importance of soil aggregates and their stability under different management practices.

13.3 IMPORTANCE OF SOIL AGGREGATES: RELATIVE PROPORTION Two well-known hypotheses regarding soil structural development follow opposite courses, being (1) bottom up and (2) top down development. The functional importance of micro- and macroaggregates, respectively, has been postulated in these two models of soil structure development. Elliott and Coleman (1988) proposed that macroaggregate characteristics define the microaggregate characteristics via anaerobic means (i.e., developing an anaerobic environment inside the macroaggregate). The redistribution of carbon from macroaggregates to microaggregates has been reported using tracer techniques (Angers et al., 1997). Oades (1984) also advocated the concept of microaggregate formation within

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macroaggregates (i.e. top-down model). On the other hand, the formation of microaggregates has been proposed to define the functional characteristics of macroaggregates in the other model (i.e. bottom-up model). This is due to the fact that microaggregates stick together with the help of cementing agents of transient or permanent natures to form the macroaggregate (Tisdall and Oades, 1982; Six et al., 2000). Moreover, the relative functional importance of micro- and macroaggregate might be a function of the ecosystem, successional stage, and management, which may define the course of soil structural development mediating scale-dependent microbial behavior and its interaction (i.e., biophysical) with soil abiotic factors (Fig. 13.2). Appropriate soil structural development is essential for enhancing soil fertility and soil C stocks (Bronick and Lal, 2005). Soil aggregate occludes the SOC physicochemically from microbial decomposition (Garland et al., 2018). Though, organic matter in aggregate fractions is beneficial for soil C storage, a potential trade-off between yield and long-term C sequestration exists (Cates and Ruark, 2017). A negative correlation between macroaggregate C and crop yield has been reported in the literature (Cates and Ruark, 2017). Aggregate size distribution and nutrient distribution among these fractions determine the microbial activity (such as denitrification) and carbon utilization rates with a change in soil moisture behavior (Ebrahimi and Or, 2016). Thus, the size of the aggregate can directly define the decomposability of organic material (Abiven et al., 2009) and nutrient availability. It has been reported that macroaggregates can physically protect the original and recently added organic matter from microbial attack and mineralization (Oorts et al., 2006; Razafimbelo et al., 2008). In contrast, these larger soil aggregates have also been suggested to quickly develop suitable internal conditions for microbial activity inside the aggregates (Diba et al., 2011), thus possibly helping in C humification and protection. However, these macroaggregates have been found to be comparatively more sensitive to climate warming (Fang et al., 2016) which has been attributed to the differential impact of climate warming on soil microbial communities and enzyme activities in various aggregate fractions that may have important implications for C cycling. This suggests that macroaggregates (including mesoaggregates) may have an important role in soil C sequestration which could be affected drastically under changing climate conditions. Organic materials are protected by the heterogeneity of the soil microenvironment which limits the access of decomposers and their enzymes to

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organic matter (Schmidt et al., 2011; Ananyeva et al., 2013). Aggregates not only protect soil organic matter, but also influence other biotic and abiotic factors which act secondarily in SOC protection such as an alteration in the microbial community structure (Mummey and Stahl, 2004), limitation on oxygen diffusion (Sexstone et al., 1985) and water movements (Prove et al., 1990), and nutrient adsorption and desorption (Linquist et al., 1997). All these factors may affect the microbial processes, which could be accessed directly or indirectly to correlate with soil carbon efflux and sequestration and to understand the nature of soil structural development. In some recent studies, the soil nitrification process has been suggested to associate with soil aggregate development and macroaggregate characteristics, which could be used as an indicator of soil carbon dynamics (Srivastava et al., 2016b,c). Agricultural management practices such as tillage and nitrogen fertilization regulate greenhouse gas (GHG) production in macroaggregates through changes in the substrates’ C/N ratio and microbial activity (Plaza-Bonilla et al., 2014). Moreover, changes in nutrient stoichiometry under different management scenarios also have considerable impact on aggregate formation, stability, and its dynamics (Srivastava et al., 2016c; Toosi et al., 2017b). In a recent experiment, Lenka and Lal (2013) suggested that the aggregate hierarchy theory could be extended to describe the effect of soil aggregation on GHG emission from the soil. An illustration of the complicated mechanisms of soil aggregate development and its close relationship with the current global environment crisis is provided in Fig. 13.5. It has been suggested that the response of soil organic matter decomposition to higher temperatures due to the changing climate might be closely associated with soil aggregation, which may complicate the responses of ecosystem C budgets to future warming scenarios (Wang et al., 2015). This may be attributed to the interactive effect of temperature and aggregation on soil organic matter decomposition. Therefore macroaggregate attributes need to be studied in interaction with the changing climate and N availability for better soil management.

13.4 AGGREGATE STABILITY According to Tisdall and Oades (1982) and Chan (2001) macroaggregate stability is controlled by the temporary form of soil organic carbon which is highly sensitive to management practices. For example, soil macroaggregate stability is higher in reduced than conventional tillage, and disintegrates into microaggregates and silt and clay fractions (Six et al., 2002).

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Macroaggregate Bottom-up model

Top-down model

Microaggregate

Differential impact across size classes

Physical pore architecture

Aggregate chemical attributes

Microbial biogeography

Soil quality

Sustainable agriculture

Figure 13.5 Role of bidirectional interaction between soil organic C and aggregate development in agricultural sustainability.

The quality of added organic residues as defined by specific biochemical characteristics (N, lignin, and polyphenol contents), may influence the rate of macroaggregate turnover (Six et al., 2001; Six and Paustian, 2014; Toosi et al., 2017b). Contrary to low-quality organic residues, high-quality residues induce a faster macroaggregate turnover (Six et al., 2001). This is associated with the enhanced mineralization of carbon and nutrients leading to their loss from the system (Haynes and Beare, 1997). Moreover, the addition of low-quality organic residues provides a short-term increase of macroaggregate SOC and N compared to high-quality residues (Chivenge et al., 2011). However, the simultaneous addition of N fertilizers has been found to nullify such effects of low-quality residues. Similarly, the addition of N fertilizers without a C source enhances macroaggregate turnover due to the enhanced decomposition of C-rich binding agents by microbes (Harris et al., 1963). Manure application has been found to improve aggregation in coarsetextured soils (Nyamangara et al., 1999); however, manure decreased the

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aggregate stability in fine-textured soils (Pare et al., 2000). Similarly, biochar has also been suggested to improve aggregate stability and, hence, could be a strategy for enhanced agro-production (Li et al., 2017). Aggregate stability has been found to reduce the carbon output from the soil as particulate organic C (POC) and dissolved organic C (DOC) (Chaplot and Cooper, 2015). Recently, a mechanistic linkage has been proposed to exist between soil’s physicochemical (such as pH, porosity, relative availability of inorganicN pools) properties and aggregate formation (Regelink et al., 2015; Srivastava et al., 2016b,c).

13.5 PRACTICES INFLUENCING SOIL AGGREGATE DYNAMICS Management practices in agriculture have a significant impact on soil aggregate stability and dynamics (Bhattacharyya et al., 2010; Huang et al., 2010) which have been identified as the major culprits in declining soil fertility and changing climate. Simultaneously, soil aggregate stability and dynamics have also received attention as an integrative soil functional trait, which may have some management implications. Management disturbances in agricultural soils play a defining role in the global carbon cycle and soil carbon dynamics affecting soil microstructures, which contains two thirds of terrestrial carbon remains sequestered within it. Therefore agronomic practices influence aggregation and, thus, SOC content. Soil aggregation is found to be affected by a variety of factors in agroecosystems, such as tillage regime (Shirani et al., 2002; Zotarelli et al., 2005), crop rotation system (Holeplass et al., 2004; Zotarelli et al., 2005), crop species (Wright and Hons, 2005), residue management (Saroa and Lal, 2003), cropping duration, and fertilization regime (Holeplass et al., 2004). Soil aggregate dynamics, particularly its size distribution and carbon characteristics is management-specific (Singh and Singh, 1995). The latter defines the extent and direction of soil aggregate development and, thus, efficiency of SOC protection from decomposition. Long-term studies under varied management practices (Tiessen and Stewart, 1983; Christensen, 1986; Dalal and Mayer, 1986; Yu et al., 2012) have shown differential aggregate size distribution and carbon characteristics. Manure application increases SOC concentration, aggregate stability. and soil biological activities which have been found to be associated with improvement in soil structure (Martens and Frankenberger, 1992; Haynes and Naidu, 1998; Nyamangara et al., 1999; Aoyama et al., 2000).

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Continuous cultivation affects the distribution and stability of soil aggregates and reduces organic carbon stock in the soil (Six et al., 2002). The inter-microaggregate binding agents are the principle component of soil organic carbon which is lost when the soil in cultivated (Tisdall and Oades, 1982). Whalen et al. (2003) and Jiao et al. (2006) suggested that significant quantities of carbon from farmyard manure was retained in whole soil and its aggregate fractions. Several researchers reported that increasing C input in soil leads to enrichment in the aggregate-associated C (Kong et al., 2005; Manna et al., 2006). Bhattacharyya et al. (2010) showed that the relationship between carbon input and carbon sequestration is dominated by the increase in SOC associated with the macro- as well as microaggregates. Aoyama and Kumakura (2001) found that the application of animal manure increased soil organic matter and the formation of macroaggregate. Also, increased soil organic matter has been attributed to the increased accumulation of macroaggregate-protected carbon. Organic carbon input into the soil generally improves the mass proportion of macroaggregate at the expense of microaggregate and the free silt and clay fraction (Liao et al., 2006; Yu et al., 2012). In contrast, long-term mineral fertilizer application decreases SOC concentration as well as mass distribution of macroaggregate. However, some long-term studies on mineral fertilization have shown an increase of SOC without significant improvement in aggregation (Yu et al., 2012). Moreover, Aoyama et al. (1999) reported that manure addition has no significant effect on SOC storage over chemical fertilization in smaller fractions. Biochar improves the resistance of aggregates to stresses, actively promotes soil C storage and, thus, provides a scientific strategy for sustainable agricultural production (Li et al., 2017; Wang et al., 2017a,b). Additionally, biochar types have been found to greatly impact soil chemical properties and microbial communities (Zheng et al., 2018). However, comparative studies on temporal change in soil aggregate dynamics under various kinds of nutrient amendments in relation to soil carbon dynamics are almost absent for tropical agroecosystems. Such studies may further provide some important understanding regarding the unifying concept of soil C accumulation.

13.6 MANAGEMENT OF BELOWGROUND INTERACTION USING SOIL AGGREGATE DYNAMICS AS SURROGATE The complex interaction in spatio-temporal dimensions is fundamental to a biological or ecological system (such as organism or ecosystem) as it provides stability, efficiency (in use of energy and nutrients), resilience, and

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sustainability to the system. Such dynamic and nonlinear interactions among the soil biotic and physicochemical environments play crucial roles in the ecosystem engineering and functioning. Currently, scientists firmly believe that reliance on agrochemicals (fertilizers and pesticides) in agroecosystems could be considerably reduced by managing the dynamic and nonlinear interactions in the soils (Médiène et al., 2011). However, the present human ability to directly manage and control these intricate microscale interactions is highly limited which is clearly visible by the escalating environmental problems due to human mismanagement of the biosphere. However, the identification of integrative macroscale indicators representing the overall function of interactions happening inside soil may help us to manage them indirectly. It would be analogous to “managing the lock (integrative macro-scale indicator) to understand and use the key (micro-scale and dynamic interaction)” functionally. The integrative indicatorswould ultimately help the agricultural and soil scientists to reap the indispensable ecological subsidies provided by the inherent interactions in the soil ecosystem (Srivastava et al., 2016a). A macroscale indicator which integrates the ecosystem interactions as a whole in itself may well indicate the sustainable or unsustainable nature of interactions in soil. For example, quantitative and qualitative characteristics of soil aggregate fractions which are regulated by microbial behavior, pedoclimatic conditions, and management have the potential to become one such (integrative and surrogate) variable (Srivastava et al., 2016b,c) (Fig. 13.5). Soil aggregate development holds crucial importance in contemporary climate change and decline in soil quality due to its primary role in soil C sequestration (see Fig. 13.5) (Holeplass et al., 2004) as it primarily controls the physical, chemical, and biological attributes (Yang and Wander, 1998). On the other hand, it has also been found to be strongly affected by the soil’s physicochemical and biological properties. Beauchamp and Seech (1990) reported that a high variability in denitrification activity was associated with aggregate size and its water stability. Soil aggregate size and stability reduces soil erosion determining the physical protection of soil organic matter from decomposition (Paul et al., 2013). Therefore aggregate stability and soil organic matter relate strongly with each other and impart synergistic effects on soil health. Therefore both are considered as indicators of soil health and agricultural sustainability with their losses representing an unsustainable management system (Carter, 2002). Soil aggregate dynamics protect and maintain the multifaceted function of soils; however, its potential to do so depends on management practices

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(Srivastava et al., 2016a). Soil aggregate development and dynamics integrates the spatiotemporally variable interactions among soils (Miller and Jastrow, 1990). It has been found that the main difference between microand macroaggregates might be attributed to the microbial community structure within each fraction, which could affect soil nutrient dynamics, especially N availability (Noguez et al., 2008). Although enhanced N availability has been found to have significant, but variable, effect on microbial biomass C and N, DOC, and DOC to inorganic N across aggregate size fractions (Yin et al., 2016; Zhong et al., 2017). However, how the interaction of microorganisms with the physical environment during soil aggregate development affects nutrient turnover and availability has hardly been explored. Such an understanding may have a significant future prospect in soil C sequestration in agroecosystems. Recently, qualitative and quantitative characteristics of soil aggregates have been found to be associated with microbial community characteristics and the consequent relative availability of inorganic N pools (Srivastava et al., 2016b,c). The relative availability of inorganic N pools has been identified as an integrative variable of significant importance in soil carbon dynamics because it indirectly indicates the microbial behavior inside the soil under complex physical constrains (Srivastava et al., 2016b,c). Therefore we propose that combining physical fractionation and chemical characterization with soil relative availability of inorganic N pools across aggregate size fractions under various management systems may help to better understand the mechanism of soil carbon sequestration, which could be helpful in the mitigation of climate change. Moreover, it may help to identify novel and integrative (physicochemical) signatures/markers of soil carbon dynamics for cost-effective and simple, onsite management of the agroecosystems for improved C sequestration. Therefore, such integrative characteristics (having a crucial role in soil carbon dynamics) as a whole, or in appropriate combinations may help us to track and manage the complex belowground interactions in an indirect manner for site-specific management of soil carbon dynamics and fertility to achieve sustainable agriculture on the one hand and the mitigation of climate change on the other.

13.7 CONCLUSION Soil multifunctionality seems to be a function of the level of physical engineering in soil aggregate development which is primarily linked with climate change due to its close association with soil carbon sequestration.

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Soil aggregate characteristics reflect dynamic and variable spatiotemporal interactions between the physical, chemical, and biological properties of the soil. Agro-management practices distinctly affect these soil biophysical interactions at each level of the hierarchy and, thus, define soil aggregate development by modifying the soil chemical environment. Some recent studies have shown that the resource (C and N) conservation mechanisms might be linked with the change in soil chemical environment (such as stoichiometric availability of inorganic-N pools), which strongly correlates with soil aggregate characteristics. In addition, various emerging soil ameliorants like biochar have a potential impact over soil aggregate dynamics which needs to be explored in relation to soil aggregate nutrient dynamics. Therefore, we strongly propose that soil aggregate quantitative and qualitative characteristics should be studied in relation to soil inorganic N pool dynamics and carbon accumulation under various land uses and management for effective climate change mitigation strategies. This would also help in the identification of some novel physicochemical signatures/markers of soil carbon sequestration related to soil aggregate characteristics. These markers as a whole (or in various combinations) can be potentially used as a surrogate indicator to indirectly manage the belowground interactions in a cost-effective and site-specific manner to increase carbon sequestration by providing dual benefits of increased soil fertility and mitigation of climate change.

ACKNOWLEDGMENT The authors gratefully acknowledge financial support from University Grants Commission (UGC), Council of Scientific and Industrial Research (CSIR) and DST-SERB, New Delhi, India. The authors also extend their gratitude to the handling editor and three anonymous reviewers for their constructive suggestions for the improvement of the manuscript.

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Shirani, H., Hajabbasi, M., Afyuni, M., Hemmat, A., 2002. Effects of farmyard manure and tillage systems on soil physical properties and corn yield in central Iran. Soil Tillage Res. 68, 101 108. Siddiky, M.R.K., 2011. Soil biota interactions and soil aggregation, Freie Universität Berlin. Singh, S., Singh, J., 1995. Microbial biomass associated with water-stable aggregates in forest, savanna and cropland soils of a seasonally dry tropical region, India. Soil Biol. Biochem. 27, 1027 1033. Singh, S., Singh, J., 1996. Water-stable aggregates and associated organic matter in forest, savanna, and cropland soils of a seasonally dry tropical region, India. Biol. Fert. Soils 22, 76 82. Six, J., Paustian, K., 2014. Aggregate-associated soil organic matter as an ecosystem property and a measurement tool. Soil Biol. Biochem. 68, A4 A9. Six, J., Elliott, E., Paustian, K., 2000. Soil macroaggregate turnover and microaggregate formation: a mechanism for C sequestration under no-tillage agriculture. Soil Biol. Biochem. 32, 2099 2103. Six, J., et al., 2001. Impact of elevated CO2 on soil organic matter dynamics as related to changes in aggregate turnover and residue quality. Plant Soil 234, 27 36. Six, J., Conant, R., Paul, E.A., Paustian, K., 2002. Stabilization mechanisms of soil organic matter: implications for C-saturation of soils. Plant Soil 241, 155 176. Six, J., Bossuyt, H., Degryze, S., Denef, K., 2004. A history of research on the link between (micro) aggregates, soil biota, and soil organic matter dynamics. Soil Tillage Res. 79, 7 31. Six, J., Frey, S., Thiet, R., Batten, K., 2006. Bacterial and fungal contributions to carbon sequestration in agroecosystems. Soil Sci. Soc. Am. J. 70, 555 569. Srivastava, P., Singh, R., Tripathi, S., Raghubanshi, A.S., 2016a. An urgent need for sustainable thinking in agriculture an Indian scenario. Ecol. Indic. 67, 611 622. Srivastava, P., Singh, R., Bhadouria, R., Tripathi, S., Singh, P., Singh, H., et al., 2016b. Organic amendment impact on SOC dynamics in dry tropics: a possible role of relative availability of inorganic-N pools. Agric. Ecosyst. Environ. 235, 38 50. Srivastava, P., Singh, P.K., Singh, R., Bhadouria, R., Singh, D.K., Singh, S., et al., 2016c. Relative availability of inorganic N-pools shifts under land use change: an unexplored variable in soil carbon dynamics. Ecol. Indic. 64, 228 236. Tiessen, H., Stewart, J., 1983. Particle-size fractions and their use in studies of soil organic matter: II. Cultivation effects on organic matter composition in size fractions. Soil Sci. Soc. Am. J. 47, 509 514. Tisdall, J., 1991. Fungal hyphae and structural stability of soil. Soil Res. 29, 729 743. Tisdall, J., Oades, J.M., 1982. Organic matter and water-stable aggregates in soils. J. Soil Sci. 33, 141 163. Toosi, E.R., Kravchenko, A.N., Guber, A.K., Rivers, M.L., 2017a. Pore characteristics regulate priming and fate of carbon from plant residue. Soil Biol. Biochem. 113, 219 230. Toosi, E.R., Kravchenko, A.N., Mao, J., Quigley, M.Y., Rivers, M.L., 2017b. Effects of management and pore characteristics on organic matter composition of macroaggregates: evidence from characterization of organic matter and imaging. Eur. J. Soil Sci. 68, 200 211. Wang, Q., Wang, D., Wen, X., Yu, G., He, N., Wang, R., 2015. Differences in SOM decomposition and temperature sensitivity among soil aggregate size classes in a temperate grasslands. PLoS One 10, e0117033. Wang, D., Fonte, S.J., Parikh, S.J., Six, J., Scow, K.M., 2017a. Biochar additions can enhance soil structure and the physical stabilization of C in aggregates. Geoderma 303, 110 117.

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FURTHER READING Kandeler, E., Tscherko, D., Spiegel, H., 1999. Long-term monitoring of microbial biomass, N mineralisation and enzyme activities of a Chernozem under different tillage management. Biol Fert. Soils 28, 343 351. Majumder, B., Mandal, B., Bandyopadhyay, P., Chaudhury, J., 2007. Soil organic carbon pools and productivity relationships for a 34 year old rice wheat jute agroecosystem under different fertilizer treatments. Plant Soil 297, 53 67. Poll, C., Thiede, A., Wermbter, N., Sessitsch, A., Kandeler, E., 2003. Micro-scale distribution of microorganisms and microbial enzyme activities in a soil with long-term organic amendment. Eur. J. Soil. Sci. 54, 715 724. Sohi, S.P., et al., 2001. A procedure for isolating soil organic matter fractions suitable for modeling. Soil Sci. Soc. Am. J. 65, 1121 1128. Stemmer, M., Gerzabek, M.H., Kandeler, E., 1998. Organic matter and enzyme activity in particle-size fractions of soils obtained after low-energy sonication. Soil Biol. Biochem. 30, 9 17.

CHAPTER 14

Climate Change: A Challenge for Postharvest Management, Food Loss, Food Quality, and Food Security Yashi Srivastava

Department of Applied Agriculture, Central University of Punjab, Bathinda, India

Contents Abbreviations 14.1 Introduction 14.2 Climatic Factors Affecting Postharvest Management, Food Loss, Food Quality, and Food Security 14.2.1 Temperature 14.2.2 Atmospheric Gases 14.2.3 Rainfall 14.3 Effects of Climate Change on Agriculture Production 14.4 Effects of Climate Change on Livestock Production 14.5 Effects of Climate Change on Food Loss and Postharvest Technology and Management 14.6 Effects of Climate Change on Food Quality 14.6.1 Carbohydrate Content 14.6.2 Mineral Content 14.6.3 Lipid Content 14.6.4 Proteins, Tannins, and Phenols 14.6.5 Mycotoxins 14.7 Food Security Under Climate Change 14.8 Conclusion References Further Reading

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ABBREVIATIONS AWD CCAFS CGIAR CSV

alternate wetting and drying Climate Change, Agriculture and Food Security Consultative Group on International Agricultural Research cluster of villages

Climate Change and Agricultural Ecosystems DOI: https://doi.org/10.1016/B978-0-12-816483-9.00019-0

Copyright © 2019 Elsevier Inc. All rights reserved.

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GDP GHG IARI IPCC IR MT UN FAO UNFCC

gross domestic product greenhouse gas Indian Agricultural Research Institute Intergovernmental Panel on Climate Change infrared million tons United Nation Food & Agriculture Organisation United Nations Framework Convention on Climate Change

14.1 INTRODUCTION The environmental changes over time which can alter the atmospheric conditions either naturally or due to human activity are called climate change (IPCC and UNFCC, 2013). It can also be explained as a process of biophysical interactions of gases in the Earth’s atmosphere which are responsible for terrestrial and ocean temperature rises (Josef and Francesco, 2007; Cruz, 2007; Hoffmann, 2011). The increase in temperature and greenhouse gases (GHGs) in the environment decrease the water and land availability, which affects crop production in terms of quality as well as quantity (Watson et al., 1996). The tendency for unreliable weather patterns can cause damage to the cultivation, processing, industrialization, and economic status of both developed and developing countries. It is a dangerous threat which threatens the very existence of mankind. Climatic change is caused by many internal and external factors (pollution, biodiversity, land use, etc.). These factors are closely related to the survival of all life on Earth and its interactions with the troposphere, ocean, and land. The environmental factors (i.e., temperature, rainfall, and wind) play an important role in the complete cycle of cultivation, while postharvest loss is mostly affected by environmental temperature. The food processing sector includes agricultural production, livestock production, processing, packaging, storage, and transportation, on which all of the above-mentioned factors play an important role. Postharvest losses occur at all levels of the food chain, from the farm to the consumer. Postharvest loss is also described as harvested fresh produce or animal products which are either spoiled or contaminated prior to reaching the customer or retailer. Globally, more than $750 billion per year of food is discarded, which is a huge loss for farmers, value chain factors, and in revenue loss in terms of gross domestic product (GDP) for governments. A 10% postharvest loss in developing countries would lead to a 14% increase in fresh produce prices, 11% hunger risk, and 4% malnutrition of children by 2050. These vast losses for the economy, environment, and

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human health illustrate why radical steps need to be taken to minimize postharvest losses (Kitinoja and Kader, 2002). The estimated increase in the world’s population to about nine billion by 2050 would need a 70% increase in global food production (Kader, 2005; Kaderand Rolle, 2004). This regular demand for an increase in global food production severely affects food quality and food security [production, distribution, sensory qualities (color, taste, texture, aroma, overall acceptability)] of all countries. Malnutrition is set to increase by 55% (east and south sub-Saharan Africa) and 62% (south Asian region) by 2050 (Lloyd et al., 2011). The Climate Change, Agriculture and Food Security and Consultative Group on International Agricultural Research (CGIAR-CCAFS) works in collaboration with rural communities to develop clusters of villages (CSVs) to create a platform of farmers, researchers, government policy dealers, and local associations in the selection and implementation of new technologies (Palanisami et al., 2015; Lipper et al., 2014). The latest agricultural and processing technologies associated with the CSVs include water, nutrient, carbon, energy, knowledge, and climate smart practices. The acceptance of the latest technologies in the agricultural and food processing sectors with the aim of reducing risks to economical yields due to climatic changes is very low in India, at about 12% over the last 40 years. The CSVs have four major components: smart technologies, information services of climate, development plans for village, and knowledge come from lab to land (Bruce et al., 2016). The perception of smart agriculture is based on climate-enhanced productivity, income, weather resilience, and mitigation (Aggarwal et al., 2013).

14.2 CLIMATIC FACTORS AFFECTING POSTHARVEST MANAGEMENT, FOOD LOSS, FOOD QUALITY, AND FOOD SECURITY There are various climatic factors that have important influences on postharvest management (PHM), food loss, food quality and food security. Automation in agricultural practices and machinery for environmental control may be able to decrease postharvest losses. The three key climate factors which impact postharvest handling operations are as: 1. Temperature; 2. Atmospheric gases; 3. Rainfall.

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14.2.1 Temperature Global warming is affecting the food sector in many ways, such as causing increases in sea levels leading to a decrease in available agricultural land and the creation of unsuitable conditions for the growth of many crops. High temperatures have a significant impact on physiological and biochemical activities including photosynthesis, transpiration and respiration, a reduction in seed germination, and the development of fresh produce (fruit and vegetable, e.g., carrots, lettuce, cucurbits, and cactus) (Kondinya et al., 2014). The photosynthesis/respiration ratio must always be greater than 1. At temperatures of about 15°C, the ratio is generally greater than 10, so plant growth is very rapidly in temperate rather than tropical zones. Higher temperatures (above 36°C) affect photosynthesis by variation in enzyme activity (e.g., polygalacturonase and cellulase negatively affect the firmness of fruit and delay ripening in tomato) and the electron transport system cycle. Damage to fresh produce is greater above a certain threshold climatic temperature (Wheeler et al., 2000). The climatic temperature plays an essential role in the physiological developmental of plant, for example, the (color, texture, chemical, sensory) changes that occur during ripening. The synthesis of tartaric acid and malic acid at the time of growth phase are affected including a 50% reduction in tartaric acid with a 10°C temperature rise (Lakso and Kliewer, 1978). Other examples include grapes and apples, as when these fruits are subjected to sunlight they ripen faster, and have a higher sugar content and lower tartaric acid than when they mature in sheltered areas (Castellarin et al., 2016; Famiani et al., 2014; Duchene and Schneider, 2005). “Fuerte” avocados in direct sunlight (35°C) were 2.5-fold tougher than those grown on the shaded side of the plant (20°C). The changes in cell wall composition, number, and turgor properties were related to the sunlight exposure. High temperature also affects the fatty acids composition, for example, there is 30% increased palmitic acid but no change in oleic acid (Woolf and Ferguson, 2000). The majority of leafy vegetables are heat-sensitive, but some are heat-tolerant due to the membrane stability of their leaves. These exceptions include Amaranthus (moderately), water spinach, and cucurbits, which can survive in hot environmental conditions (Wang et al., 2016; Ebert et al., 2007; Kuo and Chen, 1990). In Taiwan, plastic greenhouses are utilized to overcome this problem during the summer season, as they have inner temperatures of up to 40°C, which is the ideal to reduce this type of damage.

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Stern (2006) reported that temperature rises of 0.7°C (2020), 1.96°C (up to 2050), and 3.67°C (up to 2080) will decrease cereal yields (without beneficial CO2 effects) up to 10% 18% (2050) and 30% (2080) in Africa and Asia, respectively. Sinha and Swaminathan (1991) explained that an increase of 2°C in temperature could affect rice yield by 0.75 ton/ha in maximum production areas and a 0.5°C increase in winter temperature could reduce wheat production by 0.45 tons/ha. Meanwhile Saseendran et al. (2000, 2007) indicated the possibility of a 10% 40% crop production loss in India with a 1°C temperature rise by 2080 2100, in which rice production could decline by up to 6%. Recently, an IARI (Indian Agricultural Research Institute) report described the prospect of 4 5 million tons loss of wheat production in the near future with a rise in temperature of 1°C during the growth period (Anupama, 2014). The temperature rises and rainfall change will lead to water shortage, resulting in low productivity in the crop-oriented food sector (vegetables, rice, cereals, spices) in Pakistan (Menhas et al., 2016). The climatic temperature plays another important role in increasing the rate of respiration of fresh produce after harvesting causing damage/ decay. Therefore it is essential to remove field heat from fresh produce before starting any kind of postharvest handling operation. Precooling is the essential first step in the postharvest handling operation, and is generally performed to avoid microorganism development, for field heat removal and shelf life extension before packing, and for transportation and storage (Aggarwal, 2008). There are different traditional as well as modern precooling techniques (like room cooling, forced-air cooling, hydrocooling, vacuum cooling, evaporative cooling, and cryogenic cooling) applied to remove the field heat and also to extend the shelf-life two- to threefold for every 10°C temperature decrease from fresh produce. These various precooling methods demand considerable amounts of energy to run the equipment. The drop in the rate of respiration leads to less enzymatic activity, slow senescence, ripening, firmness, pathogenic inhibition, less microbial growth, and less water loss, which ultimately improves fresh produce quality. Therefore an adverse environmental temperature increases the temperature of fresh produce, requiring extra energy and cost expenditure on processing, storage, and transportation.

14.2.2 Atmospheric Gases Atmospheric gases are another important climatic factor which has a crucial affect on plant growth, fresh produce quality, processing, and storage.

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GHG is a mixture of water vapor, CO2, and other gases (CH4, N2O/ NO2, O3), which absorb the Sun’s radiation inhibiting it from leaving the Earth’s surface. CO2 and other gases absorb the Earth’s infrared radiation which traps heat and means that the CO2 concentration and atmospheric energy retention have a direct relationship with the increase in climatic temperature and are ultimately contributing to global warming. Hogy (2009) illustrated that an atmospheric CO2 (50% higher) rise would contribute to the malformation of tubers (approximately 63%) and resulting tuber greening (12%), which gave lower-quality grade processed product. Besides these changes the high CO2 level exposure resulted in reduced potassium, calcium, and protein levels in tubers, as well as nutritional and sensory quality loss of the product. Bindi et al. (1997) worked on grapevine exposure to high CO2 levels and its effect on growth. They found that there were increases of 8% and 14% in tartaric acid and total sugars contents in the wine ripening season, respectively. Skog and Chu (2001) studied the effect of ozone treatment on the quality of fruits and vegetables in cold storage. They reported that there was a threefold increment in vitamin C content observed with refrigerated storage (2°C) in an atmosphere enriched with ozone (0.35 µL/L), while in berries a 40% reduction was observed with the same temperature and in a normal atmosphere.

14.2.3 Rainfall The next important climatic factor to affect the food supply chain is annual rainfall and its distinct geographical distribution. With variations in air temperature, rainfall can increase the frequency or strength of floods and droughts. Intense rainfall can damage seeds and seedlings, decreasing plant growth and increasing plant pests and diseases during the planting season, ultimately resulting in plant stress and a reduction in yield. Severe rainfall has an intense impact on drenched soils, decreasing agricultural production and providing ideal conditions for the proliferation of various plant pathogens (Rathod and Aruchamy, 2010). Changes to rainfall pattern or rainy seasons are the most dynamic indicator of whether weather change has occurred in a certain area. Rain may occur for a shorter period with high intensity, creating problem for farmers (Mitchell et al., 1990). Jones and Kay (2007) framed a rainfall runoff model for analysis of flood frequencies in southern Africa. It has been reported that southern Africa is faced with less than 75% of its average

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rainfall, which will affect the communities of 10 countries with an estimated 70% crop failure. The changing patterns of rainfall in semiarid and subhumid regions directly affects cereal production, which is a challenging situation for billions of people and their livelihoods. In 2010 11, Uganda suffered from rainfall deficits, resulting in low production and reduced export of crops (sugar, coffee, tea, tobacco, and grains), a sign of the negative effect on the agro-industry sector and lowering GDP by up to 3.5% (Department of Disaster Management Office of the Prime Minister, 2010 11).

14.3 EFFECTS OF CLIMATE CHANGE ON AGRICULTURE PRODUCTION India has approximately 2.4% of the total land mass in the world, of which the arable land percentage is 11.2%, enabling it to feed 17.5% of the global population. India produced 22.3% of world agriculture output, illustrating that India is more competent in agriculture than other countries. The total dynamic or active population of 14.8% which is involved in agriculture, reflect that the efficiency of agricultural workforce. A total of 55% of the Indian population is totally dependent on income from agriculture, which illustrates the country’s sensitivity to this sector. Indian agriculture roughly contributes 17% to GDP and provides employment to nearly 52% of the population (Jamil et al., 2011). Moreover, agricultural production and productivity in nonindustrialized countries are fully dependent on weather changes (Nelson et al., 2009). Agriculture is a victim but also a cause of environmental changes. The environmental factors which have a direct qualitative and quantitative effect on agriculture produce or yield include wind velocity, radiation, temperature, rainfall, humidity, etc. Basically, climatic conditions or factors determine the vegetative growth of plants, which affect, the agronomic production results and their contribution to the economy (Shreedhar et al., 2016). In developing or nonindustrialized countries approximately 70% of the population live in countryside areas and agriculture or natural resources are the main source of the livelihoods of the majority of this population. This increases efforts in cultivation and climate-sensitive resources, therefore these countries may have undesirable influences on the ecological system, reducing the capacity for human development as well as adversely affecting food security. The global population increment is contributing to food production stresses and therefore

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exerting further climate change stress. Nowadays, climatic disasters are increasingly frequent due principally the growth of water- and windrelated affects. Natural hazards related to the cultivation sector contribute 22% of the harm in nonindustrialized countries. The increasing temperatures may also reduce the length of the growing season and final yields of many crops (Thornton and Gerber, 2010). These developing countries are forecast to suffer up to 25% (2080) deterioration in cultivation productivity, while industrialized and developed countries will experience a milder effect with a reduction of 6% 8% in productivity (Anupama, 2014). One study revealed that from 1989 to 2000, Indian rice production declined from 17.96% to 3.45%, while wheat production reduced from 34.37% to 3.51%, which is the clear indication of environmental change and its disastrous effect on agriculture (Jamil et al., 2011). Another study showed that environmental change has a significant effect on the yield of crops, for example, wheat (decline by 29.2% 33.5%) and maize (decline by 8.9% 18.5%) in South Asia (Ken, 2018). Overall, Verchot et al. (2007) reported that agricultural productivity for the entire world is projected to reduce by 3% 16% by 2080 due to global warming.

14.4 EFFECTS OF CLIMATE CHANGE ON LIVESTOCK PRODUCTION Globally, livestock contribute to the livelihoods of one billion of the world’s poorest and generate employment for 1.1 billion (Hurst et al., 2005). Demand for processed livestock products will increase by 100% up to 2050 globally as the standard living increases (Lance et al., 2012). Livestock products will provide 17% of energy and 33% of protein consumption (Rosegrant, 2009). Alexandratos and Bruinsma (2012) predicted that milk production would increase from 664 MT (Stern, 2006) to 1077 MT and meat production would increase from 258 MT (2006) to 455 MT by 2050. This rapidly growing demand for livestock products in developing and nonindustrialized countries will require a “livestock revolution.” Livestock production and processing have strong influences on climate due to land use, manure, feed usage for animal production, processing, and transport (Wright et al., 2012). The best quality of livestock production and processing is totally dependent on many factors including environmental temperature, water quality, reproduction, health, forage (quantity and quality), and animal diseases. The production and processing

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of livestock and its products take up 45% of the Earth’s land surface, contributing 14.5% of global GHG emissions (emissions of 7.1 gigatonnes CO2 equivalent per year) (Melissa et al., 2017). Natural or climatic threats could reduce livestock production and processing in nonindustrialized countries by as much as 37% by flood, 19% by drought, and 23% by other climatological disasters, creating a loss of approximately $93 billion. There are many direct and indirect effects of climatic change. A climatic temperature increase of 3°C will have direct effects on thermoregulatory control, biological stress, disease stress, and nutrition of wildlife. There is a direct effect of temperatures higher than 25°C on the production as well as processing of meats in terms of carcass weights, coats and colors, body size, and fat marbling (Nardone et al., 2010; Thornton and Gerber, 2010). The poultry industry is highly affected by temperatures above 30°C as it causes heat stress to birds. This heat stress affects feed intake, reproduction, carcass weight, egg quality (egg and shell weight, thickness), and the nutritional value of meat (Lucas et al., 2000; Esminger et al., 1990; Tankson et al., 2001; Novero et al., 1991; Mashaly et al., 2004).

14.5 EFFECTS OF CLIMATE CHANGE ON FOOD LOSS AND POSTHARVEST TECHNOLOGY AND MANAGEMENT Food loss is the quantity of pre- or postharvest food available for human eating that is wasted for any reason. It comprises natural losses such as moisture loss from food, mold, and pest infestation due to inadequate climate control (Rohkani, 2007). It is characterized not only by financial losses but also other resources invested during its production, processing, storage, and transport. Reasons for food loss also including improper storage and poor accessibility to markets (Buzby and Jeffrey, 2012). The 14% food leftover in 2008 in the United States generated costs of around $1.3 billion (Schwab, 2010). Postharvest technology is the set of operations between cultivation, harvesting, processing, and consumption of agricultural commodities, which aims to avert losses of food items on a qualitative and quantitative basis. PHM is systems and procedures used to increase the storage life of fresh or processed horticultural produce as well as retention of food quality right up to the end of the supply chain. PHM starts with preharvest management and the factors which influence the quality (chemical composition and morphology) of harvested produce, that is, irrigation,

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pest control, fertilizers, growth regulators, climate (temperature, wind velocity, rainfall), and tree conditions (training, age, light penetration, pruning) (Kader, 2002; Nayyer et al., 2015). Postharvest handling operations include various steps after harvesting of fruits, vegetables, and grains, including precooling (room cooling, vacuum cooling, forced air cooling, icing, evaporative, hydrocooling, cryogenic cooling), sorting (screen, barrel screen, diverging belt, roller grader, color grader), grading, washing [to remove dust, microorganisms, latex stains (mango and banana) from fresh produce by running through chlorinated water], waxing of paraffin wax, carnauba wax, bee’s wax, shellac wax, sugarcane wax, resin (dipping, spraying, brushing, fogging, foaming), curing (for bulbous vegetables, i.e., onion, garlic, yam), packing (wood pallet, stretch film, bins, wire-bound crates, corrugated fiberboard, paper and mesh bags, rigid plastic container), transportation (road, railway, air, waterway), and storage [in situ storage, pits, clamps, cellars, refrigerated storage, frozen storage, zero-energy cool chamber (ZECC), modern cold storage, modified atmosphere packaging (MAP), controlled atmosphere packaging (CAP), hypobaric storage] (Fig. 14.1). Generally, 40% 50% of crops are lost or decay after harvesting in the field and before reaching the end market in developing countries due to the physiological activity (respiration, transpiration) of the produce, improper handling, inefficient storage systems, and scarce availability of processing plants. The various reported data sources for crop losses have been up to one third (Kitinoja, 2002) or 30% 40%, that is, about 1.3 billion tons per year (Ray and Ravi, 2005; Kader, 2002; Gustavsson, 2010). Overall, postharvest loss estimations were 4.8% 21.5% for maize in Ghana. while in India it was observed that losses varied between 10.63% and 2.51% for kinnow fruit in the Punjab depending on the distance to the marketplace. This loss was 5.15% for the medium-distance markets and 8.17% for long-distance markets. Overall, postharvest losses were 14.47% and 21.91% for the Delhi and Bangalore markets, respectively (Gangwar et al., 2007). The diverse factors affecting postharvest losses were 3.82% for rice (farm level) and 3.28% for wheat (supply chain) in Karnataka (India). The level of postharvest losses of staple crops (maize, wheat, rice) during storage was mostly due to insects (Boxall and Gillett, 1982). Environmental change will have a huge impact on capacity and the ability to distribute fresh produce while reducing losses and waste.

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Harvesting

P r e c o o lin g (Room Cooling, Forced Air Cooling, Hydro Cooling, Vacuum Cooling, Icing, Evaporative. Cryogenic cooling)

Washing

Drying

Sorting (Screen, Barrel Screen, Diverging Belt, Roller Grader, Color Grader)

Grading

Curing/Waxing

Packaging (Wood Pallet, stretch film, bins, Wire-bound crates, Corrugated fiberboard, Paper & Mesh bags, Rigid plastic container)

Storage (In-situ storage, Pits, clamps, cellars, Refrigerated storage, Frozen storage, Zero Energy Cool Chamber (ZECC), modern cold storage, Modified Atmosphere storage (MAS), Controlled Atmosphere storage (CAS), hypobaric storage)

Transportation (Train, Flight, Road, Water)

Figure 14.1 General flowchart for Post-Harvest Handling Operation of Fresh Produce (Fruits/Vegetable).

Fluctuations in climatic factors will result in the development of more diseases, natural occurring toxins, and pests, making a worst situation for any food surplus. This will not only create less food availability and rising food prices but will also put pressure on the environment due to GHG production, waste management, and loss of resources for production (Siddiqui et al., 2015). Grading, the next step in the postharvest handling operation, is applicable for all types of food items (fruits, vegetable, cereals, pulses). It is an essential step in the fresh produce handling operation in well-established supply chains (Watkins, 2003). Mechanization in the harvesting process by various harvesters (including robotic harvesters) and the food procurement process can elevate the quality and reduce losses of fresh produce. The quality of postharvest technology and labor accessibility play a major role in postharvest losses (Kitinoja, 2002).

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Drying/curing is the next step in the postharvest handling operation after sorting/grading, and involves the elimination of moisture from fresh produce, hence preventing the growth of bacteria, yeasts, and molds. This method has been used since the beginning of farming to extend the shelf life and preserve the quality of fresh produce. Various types of dryers and dehydrators are used, employing sun drying, air drying, smoking, or wind drying to evaporate water from fresh crops (Sargent et al., 2000). Agricultural and postharvest losses are heavily dependent on the transport medium used (rail, flight, roads) and the distance traveled. The distance from production to consumption of food is called the “food miles.” Food miles have a direct relationship with GHG production due to the combustion of fuel required during transportation. Truck, rail, and air transport contribute to GHG production at a rate of 0.15 CO2 equivalent/t/km, 0.01 CO2 equivalent/t/km, and 1.093 CO2 equivalent/t/km, respectively (Lin et al., 2011). In addition to this, storage temperature, moisture content, processing technology, container material, and buildings contribute greatly to losses. High moisture, temperature, and storage time provide an ideal environment for microorganisms to complete their life cycles (Sargent and Treadwell, 2015). Climate change (high/low temperatures, erratic rainfall, dry seasons) is the main cause of postharvest losses for paddies in Karawang district (Indonesia). There are few technological advances which can be applied after harvesting of paddy to resist environmental changes, that is, production, investment, marketing, and network creation capability for small or major changes and capability (Ernst et al., 1998). These technological capabilities are defined as assimilating, using, adapting, and modifying the available existing technologies with the support of technological information (Kim, 2001). Szogs (2010) described technological capability as the capability to select the most suitable existing technology to be adapted for the creation of new knowledge. The technological capability of any firm showed the current status of technology learning and adaptation process. Technological capability is specified in five stages, that is, very low, low, medium, high, and very high, on the basis of its user acceptability (Bell and Pavitt, 1993). It involves improvement of currently utilized postharvest technology of agriculturalists and making policies to overcome the effects of climatic changes. Packaging of fresh/processed products is an important factor for the shelf life of the produce. The well-developed supply chains in developing countries have standardized packaging materials and machines. In addition

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to this, these packaging systems decrease the spoilage of commodities due to mechanical shock in transportation to the market. Loss of moisture contributes to food losses more prominent in poorly packaged material and unfavorable conditions. There are various postharvest factors which significantly affect fresh produce quality as listed here (Ahmad and Siddiqui, 2016): • Maturity stage; • Harvesting tools, time and methods; • Precooling; • Sorting and grading; • Packaging and cushioning materials (foam net, paper cutting, rice, straw, etc.); • Storage condition (temperature, relative humidity); • Types and condition of transportation; • Loading and unloading pattern; • Exposure to sunlight.

14.6 EFFECTS OF CLIMATE CHANGE ON FOOD QUALITY Food quality (degree of excellence) has three attributes: qualitative, quantitative, and hidden. The qualitative attributes include all sensory parameters: color, aroma, taste, texture, overall acceptability on manual perception. The quantitative attributes correspond numerical data values of these qualitative attributes like color [Commission Internationale de “E” clairage color space for L (light-dark), a (green red), and b (blue yellow), taste (sour, sweet, bitter, salt), texture (hardness, chewiness, adhesiveness, springiness, softness, cohesiveness, etc.)], while hidden attributes are associated with nutritional parameters (carbohydrate, protein, lipid, mineral, tannin, phenol, vitamins, etc.). These quantitative, quantitative, and hidden attributes generally decrease due to climatic factors, for example, a rise in the atmospheric concentration of carbon dioxide.

14.6.1 Carbohydrate Content It is seen that among all the climatic factors, temperature plays an increasing large effect on carbohydrate composition than the high concentration of carbon dioxide in the atmosphere. For example, the carbohydrate concentration or composition of soybean seeds has changed significantly with temperature increases. Williams et al. (1995) reported that a small temperature increase (2°C 4°C) can affect atmospheric CO2 concentration,

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which results in changes in the starch and protein matrix of wheat grain. Furthermore, starch gelatinization property (content, grain size, number) changes mainly due to climatic temperature and less due to the CO2 increment. Thomas et al. (2003) studied the combined effects of temperature and CO2 concentration on red kidney beans and showed that composition of the seed was not affected by increased CO2 concentration. In contrast, seeds exposed to high temperature resulted in decreased glucose (44%), and increased sucrose (33%) and raffinose (116%) compared to the low temperature-treated samples (DaMatta et al., 2010; Mudasir et al., 2017). The changes in carbohydrate content and property (as discussed above) play an essential role in product nutritional qualities, such as increase in raffinose content creating digestive problems (lacking of galactosidase is important for raffinose) in animals (nonruminants) and humans (Sebastian et al., 2000).

14.6.2 Mineral Content The elevated level of CO2 has a strong impact on food crop nutritional values, especially their mineral content. Many researchers have studied and reported that maize, wheat, soybeans, and rice have shown a reduction in mineral content (iron and zinc) due to elevated levels of CO2 in the atmosphere. Ainsworth and McGrath (2010) have shown that the concentration of protein and mineral (iron and zinc) decreased by 10% 14% and 15% 30% in nonleguminous grains, respectively. Seneweera and Conroy (1997) reported that at elevated CO2 concentrations the four elements, that is, N (14%), Fe (17%), P (5%), and Zn (28%) decreased, while the calcium concentration increased (32%) in rice. Lloyd et al. (2011) studied a slight decrease in P, Mg, Zn, N, Ca, S, and Fe, whereas K increased because of elevated CO2 in wheat. A higher CO2 concentration leads to a decrease in all micronutrients by 3.7% 18.3%, except Fe, in wheat and rice (Hogy, 2009).

14.6.3 Lipid Content The concentrations of carbon dioxide and temperature play a core role in the yield (highest at 32/22°C) of soybean oil. In the fatty acid profile, oleic acid increased with temperature, whereas linolenic acid decreased, which was associated with the nutritional quality and shelf life of oils/fats (oleic acid has a lower affinity to oxidation than linolenic acid) (Thomas et al., 2003). Namazkar et al. (2015) reported that the individual and

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combined effects of elevated carbon dioxide, ozone, and temperature reduced the oil quality and production of rapeseed. Oleic acid, linoleic acid, linolenic acid, omega-3, palmitic, and eicosenoic acids were reduced due to elevated carbon dioxide alone, while the combined effect of elevated carbon dioxide and temperature decreased the linoleic acid and linolenic acid by 45% and rapeseed oil yield by up to 10%.

14.6.4 Proteins, Tannins, and Phenols The proportions and properties of the two wheat proteins, that is, glutenin (35%) and gliadin (45%), are mainly responsible for the quality of bakery products (Mudasir et al., 2017). The concentrations of these proteins decreased at an elevated carbon dioxide concentration, but this result was completely dependent on the cultivar. There is a strong impact of environmental carbon dioxide concentration (315 400 ppm) on the protein percentage reduction from 10% to 15% of rice, wheat, and barley crops (Taub, 2010). The high carbon dioxide concentration induced a drop in protein concentration of potato tubers of up to 14%, while it was 1.4% for soybean (Hogy, 2009). Tannin, phenol, lignin. and terpenoids are mainly considered to be secondary metabolites which can alter the edible, nutritional quality and toxicity level of plants. Robinson et al. (2012) reported that the effect of these secondary metabolites is less understood and more variable than primary metabolites. The main two climatic factors, that is, elevated temperatures (drought stress) and carbon dioxide percentage are often related to increased tannin and phenol contents (Idso and Idso, 1994).

14.6.5 Mycotoxins Fungi have been present on Earth for more than 1.6 million years and they have many groups of microorganisms. The mycotoxins, lowmolecular-weight toxic complexes, are formed by filamentous fungi which can contaminate food. There are various types of mycotoxins produced by microorganisms, such as aflatoxin, fumonisins, zearalenone, deoxynivalenol, ochratoxin, etc. Environmental factors (temperature and relative humidity) have a large influence on the production of mycotoxins in plants. Garcia Solache and Casadevall (2010) reported that global warming exerts an additional pressure on fungi to increase their adaptability for heat tolerance for effective invasion of the human host. Furthermore, global warming is the cause of an approximately 5% reduction in temperature

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gradient of mammal and ambient climatic temperatures. Thus, heat tolerance affects fungal pathogen survival and favors the emergence of new diseases because of modifications to the distribution range, sequential activity, and pathogen community structure. Some species of fungi have survival threats due to global warming, like Aspergillus flavus which require optimal temperature of approximately 30°C for growth. Mycotoxin production is not only climate-dependent but is also affected by noninfectious factors, such as micronutrient bioavailability and insect damage (Ingram, 1999; Jeger et al., 2007).

14.7 FOOD SECURITY UNDER CLIMATE CHANGE The definition of food security includes availability, accessibility (economically and physically), utilization (method of use and assimilation in the human body), and stability of food. There are a number of direct effects of climate change on food security including loss of livelihoods, reduced income of rural people, impacts on marine ecosystems and terrestrial ecosystems which break down food systems. The four pillars (availability, accessibility, utilization, stability) of food security are interlocked with climate change directly or indirectly. Availability is the first pillar of food security, and gives an idea of national production, stock, supply risk, and research efforts to increase agricultural output in terms of quality and quantity. Environmental change is the main cause of shifting seasons and agriculture yields, which reduce local food availability. Climate change reduces the available arable land for agriculture, soil fertility, and water bodies, but increases pests and disease, which affect the overall crop and livestock productivity. Water availability and salinity of water bodies also affect the production of food. Economical and physical food accessibility is concerned with the livelihoods of people. Market factors like food prices and purchasing power are important for the accessibility of food. Climate change has also been a significant inference in food distribution because of the destruction of roads, bridges, and other transport media. Utilization is measured by quality of food and depends on physiological needs, water, sanitation, and hygienic and health status. The consumption of protein and micronutrients is very important as part of a balanced and nutritious diet. Environmental changes can disturb food utilization through income and consumption patterns. The low-income

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population cope with an adverse economic situation by reducing the quantity and quality of food intake. Stability is the next pillar of food security, and means that food availability, accessibility, and utilization should be stable over time. Environmental changes disturb variability in production and induce instability. The excessive high temperatures, floods, and droughts are the main threats to agricultural growth and hence to food security. Another effect of environmental change is an increase in conflict over water and land, for example, due to herdsmen and animal movement into new locations for food and shelter. The agriculture, forestry, and fishing sectors in various countries depend on the climate for the livelihood of many people. There are currently 836 million people living in poverty (less than $1.25 per day) (United Nations, 2015). People living below the poverty line in remote areas may be dependent on forests and other uncultivated lands for their needs. The strong climatic factors, that is, temperature and humidity, play a direct effect on the lives of these people by changing the values of products and floods and droughts increase crop losses, with spoilage of arable land for farming, together these factors all aggravate the threat to food safety. The European Commission described the severe impact of climate change in Africa was visible due to reducing crop yield. The crop yield reduction affects the availability of food which result in indirect pressures on production and consumption patterns of agriculture and consumer. Africa is considered to be the continental area most likely to have severe climate changes. Two-thirds of the arable land of Africa is threatened due to rainfall and famine. Severe climatic effects are clearly visible in Ethiopia and sub-Saharan Africa in the form of food insecurity. Global warming is playing a significant part in the pillar of food security, as between 5 and 170 million individuals will be in danger of starvation by the year 2080. Southern Africa is facing food insecurity at a massive level of 58% 73% (Schmidhuber and Tubiello, 2007; Hendriks, 2005; Gutu et al., 2012; Vogel, 2005).

14.8 CONCLUSION Climate change is a serious challenge for the food industry in developing as well as developed countries. The food system is affected by GHG in terms of a rise in temperature and irregularity in rainfall, which cause a negative effect on agricultural land, crop production, PHM, food loss, and

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food quality attributes (qualitative, quantitative, and hidden). This chapter has focused on the main challenges (food research pattern change, focused research outcome, research outcome inclusion in society, adaptation for changes) of food security (availability, accessibility, utilization, stability of food) based on climatic changes. Hence, in future the initiative taken by Consultative Group for International Agricultural Research (CGIAR) for CSV and climate smart practice creation (e.g., alternate wetting and drying) is giving hope to finding a solution to the problems of PHM, food loss, food quality, and food security in all countries.

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Kader, A.A., 2002. Postharvest biology and technology: an overview. In: Kader, A.A. (Ed.), Postharvest Technology of Horticultural Crops, 3311. University of California, Division of Agriculture and Natural Resources, Special Publ, pp. 39 47. Kader, A.A., 2005. Increasing food availability by reducing postharvest losses of fresh produce. Acta Hort. 682, 2169 2176. Kader, A.A., Rolle, R.S., 2004. The role of post-harvest management in assuring the quality and safety horticultural crops. FAO Agric. Serv. Bull. 152, 51. Ken, J., 2018. Grain and Feed Annual. USDA Foreign Agriculture Services. Kim, L., 2001. The Dynamics of Technological Learning in Industrialisation. The United Nations University. Institute for new technologies, UNU/INTECH, pp. 15 35. Kitinoja, L., 2002. Making the link: extension of postharvest technology. In: Kader, A.A. (Ed.), Postharvest Technology of Horticultural Crops. Publication 3311, third ed. University of California, Oakland, CA, pp. 481 509. Kitinoja, L., Kader, A.A., 2002. Small-scale postharvest handling practices: a manual for horticultural crops, Postharvest Technology Center, Postharvest Horticulture Series 8E, fourth ed. Univ. of California, Davis. Kondinya, A., Palash, S., Pandit, M.K., 2014. Impact of climate change on vegetable cultivation a review. Int. J. Agric. Environ. Biotechnol. 7 (1), 145 155. Kuo, Y.H., Chen, G.T.J., 1990. The Taiwan Area Mesoscale Experiment (TAMEX): an overview. Bull. Amer. Meteor. Soc. 71, 488 503. Lakso, A.N., Kliewer, W.M., 1978. The influence of temperature on malic acid metabolism in grape berries. II. Temperature responses of net dark CO2 fixation and malic acid pools. Am. J. Enol. Vitic. 29, 145 149. Lance, H.B., Robert, P.R., Michelle, L.R., Nicholas, G., 2012. Impact of Climate Change on Livestock Production. Environmental Stress and Amelioration in Livestock Production. Springer, Berlin, Heidelberg, pp. 413 468. Lin, B.B., Chappell, M.J., Vandermeer, J., Smith, G., Quintero, E., Bezner-Kerr, R., et al., 2011. Effects of industrial agriculture on climate change and the mitigation potential on small-scale agro-ecological farms. CAB Rev. Perspect. Agric. Vet. Sci. Nutr. Nat. Resour 6, 1 18. Lipper, L., Thornton, P., Campbell, B.M., Baedeker, T., Braimoh, A., Bwalya, M., et al., 2014. Climate-smart agriculture for food security. Nat. Clim. Chang 4, 1068 1072. Lloyd, S.J., Sari, K.R., Zaid, C., 2011. Climate change, crop yields, and undernutrition: development of a model to quantify the impact of climate scenarios on child undernutrition. Environ. Health Perspect. 119, 12. Lucas, E.M., Randall, J.M., Menses, J.F., 2000. Potential for evaporative cooling during heat stress periods in pig production in Portugal. J. Agri. Eng. Res. 76, 363 371. Mashaly, M.M., Hendricks, G.I., Kalama, M.A., Gehad, A.E., Abbas, A.O., Patterson, P. H., 2004. Effect of heat stress on production parameters and immune responses of commercial laying hens. Poult. Sci. 83, 889 894. Melissa, M.R.D., Nejadhashemi, A.P., Harrigan, T., Sean, A.W., 2017. Climatic change & livestock: impacts, adaptation and mitigation. Clim. Risk Manage. 16, 145 163. Menhas, R., Umer, S., Shabbir, G., 2016. Climate change and its impact on food and nutrition security in Pakistan. Iran J. Public Health. 45 (4), 549 550. Mitchell, J.F.B., Manabe, S., Tokioka, T., Meleshko, V., 1990. Equilibrium climate change—and its implications for the future. In: Houghton, J.T., Jenkins, G.J., Ephraums, J.J. (Eds.), Climate Change. The IPCC Scientific Assessment. Cambridge University Press, Cambridge, UK, pp. 131 172. Mudasir, A.B., Hafiza, A., Shabber, H., 2017. Climate change and its impact on food quality. Int. J. Pure App. Biosci. 5 (3), 709 725.

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Namazkar, S., Egsgaard, H., Egsgaard, H., Frenck, G., Frenck, G., Jorgensen, S.R.B., 2015. Significant reductions in oil quality and lipid content of oilseed rape (Brassica napus L.) under climate change. Proc. Environ. Sci. 29, 121 122. Nardone, A., Ronchi, B., Lacetera, N., Ranieri, M.S., Bernabucci, U., 2010. Effects of climate changes on animal production and sustainability of livestock systems. Livest. Sci. 13057 13069. Nayyer, M.A., Siddiqui, M.W., Barman, K., 2015. Quality of fruits in the changing climate. In: Choudhary, M.L., Patel, V.B., Siddiqui, M.W., Verma, R.B. (Eds.), Climate Dynamics in Horticultural Science: Impact, Adaptation, and Mitigation, vol. 2. Apple Academic Press, Waretown, NJ, pp. 269 278. Nelson, G.C., Rosegrant, M.W., Robertson, J.K.R., Sulser, T., Zhu, T., Ringler, C., et al., 2009. Climate Change: Impact on Agriculture and Costs of Adaptation. IFPRI Food Policy Report, Washington DC. Novero, R.P., Beck, M.M., Gleaves, E.W., Johnson, A.I., Deshazer, J.A., 1991. Plasma progesterone, luteinizing hormone concentration, and granulosa cell responsiveness in heat stressed hens. Poultry. Sci. 70, 2335 2339. Palanisami, K., Kumar, D.S., Malik, R.P.S., Raman, S., Kar, G., Monhan, K., 2015. Managing water management research: analysis of four decades of research and outreach programmes in India. Econ. Polit. Rev. Wkly 26 27, 33 43. Rathod, I.M., Aruchamy, S., 2010. Rainfall trends and pattern of kongu Upland, Tamil Nadu, India using GIS techniques. Int. J. Environ. Sci. 1 (2), 109 122. Ray, R.C., Ravi, V., 2005. Postharvest spoilage of sweet potato in tropics and control measures. Crit. Rev. Food Sci. Nutr. 45, 623 644. Robinson, E., Ryan, G., Newman, J., 2012. A meta-analytical review of the effects of elevated CO2 on plant arthropod interactions high-lights the importance of interacting environmental and biological variables. New Phytol. 194, 321 336. Rohkani, 2007. Reduction of postharvest losses national movement, an attempt to cope with food crisis. Agrimedia Magazine 12 (2), 21 30. Rosegrant, M.W., 2009. Looking into the future for agriculture and AKST (Agricultural Knowledge Science and Technology). In: McIntyre, B.D., Herren, H.R., Wakhungu, J., Watson, R.T. (Eds.), Agriculture at a Crossroads. Island Press, Washington, DC, pp. 307 376. Sargent, S.A., Ritenour, M.A., Brecht, J.K., 2000. Handling, cooling and sanitation techniques for maintaining postharvest quality. HS719. Horticultural Sciences Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Sargent, S.A., Treadwell, D.D., 2015. Guide for Maintaining Quality During Harvest and Handling of Organic Vegetables and Melons. HS1148. University of Florida Institute of Food and Agricultural Sciences, Gainesville. Saseendran, S.A., Malone, R.W., Heilman, P., Ahuja, L.R., Kanwar, R.S., Karlen, D.L., et al., 2007. Simulating management effects on crop production, tile drainage, and water quality using RZWQM-DSSAT. Geoderma 140, 297 309. Schmidhuber, J., Tubiello, F.N., 2007. Global food security under climate change. Proc. Natl. Acad. Sci. USA 104, 19703 19708. Schwab, J., 2010. Environmental Protection Agency (EPA), Office of Solid Waste and Emergency Response (OSWER). Personal interview with J. Buzby, Washington, DC. Sebastian, S.A., Kerr, P.S., Pearlstein, R.W., Hitz, W.D., 2000. Soybean germplasm with novel genes for improved digestibility. In: J. K. Drackely (Ed.), Soy in Animal Nutrition, 30, 56 74. Seneweera, S., Conroy, J.P., 1997. Growth, grain yield and quality of rice (Oryza sativa L.) in response to elevated CO2 and phosphorus nutrition. Plant Nutr. Sustain. Food Prod. Environ. 873 878.

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Shreedhar, M., Bhatt, D., Uhlenbrook, S., Krishna, C.P., Mukand, S.B., 2016. Adaptation to climate change impacts on agriculture and agricultural water management a review. In: Hoanh, C.T., Johnston, R., Smakhtin, V. (Eds.), CAB International, Climate Change and Agricultural Water Management in Developing Countries, CABI, 11 31. Siddiqui, M.W., Patel, V.B., Ahmad, M.S., 2015. Effect of climate change on postharvest quality of fruits. In: Choudhary, M.L., Patel, V.B., Siddiqui, M.W., Mahdi, S.S. (Eds.), Climate Dynamics in Horticultural Science: Principles and Applications, 1. Apple Academic Press, Waretown, NJ, pp. 313 326. Sinha, S.K., Swaminathan, M.S., 1991. Deforestation, climate change and sustainable nutrition security: a case study of India. Clim. Change 19, 201 209. Skog, L.J., Chu, C.L., 2001. Effect of ozone on qualities of fruits and vegetables in cold storage. Can. J. Plant Sci. 81, 773 778. Stern, N., 2006. Stern Review on the Economics of Climate Change. Her Majesty’s Treasury and the Cabinet Office, London. Szogs, A., 2010. Technology Transfer and Technological Capability Building in Informal Firms in Tanzania. Centre for Innovation, Research and Competence in the Learning Economy (CIRCLE) Department of Design Sciences Lund University. Tankson, J.D., Vizzier, T., Thaxton, J.P., May, J.D., Cameron, J.A., 2001. Stress and nutritional quality of broilers. Poult. Sci. 80, 1384 1389. Taub, D.R., 2010. Effects of rising atmospheric concentrations of carbon dioxide on plants. Nat. Educ. Knowl. 3 (10), 21. Thomas, J.M.G., Boote, K.J., Allen, L.H., Gallo-Meagher, M., Davis, J.M., 2003. Elevated temperature and carbon dioxide effects on soybean seed composition and transcript abundance. Crop Sci. 43, 1548 1557. Thornton, P.K., Gerber, P., 2010. Climate change and the growth of the livestock sector in developing countries. Mitig. Adapt. Strat. Glob. Change 15 (2), 169 184. United Nations, 2015. https://www.theguardian.com/society/2015/oct/05/world-bankextreme-poverty-to-fall-below-10-of-world-population-for-first-time Verchot, L.V., Noordwijk, M.V., Kandji, S., Tomich, T., Ong, C., 2007. Climate change: linking adaptation and mitigation through agroforestry. Mitig. Adapt. Strat. Glob. Change 12, 901 918. Vogel, C., 2005. Usable science: an assessment of long-term seasonal forecasts among farmers in rural areas of South Africa. South Afr. Geograph. J. 82 (2), 107 116. Wang, D.X., Wang, L.L., Dagang, W.H.H., Cuilin, P., 2016. Evaluation of CMPA precipitation estimate in the evolution of typhoon-related storm rainfall in Guangdong, China. J. Hydroinform. 18 (6), 1055 1068. Watkins, C.B., 2003. Principles and practices of postharvest handling and stress. In: Ferree, D., Warrington, I. (Eds.), Apples-Botany, Production, and Uses. CABI, Boston, pp. 585 615. Watson, R.T., Zinyowera, M.C., Moss, R.H., 1996. Climate Change: Impacts, Adaptations and Mitigation of Climate Change: Scientific-Technical Analyses. Cambridge University Press, Cambridge. Wheeler, T.R., Craufurd, P.Q., Ellis, R.H., Porter, J.R., Vara Prasad, P.V., 2000. Temperature variability and the annual yield of crops. Agric. Ecosyst. Environ. 82, 159 167. Williams, M., Shewry, P.R., Lawlor, D.W., Harwood, J.L., 1995. The effects of elevated temperature and atmospheric carbon dioxide concentration on the quality of grain lipids in wheat (Triticum aestivum L.) grown at two levels of nitrogen application. Plant, Cell Environ. 18, 999 1009. Woolf, A.B., Ferguson, I.B., 2000. Postharvest responses to high fruit temperatures in the field. Postharvest Biol. Technol. 21, 7 20. Wright, C., Beurs, K.M., Henebry, G., 2012. Combined analysis of land cover change and NDVI trends in the Northern Eurasian grain belt. Front. Earth Sci. 6, 177 187.

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FURTHER READING Bock, A., Sparks, T.H., Estrella, N., Menzel, A., 2013. Climate-induced changes in grapevine yield and must sugar content in Franconia (Germany) between 1805 and 2010. PLoS ONE 8(7): e69015. https://doi.org/10.1371/journal.pone.0069015. Intergovernmental Panel on Climate Change (IPCC). 2014. Summary for policymakers. In: Field, C.B., Barros, V.R., Dokken, D.J., Mach, K.J., Mastrandrea, M.D., Bilir, T.E. et al. (Eds.), Climate Change 2014: Impacts, Adaptation, and Vulnerability. Part A: Global and Sectoral Aspects. Contribution of Working Group II to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY. Iswari, K., 2012. Readiness of harvest and post-harvest technology of paddy to minimize yield loss and improve the quality of rice. Agric. Res. Dev. J. 31 (2), 58 67. Johnstone, S., Mazo, J., 2011. Global warming and the Arab spring. Survival: Glob. Polit. Strategy 53, 11 17. Setyono, A., 2010. Postharvest technology improvement to minimize paddy yield los. Agric. Innovat. Dev. J. 3 (3), 212 226.

CHAPTER 15

Impact of Climate Change on Soil Carbon Exchange, Ecosystem Dynamics, and Plant Microbe Interactions Mohd Aamir1, Krishna Kumar Rai1,2, Manish Kumar Dubey1, Andleeb Zehra1, Yashoda Nandan Tripathi1, Kumari Divyanshu1, Swarnmala Samal1 and R.S. Upadhyay1 1

Laboratory of Mycopathology and Microbial Technology, Centre of Advanced Study in Botany, Institute of Science, Banaras Hindu University, Varanasi, India 2 Division of Crop Improvement and Biotechnology, Indian Institute of Vegetable Research, Indian Council of Agricultural Research (ICAR), Varanasi, India

Contents 15.1 Introduction 15.2 Climate Change, Plant Diseases, and Host Pathogen Interaction 15.3 Climate Change: Impact on Carbon Exchange and Microbial Activities 15.4 Microbial Response Mechanism to Climate Change 15.5 Impact of Climatic Change on Plant Microbe Interactions 15.6 Climate Change and Abiotic Stress: Microbe-Mediated Stress Alleviation 15.7 Microbial Attenuation of Abiotic Stress 15.8 Mechanism of Microbe-Mediated Stress Tolerance 15.9 Conclusion Acknowledgments Author Contributions Statement Conflict of Interest References Further Reading

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15.1 INTRODUCTION Global climate change is a matter of debate among researchers, scientists, and environmentalists. The changing climate in different regions has affected many natural phenomena including weather patterns and sea levels, as well as modulating the lifestyles and biogeography of flora and Climate Change and Agricultural Ecosystems DOI: https://doi.org/10.1016/B978-0-12-816483-9.00020-7

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fauna. The Intergovernmental Panel on Climate Change (IPCC) estimated that in order to limit global warming to 1.5°C above preindustrial levels, CO2 emissions must be reduced by approximately 45% from 2010 levels by 2030, and reach net zero around 2050 (Weigmann, 2019). Furthermore, according to one report, it was projected that the temperature and CO2 concentration may increase by 3.4°C and 1250 ppm by 2095, respectively (Savary et al., 2012), which would cause more variabilities in climatic conditions and adverse weather events (Pachauri and Reisinger, 2007) affecting agriculture, forest, flora, fauna, and their existing co-interactions drastically (Pathak et al., 2018). The IPCC predicted that, by 2100, the resilience of the majority of the natural ecosystem would be likely to be exceeded by the combinations of climate change, and the associated disturbances including high temperature, drought, salinity, flooding, wildfire, ocean acidification, insects, and other factors regulating the global climate change such as deforestation, habitat destruction, overexploitation of resources, land use change and pollution. Several natural causes that have been reported for changed climatic conditions including modification in solar activity, ice cap distribution, volcanic eruption, and waves. In contrast, the artificial causes include various anthropogenic activities, such as CO2 emissions from various industrial areas, deforestation, acid rain, depletion of the ozone layer, and increased evolution of greenhouse gases (GHGs) (Presidential Advisory Council on Education, Science, and Technology: PACEST, 2007). However, since the mid-20th century, the main causes of global warming and climate change have been the unrestrained release of GHGs, particularly atmospheric CO2 (Serrano et al., 2019). The greenhouse effect on the Earth is mainly contributed to by deforestation, burning of fossil fuels, and intensification of agriculture sector. Past reports have predicted that about 20% 35% of total GHGs emission are contributed by agriculture (Thangarajan et al., 2013; Zhou et al., 2016). According to one report, during 2010 13 about 12,000 tons of CO2 was released mainly from deforestation, management of nutrients, soil emission, and livestock production (IPCC, 2014). The continuum change in climatic conditions is the result of changes in the carbon flux distributed between the land, oceans, and atmosphere. Climate change affects the soil microbial activities in both a direct and indirect manner. Therefore, the soil microbiota reactions to GHGs in the atmosphere play an important role in global warming. Some indispensable climatic parameters including temperature, humidity, precipitation, solar radiation, and air motion are directly or indirectly involved

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in the regulation of energy and water balance in a geographical region. However, the cumulative effects of these variables cannot be avoided as there is considerable potential for mutual interactions between these variables that may cause additive or antagonistic effects on soil microbiota and, therefore, activities engaged in the production of GHGs (Shaw et al., 2002; Mikkelsen et al., 2008). The most speculated on and direct effects of climate change include the warming effects or increase in temperature, extreme climatic episodes, and precipitation changes, whereas indirect effects include complexities observed in the diversity of the microbial population in the soil that alter the physiochemical conditions of soil and thereby affect plant productivity (Bardgett et al., 2008). Additionally, the fluctuations observed in climatic trends are also affected by carbon allocation to the microbial community which overall affects the community structures and dynamics of the microbial system, playing a crucial role in the decomposition of organic matter. Moreover, the biological mechanisms responsible for regulating this exchange and distribution of carbon between interdependent community systems affects climate change through climate-ecosystem feedback and could augment or the longlasting effects of regional or global climate (Heimann and Reichstein, 2008). The distribution and circulation of carbon between the terrestrial ecosystems function as a global carbon sink through C (carbon) accumulation in the living vegetations, microbial biomass, and in soil (including litter, detritus, and humic components), while releasing and absorbing GHGs such as methane, nitrous oxide, and CO2, and thereby regulating global climate feedback trends. The natural processes and anthropogenic disturbances of modern industrialization including increased deforestation, habitat destruction, GHG emissions, burning of fossil fuels, release of ozone-depleting substances, N2 enrichment (Beedlow et al., 2004), and sulfur deposition (Monteith et al., 2007) have been reported to cause a major impact on the sink activity of the terrestrial ecosystem (Bardgett et al., 2008). Since CO2 is a primary substrate utilized as metabolic fuel by plants, atmospheric CO2 affects the allocation of carbon below the ground level and also influences root exudation chemistry. All these changes potentially affect the rhizospheric interaction of plants with beneficial microbes (Williams et al., 2018). In all cases, the overall carbon budget of the ecosystem under the influence of fluctuating climatic conditions is determined through the balance between respiration and photosynthesis. The diversity and productivity of plants of a particular region are well established and determined through interactions with rhizospheric

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microbiota and decomposers, thereby the circulating the carbon flux that overall affect the climatic conditions is dependent on complex interactions and feedback between the biotic and abiotic components, including freeliving heterotrophic microbes, and symbiotic and nonsymbiotic associative partners which overall influence and determine the quality and quantity of carbon flux within the ecosystem. In a natural ecosystem, the community compositions and structures are highly dependent on multiple parameters that include the type of organisms, life history traits, different thermal tolerances, and most importantly their dispersal abilities. Moreover, the community dynamics are decided by interactions occurring between the species (interspecific) or among the species (intraspecific) and are well determined by the complexity and diversity of occupying species in terms of different attributes and parameters, as mentioned above. The complexities observed in natural communities are highly dependent on interaction types and mechanisms that could be positive, negative, or have no functional roles. Moreover, the changed climatic conditions have also altered the distribution of species and therefore, their possible existing interactions (Wookey et al., 2009; Vander Putten, 2012). The rise in atmospheric concentration of GHGs, in particular CO2, has been reported as a major component triggering a rise in global average temperature and consequently affecting the distribution of rain and climatic conditions worldwide (Vasskog et al., 2015; Castello and Macedo, 2016; Worm and Paine, 2016; Runting et al., 2017). Due to the changed climatic conditions, crop plants suffer from adverse environmental conditions such as high temperature (or temperature changes from freezing to scorching), drought (water stress), variable light conditions that affect photomorphogenetic responses, and nutrient deprivation in soil, which directly influence the growth, morphology, physiology, and developmental aspects of plants (Aamir et al., 2017). The most detrimental effects of the increased CO2 level in the atmosphere on plants are the altered photosynthetic rate, and disturbed metabolism (Jia and Zhou, 2012; Philippot et al., 2013), which ultimately cause changed physiological processes in plants. Higher biomass accumulation and differential patterns of occurrences of pests and diseases are also challenging issues (Mendes et al., 2013). Furthermore, the distribution of assimilated carbon to decomposers is an important component of the ecosystem’s function. The altered physiological mechanism due to climate change disturbs this partition of assimilated carbon to microbial entities associated with rhizospheric soils and, therefore, affects the relationships between plants and microbes.

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15.2 CLIMATE CHANGE, PLANT DISEASES, AND HOST PATHOGEN INTERACTION The climatic parameters allow interpretation of physical processes in upper soil layers and the lower atmospheric region plays a crucial role in determining the climate for the local or regional biosphere (Monteith and Unsworth, 2007). It has been suggested that precipitation changes and rising temperature due to changing climate have resulted into the increased occurrence of the plant diseases which could be attributed to the increased use of pesticides (Chen and McCarl, 2001). It is also conceivable that the rising temperature, change in rainfall duration, and changes in relative humidity have not only affected agricultural productivity but also have a significant impact on the severity of plant diseases (Chakraborty and Newton, 2011). Furthermore, it has been found that climatic change could have severe repercussions on developmental stages and processes of pathogens, affecting host development, changes in the morphophysiological processes of plants, and therefore, affecting the host pathogen interaction (Coakley et al., 1999). Moreover, host susceptibility toward diseases caused by pathogens is also influenced in a stressed environments (Gassmann et al., 2016). Host susceptibility is determined through a plethora of molecular processes and signaling mechanisms including reactive oxygen signaling (ROS)-mediated defense signaling (Baxter et al., 2013), hormonal-induced cell signaling (Nguyen et al., 2016), calcium sensors (Ranty et al., 2016), and molecular priming (Thevenet et al., 2017), all of which modify the transcriptional processes, cellular mechanisms, and physiological responses. Agricultural productivity is largely determined by the presence of pathogens and the status of plant diseases in any environment. In changing environments, the condition of occurring diseases in crop plants is boosted due to a change in distribution pattern, an evolution of the new races and pathotypes, and epidemic development (Yáñez-López et al., 2012). Furthermore, the rapidly fluctuating environment has influenced many inoffensive pests to become more virulent, resulting in a greater number of pest occurrences. Nevertheless, the disease severity of plants is significantly influenced by increased temperature and its exposure duration to the plants, as it was suggested that epidemics of plant diseases are highly dependent on temperature variations (Elad, 2009) and stability of plants in a particular environment (Evans et al., 2008). Temperature changes, particularly optimum temperatures, influence the development of hosts,

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physiological attributes of microbial pathogens, and therefore determine the incidence of disease development (Suzuki et al., 2014; Ashoub et al., 2015). In addition, alterations observed in the biology of hosts and pathogens due to their temperature dependences corresponds to a difficult and complex nature of disease prediction outcomes. Microbial populations with short life cycles adapt the re-occurring changes in the environment with faster reproductive processes and dispersal mechanisms (Coakley et al., 1999) and become more complicated in physiological attributes in a stressed environment developed through changed climatic events (Sturrock et al., 2011). The prolonged effect of climate change results in divergence in the geographical distribution of hosts and pathogens, and altered crop losses, and therefore, difficulty in managing plant diseases. Chakraborty et al. (2012) have emphasized that climate change affects both the flora and fauna with their multitrophic interactions. It has been documented that both abiotic and biotic stresses effectuated from the changed climatic condition have influenced the host pathogen interaction in multiple ways (Vandenkoornhuyse et al., 2015). The interactions between soil microbes and plants assist in the regulation and maintenance of ecosystem properties (Classen et al., 2015). In a changed environment, the interaction networks between species are altered and, therefore, change the dynamical aspects of ecosystem properties. Moreover, altered the host pathogen relationship complicates the relation required for predicting the risk of disease development. However, disease prediction depends on several factors that include the shifting of the pathogenic races or change in host abundances in an altered environment. Mounting amount of research conducted over the last few years has demonstrated the shifting of species interactions in changing climate which has altered the function of the ecosystem, and affected the biodiversity (Walther et al., 2002; Gottfried et al., 2012; Langley and Hungate, 2014). In fact, the interactions between the microbial populations of soil, and their interactions with plants determine the composition, abundance, distribution, and diversity of the species as well as the landscape patterns of animals and plants (Berg et al., 2010; Vander Putten et al., 2013). The impact of climate change on plant pathogen interactions and diseases is an interesting and challenging research field among researchers, but the information on the topic is scanty. In recent years, the ongoing research in the field has shifted the direction of study to evaluate the effect of a single meteorological variable such as elevated concentration of atmospheric CO2, temperature, and precipitation on the life cycle of the host,

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pathogens, and the effect of interactions occurring between these variables on host cell physiology, pathogen response, and disease epidemiology in a controlled environment. The climate induced increased CO2 concentration and rising temperature levels could affect the sensing and response mechanisms of microbial communities existing in soil as well as the behavioral responses of plants (Ainsworth and Rogers, 2007; Chakraborty et al., 2012) and therefore, will definitely modulate the plant pathogen interaction (Ferrocino et al., 2013). It has been reported that under the conditions of the rising CO2 levels and temperatures the morphophysiological attributes and the metabolic performances will be influenced and, therefore, affect agricultural productivity. The continual changes in climate have affected plant microbe interactions under the extremities of abiotic stress condition. Therefore, it is imperative to understand the role beneficial microbes in the alleviation of stress response that has been resulted from the changed climatic condition, and its translation to enhanced agricultural production.

15.3 CLIMATE CHANGE: IMPACT ON CARBON EXCHANGE AND MICROBIAL ACTIVITIES Due to their vast diversity, complexity, and extreme genetic potential, microbes affect the global exchange of carbon (C) between land and atmosphere and the C cycle-climate feedback in many ways. However, one could measure their metabolic activities either in the form of atmospheric carbon gain or carbon loss from the soil ecosystem (Bardgett et al., 2008). The increased human interferences for the release of GHGs and other global changes have affected climatic conditions and, thereby, affected the exchange of C between the land and the atmosphere. The changed climatic conditions affected the microbial activities for the breakdown of organic matter. The microbial system plays an indispensable role in regulating the C flux between land and atmosphere. The direct impact of climate change on the microbial system and their functions have been well evidenced (Blankinship et al., 2011; Henry, 2013; Manzoni et al., 2012; A’Bear et al., 2014). The disturbed C flux in the changed climatic scenario has also affected the microbial response and mechanisms for circulating C and, therefore, explicit consideration of both positive and negative roles of the microbial system in the C cycle which explain both the positive and negative impacts of global climate change on microbial physiology, C transfer, and assimilation in a terrestrial ecosystem. In the

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majority, the C gain could be assessed from the uptake of C in the form of CO2 and methane (methanogens) from the atmosphere, whereas the C loss could be analyzed through the C loss from the soil via respiration and methanogenesis (methanogens) processes. The temperature-dependent microbial processes directly affect the microbial soil respiration rates. Furthermore, the rise in warming temperature has influenced microbial metabolism and received considerable attention in the last few years (Bradford, 2013; Frey et al., 2013; Hagerty et al., 2014; Karhu et al., 2014). Moreover, previous research has indicated that the microbial activity in feeding back GHG to the atmosphere causes the actual warming effect (Bardgett et al., 2008). The direct effect of climate change on microbial activities, response mechanisms, and their functional profile could be interpreted from the relative abundances and diversity of microbial communities in soil. Furthermore, the differential behavior could be explained based on their different growth rates, temperature sensitivity, and other physiological attributes (Castro et al., 2010; Gray et al., 2011; Lennon et al., 2012; Whitaker et al., 2014; Briones et al., 2014; DelgadoBaquerizo et al., 2014). In contrast, the indirect effects include the change in the physiochemical profile of soil that alters the diversity of microbial populations and overall affect the productivity of crop plants. The influx of C to soil affects the activity and dynamics of microbial communities, as reflected from microbial respiration, decomposition processes, and release of C from the soil. One of the most important contributions of the soil microbiota is the decomposition of organic matter and an increased warming effect that definitely increases microbial activity for organic matter decomposition. The structure of microbial communities is also affected by prolonged drought conditions (Evans and Wallenstein, 2012), which might result in decreased efficiency for C uptake and its utilization (Göransson et al., 2013), decoupling microbial growth and respiration (Meisner et al., 2013a), or might have positive or negative feedbacks to plant communities (Meisner et al., 2013b). Explicitly,the increased decomposition would generate a large amounts of GHGs and, therefore, have increased efflux of CO2 to the atmosphere and export of the dissolved organic C by the process of hydrologic leaching (Jenkinson et al., 1991; Davidson and Janssens, 2006). It is noteworthy that the since soil microbes play a crucial role in soil C cycling and other ecosystem processes, a comprehensive study is required to understand the biological processes and mechanisms involved in microbial processing of organic matter to the release of the atmospheric CO2 and other GHGs under the

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scenario of future climatic changes. When environmental conditions are disturbed by climate change-induced events it will affect the microbial processes through observed changes in activities like soil respiration, enzyme activities, and decomposition of litter (Keitt et al., 2016). Under the effect of changed environment, microbial activities will be influenced as reflected from observed changes in functions like microbial enzymic activity, soil respiration, and decomposition of litter. However, the exact mechanism through which these changes occurred is not known. Further, it has been reported that whole soil, aggregate functional responses result from the individual activities of a diverse community of soil microbes may involve different mechanism working simultaneously to create the observed function. The most common response mechanism includes microbial physiology, evolution, community composition, and feedbacks (Keitt et al., 2016) as microbial traits that correlate physiological attributes with environmental performances and fitness of microbial species that lead to sorting of species and compositional change over gradients (Leibold et al., 2004). This could be interpreted as mean annual air temperatures, and mean annual precipitation could be positively correlated with the rate of soil respiration (Raich and Schlesinger, 1992). Moreover, the mean annual net primary productivity of the ecosystem shows strong correlations with mean annual respiration rates, as reported the soil respiration rate is found to be 24% higher than the mean annual NPP (Keitt et al., 2016). However, the temperature dependence and strong response of soil respiration rates over productivity (Jenkinson et al., 1991; Schimel et al., 1994) may result in a net transfer of carbon from land to the atmosphere, which would generate positive feedback on climate change. In contrast, on increasing temperature, the effect on heterotrophic microbial respiration (during temperature-dependent microbial decomposition of substrates) and its potential feedback to climate change is uncertain (Davidson and Janssens, 2006; Trumbore, 2006). This uncertainty may be due to the chemical complexity and diversity of the substrates that would affect the temperature-dependent biochemical decomposition of organic matter by heterotrophic organisms (Davidson and Janssens, 2006). Furthermore, other parameters that decide the microbial response to contribute to global warming include the decomposition of organic materials. In some cases, the decomposition rate is either hindered by the type of material (recalcitrant or susceptible to microbial attack) or has some physical or chemical barrier (for preventing microbial action), therefore the decomposition process is relatively very slow. The indirect feedback of climate

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change on the microbial system affects the potential abilities and functioning of microbes through their impact on plant growth and composition of vegetation. The indirect effects of climate change to the microbial system are regulated through different feedback loops that incorporate plant microbe interactions, microbe microbe interactions, soil mineralization events, plant chemistry, and plant composition, and the most likely shifting in other ecosystem interactions that mediate the other functions of the ecosystem (Gilman et al., 2010; Adler et al., 2012; Steinauer et al., 2015). The increased concentration of atmospheric CO2 causes an increased rate of photosynthesis, production of food products, and the transfer of photosynthate C to heterotrophic microbes and symbiotic mycorrhizal fungi (Bardgett, 2005; Johnson et al., 2005). C is distributed through the secretion of sugar-rich exudates, amino acids, and organic acids (Diaz et al., 1993; Zak et al., 1993). The results of increased carbon flux from vegetation to the soil and the microbial biomass is unpredictable due to its dependence on multiple factors such as the status of the soil health and properties, soil food web interactions, and other ecosystem functions. However, the most accurate outcome of such C transfer is the loss of C from the soil through microbial respiration and its dissolution in water bodies due to stimulation of microbial activities and enhanced mineralization of soil organic carbon. Furthermore, other possible results include stimulation of microbial biomass and immobilization of soil nitrogen, thereby delimiting the N availability to plants, creating negative feedback that constrains future increases in plant growth and carbon transfer to the soil (Diaz et al., 1993). Moreover, a deficiency in the soil N content could result in increased competition between microbes and plants to obtain soil N. The deficient condition for soil N in microbes affects the microbial decomposition, and therefore causes enhanced accumulation of soil C.

15.4 MICROBIAL RESPONSE MECHANISM TO CLIMATE CHANGE It has been reported that microbial contributions to the warming effect are further dependent on the microbial response mechanism and are decided by a plethora of mechanisms including microbial physiology, community compositions, feedback, and evolution. The microbial physiology and susceptibility for varying environmental conditions may lead to the sorting of species and compositional changes over gradients

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(Leibold et al., 2004). In many cases, it has been reported that that under the climate extremes such as freezing and drought have explicated greater changes in microbial activities compared to overall changes in temperature and precipitation as well as functional plasticity observed during the shortterm changes in this entire process. Furthermore, the community compositions are decided by differential abilities of the microbial systems to perform well in a changed environment either through adopting the particular niche by relatively abundant taxa or through dispersal of new taxa, which ultimately results in the directional selection of species in changing environmental conditions. It was reported that shifting in the composition of microbial communities could be directly linked to altered ecosystem functioning when soil organisms differ in their functional traits or control a rate-limiting or fate-controlling step (Schimel and Schaeffer, 2012). To better understand this phenomenon, we can assume that under the effect of climate change the relative abundances of the microbial population is affected and therefore, the crucial ecological functions such as nitrogen fixation, nitrification and de-nitrification and methanogenesis associated with them are also disturbed (Bakken et al., 2012; Salles et al., 2012; Bodelier et al., 2000). The positive feedback of the microbial community under the effect of climate change is attributed by the initial resistance to change followed by a rapid shift or collapse as the extent of change progresses. However, the microbial communities existing in microbial consortia under the effect of changed climate initially show resistance to the change, resulting in frequency-dependent selection but finally leading to resilience when an abrupt state shifting occurs.

15.5 IMPACT OF CLIMATIC CHANGE ON PLANT MICROBE INTERACTIONS Climate adversities like the elevated levels of atmospheric CO2, the rise in global temperature, drought have affected the ecology and physiology of both plants and microbes. Since plants distribute some the assimilated carbon to feed microbial populations associated with them, the interruption in the C assimilation pathway under the effect of climate change would definitely influence plant microbial interactions (Jia and Zhou, 2012). It has been reported that microbial diversity and abundances are highly susceptible to climate change events (Maestre et al., 2015a,b). The beneficial microbes associated with plants have a large impact on host cell physiology and protect their host from disease and various abiotic stress factors.

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Changed climatic events have altered the interactions and dynamics of the plant microbe response and affected the microbial communities associated with plants, and therefore, altered their establishment and performances to regulate the soil N and C dynamics. Further, under the effect of increased temperature, the species are moving to higher altitudes and latitudes. The reproductive physiology of the host plant under the warming effect has been found to be changed by early leafing and flowering time in the growing season (Wolkovich et al., 2012). This has resulted into an alteration in the functional traits of plants (Hudson et al., 2011; Verheijen et al., 2015) and therefore affected the multiple properties of the ecosystem (Valencia et al., 2016; Butler et al., 2017). The rise in temperature has affected the community structures and dynamics, with the conversion of grasses and forbs with shrubs which disturb the ecosystem functions and processes through the large impact on carbon exchange between land and atmosphere (Lawrence and Swenson, 2011; Pearson et al., 2013). The elevated CO2 level in the changed climatic scenario has affected the biomass accumulated by C3 and C4 plants (Poorter and Navas, 2003). Differences in the C allocation pattern and host physiology under the warming effect caused maximum accumulation of biomass above ground level (45%) by C3 plants compared to C4 (12%) plants. The differences observed in the biomass accumulation level could be interlinked with the association of the host with beneficial microbes, particularly arbuscular mycorrhiza (AM) fungi. C4 plants allocate more C to AM fungi to gain benefits from these fungi and therefore selection force favors the growth of AM fungi in the case of C4 plants rather than the accumulation of biomass by C3 species. Overtly, these results explain how warming resulted in the beneficial association of the host with the AM fungi. The association of Glomus intraradices and Glomus mossae with the host under the effect of increased temperature has been experimentally demonstrated (Baon et al., 1994; Monz et al., 1994). Under the effect of rising drought the plant growth was affected (both roots and shoots) and therefore there was an interruption in the allocation of photosynthetic food products to the rhizospheric microbes and AM fungi. However, the density of ectomycorrhizal fungi is not disturbed during prolonged drought conditions. The consequences of climate change on plant microbe, microbe microbe, and ecosystem functions are not fully understood. One of the major limitations in this context is that to date the effect of global climate change on plant microbe interactions has been considered with a focused

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approach on a single trophic level (either a specific plant or microbe), ignoring the interactive aspects of plant microbe and microbe microbe interactions occurring at different or the same trophic levels observed for aboveground or belowground organisms in a natural ecosystem. This limitation has generated a gap in our understanding of the direct and indirect impacts of changes in climate and biodiversity on the terrestrial ecosystem and therefore an accurate prediction for ecological consequences of global change.

15.6 CLIMATE CHANGE AND ABIOTIC STRESS: MICROBEMEDIATED STRESS ALLEVIATION Global warming may result in an increase in average global temperature, severe droughts, increased CO2 level, extreme rainfall, floods, cyclones, etc. All these factors will together cause a detrimental effect on crop growth and yields and will also impose severe pressure on land and water resources. According to one report, it was predicted that the climatic conditions will undergo drastic changes in the 21st century and will affect various parameters such as increased global mean surface temperature that may result in an unbalanced and disturbed rainfall in a particular regime, and therefore, will agricultural productivity will be affected (Shah and Srivastava, 2017). The sector which is considered to be most vulnerable to climate change is agriculture. Global productivity has been severely affected under the changed climatic events which have influenced plant susceptibility to diseases (Prasch and Sonnewald, 2013; Narsai et al., 2013; Atkinson et al., 2013; S