Handbook of Flowering: Volume III [1 ed.] 9781315893457, 9781351072557, 9781351089456, 9781351097901, 9781351081009

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Handbook of Flowering: Volume III [1 ed.]
 9781315893457, 9781351072557, 9781351089456, 9781351097901, 9781351081009

Table of contents :

Volume III: F � O

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CRC Handbook of Flowering Volume III Editor

Abraham H. Halevy

Professor Department of Ornamental Horticulture The Hebrew University of Jerusalem Rehovot Israel

Boca Raton London New York

CRC Press is an imprint of the Taylor & Francis Group, an informa business

First published 1985 by CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 Reissued 2018 by CRC Press © 1985 by CRC Press, Inc. CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright.com (http://www.copyright. com/) or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a notfor-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Library of Congress Cataloging in Publication Data (Revised for Volume 3) CRC handbook of flowering. Includes bibliographies and indexes. 1.  Plants, Flowering of—Handbooks, manuals, etc. 2. Plants, Cultivated—Handbooks, manuals, etc. I. Halevy, A. H. (Abraham H.), 1927-  . II. C.R.C. handbook of flowering. III. Handbook of flowering. SB126.8.C73 1985  635.9   83-21061 ISBN 0-8493-3913-8 Publisher’s Note The publisher has gone to great lengths to ensure the quality of this reprint but points out that some imperfections in the original copies may be apparent. Disclaimer The publisher has made every effort to trace copyright holders and welcomes correspondence from those they have been unable to contact. ISBN 13: 978-1-315-89345-7 (hbk) ISBN 13: 978-1-351-07255-7 (ebk) Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com

FOREWORD Life would be awfully dull without flowers. The love of flowers is common to all people all over the world, no matter how different their cultural patterns may be. Artistically, flowers play an important role in poetry and in decorative arts for their attractive structure, color and odor in endless variation: for their eternal beauty. Agriculturally, and perhaps still more so horticulturally, flowers play an essential part in the production of all crops. This is, of course, most evident in floriculture, where the flower is the final product. In all fruit crops, whether deciduous or evergreen, and in crops like grains and the fruit-vegetables, the fruit is the final product, but flowers are the indispensable introductory step to fruit formation. For the many crops which are grown for their seeds, including all breeding projects, flowers and fruits cannot be missed for seed formation. On the other hand, there are crops where flowers and fruits are undesirable — for instance, those which are grown for their roots, stems, or leaves. However, in seed growing and breeding of these crops, flowers again are indispensable. The above implies that control of flowering, whether positively promoting or negatively preventing flowering, is a cultural measure which has to be applied in the growing of most, if not all, crops. No wonder that in several cases more or less sophisticated methods for controlling flowering are rather well known. Quite generally, such empirically developed methods are genetical and ecological, but an endless variation occurs in the requirements for flowering among different species and even among the cultivars of one species. The human mind is not satisfied by answering the question as to which factors influence flowering — as approached empirically — it also wants to know the physiological background of the actions of these factors. This is a much more difficult problem, and disciplines like biophysics, biochemistry, and molecular biology enter the picture. It is self-evident that this approach has made much less progress than the empirical one. However, it has become clear that also in this respect an endless variation occurs among different species and that a general mechanism of action — if it exists — has not been found yet. Be that as it may, the study of flowering, whether empirical or — on a higher level — physiological, contributes to the development of the human mind by the attempt to understand some of the mysteries of Life. In the foregoing I used "endless variation" no less than three times, and students of flowering, who concentrate on their own plant(s), run the risk of becoming one-sided. It is, therefore, highly important to provide them with a catalogue of the flowering behavior of as many other plants as possible. The present Handbook fills the existing gap by bringing together our knowledge of the individual cases. The "Table of Contents" illustrates that it was no simple effort to compose this edition. A. H. Halevy, "Abe" to his friends, has had the courage and the energy to undertake this task. All those who are interested in flowering in some or other way owe him their gratitude. S. J. Wellensiek

PREFACE Rise up, my love, my fair one, and come away. For, lo, the winter is past, The rain is over and gone; The flowers appear on the earth; The time of singing is come, And the voice of the turtledove is heard in our land; The fig tree puts forth her green figs, And the vines in blossom give forth their fragrance. (Song of Songs 2:10—13) From antiquity, poets have expressed humanity's association of flowering with spring, renewal, singing, beauty, fragrance, and love. This book deals with the more prosaic aspects of flowering: flower formation and development and the environmental and physiological factors which regulate them. Several excellent reviews and books on flowering have been published in the last 25 years. These include Lang's6 and Schwabe's 9 chapters in general encyclopedias of plant physiology, Evan's opening and concluding chapters in his book,4 the books by Salisbury, 8 Vince-Prue," and Bernier et al. 1 and the several review articles in the Annual Reviews of Plant Physiology.2-3-^-7-10-12 With the exception of Evans, 4 these authors have presented a general review of the flowering process and attempted to integrate the data into a unifying theory. Such unifying theories have generally suffered from the disadvantages noted by Evans in the preface to his book;4 they deal primarily with the earliest events of the flowering process, and they are based on data obtained from a small number of "model" plants. Evan's book, which contains flowering "case histories" of the majority of these species, includes chapters on only 20 plants, of which only one is a woody plant, two are monocotyledons, and none is a gymnosperm. A great wealth of data on the regulation of flower formation and development can be found in the practical literature of agriculture, horticulture, and forestry. Much of this has been often ignored by flowering physiologists. The flowering process is indeed much more diverse than that revealed in the 20 "model" plants presented in Evan's book. It was my aim in planning this book to make a more comprehensive view of the flowering process possible by presenting relevant data from as many plants as possible. This includes the majority of the cultivated plants on which such information is available: field crops, fruits, vegetables, ornamentals, industrial plants, and forest trees, not only of the temperate regions, but also of subtropical and tropical climates. To accomplish this goal I have invited scientists from all over the world to contribute chapters on specific plants or groups of plants. Many of the authors have not only reviewed the available literature, but have also included previously unpublished data. Many of the chapters present the first general review of the flowering process in their specific subject area. The book deals with all aspects of flowering, including juvenility and maturation, flower morphology, flower induction, and morphogenesis to anthesis. Flower morphogenesis has been taken to include also development of individual flower parts, sex expression, and flower malformations. When possible the authors have attempted to present information on all stages of the flowering process. In many cases, however, this has not been feasible, since little or nothing is known about some of the stages. In most cases the "flowering story" is terminated at anthesis. In some plants, however, flower structure and anthesis are directly related to pollination, and in these cases pollination is also included. In some commercial

food crops the description is also extended somewhat beyond anthesis to include important factors in crop production. In many cultivated plants, mostly ornamentals, practical methods for manipulation of flowering are included. The length of the individual chapters and the emphasis on specific aspects depends in most cases on the availability of experimental information and not on the importance of the plant as a crop or the significance of the physiological stage described. Some important economic plants are absent from this book since little or no information is available on their flowering. In some cases most of the chapter is devoted to a single aspect of flowering, such as juvenility, flower induction, flower development, sex expression, cleistogamy, development of certain flower parts, or flower opening. Some chapters concentrate on physiological aspects, others on ecological, morphological or genetic ones. Other aspects are covered only briefly or even absent, not because they are not important but because they have not been studied in detail. I am well aware that this book is far from being a comprehensive encyclopedia of flowering. I would greatly appreciate comments from readers on errors found in articles, missing information, and plants not included in the book, whose flowering process have been documented. The handbook consists of five volumes: • • • • •

Volume I — contains general chapters on groups of plants, and individual chapters on plants beginning with the letter A Volume II — contains plants of letters B to E Volume III — contains plants of letters F to O Volume IV — contains plants of letters P to Z Volume V — will be a supplementary volume and will include chapters on plants not included in Volumes I to IV, listed in alphabetical order, and will appear after the first 4 volumes

The merit of the book rests upon the work of the individual authors. I am grateful to them for their efforts, cooperation, and forbearance. I would like to thank my colleagues in the Editorial Board who helped me to select the authors and to review the chapters, and the many other colleagues who helped in reviewing (and sometimes rewriting) specific chapters. I hope that this Handbook will serve as a reference and source book for scientists interested in the flowering process of particular plants, and will draw their attention to the lack of information on important aspects of the flowering process in many important plant species. I also hope that the wealth of information accumulated here will be useful in future attempts to synthesize general theories of the physiology of flowering.

REFERENCES 1 . Bernier, G., Kinet, J. M., and Sachs, R. M., The Physiology of Flowering, CRC Press, Boca Raton, Fla.. 1981. Vol. 1, 149; Vol. 2, 231. 2. Chouard, P., Vernalization and its relation to dormancy, Anna. Rev. Plant Physiol., 11, 191—238, 1960. 3. Doorenbos, J. and Wellensiek, S. J., Photoperiodic control of floral induction, Annu. Rev. Plant Ph\siol., 10, 147—184, 1959. 4. Evans, L. T., The Induction of Flowering. Some Case Histories, Evans, L. T., Ed., Macmillan, Mellbourn, 1969, 328. 5. Evans, L. T., Flower induction and the flowering process, Annu. Rev. Plant Physiol., 22, 365—394,

6. Lang, A . , The physiology of flower initiation, in Encvcli>[mlUi of Plant Plivsiologv, Vol. 15 (Part I). Ruhland. W.. Eds.. Springer-Verlag. Berlin, 1965. 1380—1563. 7. Salisbury, F. B., The Flou-erinx Proces.s. Pergamon Press, Oxford, 1963. 234. 8. Salisbury, F. B., Photoperiodism and the flowering process, Anna. Rev. Plain Phv.siol., 12. 293—326. 1961. 9. Schwabe, W. VV., Physiology of vegetative reproduction and flowering, in Plant Ph\.siolof>\. A Treatise. Steward. F. C.. Ed., Vol. V I - A . Academic Press. New York. 1971. 233^1 1. 10. Searle, N. E., Physiology of Flowering. Anna. Rev. Plant Physiol.. 16. 97—118. 1965. 1 1 . Vince-Prue, D., Photopcriodixm in Plants, McGraw H i l l . London. 1975. 444. 12. Zeevaart, J. A. D., Physiology of flower formation, Aiwu. Rev. Plant Physiol.. 27. 321—348. 1976.

Abraham H. Halevy

THE EDITOR Abraham H. Halevy, Ph.D., is a professor of Horticulture and Plant Physiology at the Hebrew University of Jerusalem, Rehovot, Israel. Dr. Halevy obtained his M.Sc. and Ph.D. degrees from the Hebrew University in 1953 and 1958, respectively. Dr. Halevy was a Research Fellow at the U.S. Department of Agriculture Research Center at Beltsville, Md. (1958 to 1959) and has been a Visiting Professor at Michigan State University (1964 to 1965) and the University of California, Davis (1970 to 1971, 1976, and 1982 to 1984). He has twice received the Alex Laurie award of the American Society of Horticultural Science, and was recently (1983) nominated as a fellow of the society. Dr. Halevy has published over 150 scientific papers in international journals, and has been a guest invited lecturer in numerous symposia around the world. His current research interests are the regulation of flowering and of flower senescence.

ADVISORY BOARD K. E. Cockshull, Ph.D. Physiology and Biochemistry Division Glasshouse Crops Research Institute Littlehampton, West Sussex United Kingdom

M. Pinthus, Ph.D. Faculty of Agriculture The Hebrew University of lerusalem Rehovot Israel

E. Goldschmidt, Ph.D. Faculty of Agriculture The Hebrew University of Jerusalem Rehovot Israel

Y. Rudich, Ph.D. Faculty of Agriculture The Hebrew University of Jerusalem Rehovot Israel

CONTRIBUTORS Taiji Adachi, Dr. Agric. Department of Agronomy Miyazaki University Miyazaki Japan Amos Blumenfeld, Ph.D. Department of Subtropical Horticulture Volcani Center Agricultural Research Organization Bet Dagan Israel K. E. Cockshull, Ph.D. Physiology and Biochemistry Division Glasshouse Crops Research Institute Littlehampton, West Sussex England Richard A. Criley, Ph.D. Department of Horticulture University of Hawaii Honolulu, Hawaii Gerald F. Deitzer, Ph.D. Environmental Research Center Smithsonian Institution Rockville, Maryland E. Julian Duncan, Ph.D. Department of Biological Sciences University of the West Indies St. Augustine Trinidad Bernard Durand, State Doctor Department of Plant Biology Orleans University Orleans France Rayrnonde Durand, State Doctor Department of Plant Biology Orleans University Orleans France

C. Dean Dybing, Ph.D. Research Plant Physiologist Agricultural Research Service U.S. Department of Agriculture Brookings, South Dakota L. T. Evans Division of Plant Industry Commonwealth Scientific and Industrial Research Organization Canberra City Australia Terry L. Gilbertson-Ferriss, Ph.D. Department of Plant and Earth Science University of Wisconsin-River Falls River Falls, Wisconsin Richard I. Greyson, Ph.D. Department of Plant Sciences University of Western Ontario London, Ontario Canada Arye Gur, Ph.D. Department of Horticulture Faculty of Agriculture The Hebrew University of Jerusalem Rehovot Israel Chester G. Guttridge, Ph.D. Department of Agriculture and Horticulture Research Station Long Ashton, Bristol England Wesley P. Hackett, Ph.D. Department of Horticultural Science and Landscape Architecture University of Minnesota St. Paul, Minnesota

Abraham H. Halevy, Ph.D. Department of Ornamental Horticulture The Hebrew University of Jerusalem Rehovot Israel

Shimon Lavee, Ph.D. Volcani Center Agriculture Research Organization Bet Dagan Israel

Harrell L. Hammett, Ph.D. (deceased) Sweet Potato Research Station Louisiana State University Chase, Louisiana

Charles Lay, Ph.D. Plant Science Department South Dakota State University Brookings, South Dakota

Harold Hildrum, Ph.D. Gartnerhallen Eliteplantestasjon Sauherad Akkerhaugen Norway

C. R. McDavid, Ph.D. Biological Sciences Department University of the West Indies St. Augustine Trinidad

R. G. Hurd, Ph.D. Crop Science Division Glasshouse Crops Research Institute Littlehampton, West Sussex England Yair Israeli Jordan Valley Banana Experiment Station Jordan Valley Regional Council Zemach Israel Gerard Jacobs, Ph.D. Department of Horticultural Science University of Stellenbosch Stellenbosch Republic of South Africa Klaus Junker, Dipl. Ing. Agr. Benary Seed Growers, Ltd. Hannoversch-Miinden Federal Republic of Germany

Avishalom Marani, Ph.D. Department of Field and Vegetable Crops Faculty of Agriculture The Hebrew University of Jerusalem Rehovot Israel Ian C. Murfet, Ph.D. Botany Department University of Tasmania Hobart, Tasmania Australia Takashi Nagatomo, Dr. Agric. Department of Agronomy (Emeritus) Miyazaki University Miyazaki Japan

Riklef Kandeler, Ph.D. Botanical Institute University of Agriculture Vienna Austria

K. K. Nanda, Ph.D., F.N.A. (deceased) Department of Plant Physiology, and Department of Botany Panjab University Chandigarh India

Rod W. King, Ph.D. Division of Plant Industry Commonwealth Scientific and Industrial Research Organization Canberra City Australia

Jeshajahu Nothmann, Ph.D. Department of Vegetable Crops Volcani Center Agricultural Research Organization Bet Dagan Israel

Festus I. O. Nwoke, Ph.D. Department of Botany University of Nigeria Nsukka Nigeria

Walter Schuster, Dr. Agr. Department of Plant Breeding Justus-Liebig University Giessen West Germany

Dan Palevitch, Ph.D. Department of Field and Garden Crops The Hebrew University of Jerusalem, and Volcani Center Agricultural Research Organization Bet Dagan Israel

Walter W. Schwabe, D.I.C., Ph.D., D.Sc., D.Ag.h.c.(As), F.I.Biol. Department of Horticulture Wye College (London University) Ashford, Kent England

Andrew Picken Physiology and Chemistry Division Glasshouse Crops Research Institute Littlehampton, West Sussex England A. H. Pieterse, Ph.D. Department of Agricultural Research Royal Tropical Institute Amsterdam The Netherlands K. Raman, Ph.D. Division of Plant Physiology UPASI Tea Research Institute Cinchona, Coimbatore South India John J. Ross, Ph.D. Department of Botany University of Tasmania Hobart, Tasmania Australia Walter Riinger, Ph.D. (deceased) Ecology Institute Technical University of Berlin Berlin West Germany Edward J. Ryder, Ph.D. Agricultural Research Station U.S. Department of Agriculture Salinas, California

Ruth Shillo, Ph.D. Department of Ornamental Horticulture The Hebrew University of Jerusalem Rehovot Israel Pinhas Spiegel-Roy, Ph.D. Department of Fruit Breeding and Genetics Volcani Center Agricultural Research Organization Bet Dagan Israel Vini Sood, Ph.D. Department of Botany MCMDAV College for Women Chandigarh India C. Srinavasan, Ph.D. University of Florida Agricultural Center Homestead, Florida William B. Storey, Ph.D. Professor of Horticulture, Emeritus Department of Botany and Plant Sciences University of California-Riverside Riverside, California Erling Str0mme, Ph.D. Department of Floriculture Agricultural University of Norway Aas Norway

Graham G. Thomas, Ph.D. Department of Hop Research Wye College (London University) Ashford, Kent England

S. J. Wellensiek, Dr. Hort. Sci. Department of Horticulture Agricultural University Wageningen The Netherlands

Marie Tran Thanh Van, Ph.D. Labratoire du Phytotron CNRS Gif-sur-Yvette France

Thomas W. Whitaker, Ph.D. Biology Department University of California, San Diego La Jolla, California

Peter A. Van de Pol, Ph.D. Department of Horticulture Agricultural University Wageningen The Netherlands Benito S. Vergara, Ph.D. Plant Physiology Department International Rice Research Institute Manila Philippines Daphne Vince-Prue, Ph.D., N.D.H., D.Sc. Physiology and Chemistry Division Glasshouse Crops Research Institue Littlehampton, West Sussex England Israela Wallerstein, Ph.D. Department of Floriculture and Ornamental Horticulture Volcani Center Agricultural Research Organization Bet Dagan Israel

Harold F. Wilkins, Ph.D. Department of Horticultural Science and Landscape Architecture University of Minnesota St. Paul, Minnesota Eliezer Zamski, Ph.D. Department of Agricultural Botany The Hebrew University of Jerusalem Rehovot Israel Naftaly Zieslin, Ph.D. Department of Ornamental Horticulture The Hebrew University of Jerusalem Rehovot Israel Karl Zimmer, Dr. rer. hort. Institute of Ornamental Horticulture Hannover Federal Republic of Germany

TABLE OF CONTENTS Volume III Fagopyrum esculentum Ficus carica Fragaria x ananassa Freesia X hybrida Fuchsia X hybrida Gerbera Gesneriaceae Geum urbanum Gladiolus Gloriosa Gossvpium Gypsophila paniculata Hedera helix and Hedera canariensis Helianthus annuus Helianthus tuberosus HeHconia Hemerocallis fulva Hibiscus cannabinus and H. sabdariffa Hibiscus esculentus Hordeum vulgare Humulus lupulus Hydrangea macrophylla Hydrilla verticillata Hydrocharitaceae (Sea Grasses) Hvpoestes Impatiens balsamina Ipomoea batatas Ixia Juglans regia and Related Species Kalanchoe blossfeldiana Kalanchoe porph\rocalyx Lactuca saliva Lathyrus odoratus Lemnaceae Leptospermum scoparium Leucospermum Liatris Limonium sinuatum Linum usitatissimum Lolium temulentum Lunaria annua Lycopersicon esculentum Macadamia Mammillaria Mandevilla sanderi Manihot esculenta

1 9 16 34 38 43 48 53 63 71 77 83 89 98 122 125 130 133 140 150 167 173 178 181 183 187 200 205 207 217 236 241 246 251 280 283 287 292 302 306 324 330 347 356 358 360

Matthiola incana Melandrium Metha piperita Mercurialis Moms Musa Nerium oleander Nigella Notocactus Olea europea Orvza saliva Oxalis

363 368 371 376 388 390 411 413 420 423 435 442

Abbreviations and Alternative Names of Chemicals

445

Index

447

Editor's note — The following chapters in letters F through O will appear in Volume V: Forsvthia, Gardenia, Hevea, Hibiscus rosa-sinensis, Hyoscyamus, Lagerstroemia, Lamium, Litchi, Mangifera, and Nicotiana.

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FAGOPYRUM

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ESCULENTUM

En. Buckwheat; Fr. Ble sarrasin; Ge. Buchweizen; Sp. Trigo sarraceno Takashi Nagatomo and Taiji Adachi

INTRODUCTION The genus Fagopyrum (Polygonaceae) is classified into three species: F. esculentum, F. cymosum, and F. tataricum, common, perennial, and Tartary buckwheat, respectively. 1 Somatic chromosome numbers are all 2n= 16.2 The buckwheats originated in temperate eastern Asia. The perennial species, the probable ultimate source of both common and Tartary buckwheat, is native to northern India and China. Wild forms of F. esculentum are found in China and Siberia. Tartary buckwheat is reported as growing in the Himalayas of northeastern India and China under cooler and harsher climatic conditions, to which it is better adapted than common buckwheat. Buckwheat plants are not grasses, but the seeds (strictly, achenes) are usually classified among the cereal grains because of similar usage. In many countries, including Japan, only common buckwheat is usually cultivated, while Tartary buckwheat is cultivated in limited amounts by the name of India-wheat or Siberian buckwheat. The perennial buckwheat is also cultivated in part as vegetable buckwheat because the young leaves are edible. The flour is generally used in pancakes, biscuits, noodles, and soups, and dehulled groats are cooked as porridge. The protein of buckwheat is of excellent quality and this, coupled with the ability of the plant to do well on the poorer soils, perhaps accounts for its widespread usage. 1 The following description on flowering is restricted to common buckwheat.

FLORAL MORPHOLOGY The inflorescence is a compound raceme which may be either paniculate or corymbose; it is terminal and axillary, many-flowered and erect or slightly drooping. The flowers are usually white, but some varieties have flowers tinged with pink. There are no petals, but there is a 5-parted corolla-like calyx which remains attached to the base of the fruit. Each member is oval shaped. There are 8 stamens with glabrous filiform filaments and oblong anthers. Of the stamens, 3 closely surround the styles and dehisce outward, while the 5 others are inserted outside these 3, and dehisce inward. The single ovary is 1celled and 1-ovuled and bears 3 style branches, which are bent back in fruit. 3 Common buckwheat has dimorphic flowers, specifically, heterostylic, as shown in Figure 1. One of these forms has long styles and short stamens, pin (a); and the other short styles and long stamens, thrum (b). The frequency of the two types of flowers in the population is normally equal. Buckwheat shows a sporophytic self incompatibility caused by heterostyly. Seed set is, therefore, dependent on cross-pollination between pin and thrum, the legitimate cross. Either self-pollination or cross-pollination between flowers of the same types (the illegitimate cross) will not set seeds.4 Sometimes flowers with anther and stigma on the same level can be found (Figure Ic) in both diploid 5 and induced autotetraploids. 6 In the autotetraploid there tends to be an increased number of autogamous homomorphic plants. Buckwheat bears many incomplete developing flowers, which are mainly deficient in pistils under the conditions of LD and high temperature. 7 Incomplete pistils vary over a wide range in their morphology, as shown in Figure 2.8 An abortive embryo may be formed sometimes. This condition was named enervative sterility."'

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CRC Handbook of Flowering

FIGURE 1. Morphology of buckwheat flowers, (a) Pin-type (long styles and short stamens); (b) thrum-type (short styles and long stamens); (c) homostyled variant (autogamous, styles are shorter than pin, stamens shorter than thrum). (From Nagatomo, T., Rep. Lab. Plant Breed, Miyazaki Univ., No. 1, 1961. With permission.)

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N

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N

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0 I

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II b

0 II

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FIGURE 2. Morphology of complete and incomplete developing pistils in common buckwheat. Upper row shows pin pistils, bottom thrum. N, normal completely developed: I-11I, incompletely developed in different levels. (From Nagatomo, T., Rep. Lab. Plant Breed. Miyazaki Univ., No. 1, 1961. With permission.)

FLOWERING HABITS The main stem is quite succulent and smooth, except at the nodes, and the branches are strongly grooved when not crowded. The inflorescence consists of compound racemes, which are borne on peduncle-like branches arising from leaf axils. It may appear at the earliest at the 3rd node above the cotyledons, but normally at the 4th or 5th node. The branching and flowering order are shown in Figures 3s and 4. 7 Racemes on the primary and secondary lateral branches are more delayed in flowering than that of the main stem. Each floret forms on a fine pedicel of ca. 2 to 5 mm length. Florets are clustered around the raceme, which is enclosed in a small bract, and come into blossom indeterminately upward. Each raceme is arranged spirally at a pitch of 120°. The compound racemes are shown diagrammatically in Figure 5.8 Using a variety 'Takasago' in Formosa, Jo 10 suggested that the extension of the vegetative phase (the period from sowing to the first anthesis) was promoted by both increase of day length and temperature, though the shortening of the vegetative phase depends mainly on day length, as shown in Figure 6. Eighty percent or more of the flowers open in the morning, normally from 6 to 9 a.m. during summer and autumn. In winter they open at mid-day. Flowers do not bloom either in the afternoon or at night. Uehara and Taguchi also reported ecological studies on the inflorescence and the fruiting of buckwheat.' 2 FLOWER INITIATION AND DEVELOPMENT Initiation and differentiation of flower buds depends mainly upon daylength, although photoperiodic response is affected by temperature. Buckwheat is generally considered a SD plant, but some reports define buckwheat to be day neutral, since it produces flowers over a wide range of photoperiods.13 The process of flower bud differentiation has been divided into seven stages. Each stage

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CRC Handbook of Flowering

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FIGURE 3. Diagrammatic description of compound racemes borne on a single plant. S,,, main stem; S,—S m , primary branches; S, ,, S,, ,, S m , (etc)., secondary branches; S,,.,.,, S,,.,.,, S,,.,.,, S m ,.,, (etc.), tertiary branches. (Modified from Nagatomo.*)

and its features are shown in Figure 7.8 The critical dark period for photoperiodic response was observed by using both summer and autumn types. Seedlings were exposed to continuous light for 7 days after emergence. Then, alternate light and dark periods in 24-hr cycles were repeated for 10 days. It was found that flower buds were formed when more than a 9- or 10-hr dark period was employed for summer and autumn types, respectively.8 The critical daylength is therefore 15 hr for the summer and 14 hr for the autumn type. Onda et al.14 studied phenology of several varieties for the response to daylength. He grouped the Japanese original varieties into three types. The Southern type is delayed in flowering by LD more than Northern and intermediate types. Inoue 15 studied the flowering behavior of seedlings cultured in vitro in total darkness. Flower initiation occurred about 40 days after sowing in total darkness at 15°C. Light was not necessary for flower initiation in buckwheat. 2-Thiouracil promoted flower initiation at high temperature in total darkness, though it inhibited flowering in light. Sucrose promoted flower initiation at higher temperature than 15°C. But IAA, GA, and kinetin had no effect on flower initiation in total darkness.

Volume IH

PRIMARY BRANCH x . V\

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16

20

FIGURE 4. Changes in flower number per day form the beginning of flowering. (From Sugawara, K. and Sugiyama, K... Bull. Fin: Arts. I ware Univ., 6, 55—68, I954. With permission.)

In summary, common buckwheat is a SD plant, but its photoperiodic response can be modified by temperature and some chemicals.

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CRC Handbook of Flowering

FIGURE 5. Diagrammatic description of spiral arrangement of florets in compound racemes. Numerals indicate each raceme, which are shown in ascending order. In each raceme, florets bloom in the order of size. (Modified from Nagatomo.8)

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7

FIGURE 6. Seasonal fluctuation of vegetative period (days to anthesis) and sunshine hours and temperature. A: vegetative period, B; sunshine hours, C; temperature. D: standard deviation in anthesis. (From Jo, K., Nogvo Oyohi Engei (Agriculture and Horticulture), 13. 1601 —1612. 1938. With permission.)

FIGURE 7. Anatomical description 01 each stage in flower bud differentiation. N, flower bud primodia are not differentiated; D,, early stage of differentiated raceme (duration to anthesis is 18.0 days); D,, 1st and 2nd racemes develop laterally on the vegetative cone (17.5 days): D,, central cone becomes lower than another lateral meristem (16.0 days); D4, early developing stage of small compound racemes (14.5 days); D5, late developing stage of compound racemes, active in differentiation of compound racemes (13.0 days); D6, stage of development for each floret (12.0 days). (Modified from Nagatomo.8)

8

CRC Handbook of Flowering REFERENCES 1 . Campbell, C. G., Buckwheat, in Evolution of Crop Plants, Simmonds, N. W., Ed., Longman, London, 1976, 233-237. 2. Darlington, C. D. and Wylie, A. P., Chromosome Atlas of Flowering Plants, 2nd ed., George Allen & Unwin, London, 1955, 72 pp. 3. Robbins, W. W., Botany of Crop Plants, 2nd ed., Blakiston's Son & Co., Philadelphia, 1924. 4. Stevens, N. E., Observations on heterostylous plants, Dot. Gaz., 53. 277-308. 1912. 5. Marshall, H. G., Isolation of self-fertile, homomorphic forms in buckwheat, Fagopyrum sagittatum Gilih., CrupSci., 9, 651-653, 1969. 6. Adachi, T., Yabuya, T., and Nagatomo, T., Inheritance of stylar morphology and loss of self-incompatibility in the progenies of induced autotetraploid buckwheat. Jpn. J. Breed., 32, 61-70, 1982. 7. Sugawara, K. and Sugiyama, K., Studies on the ecology of the inflorescence and the fruiting ol buckwheat, Fagopvrum escu/entum Moench. Bull, Fuc. Arts, Iwule Univ., 6, 55-68, 1954. 8. Nagatomo, T., Studies on physiology of reproduction and some cases of inheritance in buckwheat. Rep. Lab. Plant Breed., Miyazaki Univ., No. 1, 1961 (in Japanese). 9. Nakamura, M. and Nakayama, H., On the enervative sterility in buckwheat, Proc. Crop Set. Jpn.. 19, 122-125, 1949. 10. Jo, K., Studies on the effects on seasonal change of sunshine hours and temperature on the reproductive period in crop. II. Flowering time and its uniformity in buckwheat, Nog\o Oyobi Engci (Agriculture and Horticulture), 13, 1601-1612, 1938 (in Japanese). 1 1 . Nagatomo, T., Studies on the breeding of buckwheat. I. Location of flowers on the stalk and flowering habit, Jpn. J. Breed., 8, 238-246, 1958. 12. Uehara, S. and Taguchi, R., Influences of different day-length upon the growth and reproduction of the buckwheat grown in different seasons. Bull. Fac. Textiles and Sericulture, Shinshu Univ., 5, 31-35, 1955. 13. Jo, K., Review article buckwheat, Field Crop Abst.. 25, 389-396. 1972. 14. Onda, S., and Takeuchi, T., On the ecotype of Japanese varieties of buckwheat, Nogyo Oyobi Engei (Agriculture and Horticulture), 17.971-974, 1942. 15. Inoue, J., Flower initiation in total darkness in a quantitative short day plant. Fagopyrum esculentum Moench., Plant Cell Ph\siol., 6, 167-177, 1965.

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F1CUS CAR1CA En. Fig: Fr. Figue; Ge. Feige; Sp, Higo W. B. Storey

INTRODUCTION Ficus carlca L., the cultivated edible fig, is a gynodioecious species of fairly long-lived, medium-sized deciduous subtropical trees in the mulberry family Moraceae. The genus Ficus includes at least 1800 species of tropical and subtropical evergreen or deciduous trees, shrubs, and vines. 1 The center of origin of the fig is thought to be southern Arabia where it was first brought into cultivation. 2 In ancient times it spread in cultivation throughout the entire Mediterranean region. Characteristically, F. carica trees have milky latex which contains the proteolytic enzyme ficin, petiolate palmately lobed leaves, and a complex inflorescence called a "syconium" (Figure 1). The generic name Ficus comes from the ancient Roman name for the fig tree; it may have derived from an earlier Hebrew name faga (paga) or Indian stem name "fag". 2 The specific epithet carica refers to Caria, a district in Asia Minor where excellent drying figs were grown in ancient times. Common names for the fig are English, fig; Greek, sykon; Hebrew, teena; Italian, fico; Portuguese, figo; Spanish, higo; German, Feige; Dutch, vijg; French, figue; Arabic, tin; Aramaic, tenna; and Persian, anjir.

FLORAL MORPHOLOGY F. carica is characterized by three types of unisexual flowers: A, staminate; B, shortstyled pistillate; C, long-styled pistillate (Figure 2). The perianths of all three are similar. Usually they are 3 to 6 lobed. They are connate at the base and are hyaline at maturity. The pistils of the pistilate flowers consist of a single carpel. Ovaries of fertilized flowers develop into juicy drupelets, each containing a single seed having a small curved embryo surrounded by endosperm. The unfertilized ovaries of many cvs develop parthenocarpically into "cenocarps", which are called "seed-like bodies" by growers and processors. Occasional staminate flowers may have a vestigial pistil, and occasional pistillate flowers may have vestigial stamens. Staminate Flower (Figure 2A) The floral organs of the staminate flower are borne on a pedicel 4- to 6-mm long. Usually, a rudimentary pistil is present. Generally, there are 5 2-loculed introrse anthers on filaments 1.5- to 2.0-mm long, but occasional flowers may be seen having 4, 6, or 7 stamens. Short-Styled Pistillate Flower (Figure 2B) The pedicel of the short-styled pistillate flower is slightly shorter than that of the longstyled pistillate flower, measuring 1.2 to 1.5 mm. The style measures 0.55 to 0.70 mm in length and is suitable for oviposition in the embryo sac by the fig wasp Blastophaga psenes L., which has a 1.0- to 1.2-mm long ovipositor. The apex of the style is funnelform, generally with one stigmatic lobe longer than the other. Ovaries of flowers that have been pollinated develop normally into drupelets containing viable seeds. Ovaries in which fig wasps have laid eggs develop into "psenocarps", i.e., drupelets in which wasp larve develop by consuming the embryo, endosperm, and integuments. Commonly, these are called "gall flowers".

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CRC Handbook of Flowering

FIGURE 1 . F. caricu syconia: (A) caprifig, showing staminate flowers in anthesis and mature ovaries of shortstyled pistillate flowers containing adult fig wasps (Blaslophaga pxenes) just prior to hatching (psenocarps); (B), fig with long-styled pistillate flowers in anthesis.

FIGURE 2. Morphology of F. curica flowers: (A) staminate; (B), short-styled pistillate; (C) long-styled pistillate. Abbreviations: ped, peduncle; rec, receptacle; per. perianth;.««, stamen; emb, embryo; vc, ventral carpellary bundle; dc, dorsal carpellary bundle; .vrv, style; sti, stigma.

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Long-Styled Pistillate Flower (Figure 2C) The floral organs of the long-styled pistillate flower are borne on a pedicel 1.25- to 1.60mm long. The style is 1.5- to 2.0-mm long, terminating in a bilobed papillate stigma. It is too long to permit oviposition in the embryo sac by the fig wasp.

THE SYCONIUM The inflorescence of F. carica is a complex many-flowered structure called a "syconium". Essentially, it consists of several laterally fused secondary peduncles turned outside-in forming a pyriform, ovoid, globose, or oblate spheroidal structure bearing the many flowers on the inner wall. 2 - ' 5 It is entirely closed except for a slender apical canal (ostiole) connecting the cavity with the outside air. The ostiole is lined with numerous small scale-like bracts. The visible portion of the ostiole is the umbilicus or "eye". The fig fruit is the fully developed ripened syconium. It is classified botanically as a collective or multiple fruit; it is an accessory or spurious fruit. The true fruits are the drupelets that develop from the ovaries of the flowers within.

FLORAL INDUCTION Caprifig The caprifig tree bears three crops of syconia annually; (1) mamme, the over-wintering crop; (2) profichi, the late spring crop; (3) mammoni, the late summer and fall crop. Each of the crops supports a generation of B. psenes. Mamme Crop The mamme crop is initiated on current growth in early fall and develops well along toward maturity until the tree is forced into dormancy by the onset of cold weather. The female wasps migrate from the previous crop (mammoni) to the mamme crop when the short-styled flowers are in anthesis and oviposit in their embryo sacs. Presence of immature wasps in the flowers is essential for retention of a syconium on the tree. In early spring the syconia mature and the wasps complete their development. When the syconium ripens, the wasps issue forth through the ostiole and enter the syconia of the profichi crop which follows. Caprifig flowers are protogynous, and the pistillate flowers are in anthesis at about the time the staminate flowers are being initiated. The staminate flowers of the mamme crop never develop to maturation. Profichi Crop Syconia of the profichi crop are initiated in autumn. They push out in early spring (March to April in California) and are entered by fig wasps from the mamme crop when the pistillate flowers are in anthesis. It is at this time that the staminate flowers are initiated. The syconia mature in late spring and early summer (June and July in California). At this time mammoni syconia are being initiated and becoming receptive to habitation by the wasp. When the profichi syconia ripen, the staminate flowers are in anthesis and shedding pollen. At the same time, the flowers of the fig trees are in anthesis and receptive to pollination. Wasps leaving the syconia are covered with pollen. Profichi syconia are placed in trees of Smyrna-type figs which require pollination (caprification) to set a crop. The profichi crop is the main one in which the syconia contain fully developed pollenshedding staminate flowers.

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Mammon: Crop The mammoni crop reach the receptive stage in summer when the profichi crop is ripe and becomes inhabited by the fig wasp. It matures in late fall at the time syconia of the mamme crop become receptive. Wasps from this crop enter the mamme crop and complete the annual cycle. Fig Depending on cv, a fig tree may have only a single crop of fruit which matures in summer, or two crops, one maturing in spring, the other in summer. In the U.S., the spring crop usually is simply called the "first crop" and the summer crop the "second crop" or "main crop''. However, the Spanish designations' 'breba'' and ' 'higo" also are used rather commonly. The breba crop is initiated on current growth in late fall but remain as latent buds when cold weather causes the tree to become dormant. Development resumes in early spring, and the syconia mature in late spring and early summer (June in California). The main crop is initiated on current growth in late spring and early summer (late May and early June in California). Fruit ripening begins in early August. Some cvs, e.g., 'Sari Lop', cease vegetative growth after a few weeks when the crop is maturing; consequently, they do not initiate a breba crop. Other cvs, e.g., 'Beall', have a long period of summer growth and floral initiation (FI) which ceases only with the onset of cold weather. Such cvs mature good breba crops as well as main crops. Cvs are classified into three horticultural types on the basis of FI and fruit setting: (1) Smyrna-type; (2) San Pedro-type; (3) Common-type.

Smyrna-Type Figs Generally, figs of the Smyrna-type have no breba crop. Unless the flowers of the main crop are caprified, i.e., pollinated, by the fig wasp, the syconia are caducous and fall from the tree soon after anthesis. This type is exemplified by the cvs 'Sari Lop' (syns. 'Lob Inger', 'Calimyrna'), 'Kalamata', 'Taranimt', and 'Zidi'. San Pedro Type Figs of the San Pedro type produce a good crop of brebas which is the more important of the two crops. Main crop syconia of most cvs either drop from the tree when uncaprified or develop into fruit of inferior quality when retained. Some cvs of this type are 'San Pedro', 'Dauphine', 'Gentile', 'Lampeira', and 'King'. Common-Type Figs Figs of this type have good main crops of fruit. The syconia on this type of tree are persistent (usually but erroneously called parthenocarpic) and develop to maturity and ripening without caprification. They may contain parthenocarpic drupelets (cenocarps or seedlike bodies) or may not, in which case they are said to be seedless. Common-type cvs may or may not produce breba crops, depending on their growth habit up to the time cold weather sets in. Cvs that begin to go dormant early usually lack brebas. Cvs that continue vegetative growth until forced into dormancy by cold weather tend to have brebas. Some cvs which produce no brebas in California are 'Celeste', 'Troiano', 'Verdal', 'Vernino', and 'Verte'. Cvs that normally have good crops of brebas are 'Beall', 'Dottato', 'Franciscana', 'Genoa', 'San Piero', and 'Tena'.

SEX DETERMINATION F. carica is a gynodioecious species characterized by two sex forms of trees: (1), caprifig which is monoecious; (2), fig which is strictly pistillate.

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Table 1 SEX SEGREGATION RATIOS IN PROGENIES RESULTING FROM VARIOUS MATINGS OF F. CARICA Pollination CC X CC or scli'cd CC X Cc and reciprocal Cc X Cc or sell'cd cc X CC

cc x Cc

Progeny genotypes

Progeny phenotypes

All CC 1 CC: I Cc

All capril'igs All capril'igs

1 CC: 2 Cc: 1 cc All Cc 1 Cc: 1 cc

3 Capnligs: 1 t i g All capritigs 1 Caprilig: 1 fig

Note: Homozygous caprifig. CC: heterozygous caprilig. Cc: tig, cc.

The pistillate flowers of the caprifig are exclusively the short-styled type. The pistillate flowers of the fig are the long-styled type exclusively. Sex is determined by a pair of homologous chromosome segments consisting of a number of tightly linked genes which are transmitted as units. 1 3 - 1 4 The results are the same as if phenotypic expression were due to a single pair of alleles with pleiotropic effects. ls The characters that typify the caprifig are dominant over those that typify the fig. The complex representing the characters of the caprifig is symbolized as C, of the fig as c. Caprifigs are represented by two genotypes: (1) homozygous CC: (2) heterozygous Cc. The fig is homozygous cc. The pollination combinations possible and the sex segregations in the resulting progenies are shown in Table 1. Because of the rudimentary nature of the stamens in the mamme and mammoni crops of the caprifig, pollen is obtainable only from the profichi crop. And because of protogyny, self-pollination within a syconium is impossible. However, self-pollination can be achieved by using pollen from profichi syconia to pollinate flowers of the mammoni crop. F. carcia has 26 somatic chromosomes. To date, nobody seems to have reported recognizing a pair that conceivably might be sex chromosomes. Nor has anyone reported finding a sex-linked vegetative character that would be useful in separating seedling figs from caprifigs in an early stage of development. Consequently, one must wait until the first syconium appears on a seedling tree and he can recognize the type of pistillate flowers to identify its sex.

TEMPERATURE The flowering patterns of the fig and caprifig are genetically controlled responses to temperature. In the subtropics where the trees are forced into dormancy by cold, cvs such as 'Beall', 'Gentile', 'Franciscana', etc. follow the pattern of having a breba crop on the previous season's wood, and the main crop on current summer growth. In truly tropical climates where winter temperatures seldom or never fall below 12°C, fig trees produce no breba crop. Instead, there tends to be two or sometimes three crops on current increments of growth, with brief resting stages in between. PHOTOPERIOD Photoperiod seems to exert no influence on floral formation in F. carica.

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JUVENILITY Fig seeds germinate readily in 4 to 6 weeks after sowing. The seedlings grow rapidly, but go through a period of juvenility which generally lasts 4 to 7 years, or longer, before they produce their first syconia. The long juvenile period can be circumvented by grafting seedlings on branches of older trees. Seedlings germinated in August are thick enough for grafting in the following April or May. They develop rapidly. Many flower in the first year after grafting and most flower in the second year.

GROWTH REGULATOR A number of workers have experimented with various plant growth regulators on figs to study their effects on floral induction, development, and maturation. None of the materials actually effected floral induction, but most hastened development after the syconium had been initiated normally. 7 "" Crane and Blondeau4 reported that an IB A spray of 1000 ppm fostered continued development and maturation of syconia. Stewart and Condit 12 reported that mature, edible seedless fruit could be developed from noncaprified 'Sari Lop' syconia by spraying with 2,4-D or 2,4,5-T sprays at concentrations of 250 ppm or more. Treatment with 2,4-D or 2,4,5-T hastened maturity when applied after the syconia had nearly attained maximum size, but at concentrations of more than 250 ppm damaged young syconia. The studies of Crane et al. 5 - 6 on the influence of auxins, cytocinins, and ethephon point to release of ethylene as the stimulating factor. SUMMARY Temperature is the determining environmental factor for floral induction in F. carica. In caprifig, mamme crop syconia are initiated on current growth in fall but fail to develop fully when the trees become dormant with the onset of cold weather. They resume development and ripen when the weather warms in early spring. Syconia of the profichi crop are initiated on late fall growth and reach maturation in late spring and early summer. Mammoni crop syconia are initiated on current growth in summer and mature in early fall. Common-type and San Pedro-type fig trees may initiate a breba crop on late fall growth if the onset of cold weather is abrupt. These remain as latent buds when cold weather forces the trees into dormancy. They resume development in early spring and reach maturation in late spring. The main crop syconia are initiated on new growth in late spring and ripen in midsummer. Syconia may be initiated and reach full development into late fall as long as current growth continues on the tree. None of a number of plant growth regulators that have been tried have been effective in actually inducing flowering. However, several have hastened development, maturation, and ripening of the fruit when applied after the syconia have passed a certain critical early stage.

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REFERENCES 1 . Condit, I. J., The structure and development of flowers of Ficus carica L., Hilgardia, 6, 443—481, 1932. 2. Condit, I. J., The Fig, Chronica Botanica Company, Waltham, Mass., 1947. 3. Condit, I. J., Ficus: The Exotic Species, Division of Agricultural Science, University of California, Berkeley, 1969. 4. Crane, J. C. and Blondeau, R., Seedless calimyrna figs, Calif. Agric., 2, 7—8, 1948. 5. Crane, J. C., Bradley, M. V., and Luckwill, L. C., Auxins in parthenocarpic and nonparthenocarpic figs, J. Hortic. Sci. (London), 34, 142—153, 1959. 6. Crane, J. C. and van Overbeek, J., Kinin-induced parthenocarpy in the fig, Ficus carica L., Science, 147, 1468—1469, 1965. 7. Crane, J. C., Marei, N., and Nelson, M. M., Growth and maturation of fig fruits stimulated by 2chloroethylphosphonic acid, J. Am. Soc. Hortic. Sci.. 95, 367—370, 1970. 8. Crane, J. C., Marei, N., and Nelson, M. M., Ethrel speeds growth and maturity of figs, Calif. Agric., 24, 8—10, 1970. 9. Gerdts, M. and Obenauf, G., Effects of preharvest applications of ethephon on maturation and quality of calimyrna figs, Calif. Agric., 26, 8—9, 1972. 10. Maxie, E. C. and Crane, J. C., Effect of ethylene on growth and maturation of the fig, Ficus carica L., Proc. Am. Soc. Hortic. Sci.., 92, 255—267, 1978. 1 1 . Saad, F. A., Crane, J. C., and Maxie, E. C., Timing of olive oil application and its probable role in hastening maturation of fig fruits, J. Am. Soc. Hortic. Sci.. 94, 335—337, 1969. 12. Stewart, W. S. and Condit, I. J., The effect of 2,4-dichlorophenoxyacetic acid and other plant growth regulators on the calimyrna fig, Am. J. Bot., 36, 332—335, 1949. 13. Storey, W. B. and Condit, I. J., Fig (Ficus carica L.) in Outlines of Perennial Crop Breeding in the Tropics, Ferwerda, F. P. and Wit, F., Eds., H. Veenman and Zonen N. V., Wageningen, 1969, 259— 267. 14. Storey, W. B,, Figs in Advances in Fruit Breeding, Janick, J. and Moore, J. M., Eds., Purdue University Press, West Lafayette, Ind., 1975, 568—589. 15. Valdeyron, G., Sur le systeme genetique du figuier, Ficus carica L.; essai d'interpretation evolutive, Ann. Inst. Nati. Agron., 5, 1—164, 1967.

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CRC Handbook of Flowering FRAG ARIA X AN AN ASS A En. Strawberry; Fr. Praise; Ge. Erdbeere; Sp. Freson C. G. Guttridge

INTRODUCTION AND GROWTH HABIT Cultivated strawberries (Fragaria x ananassa Duch., Rosaceae) derive from hybrids between the two American octoploid (2n = 56) species F. chiloensis Duch. and F. virginiana Duch., with both of which they are fertile.' The diploid F. vesca (2n = 14) with its much smaller fruit is also occasionally cultivated, especially the bushy, nonrunnering, everbearing form, F.v. semperflorens. Darrow' describes most of the wild species, including tetraploids and hexaploids, and he2 and Wilhelm and Sagen 1 describe the history and breeding of cultivated strawberries. In common with several other rosaceous genera, the strawberry forms both long and short shoots. The perennial crown or rootstock has short internodes and a rosette habit; axillary buds may remain dormant or form branch crowns or stolons which produce two long internodes before rosetting. The perennial crown is terminated by a bud generally containing 5 to 7 developing leaves enclosed within the stipules of the last emerged leaf.4"6 The inflorescence is formed terminally and the uppermost axillary bud then continues the vegetative extension of the crown, reassuming dominance over lower laterals, later displacing the inflorescence to one side. Axillary buds, including the uppermost, may also form terminal inflorescences after initiating 2, or usually 3 or 4 leaf primordia. The growth habit is thus sympodial, although the lateral displacement of the inflorescence superficially suggests a monopodium. The growth habit of tops, 5 - 7 buds, 4 - 8 crowns and inflorescences, 6 and roots9 have been described. Figure 1 illustrates the structure of a strawberry crown. Inflorescences emerge from the buds some 5 or 6 plastochrons after initiation; the flowers anthese about 2 plastochrons later, and fruit ripens some 3 plastochrons after anthesis. In temperate summer climates plastochrons are some 8 to 12 days, longer in autumn, depending on temperature. Inflorescences borne on the primary axis and on the uppermost axillary axis are usually the largest and most fruitful. Those borne on lower axillary axes normally mature into fruiting trusses only if the axillary buds bearing them produce leaves and become branch crowns. Thus fruitfulness depends not only on the initiation of a sufficient number of inflorescences but also on the vegetative development of the plant, and especially on the number of branch crowns. The so-called fruits, for which the strawberry is cultivated, are swollen receptacles, and the seeds borne on the surface are achenes. Vegetative growth, as well as floral induction, is sensitive to the environment; and increases in photoperiod, temperature, chilling, and nutrition, result in increased plant size, leaf area, petiole and inflorescence length, and induce runnering if thresholds are passed. Petioles are particularly responsive, mature lengths ranging from about 3 to 40 cm or more, depending on the environment. All these responses are quantitative and cvs differ in sensitivity. The resulting complexity demands care in the management of experiments and recording and interpretation of results.

FLORAL MORPHOLOGY The inflorescence is basically a dichasial cyme, but very variable in detailed structure. 10 -' 2 The primary or terminal flower is initiated first 13 and is the largest. In modern cvs

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FIGURE 1. A schematic diagram of a strawberry crown, sampled and dissected when dormant in winter. Leaves arise in 2/5 spiral phyllotaxis, but are here represented alternately for convenience. Inflorescence S flowered in the previous summer. After producing 13 axillary stolons, stolon formation gave place to axillary branch buds (B to G), which were of increasing size from C to G. Only the oldest and largest (G) is illustrated. The axes terminated by inflorescence 1, 2, and 3 would have formed the main continuing crown. Some of axes B to G would have grown out as branch crowns (certainly G), while others would have remained dormant depending on environmental conditions and nutrition in the following spring. Inflorescence 1 was initiated in September (Northern Hemisphere) and would have emerged from the bud and flowered early in the spring, followed in sequence by inflorescences 2 and 3, which were initiated later. Inflorescences 4 and 5 on the branch crown would have emerged along with 2 and 3, respectively, probably together with inflorescences on any of buds B to F that might have grown out; 1 year later inflorescence 3 would have become inflorescence S. (From Guttridge, C. G., J. Honic. Sci., 30, 1—11, 1955. With permission.)

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a secondary flower develops terminally on each of the 2 or 3, occasionally 4 branches of the main floral axis beneath the primary flower.' 4 l s Thereafter the structure usually becomes strictly dichasial, 2 tertiaries forming on each secondary branch and 2 quaternaries on each tertiary, and so on. Well-developed inflorescences usually, but not always, 14 l6 have a full complement of tertiary flowers but not of quaternaries. 15 -"' Quinaries may be present even when some quaternaries are missing. l s - 1 6 Anderson and Guttridge ls found all primaries and secondaries anthesed, as did about 80% of tertiary and 50% of quaternary rank flowers, but variation between cvs and in different environments is to be expected. Both the flowers themselves and the resulting fruits decline in size with increasing rank number."- 1 4 '" | l ) There is little detailed information on the influence of environment on the number and size of flowers in inflorescences, most reports being of horticultural experiments usually recording crop yields, and sometimes numbers of berries per plant. Even when number of inflorescences are also reported, ratios of numbers of flowers or, even less reliably, of berries per inflorescence, can be misleading because different aged trusses may have been sampled. Some evidence suggests that the potential number of flowers likely to attain anthesis is determined before truss emergence. For example, sprays of GA increased flower numbers only when applied during October in Scotland, that is before the onset of dormancy, and not when applied later, either to dormant or chilled plants 2 "--' (see Chemical Activity). These results imply that flower numbers are beyond promotive i n f l u e n c e some 4 to 5 plastochrons after initiation or 1 to 2 before emergence. These timings, which represent stages of truss ontogeny, are supported by work in England, in which flower numbers per truss were increased when plants were brought from outdoors into a growth room in October but not when brought in when dormant in December. ls Whether the autumn-created flowering potential is realized in spring depends on winter damage being avoided, '-^ on providing sufficient light, 2 ' and no doubt generally on the provision of a reasonably supportive environment for growth. The basic floral whorl number is 5 as exemplified in F. vesca, there being 10 sepals, 5 petals, 20 anthers, and numerous carpels on a fleshy receptacle, but numbers vary even in the simple diploid. 23 Initiation of floral organs proceeds centripetally from sepals to petals, stamens and carpels,6 the last carpels becoming visible, under microscopic dissection, at the tip of the receptacle some 5 plastochrons after the original initiation. Flowers of octoploid cultivars are larger than those of the diploid usually having more than the basic pentamerous multiples (Table 1). Fruit size is related to the numbers of floral parts, 24 especially of carpels. 17 - 18 ' 25 Poor anther quality and insufficient pollen24 in some cvs are horticultural problems which probably have their origin in the dioecious tendency in some clones of F. chiloensis and F. virginiana. However, environment exerts affects, light intensity, 22 - 26 the avoidance of winter damage,27 and nutrition28 being known to influence anther quality against a weak genetic background.29

GENETIC INFORMATION The distinct everbearing and noneverbearing, or seasonal fruiting forms of F. vesca differ in a single gene allele, the recessive being everbearing, representing a loss of photoperiodic and thermal control over flower induction. 33 Polyploidy complicates genetic analysis of F. x ananassa but results of Ourecky and Slate34 appear to be consistent with their suggestion that the everbearing character in octoploid cvs is determined by two complementary dominant interacting genes, although other explanations are plausible. If the everbearing character is dominant in the octoploids then the basis of their "perpetual" fruiting is different from that in F. vesca. More recently Bringhurst and Voth35 have described cvs of everbearing habit derived from crosses between F. vir-

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Table 1 MEAN NUMBERS OF FLORAL PARTS IN CULTIVATED STRAWBERRIES Floral part Sepals

Petals

Stamens

Carpels

•' h c J

Primary flowers

Secondary flowers

16.0 ± 1.8"

12.6 ± 1.0"

15.6

12.5

Tertiary flowers

11.0

8.5 ± 1.3

6.4 ± 0.7

8.1

6.3

5.2

6.2(5— 7) h

5.4(5—7)"

7.3 (7—9)" 38.2 ± 7.5"

30.8 ± 5.2"

35.1

27.6

24.2

38

35

— 149 ± 48C 146d

347 ± I 1 7 C 296'1

189 ± 78C 209°

Cv Royal Sovereign Pusa Early Dwarf Royal Sovereign Pusa Early Dwarf Redgauntlet Royal Sovereign Pusa Early Dwarf Redgauntlet Redgauntlet Cambridge Favourite

Ref.

23 30 23 30 31 23 30 32 17 17

% Standard error. Range. Standard error. Derived data.

giniana glauca (F. ova/is) and noneverbearing cvs as day neutral plants (DNP) but there appears to be no physiological evidence to suggest that the genetic basis for long-season fruiting is different in the DNPs and everbearers, although it may be. Everbearers and noneverbearers hybridize freely and give rise to mixed populations.

PHOTOPERIODIC EFFECTS Noneverbearing strawberries behave as facultative short-day plants (SDP) requiring daylengths shorter than about 14 hr or temperatures less than about 15°C for flower induction. 36 At greater temperatures than this, photoperiod becomes critical, and the threshold is variously reported as being between 12 and 16 hr," w 12 and 14 hr, w 11 and 14 hr, 40 11 and 15 hr, 41 13 and 15 hr, 42 and 13 and 16 hr. 41 Initiation dates in September outdoors in northern temperate climates suggest a threshold of about 14 hr. The optimum, in contrast to the critical, daylength probably lies between 8 and 11 hr."- 18 - 40 - 41 Lack of assimilates may limit the flowering response in shorter photoperiods or cause early abortion of young inflorescences. 40 - 41 - 44 The minimum number of SD cycles needed to induce flowering in octoploid cvs usually lies between 7 and 14,^x^ 9 - 44 - 47 but can be up to 23.40-47-49 Temperature affects the minimum cycle number by about 2 days (see Temperature Effects), time of year or pretreatment by up to about 5 days,18 while prior chilling may delay induction by as much as 9 weeks (see Chilling). For inducing an early second flowering in commercial crops of cv Redgauntlet in Northern Europe 20 to 25 SD cycles are suggested. 3V - 50 F. vesca needs 4 to 5 weeks of SD at moderately cool temperatures for induction. 31

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Continued exposure to inductive cycles induces further initiation, the new inflorescences forming terminally on axillary axes, including on the uppermost or crown-extension axis after 2 or, more usually, 3 or 4 nodes.5 Further initiation ceases if the conditions cease to be inductive, although existing inflorescences continue to develop. However, after prolonged exposure to SD, a resting or dormant condition is established 3951 in which the induction of further new inflorescences is much less easily inhibited by LD or high temperature than it is in nonresting plants (see Rest or Dormancy). Apparently incongruously, induction has been promoted by continuous light52-53 but perhaps this was in response to some unrecognized stress arising from the treatment. Inflorescences are responsive to photoperiod, being shorter in SD than in long, the peduncle often being the most affected part of the branching system.3" In some of the species, inflorescences are extremely diminished when they develop entirely from initiation to anthesis in SD, without a chilling intervention. Peduncle and secondary branches may then be reduced to a few millimeters in length, and in extreme cases only the primary and perhaps one or two secondaries may reach anthesis on pedicels no more than 3 to 5 cm in length. SD suppression of inflorescence growth is much less severe in some strains of F. chiloensis (presumably those native to near equatorial latitudes) and in cultivated octoploids, and in the latter their complement of flowers may not be much less in SD than in LD. Trusses are probably most sensitive to photoperiodic influence on length from about 2 plastochrons before to 1 after emergence from the bud and on flower number probably 1 to 2 plastochrons earlier. After chilling in temperate climates photoperiodic effects on flower numbers per truss are probably small, 54 and, on evidence referred to in the section Floral Morphology, mainly result from effects on the realization of the potential established before truss emergence. Trusses initiated after chilling and developing through to anthesis in warm and favorable environments, as provided by glasshouses in spring, are reported to have larger numbers of flowers than trusses developing in autumn. 15

INHIBITION OF FLOWERING Although ostensibly a facultative SDP, flowering in noneverbearing strawberries may in fact be regulated by an inhibitory LD process. Using donor-receptor pairs of runner plants, joined by the stolon, Guttridge found that donor plants in LD delayed and sometimes inhibited flower formation in receptor plants in SD, while donors in SD failed to induce flowering in receptors in LD.55 57 Earlier studies46 purporting to demonstrate the transmission along the stolon of a flowering stimulus generated in SD have not been confirmed.7-58 Other evidence supports control by transmissible inhibitors, 45 - 58 59 some of it having been discussed previously. 7 However, some of the effects transmitted along the stolon may be nutritional. 58 Further indirect evidence comes from the sensitivity of strawberry to R and FR irradiation applied separately, the pattern of which is characteristic of LDPs, not of SDPs,60 but with floral inhibition in strawberry replacing promotion in LDPs.47 These responses suggest that strawberry could be described as a negative LDP. This subject is further discussed in the section Spectral Responses. Flower formation is promoted, at least in some circumstances, by defoliation, especially the removal of old leaves, a response consistent with the inhibitor hypothesis. 59 Defoliation in the field in Scotland promotes flowering in cultivars which otherwise tend to be deficient in truss numbers. 61 - 62 However, even in those which respond, the time relationships between defoliation and response are hardly consistent with, but do not entirely exclude, removal of the source of inhibitors as an explanation. Consideration of his results led Mason62 to propose that inflorescence formation depends on promotors and inhibitors that are balanced differently

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in different cvs (see also Relationship between Low Temperature Flower Induction and Chilling).

NIGHT BREAKS (NB) AND CRITICAL LIGHT FLUX DENSITY In otherwise short inductive photoperiods, and when other environmental factors are not limiting, flowering is inhibited by NBs or daylength extension of sufficient duration with tungsten incandescent light of relatively low flux density. Incandescent light is inhibitory at irradiances of about 10 (juW/cm 2 (about 20 Ix),-™- 63 although most workers found greater irradiances were necessary (e.g., 240 jjiW/cm 2 ) 45 or used them effectively (e.g., 100 to 200 jjiW/cm 2 ; 150 |o,W/cm 2 ). 4 " 42 Fluorescent light is reported to be at least ten times less active than incandescent, when used for daylength extension to natural days,45 but the significance of this difference is in doubt following the more recent discovery that spectral sensitivity of the plants changes with lapse of time from the end of the main light period (see Spectral Responses).64 At 14°C, Went44 found daylength extension with 8000, but not 4000 Ix of mixed fluorescent and incandescent light inhibited flowering, but at 10°C even 12 klx was insufficient. These luminosities are much greater than other workers have found necessary, probably because photoperiod is unlikely to have been the critical factor at these temperatures (see Temperature Effects). NBs with incandescent lighting need to be of 1 to 3 hr duration to inhibit flowering,40-65 20 or 30 min being either less effective40 or virtually ineffective. 65

TEMPERATURE EFFECTS Flowering is induced (i.e., inhibition fails) in noneverbearing cvs in long daylength if temperatures are less than about 15°C.36 The critical temperature has since been reported to be between 15 and 21°C,66 10 and 14°C,44 9 and 17°C,38 and 12 and 18°C.43 Thus flower formation becomes less likely as either the daylength or the temperature increases (Table 2). The critical day length-temperature combinations are similar for the cvs Marshall, Robinson, Senga Sengana, and Abundance, but the three cvs which flower early in spring in Norway (69° 39'N) clearly form a different group, needing higher temperatures for inhibition. 41 The everbearers were not inhibited even at 26°C and 24-hr daylength, 67 but nevertheless may be only an extreme form requiring temperatures greater than 26°C for inhibition. 67 When the critical number of daily cycles necessary to induce flowering was used as a criterion of the inductive strength of the environment, there were clear cut-offs between 9 and 17°C in LD, between 24 and 30°C in SD and between 12 and 16 hr when daylength was critical (Figure 2). At 3 to 5°C, Jonkers45 obtained flowering after 4 weeks in LD. Rather similar numbers of inductive cycles are apparently needed for flowering whether the stimulus is short photoperiods or low temperatures, the minimum being between 9 and 10 days both for SD induction at high temperature (24°C) and for low temperature induction in LD (Figure 2). Only 2 days fewer were required at the optimum combination of 8-hr photoperiod and 17°C. A similar 2-day difference between the optimum temperature of 17°C and 3° either side of it was also found in rather similar circumstances.44 Ito and Saito38 found that a minimum of 16 hr/day at 9°C was necessary to induce flowering in a cyclic temperature regime when 24°C was held for the remaining daily period. When the number of hours at 9°C was increased to 20 or 24, the minimum number of inductive cycles was decreased from 16 to 12 and 10, respectively. The minimum cycle number of eight for induction at 17°C is close to the leaf plastochron at that temperature, but the 10-day inductive period at 9°C is well within the then longer

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Table 2 INTERRELATIONSHIP BETWEEN TEMPERATURE AND PHOTOPERIOD ON FLOWER INDUCTION IN VARIOUS STRAWBERRY CVS Daylengths (hr) Cv

Marshall

fC)

4

6 10 14 17 20

8

10

12

+ + + + +

+ + 0 0

+ + + 0

+ + + 0

14

+

9 17 24 30

Senga Sengana and Abundance

12 18 24

+ + +

+ + +

+ + 0

12 18 24

+ + +

+ + +

+ + +

Zefyr," Jonsok," and Glima"1

Revadah and Rabunda11

14 20 26

+ + +

20

+ + 0 0 0

Robinson

+ + 0

16

24

Ref.

+ 0 0

44

0 + 0

38

+ 0

0 0 0

43

+ +

43

+

+ + 0

+ + +

+ + +

67

0 0 0

0

+

Note: + , flowering initiated: O, flowering inhibited. " h

Cvs adapted to early fruiting in Norway (69°C 39'N). European everbearing cvs.

plastochron. Induction within the plastochron at 9°C suggests that the cool days were in fact more inductive than warm 17°C days on a growth per day comparison. The fact that flowering was so rarely induced in only some of the three plants per treatment that Ito and Saito38 used, suggests that phasing of induction with the stage of apical leaf initiation is unimportant. Thermic as well as photoperiodic responses are important in the climatic adaptation of cvs

43,61.66.68,69

SPECTRAL RESPONSES When R and FR spectral bands were applied separately and at different times to extend an 8-hr main photoperiod of daylight or of fluorescent + incandescent light, flowering was delayed or inhibited by FR rather more effectively when it was given during the 8.5 to 9 hr following the main light period than later and by R light, only when it was given in the 8.5 to 9 hr preceding the main lighting (Table 3).47 This pattern of sensitivity to R from near midnight to dawn is typical for LDP but unusual for SDP.60 Another spectral characteristic of LDP is an enhanced response when FR is added to a R day extension or NB. 70 The fact that strawberries are markedly sensitive to incandescent light, which contains both R and FR emissions, and responds to R and FR mixtures either following or preceding the main light period (Table 3), is therefore further evidence of its LD sensitivity.

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FIGURE 2. The critical number of daily cycles for flower induction in Robinson strawberry (based on data from Ito and Saito-"1). The first figure is the largest number of daily cycles not inducing flowering, and the second is the smallest number that induced flowering. A single figure denotes that a proportion of plants flowered after that number of cycles.

On this evidence strawberry exhibits the LD processes and spectral sensitivity patterns 70 that are characteristic of LDP, although the floral response is opposite, LD inhibiting flowering in strawberry, promoting it in LDP. It is therefore misleading to describe strawberry as requiring long nights for flowering even though most SDPs have this requirement. 70 7 I Thus, strawberry is induced to flower in SD apparently because the absence of LD releases it from floral inhibition. R/FR effects on flower truss extension have not been examined but because of the similarity of truss and petiole responses to photoperiod,58 temperature, and chilling, similarities in their spectral responses might also be expected. If so, then peduncles and pedicel of the truss might be expected to increase in length in response to end of day FR, to prolonged daylength extension or NBs with R, FR, and mixtures of the two and to incandescent irradiation and especially to R light from about midnight to dawn, as found for petioles.72 Relative response of peduncles and pedicels would be expected to depend on the stage of truss ontogeny when irradiated, peduncles being sensitive before pedicels. Israeli workers found that filtering sunlight through a blue filter (Mazzuchelli 1654LL) which removes R light but transmits FR, delayed flowering,73 a result since confirmed 53 with Setocyanine,74 a blue filter having similar transmission characteristics. When R and

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Table 3 PERCENTAGES OF STRAWBERRY PLANTS INHIBITED FROM FLOWERING BY IRRADIATION (A) OF DIFFERENT SPECTRAL REGIONS, 47 AND (B) FROM DIFFERENT LAMP TYPES,64 GIVEN AT DIFFERENT TIMES AFTER A SD Experiment No.

Dusk to midnight

Midnight to dawn

Control SD

LD"

Ref.

0

100

47

A. Similar Energy Fluxes (0.45 J m~ 2 sec~')

1 2

3

R FR R + FR R FR R + FR (2 x 0.45 J m~ 2 sec ')

0 33 60 0 53 60

100 20 100

0

100

47

R F R R + FR (2 x 0.45 J m^secr 1 )

0 0 52

40 0 80

0

100

47

B. Similar Lamp Inputs (ca. 50 W m 2) 4

5

6

Incandescent Fluorescent (Phillips TL32) Red (TL32 filtered) Fluorescent (TL32 unfiltered) R (fluorescent filtered) FR (incandescent filtered)

100 0

20

100 100

0

64

100 100

0

64

99

0

64

Note: R. red, 600 to 700 nm. FR. far-red, 700 to 800 nm. "

Daylength extended to 24 hr with incandescent lighting.

FR were filtered out by using a Ferric Ferrocyanide filter, plants flowered earlier than controls under a neutral filter. 73 Interestingly, Japanese work75 showed a red-absorbing filter advanced flowering, even though FR was transmitted, but the apparent anomaly may be because the Japanese work was done in filtered artificial light rather than sunlight and so the spectrum may also have lacked FR. Variations in the amount of FR irradiance may take on greater significance when, as in all these experiments, R light is substantially absent. The usual negative correlation between vegetative growth, especially petiole length, and the flowering response occurred in all these spectral studies, including those with blue filters.

REST OR DORMANCY After a prolonged inductive period of SD, the duration of which has not been quantified but may be around 4 to 6 weeks, strawberry plants gradually become dormant or resting. 39 - 51 Judged by the subsequent vegetative growth of plants transferred into warm glasshouses at different times in autumn and winter, deepest dormancy is attained in November in northern latitudes. 4554

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Dormancy is only quantitative, and dormant plants retain green leaves and a growth capability if the environment remains or becomes supportive. However, the vegetative development of dormant plants is much restrained; 54 - 76 - 77 petioles are short, laminae small, emerging inflorescences small, no stolons are formed and as continuing growth provides new sites further new inflorescences are formed. These growth responses occur in dormant plants under conditions of temperature, light, and daylength that would give large plants, induce stolon formation, and inhibit further flower formation in nondormant plants.54-58-78-7'1 During prolonged exposure to LD dormant plants gradually increase in size, but even after several months, recovery may still be incomplete 76 and new inflorescences may still be occasionally initiated. Dormant plants respond to changes in daylength and temperature in the same general manner as nondormant plants, but to a lesser degree.80-81 Inflorescence growth is sensitive to dormancy. Flower trusses develop poorly in dormant plants growing in SD but a little better in LD. In some species, e.g., F. moschata, F. vesca, F. virginiana, inflorescences developing on dormant plants are extremely small, sometimes only the primary flowers attaining anthesis, although in F. chilloensis they are larger. Dormant plants of cultivated varieties of strawberry generally produce small but reasonably developed inflorescences in LD unless the trusses are damaged. Avoiding chilling by overwintering plants in a heated glasshouse before planting them outdoors induced second cropping in summer in the U.K. 77 CHILLING Exposure to low temperatures reverses the effect of dormancy, promoting subsequent vegetative growth and inhibiting new inflorescence formation. After chilling, petioles grow longer, laminae larger, and emerging inflorescences taller; stolons are more readily initiated in new axillary sites and new inflorescences are inhibited or more easily inhibited than in dormant plants. S6 - 39 - 76 - 78 - xo A subsequent reduction in number of new flower trusses formed or their total suppression after chilling is widely reported. 3954787y The delay in re-induction of flowering in short photoperiods (12 hr) after chilling may be quantified in terms of numbers of leaves formed before the next inflorescence. In such a study 79 further inflorescence initiation was delayed by outdoor winter chilling in Scotland by 6.8 and 2.7 leaves (plastochrons) in cvs Royal Sovereign and Auchincruive Climax, respectively. These plastochrons represent about 65 and 25 SD cycles; considerably more than are required to induce flowering in plants not recently chilled (see Photoperiodic Effects). In another experiment, 36 or 72 days chilling at 2 to 4°C delayed re-induction by 3.1 and 5.2 plastochrons in the same two cvs. 79 Changes in vegetative growth induced by chilling are striking and resemble the effects of increasing the day lengths. The responses to chilling and to long daylengths are additive or more, and neither substitutes for the other, at least not for petiole responses.82 Chilling increases the plants response to daylength.1" Apparently temperatures of -2°C,1" - 1°C,78 0°C,81-84 and 3°C76 all chill effectively, 6.5°C is almost as effective,81 but 9.5°C81 and 10°C76 are much less so, and 14°C is ineffective.76 Cvs vary widely in the amount of chilling required to release them from dormancy. Although the response is quantitative, it is usually possible to identify a time when dormancy may be said to be broken. Small increases in subsequent vegetative growth are seen in cvs that have little chilling requirement (e.g., Tioga) after about 2 weeks at 0°C,84 or at 3°C,76 or outdoors54 and their chilling requirements are more or less satisfied by about 4 weeks.848 "Other cvs need much longer, e.g.. Sequoia > 30 days at 0°C,84 Glasa 5 to 6 weeks, 85 Gorella 8 weeks, 85 and Redgauntlet > 8 weeks. 85 One result shows that 2 months at — 1°C in a cold store did not completely eliminate subsequent initiation for a second flowering in LD but 4 months did, and nearly did in SD.78

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Q 22 NOVEMBER

4 20 DECEMBER

1 17 2931 14 2628 1214 FEBUARY MARCH JANUARY

26

9 APRIL

FIGURE 3. Mean length of peduncles and branches on extended inflorescences of strawberry plants which had been transferred from outdoors at Long Ashton Research Station into growth cabinets on different dates (cv Redgauntlet). (A) Length of peduncles only, winter 1980 to 1981 (closed circles). (B) Length of peduncles + mean length of the two longest secondary branches, winter 1978 to 1979 (open circles). Growth cabinet conditions were 18/15°C, dry bulb and 15.5/14.6°C wet bulb, day/night, respectively, with a photoperiod of 15 hr, fluorescent and incandescent lamps at about 15,000 Ix. Inflorescences on plants transferred in November were longer than those transferred in December, partly because, being younger at the time of transfer, more of their development occurred in the warm growth cabinets. Plants transferred from December to mid-January were dormant or resting. Dormancy was partially broken by end-January and fully so by mid-February, depending on the year, causing the inflorescences to be larger. The inflorescences were again shorter in batches transferred in late March or early April, because spring growth had started before transfer and the inflorescences had developed for a longer period in the cooler outdoors. The inflorescences were longer in the A series because they were initiated later in autumn than in the B series, and consequently were in the growth cabinet for a greater proportion of their ontology. (Data supplied by H. M. Anderson, Long Ashton Research Station, Long Ashton, Bristol, England.)

Horticulturalists are often more concerned with the subsequent development of existing fruit trusses, which are increased in size by chilling, than with subsequent flower induction. The length of peduncles, secondary branches and pedicels are increased by chilling39 (Figure 3). The magnitude of the chilling response depends on the stage of development reached by the truss at dormancy. Trusses initiated late in autumn are younger than those initiated early, and hence normally complete a greater proportion of their growth after chilling. Because chilling stimulates subsequent elongation, late-formed trusses characteristically have long peduncles and early-formed trusses short ones at maturity. Pedicels appear to complement peduncle and branch length, being shortest when peduncles and branches are longest. There is little data on effects of chilling on flower or fruit numbers per inflorescence. Numbers of flowers per truss are probably determined before dormancy outdoors in temperature climates (see Floral Morphology). At least partial satisfaction of the chilling requirement is probably necessary for sufficient growth and development of the truss for commercial fruit production whether in mild Mediterranean and Californian type climates or in glasshouses. The less ready induction of inflorescences after chilling probably explains why SD in spring fail to initiate new flower trusses in temperate regions, while similar daylengths in autumn do so. Again in temperate regions cold-stored and therefore fully chilled plants for the same reason do not initiate new inflorescences for several weeks after planting in summer. Consequently such plants must be planted for commercial fruit production before August in the U.K. and northern Europe if they are to fruit with certainty the following year, although some cvs are more tolerant of late planting than others.

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RELATIONSHIP BETWEEN LOW TEMPERATURE FLOWER INDUCTION AND LOW TEMPERATURE CHILLING As already described (see Temperature Effects) temperatures less than about 15°C induce flower formation even in long photoperiods. On the other hand, temperatures of less than about 10°C result in a chilling response, one consequence of which is an inhibition, reduction, or delay in further flower induction. This apparent anachronism, low temperatures first inducing flowering but subsequently inhibiting it, has been noted45 but not studied in detail. It seems that the first process of flower induction which occurs even at 3 to 5°C,45 is gradually overtaken by the second process which diminishes induedveness. Such circumstances could lead to flower formation in some plants while the first process predominates, but not in others in the same batch or field where the second process may have taken over sooner, and might perhaps partly explain why some plants or crowns sometimes fail altogether to initiate inflorescences in the cool Scottish autumn (see Inhibition of Flowering). Because of the importance of the dormancy and chilling characteristics of different cvs for their worldwide and regional adaptation, more research into the subject might be rewarding. Kroenberg et al.76 showed that plants maintained at 10°C in a temperature-controlled glasshouse in naturally declining autumn daylengths in Holland did not subsequently respond to chilling at 3°C, indicating that the plants were not dormant, whereas plants held at 14°C in autumn did respond. They used petiole length to assess responses. In a similar manner, Plancher87 found that temperatures between 0 and 7°C did not induce dormancy. The method of using chilling to assess the dormancy-inducing effect of a previous environment could be used to study dormancy induction and might reveal hitherto unrecognized genetic differences between cvs of possible importance in plant breeding and regional adaptation. MINERAL NUTRITION AND WATER SUPPLY Nutrition of strawberry is a complex subject.88 Growth-stimulating doses of mineral nutrients apparently have a tendency to inhibit flower formation per se. However, the total numbers of inflorescences on a plant may be increased if the dominant response to nutrients is the production of more crowns and therefore flower sites. These contrary effects are sometimes recognized but seldom analyzed in horticultural literature on nutrition, which is extensive. Flower inhibition can sometimes be inferred when nitrogen feeds depress fruit yields89 or reduce truss numbers,90 but flower promotion per se, if it occurred, would be unrecognized in such circumstances. Increasing the nutrient supply from a low base can be expected to increase fruit yields, but too much, especially of nitrogen, can inhibit flower formation.91 Extra nitrogen has been reported to reduce summer flower initiation for the autumn crop in a double-cropping variety in England.92 Withholding nitrogen or phosphorous, however, may not increase it.93 Other reports show nitrogen feeds reduce flower initiation,68-94 and another indicates that nitrogen, potassium, and phosphorous can all delay natural autumn flower initiation, suggesting,that the inhibition of flowering is related to stimulation of vegetative growth generally rather than to the supply of any one of these major elements specifically.7 Horticulturalists recognize that drought occasionally induces flower formation, but seemingly the only report on water supply is of generous applications in August, shortly before natural photoperiodic induction reducing flower formation.95 Thus, although there are few definitive examples, restricted nutrition or water supply probably can promote flower induction per se in some circumstances as well as depressing vegetative growth, and excesses of some nutrients can inhibit, delay, or reduce it. Because of these responses, there is a risk of spurious results, especially with container-grown plants, if mineral nutrients are depleted at different rates in different treatments. Thus treatments

28

CRC Handbook of Flowering

(e.g., of growth regulators) that stimulate vegetative growth may cause rapid depletion of one or more nutrient elements or even of water, so that vegetative growth is later depressed and in consequence flower induction may then occur sooner than in less stimulating treatments, which may in fact be more inductive. Growth of anthers seems to be particularly dependent on an adequate supply of calcium at the time of inflorescences emerge from the bud, although other divalent ions may substitute.28 Truss size, flower size, and carpel numbers are undoubtedly reduced by poor nutrition, but there is little precise information and responses may be small.96 One report shows numbers of flowers per truss increasing steadily from about 8 to about 11 with added increments of nitrogen.88

CHEMICAL ACTIVITY Since the original discovery,97 applications of GA, have been repeatedly shown to inhibit flower formation in both the SD,78-98-99 and everbearing or day-neutral octoploid strawber9 r j es> 98,100,101 an( j a j so jn tne everbearing F. vesca semperflorens. * Some other gibberellins are also effective.98 Both purified alcoholic extracts of actively growing78 strawberry tissues and diffusates collected on agar have gibberellin-like activity in various bioassays,78-"5-"6 especially when the plants were grown in long day lengths."6 Whether native GAs mediate in natural LD inhibition of flowering is undecided but seems possible.102 The difficulty in accepting the simple supposition that the native transmissible factor which evidently exists in an active GA, is that exogenous GAs when applied in active amounts induce elongation of the main axis of the stem, thereby destroying the compact rosette habit,97-98 so it seems more likely that the native agent is a precursor. However, increasing photoperiod and increasing doses of exogenous GA apparently inhibit flowering additively, when either factor is present in marginal amounts. 102 GA applied to strawberry plants with inflorescences developing within the bud, greatly increases the final mature height of the trusses, and if supplied in sufficient amounts causes the flowers to be borne well above the foliar canopy. Peduncle, branch, and pedicels may all be increased in length, and truss emergence and flowering are advanced. "-20-21 Thompson20 found that twice weekly sprays of 3.12 or 6.25 ppm GA in October in Scotland doubled and trebled, respectively, the number of flowers that eventually anthesed, although none set fruit. Similar sprays applied later, in December, in a glasshouse, when the plants were resting, or in January after chilling, did not increase flower numbers over controls, although they advanced flowering and increased truss height, as did the October applications.20-2' It is evident from these results that the numbers of flowers on the inflorescences in these experiments of Thompson's had been determined outdoors in Scotland before November 23 in that year. Thompson went on to show that the sterility induced by the GA was caused by carpel, rather than pollen failure.20 Although there are reports of GA advancing and increasing flowering and fruiting, results have been variable probably because of difficulties in correctly timing applications'03-'04 and avoiding sterility. Because GA inhibit flower formation, growth retardants might be expected to promote it, but there is little evidence for this. Promotion has been reported with chlormequat,105 but other investigations failed to confirm this.102-106 However, an indirect increase in fruit yield may result from the suppression of stolon formation and a consequent increase in branch crown numbers and therefore of floral sites.'07 Daminozide is similarly nonpromotive or only marginally promotive,102-108 but both this chemical and chlormequat suppress vegetative growth and truss development. Interestingly, applications of daminozide, but not of chlormequat, induced flowering in the otherwise shy-flowering long-stemmed mutant of F. vesca.1"

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Amo 1618 and Phosphon D are rather ineffective on strawberry. I0y Repeated sprays of ABA are reported to promote flower induction in strawberry 105 but again this is not confirmed87 for practical use. 101 Auxins do not affect flowering in strawberry, except indirectly, 99 and the ethylene-releasing compound CEPA is reported as producing only indirect suppression of flowering. 99 On the other hand, Thompson110 found that maleic hydrazide delayed or suppressed inflorescence initiation specifically, when applied at rates which severely depressed petiole length and induced foliar chlorosis. EVERBEARING HABIT Although everbearers or perpetual fruiting cvs produce more inflorescences in LD than in SD, 42 ' 1 " there is no absolute requirement for long photoperiods, and they are probably better described as day neutral, lacking or having only an impaired photoperiodic regulation of flowering. Increases in flowering that result from long daylengths probably mainly arise indirectly from better growth, one consequence of which may be an increase in suitable sites for flower initiation, but site occupancy may also be improved. Such increases in plant growth and site numbers may, of course, result from photoperiodic effects on vegetative growth as well as from improved assimilation. More specifically, Smeets67 found that long photoperiods failed to inhibit flower formation even at 26°C in the two everbearing cvs he studied (Table 2). The vegetative responses of everbearers are similar to those of noneverbearers; increasing photoperiods promoting leaf area, petiole length, etc. Thus only the ability to be inhibited from flowering by LD and high temperatures is lacking in everbearers. Because everbearers hydridize with noneverbearers it is possible that intermediate forms exist or can be bred which may be inhibited from flowering if the LD-temperature regime is sufficiently stimulating. In the diploid, the everbearing habit of F. vesca semperflorens is sharply distinct from the noneverbearing photoperiodically/thermally controlled flowering of the type, as expected from the single-gene difference.33 At least in some strains of F. v. semperflorens, an inflorescence is regularly formed sympodially after each 2 nodes. In marked contrast to the diploid, the difference between everbearing and noneverbearing octoploids is imprecise, suggesting that environment-sensitive processes of flower regulation are genetically impaired to different degrees in various cvs. In particular, the double-cropping "remontant" strawberries of northern Europe represent an intermediate condition in which plants fruit twice a year, first in summer on inflorescences initiated in autumn and second in autumn on trusses initiated in summer. Flower formation in these double-cropping types, of which the Scottish cv Redgauntlet is an outstanding example, is sensitive to photoperiod, but LD inhibition is apparently readily overcome by the growth check associated with the fruit burden in summer. When deblossomed in summer such cvs fail to produce autumn fruit." 2 SD, noneverbearing cvs exhibit an everbearing habit in climates having mild winters.

FLORAL ABNORMALITIES Fasciation, especially of the inflorescence, occurs in some cvs including everbearers, mainly following prolonged growth in SD.42-"3 The tendency to fasciate is heritable."3 Coxcomb-shaped fruits may be a weak expression of fasciation. Phyllody is seen occasionally in some cvs (e.g., Cambridge Favourite) again mainly after prolonged experimental periods of SD treatment. Phyllodious flowers have excessively leafy sepals and bract-like carpels on receptacles which nevertheless may swell and ripen (see Thompson,"4 for illustration). Petallous stamens are occasionally seen.

30

CRC Handbook of Flowering REFERENCES

1. Otterbacher, A. G. and Skirvin, R. M., Derivation of the binomial Fragaria x ananassa for the cultivated strawberry, HonSdence, 13, 637—639, 1978. 2. Darrow, G. M., The Strawberry, Holt, Rinehart & Winston, New York, 1966. 3. Wilhelm, S. and Sagen, J. E., A History of the Strawberry, University of California, Berkeley, 1974. 4. Arney, S. E., Studies of growth and development of the genus Fragaria. II. The initiation, growth and emergence of leaf primordia in Fragaria, Ann. Bot., 17, 477—492, 1953. 5. Guttridge, C. G., Observations on the shoot growth of the cultivated strawberry plant, J. Hortic. Sci., 30, 1—11, 1955. 6. Jahn, O. L. and Dana, M. N., Crown and inflorescence development in the strawberry, Fragaria ananassa, Am. J. Bot., 51, 605—612, 1970. 7. Guttridge, C. G., Fragaria, in Induction of Flowering, Evans, L. T., Ed., Macmillan, Melbourne, 1969, chap. 10. 8. Arney, S. E., Studies of growth and development in the genus Fragaria. IV. Winter growth, Ann. Bot., 19, 265—276, 1955. 9. Mann, C. E. T., Studies in root and shoot growth of the strawberry, Ann. Bot., 44, 55—86, 1930. 10. Darrow, G. M., Inflorescence types of strawberry varieties, Am. J. Bot., 16, 571—585, 1929. 11. Foster, J. C. and Janick, J., Variable branching patterns in the strawberry inflorescence, J. Am. Soc. Hortic. Sci., 94, 440-^43, 1969. 12. Robertson, M. and Wood, C. A., Studies in the development of strawberry. I. Flower-bud initiation and runner development in early- and late-formed runners in 1951 and 1952, /. Hortic. Sci., 29, 104—111, 1954. 13. Ruef, J. U. and Richey, H. W., A study of flower bud initiation in the Dunlop strawberry, Proc. Am. Soc. Hortic. Sci., 22, 252—260, 1926. 14. Sachs, M. and Izsak, E., The effect of flower position in the inflorescence on subsequent fruit development and size in Fresno and Tioga strawberries, Acta Hortic., 30, 107—114, 1973. 15. Anderson, H. M. and Guttridge, C, G., Strawberry truss morphology and the fate of higher order flower buds, Crop Res., 22, 105—122, 1982. 16. Sachs, M. and Izsak, E., Inflorescence formation and fruit development in winter grown Suprise des Halles strawberry, Isr. J. Agric. Res., 23, 137—139, 1974. 17. Webb, R. A., Terblanche, J. H., Purves, J. V., and Beech, M. G., Size factors in strawberry fruit, Sci. Hortic., 9, 347—356, 1978. 18. Janick, J. and Eggert, D. A., Factors affecting fruit size in the strawberry, Proc. Am. Soc. Hortic. Sci., 93, 311—316, 1968. 19. Sherman, W. B. and Janick, J., Greenhouse evaluation of fruit size and maturity in strawberry, Proc. Am. Soc. Hortic. Sci., 89, 303—308, 1966. 20. Thompson, P. A., Physiological investigations on the strawberry. Development of the flower truss, 9th Annu. Rep. Scot. Hortic. Res. Inst., pp. 41—43, 1962. 21. Thompson, P. A., Physiological investigations on the strawberry. Development of the flower truss, Wth Annu. Rep. Scot. Hortic. Res. Inst., pp. 43—44, 1963. 22. Smeets, L., Effect of the light intensity during flowering on stamen development in the strawberry cultivars 'Karina' and 'Sivetta', Sci. Hortic., 12, 343—346, 1980. 23. Haskell, G. and Williams, H., Biometrical variations in flowers of a polyploid series of strawberries, J. Genet., 52, 620—630, 1954. 24. Valleau, W. D., Sterility in the strawberry, J. Agric. Res., 12, 613—669, 1918. 25. Moore, J. N., Brown, G. R., and Brown, E. D., Comparison of factors influencing fruit size in largefruited and small-fruited clones of strawberries, J. Am. Soc. Hortic. Sci., 95, 827—831, 1970. 26. Smeets, L., Effect of light intensity on stamen development in the strawberry cultivar 'Glasa', Sci. Hortic., 4, 255—260, 1976. 27. Anderson, H. M. and Guttridge, C. G., unpublished, 1982. 28. Guttridge, C. G. and Turnbull, J. M., Improving anther dehiscence and pollen germination in strawberry with boric acid and salts of divalent cations, Hortic. Res., 14, 73—79, 1975. 29. Guttridge, C. G., Reeves, J. F., and George, W. W., Scanning electron microscopy of healthy and aborted strawberry anthers, Hortic. Res., 20, 79—82, 1980. 30. Rujput, C. B. S., Studies on the inflorescence of strawberry, Trap. Agric. (Ceylon), 129, 47—51, 1973. 31. Anderson, H. M., unpublished data, 1982. 32. Guttridge, C. G. and Anderson, H. M., Surveys of anther number and quality in 'Redgauntlet' strawberry in England, Hortic. Res., 16, 19—27, 1976. 33. Brown, T. and Wareing, P. F., The genetical control of the everbearing habit and three other characters in varieties of Fragaria vesca, Euphytica, 14, 97—112, 1965.

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34. Ourecky, D. K. and Slate, G. L., Behaviour of the everbearing characteristics in strawberries, Proc. Am. Sot. Hortic. Sci., 91, 236—241, 1967. 35. Bringhurst, R. S. and Voth, V., Breeding strawberries for high productivity and large fruit size, in The Strawberry, Childers, N. F., Ed.. University of Florida, Gainesville, 1981, 156—163. 36. Darrow, G. M. and Waldo, G. F., Responses of strawberry varieties and species to duration of the daily light period, Tech. Bull. U.S. Depi. Auric., p. 453, 1934. 37. Benoit, F., Further observations on the induction of a second flowering in the strawberry cultivar Redgauntlet, Agriculture!. 23, 29—35. 1975. 38. Ito, H. and Saito, T., Studies on the flower formation in the strawberry plants. I. Effects of temperature and photoperiod on the flower formation. Tohoku. ./. Agric. Res., 13, 191—203, 1962. 39. Muijzenberg, E. W. B. van den, The influence of light and temperature on the photoperiodic development of the strawberry and its significance in cultivation, Mecled. Lab. TuinhPlTee.lt, Wageningen, 37, 1942 (in Dutch). 40. Borthwick, H. A. and Parker, M. W., Light in relation to flowering and vegetative development, 13th Int. Hortic. Congr., 1953. 801—810. 41. Austin, M. E., Shutak, V. G., and Christopher, E. P. Response of Sparkle strawberry to inductive cycles. Proc. Am. Soc. Hortic. Sci., 77, 372—375, 1961. 42. Downs, R. J. and Piringer, A. A., Differences in photoperiodic response of everbearing and June-bearing strawberries, Proc. Am. Soc. Hortic. Sci., 66, 234—236, 1955. 43. Heide, O. M., Photoperiod and temperature interactions in growth and flowering of strawberry, Physiol. Plant. 40, 21—26. 1977. 44. Went, F. W., The Experimental Control of Plant Growth, Vol. 17, Chronica Botanica. Waltham, Mass., 1957, chap. 9. 45. Jonkers, H., On the flower formation, the dormancy and the early forcing of strawberries. Meded. Landhouwhogesch. Wageningen, 65—66, I—59, 1965. 46. Hartmann, H. T., Some effects of temperature and photoperiod on flower formation and runner production in the strawberry, Plant. Physiol., 22, 407—420. 1947. 47. Vince-Prue, D. and Guttridge, C. G., Floral initiation in strawberry: spectral evidence for the regulation of flowering by long-day inhibition, Plunta (Berlin), I 10, 165—172, 1973. 48. Gosselink, J, G., The Effect of Photoperiod and Light Quality on the Vegetative and Reproductive Growth of the Strawberry, Ph.D. thesis. Rutgers University. New Brunswick, N.J., 1959. 49. Moore, J. N. and Hough, L. F., Relationships between auxin levels, time of floral induction and vegetative growth of the strawberry, Proc. Am. Soc. Hortic. Sci.. 81. 255—264, 1962. 50. Benoit, F., Induction of second flowering in strawberry cultivar Redgauntlet, J. Hortic. Sci., 47, 429— 439, 1972. 51. Darrow, G. M. and Waldo, G. F., Photoperiodism as a cause of the rest-period in strawberries, Science, 77, 353—354, 1933. 52. Collins, W. B. and Barker, W. G., A flowering response of strawberry to continuous light, Can. J. Bot., 42, 1309—1311, 1964. 53. Guttridge, C. G., unpublished, 1982. 54. Lee, B. Y., Takahashi, K., and Sugiyama, T., Studies on dormancy in strawberry plants. 11. Vegetative and flowering response in Donner variety transferred from the open to a greenhouse at different dates in autumn and winter, J . Jpn. Soc. Hortic. Sci., 39, 232—238, 1970 (Japanese, English summary). 55. Guttridge, C. G., Photoperiodic promotion of vegetative growth in the cultivated strawberry plant. Nature (London), 178,50—51, 1956. 56. Guttridge, C. G., Evidence for a flower inhibitor and vegetative growth promoter in the strawberry, Ann. Bot.. 23, 351—360, 1959. 57. Guttridge, C. G., Further evidence for a growth-promoting and flower inhibiting hormone in strawberry, Ann. Bot., 23, 612—621, 1959. 58. Jahn, O. L. and Dana, M. N., Dormancy and growth of the strawberry plant, Proc. Am. Soc. Hortic. Set., 89, 322—330, 1966. 59. Thompson, P. A. and Guttridge, C. G., The role of leaves as inhibitors of flower induction in strawberry, Ann. Bot., 24, 482-^90, I960. 60. Vince-Prue, D., Photoperiodism in Plants, McGraw-Hill, Maidenhead, 1975, 141. 61. Mason, D. T., Inflorescence initiation in the strawberry. I. Initiation in the field and its modification by post-harvest defoliation, Hortic. Res., 6, 33—44, 1966. 62. Mason, D. T., Inflorescence initiation in the strawberry. II. Some effects of date and severity of postharvest defoliation, Hortic. Res., 7, 97—104, 1967. 63. Ueno, Y., Flowering and vegetative growth of strawberry. III. Influence of intensity of supplemental light on the floral initiation, J. Jpn. Soc. Hortic, Sci., 31, 223—226, 1962 (Japanese, English summary).

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CRC Handbook of Flowering

64. Guttridgc, C. G., Growth, flowering and runncring of strawberry after daylength extension with tungsten and fluorescent lighting, Sri. Hortic., 4. 345—352, 1976. 65. Ueno, Y., Ito, M., and Matsukawa, J., Flowering and vegetative growth of strawberry. II. Influence of the 'light break' effect, J. Jpn. Soc. Hortic. Sci.. 31, 81—85, 1962 (Japanese, English summary). 66. Hartmann, H. T., The influence of temperature on the photoperiodic response of several strawberry varieties grown under controlled environmental conditions. Proc. Am. Soc. Hortic. Sci., 50, 243—245, 1947. 67. Smeets, L., Effect of temperature and daylength on flower initiation and runner formation in two everbearing strawberry cultivars, Sri. Hortic., 12, 19—26, 1980. 68. Arney, S. E., Studies of growth and development of the genus Fragaria, IX. An investigation of floral initiation under natural conditions, Phyton, 7, 89—102, 1956. 69. Zeller, O., Flower development and runner formation of Fragaria ananassa Duch. at various altitudes on the island of Ceylon, An/few. Bat., 43, 159—173, 1969 (German, English summary). 70. Vince-Prue, D., Phytochrome and photoperiodism, in Light and Plant Development, Smith, H., Ed., Butterworths, London, 1976, 347—369. 71. Vince-Prue, D., Effects of photoperiod and phytochrome in flowering: time measurement, in La Physiologic de la Floraison. No. 285, Editions du Centre National de la Recherche Scientifique. Paris, 1979. 9—127. 72. Vince-Prue, D., Guttridge, C. G., and Buck, M. W., Photocontrol of petiole elongation in light-grown strawberry plants, Planta (Berlin), 131, 107—114, 1976. 73. Kadman-Zahavi, A. and Ephrat, E., Opposite response groups of short-day plants to the spectral composition of the main light period and end-of-day red or far-red irradiations. Plant Cell Physiol., 15, 693— 699, 1974. 74. Gurr, E., Synthetic Dyes in Biology, Medicine and Chemistry, Academic Press, London, 1971. 118— 119. 75. Yamada, H., Nakamura, H., and Shimi/u. T., Studies on the effect of light quality on the growth and development of vegetable crops. I. Effects of light quality from white light, after removal of various spectra on the growth of several vegetable crops, Bull. Veg. Orn. Crops Res. Stn. A (Ishinden-Ogoso, Tsu), 3, 43—61, 1977 (Japanese English summary). 76. Kronenberg, H. G., Wassenaar, L. M., and Lindeloof, C. P. J. van de, Effect of temperature on dormancy in strawberry, Sri. Hortic., 4, 361—366, 1976. 77. Guttridge, C. G. and Anderson, H. M., Promoting second cropping in strawberry by avoiding chilling or advancing spring growth, J. Hortic. Sci., 51, 225—234. 1976. 78. Avigdori-Avidov, H., Goldschmidt, E. E., and Kedar, N., Involvement of endogenous gibberellin in the chilling requirements of strawberry (Fragaria x ananassa Duch.}, Ann. Hot., 41, 927—936, 1977. 79. Guttridge, C. G., The effects of winter chilling on the subsequent growth and development of the cultivated strawberry plant, J. Hortic. Sci., 33, 119—127, 1958. 80. Bailey, J. S. and Rossi, A. W., Effects of fall chilling, forcing temperature and daylength on the growth and flowering of Catskill strawberry plants, Proc. Am. Soc. Hortic. Sci., 87, 245—252, 1965. 81. Takai, T., Effective temperature for chilling, and interaction of chilling and photoperiod on growth response of strawberry varieties, Bull. Hortic. Res. Stn. Morioka, Jpn. Ser. C., 6, 91—101, 1970 (Japanese, English summary). 82. Guttridge, C. G., Interaction of photoperiod, chilling and exogenous gibberellic acid on growth of strawberry petioles, Ann. Bot., 34, 349—364, 1970. 83. Voth, V. and Bringhirst, R. S., Fruiting and vegetative response of Lassen strawberries in southern California as influenced by nursery source, time of planting and plant chilling history, Proc. Am. Soc. Hortic. Sci., 72, 186—197, 1958. 84. Craig, D. L. and Brown, G. L., Influence of digging date, chilling, cultivars and culture on glasshouse strawberry production in Nova Scotia, Can. J. Plant Sci., 57, 571;—576, 1977. 85. Kronenberg, H. G. and Wassenaar, L. M., Dormancy and chilling for early forcing, Euphytica, 21, 454—459, 1972. 86. Takai, T., The growth response of strawberry varieties to chilling, Bull. Hortic. Res. Stn. Morioka, Jpn. Ser. C, 4, 73—86, 1966 (Japanese, English summary). 87. Plancher, B., Research on the Effects of Temperature and Daylength on the Arrested Growth of Strawberry, Ph.D. thesis, University of Hanover, Germany, 1974 (German). 88. Breen, P. J. and Martin, L. W., Vegetative and reproductive growth responses of three strawberry cuitivars to nitrogen, J. Am. Soc. Hortic. Sci., 106, 266—272, 1981. 89. Lineberry, R. A., Burkart, L., and Collins, E. R., Fertilizer requirements of strawberries on new land in North Carolina, Proc. 'Am. Soc. Hortic. Sci., 45, 283—292, 1944. 90. Whitehouse, W. G., Nutritional studies with the strawberry, Proc. Am. Soc. Hortic. Sci., 25, 201—206, 1928.

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9 1 . Stadelbacker, G. J., Why so much variation in strawberry fertilizer recommendations and practices. '/'/;err\ Con/'.. Rutgers University. New Brunswick. N. J . . 1963. 81—85. 92. Way, D. W. and White, G. C., The influence of vigour and nitrogen status on the fruitfulness of Talisman strawberry plants, J. Hortic. Sci., 43. 409—419, 1968. 93. Abbott, A. J., Growth of the strawberry plant in relation to nitrogen and phosphorous nutrition,./. Horiic. Sci.. 43, 491—504, I96X. 94. Mason, D. T., Physiological investigations on the strawberry. Nutrition, 9th Anna. Rep. Scot. Hortic. Res. lust., p. 43, 1962. 95. Naumann, von W-D., Timing of irrigation and fruit bud differentiation of strawberry. Gartenhauwissenschaft, 29, 21—30, 1964 (German). 96. Webb, R. A., Purves, J. V., Beech, M. G., and Terblanche, J. H., Nutrition of strawberries, Anna. Rep. Long Ashton Res. Stn., p. 43—44, 1976. 97. Thompson, P. A. and Guttridge, C. G., Effect of gibberellic acid on the initiation of flowers and runners in the strawberry. Nature (London), 184. B.A. 72—B.A. 73, 1959. 98. Guttridge, C. G. and Thompson, P. A., The effect of gibberellins on growth and (lowering of Fragaria and Duchesnea, J. Exp. Bot., 15, 631—646. 1964. 99. Tafazoli, E. and Vince-Prue, D., A comparison of the effects of long days and exogenous growth regulators on growth and flowering in strawberry, Fragaria X (mantissa Duch., J. Hortic. Sci.. 53, 255—259, 1978. 100. Dennis, F. G., Jr. and Bennett, H. O., Effects of gibberellic acid and deflowering upon runner and inflorescence development in an everbearing strawberry, J. Am. Soc. Hortic. Sci.. 94, 534—537, 1969. 101. Render, W. J., Carpenter, S., and Braun, J. W., Runner formation in everbearing strawberry as influenced by growth-promoting and inhibiting substances, Ann. Bot., 35, 1045—1052, 1971. 102. Guttridge, C. G., Hormone physiology of growth regulation in strawberry, in Plant Growth Regulators, Monograph No. 31, Society of Chemical Industry, London, 1968, 157—169. 103. Smith, C. R., Soczek, Z., and Collins, W. B., Flowering and fruiting of strawberries in relation to gibberellins, in Adv. Chem. Ser., 28, 109—115, 1961. 104. Guttridge, C. G., Physiological investigations on the strawberry, Gibberellic acid. 9th Annit. Rep. Scot. Hortic. Res. Inst., 39—41, 1962. 105. El-Antably, H. M. M., Wareing, P. F., and Hillman, J., Some physiological responses to d.l. abscisin (Dormin), Planta (Berlin), 73, 74—90, 1967. 106. Guttridge, C. G., The effects of (2-chloroethyl) trimethylarnmonium chloride on growth and runnering of strawberry plants, Hortic. Res., 3, 79—83, 1964. 107. Guttridge, C. G., Anderson, H. M., and Stewart, W. S., The control of strawberry runners in the field with CCC, Exp. Hortic., 15, 92—95, 1966. 108. Mason, D. T., unpublished, 1963. 109. Guttridge, C. G., Physiological investigations on the strawberry. AMO 1618 — a growth retardant. 9th Annu. Rep. Scot. Hortic. Res. Inst., 48—49, 1962. 110. Thompson, P. A., Some factors regulating the use of maleic hydrazide for runner control in strawberry plantations, Hortic. Res., 1, 29—36, 1961. 1 1 1 . Dennis, F. G., Jr., Lipecki, J., and Kiang, C. L., Effects of photoperiod and other factors upon flowering and runner development of three strawberry cultivars, J. Am. Soc. Hortic. Sci., 95, 750—754, 1970. 1 1 2 . Mason, D. T., Field investigations on the strawberry. Seasonal growth patterns, 10th Annu. Rep. Scot. Hortic. Res. Ins!., 51, 1963. 113. Darrow, G. M. and Borthwick, H. A., Fasciation in the strawberry, J. Hered., 45, 298—304, 1954. 114. Thompson, P. A., Evidence for a factor which prevents the development of parthenocarpic fruit in strawberry, J. Exp. Bot., 12, 199—206, 1961. 1 1 5 . Goodwin, P. B. and Gordon, A., The gibberellin-like substances and growth inhibitors in developing strawberry leaves, J. Exp. Bot., 23, 970—979, 1972. 116. Eberhardt, U., Studies on the Role of Endogenous Gibberellins in Strawberries, Doctorate thesis, University of Hohenheim, Germany, 1977 (German). 1 1 7 . Staudt, G. and Hummel, A., Flower formation in Fragaria vesca mutation arhorea after treatment with N,N-dimethylaminosuccinamic acid, Z. Pflanzenphysiol., 67, 297—304, 1972 (German, English summary).

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CRC Handbook of Flowering

FREESIA X HYBRIDA En., Fr., Ge., Sp., Freesia Terry L. Gilbertson-Ferriss

INTRODUCTION Freesia x hybrida was developed from species native to the Cape Province area of South Africa. Most of the species are found around the 33rd parallel. Freesias are endemic to a wide variety of environments. They are found growing from dry sandy plains to the edges of rivers.15 In their native habitat, freesias sprout in the autumn (February to March), and flower in winter (July to August) at 8 to 10°C. After flowering the plants die down and the corms "ripen" during the hot (3I°C), dry summer. 1 ' 2 Freesias first appeared in Europe about 1759 and became one of the more popular plants in horticulture in the last half of the 19th Century. Klatt established the genus in 1866.' Freesia x hybrida probably originated from several species but which species are in its genetic background is rather controversial.7 The plant has a wide range of flower colors including white, yellow, red, and blue. The flowers have a strong but pleasant scent. Interestingly, one person in nine is unable to smell freesias even though they are highly scented.7 Freesia is predominantly grown as a greenhouse cut flower crop. Outdoor growing of some cultivars was popular for a time, but disease problems have substantially decreased the commercial popularity of garden freesias. During the 1900s freesia breeders significantly improved the stem strength, flower size, color range, and disease resistance of greenhouse cvs. Uniform quality was not easily attained as freesias are essentially self-sterile and the offspring from cross-fertilized parents are somewhat variable. 7 However, several diploid, triploid, tetraploid, double-flowered and F, hybrid cvs were introduced that could be propagated from seed or corm. Beginning in the 1970s breeders have also worked to develop cvs conducive to year-around flowering rather than just the traditional winter and spring flowering periods.7

PLANT STRUCTURE The leaves are lanceolate with an acute apex, generally erect and arranged in a fan. The rootstock is a tunicate corm of several internodes.

FLORAL MORPHOLOGY The freesia inflorescence is a cymose spike with sessile flowers borne along the upper part of the scape. The scape bends at a semiperpendicular angle at the base of the inflorescence (Figure 1). Each floret on the spike has membranous or dark green bracts, 6 tepals which are arranged in a broadly funnel-shaped tube, 6 stamen, and a bifid branched style. The flowering stalk may be branched.

CULTURE Freesia for commercial cut flower production can be grown from seed or corms. Seedraised freesias require 7 to 8 months from seed to flowering; corms require about 5 months from planting to flowering.

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FIGURE 1.

35

Frecsia inflorescence.

Seeds are germinated at 15 to 18°C media temperature in the dark for rapid, uniform germination in about 21 days; 4 to 5 weeks after seeding, when seedlings are about 5 to 6 cm tall, they are transplanted into deep flats, benches, or pots for forcing. Grow at 21°C days and 18.5°C nights until there are 5 to 7 visible leaves. Then lower to continuous 13°C for floral initiation (FI) and flowering. Some growers have used night temperatures down to 7°C. The planting of corms can be staggered over several weeks to provide a longer flowering period. After corm planting, freesias are grown at 21°C days and 18.5°C nights until the plants develop 6 to 7 visible leaves, then at continuous 13°C for FI and development. Freesia corms can be held over for the next year's production. After flowering, gradually withhold water and fertilizer, thus allowing the plants to die down naturally. After digging, corms are then stored for 13 weeks at 31°C. Many European growers store the corms at 13°C for 3 weeks after the 31°C storage to hasten flowering. 7 " This 13°C storage treatment has not always been as successful in the U.S., however. 4

DORMANCY The dormancy of freesia corms does not directly affect flowering since the flower is initiated after sprouting. However, corm dormancy is important to flowering as corm sprouting is a prerequisite for flowering. Upon completion of flowering the plant gradually senesces. Traditionally, the corms are then dug and held at 38 to 31°C for 10 to 13 weeks to insure rapid shoot emergence when replanted. 2 Storage at 3 to 5°C immediately after harvest maintains corm dormancy. 2 When stored at 13°C, an interesting physiological phenomena called pupation occurs2 in which a new corm forms on top of the old corm. This process takes about 8 months and results in a corm which is also dormant. Gilbertson-Ferriss et al. 4 suggest that freesia corms exhibit a physiological dormancy for 4 to 6 weeks which can be broken by storage at 30°C. In Japan, the exposure of corms to smoke from burned rice husks for 3 days during the high temperature storage has been commonly used to uniformly release corms from dor-

36

CRC Handbook of I-'lowering

mancy. The C,H4 and CO present in the smoke are believed to be the effective gaseous components. Masuda and Asahira 1 3 - 1 4 reported that exposure of corms to CH4 and/or CO during or after a partial high temperature treatment promoted corm sprouting.

EFFECT OF TEMPERATURE Temperature is the causal factor in flower initiation and development in F. X hybrida. Hartsema* has described the apical changes associated with FI and subsequent inflorescence development. Complete differentiation requires 6 to 9 weeks at 12 to 15°C, depending upon environmental conditions, 9 and can be assumed to be irreversible after 4 weeks at 13°C. I() Freesias will initiate flowers over a range of temperatures (5 to 20°C) with 12 to 15°C being the optimum, 1 '• 1?-

t£ I jf.

•J 1

'' ,'. JWj

FIGURES I to 6. Development of the vegetative shoot apex of cv Sunfola 68/2 during first 16 days of growth. Figures 1 and 2 — 4 days after planting, opposite leaf primordia, small apical meristem; Figure 3. X days — leal primordia arranged at an angle giving rise to spiral phyllotaxis, large cells at central zone of apical meristem: Figure 4. 12 days — Hanks angular due to emerging leaf primordia, cells between young leaf primordia are arranged in radial files; Figures 5 and 6 — 16 days, vegetative apex with a flat apical meristem, differentiation of a hairy midrib and a marginal meristem on largest leaf primordium. A, apical meristem; L, leaf primordium. (Photographs courtesy of Drs. i. Marc and J. Palmer, School of Biological Science, University of Sydney, Sydney. Australia.)

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CRC Handbook of Flowering



'" '



;. Bol.. 31. 1657, 1980. With permission.

initiated at nodes higher up the stem at warmer temperatures. In the 'H.10', flower buds appeared on the plants in both photoperiods at the lowest temperatures of 25/20° only. Despite the fact that floral buds were initiated in all three varieties, flower opening occurred in 'Pusa Sawani' only and the time of flower opening was generally less at warmer temperatures of 35/30° than either 30/25 or 25/20° in the 12-hr photoperiod; and those of 30/ 25° than 25/20° in the 15-hr photoperiod. Flower opening was completely prevented at the warmest temperatures of 35/30° in the 15-hr photoperiod. Similarly, Oyolu29 showed that at a given photoperiod flower opening was delayed by exposure to warmer temperatures; and if warmer temperature was combined with longer photoperiod, flower buds were either not initiated or those initiated abscised without opening. When okra plants at the pre-flowering stage were subjected to low temperatures of 5° and 10°C for less than 96 hr, they suffered chilling injury, and when the low temperature treatments were prolonged to 144 hr, the plants died back to the lower nodes and the survivors among them had delayed flower bud initiation as well as reduced number of flower buds. 28

EFFECTS OF PLANT AGE The node at which the first flower bud appeared increased with increasing plant age in plants subjected to SD (Table 2). However, the time to appearance of the first flower bud, the total number of flower buds produced on each plant, as well as the time to first flower bud opening were not affected by the plant age (Table 2). The minimum number of 10-hr SD cycles required for the initiation of flower buds in all treated early okra plants was 6 if the SD treatment started at the time of cotyledon release following seed germination.26 However, if the SD started when the plants were 5 and 10 or more days old, a minimum of 5 and 4 SD cycles were needed, respectively.27 Likewise, the effectiveness of the inductive

Volume III

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t e m p e r a t u r e s , °C

FIGURE 4. Effect of photoperiod and temperature on the number of days from seeding to flower bud appearance in three cvs of okra. ('H.10' failed to show floral buds in the photoperiod: day/night temperature regimes of 12: 35/30; 15: 30/25; and 15: 35/30, at the end of 56 days.) (From Arulrajah, T. and Omrod, D. P., Ann. Bot.. 37, 331, 1973. With permission).

SD cycles in supporting flower bud development increases with plant age. For example, even though flower buds were initiated in early okra plants given 20 SD (10-hr photoperiod) cycles right from the time of cotyledon release, these buds underwent little or no development and abscised eventually. 26 However, if the same 20 SD cycles were given to plants aged 10 or more days, the first formed flower bud, and sometimes the next one also, would develop and eventually open in all the plants. 27 GENETIC EFFECTS Along with other characters such as plant height, and fruit number and weight, the number of days to flower formation and opening has high heritability30 and has been variously assessed in diverse parents, their F, and F2 hybrids, as well as their backcrosses (BC, and BC2) with a view to developing high yielding lines by selection and exploiting hybrid vigor. In general, F, hybrids exhibited positive heterosis for precocity of flowering (flower opening)14-39-48 and the hybrid variance was found to be highly significant for days to flower opening in the F2, while the variances for the specific combining ability (SCA) in the F2 and BC2 generations and for the general combining ability (GCA) in the BC, generation were also highly significant for days to flower opening. 20 However, Singh and Singh42 obtained limited heterosis for days to flower opening in studies involving 125 F, hybrids derived from crossing 25 female lines with 5 pollen parents. Studies aimed at estimating the gene effects in crosses of H. esculentus showed that both additive and dominance gene actions are important in controlling days to flower opening. 2,19,21,41.46 Also, a diallel cross analysis involving 7 varieties or lines and their 21 F,

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Table 2 EFFECT OF PLANT AGE ON THE INITIATION AND DEVELOPMENT OF FLOWER BUDS IN EARLY OKRA PLANTS SUBJECTED TO SD (10 HR) TREATMENT THROUGHOUT Plant age at start of SD treatment (days) 5

0

Time to first flower bud appearance (days) Node of first flower bud No. of flower buds per plant Time of first flower bud opening (days) •'

20 + 1.,

20

-+- 7

10

20

30

18 + 2

20 -+- 2

19 -j- 2

12.1 ± 0.3

13.4 ± 0.2 4.8 ± 0.2

60.0 ± 0 4.8 ± 0.2

7.4 ± 0.2 4.9 ± 0.2

9.1 ± O . I 4.9 ± 0.1

44 ± 1

43 ± 1

42 ± 1

4.8 ± 0.1

44 ±

1

45 ± 2

Mean ± SE.

From Nwoke, F. I. O., unpublished.

hybrids (excluding reciprocals) showed that both additive and nonadditive variances were highly significant for all the 5 components of yield examined including days to 50% flower opening, and whereas the estimate of dominance showed a partial dominance for days to 50% flower opening, the estimated heritability was also found to be high for days to 50% flower opening, indicating that this character could be improved in favor of earliness through selection.13 However, Kulkarni et al. ' 9 found that dominance gene action was more important than the additive type of gene action for the three characters they examined including days to flower opening. Polyploid plants of H. esculentus obtained by treatment with colchicine exhibited delayed flower opening,32 and this confirms the well-known fact that polyploids tend to have a longer vegetative phase than their diploid progenitors, due to their slower growth rate.7 EFFECTS OF GROWTH REGULATORS Sex expression in H. esculentus is affected by growth regulators in such a way that treatment with auxins, ethephon, and growth retardants tends to shift the balance of sex towards femaleness. 5 - 6 -"- 50 Sprays of auxins, 2,4-D, NAA, and 2,4-dichlorophenoxybutyric acid induced partial or complete male sterility,'' -50 while ovular sterility was also temporarily reduced." Maleic hydrazide (4000 ppm) most effectively produced complete male sterility no matter whether it was applied before or after flower bud initiation. 50 Ethephon (100 to 300 ppm) suppressed the growth and number of stamens and induced the formation of pistillate flowers at concentrations of 400 and 500 ppm. 5 Chlorflurenol sprays suppressed the growth of male sex organs, and in some cases complete abortion of the stamens occurred leading to the formation of pistillate flowers.6 The number of such pistillate flowers increased as the concentration of chlorflurenol was increased from 1 to 1000 ppm. However, these pistillate flowers always abscised two or three days after opening. Sankhla37 has suggested that morphactin interferes with auxin transport18 in such a way as to maintain a raised auxin level in the developing flower buds leading to the female tendency of flowers. The growth retardant, chlormequat, applied either to the soil23 or by soaking the seeds in solutions of CCC for 24 hr before sowing,16'24 delayed flower bud initiation. Similarly, treatment of seeds with MH before sowing was found to delay flower opening.10 On the other hand, Alar (daminozide) was shown to hasten flower bud initiation as well as flower

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opening no matter whether it was applied once or twice to either stopped (apical bud removed to generate side branches and produce more flowers) or nonstopped plants. 13 Foliar application of various concentrations of NAA ranging between 5 and 50 ppm was shown to delay flower opening by 3 or more days,41 while seed treatment with IAA or NOA ((3-naphthoxyacetic acid) (10 to 400 ppm) before sowing, showed that the number of flower buds increased as the concentration of the auxins was increased. 31 The number of branches was also observed to increase with increasing concentration of the auxins and, therefore, the parallel increase in the number of flower buds could probably be attributed to the presence of additional sites for flower buds on these branches. Application of GA3 to seeds before sowing hastened flower opening10 and brought about an increase in the number of flower buds due, possibly, to an increase in the growth of branches. 10 - 31

REFERENCES 1. Arulrajah, T. and Omrod, D. P., Responses of okra (Hibiscus esculentus L.) to photoperiod and temperature, Ann. Bot., 37, 331—340, 1973. 2. Arumugam, R. and Muthukrishnan, C. R., Gene effects on some quantitative characters in okra, Abelmoschus esculentus, Indian J . Agric. Sci., 49, 602—604. 1979. 3. Benjamin, H. B., Ehrig, H. K., and Roth, D. A., The use of okra as a plasma replacement, Rev. Canadienne Bwl.. 10,215—220. 1951. 4. Benjamin, H. B., Ehrig, H. K., and Roth, D. A., The use of a plasma extender in animals, Proc. Am. Vet. Med. Assoc.. 1951, 193—196. 5. Bisaria, A. K., Growth, sex expression and yield of okra (Abelmoschus esculentus) treated with ethephon, Philipp. Agric.. 62. 316—320, 1979. 6. Bisaria, A. K. and Bisaria, A. K., Jr., Effect of chlorflurenol on sex expression, fruit set and yield in Abelmoschus esculentus (L.) Moench., J. Exp. Bot.. 27, 337—340, 1976. 7. Briggs, F. N. and Knowles, P. F., Introduction to Plant Breeding, Reinhold, New York, 1967, 268— 270. 8. Churata-Masca, M. G. C., Efeito do fotoperiodo controlado no quinbeiro (Abelmoschus esculentus (L.), Cientifica, 3, 81—86, 1975. 9. Cobley, L. S. and Steele, W. M., An Introduction to the Botany of Tropical Crops, 2nd ed.. Longman, London, 1976, 146—148. 10. Das, R. C. and Pattanaik, A., Studies on the effect of growth regulator treated okra seeds (Abelmoschus esculentus L., M.) with respect to growth and subsequent development, Indian J. Hortic., 28, 293—295, 1971. 1 1 . Dubey, R., Embutox (2,4-dichlorophenoxybutyric acid) — a new phytogametocide, Curr. Sci., 41, 297— 298, 1972. 12. Epenhuijsen, C. W., van.. Growing Native Vegetables in Nigeria, Food and Agriculture Organization, Rome, 1974, 6 and 65—67. 13. Godffrey -Sam-Aggrey, W. and Ndoleh, A. S., Effects of Alar on growth, flowering and yield of okra, Expl. Agric., 14, 121—128. 178. 14. Jalani, B. S. and Graham, K. M., A study of heterosis in crosses among local and American varieties of okra (Hibiscus esculentus L.), Malaysian Agric. Res.. 2. 1—14, 1973. 15. Joshi, A. B., Singh, H. B., and Joshi, B. S., Why grow 'Pusa Sawani'?, Indian Fmg., 10, 6—7, 1960. 16. Khan, K. A. and Haq, A., Effects of 2-chloroethyltrimethyl ammonium chloride on the yield of okra (Abelmoschus esculentus), Pak. J. Bot., 10, 157—160, 1978. 17. Khanna, K. L. and Charcravarti, A. S., Studies in the clarification of sugarcane juice in gur. manufacture, Indian J. Agric. Sci., 19. 137—161. 1949. 18. Krelle, E. and Libbert, E., Wirkung eines Morphaktins aut die Amylase-Synthese im Gerstenendosperm, Planta. 76, 179—181, 1967. 19. Kulkarni, R. S., Rao, T. S., and Virupakshappa, K., Genetics of important yield components in bhindi, Indian J. Genet. Plant Breed.. 38. 160—162, 1978.

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20. Kulkarni, R. S., Rao, T. S., and Virupakshappa, K., Combining ability in bhindi. Genet. Agrar., 32. 245—258. 1978. 21. Kulkarni, R. S., Swamy Rao, T., and Virupakshappa, K., Note on the inheritance of quantitative characters in okra, Indian ,/. Agric Sci., 48, 499—501. 1978. 22. Martin, F. W. and Ruberte, R., Okra. Abelmoschus esculentus: Vegetables for the hot. humid tropics. Pt. 2, U.S. Department of Agriculture, New Orleans, Louisiana, 1978, I—22. 23. McGinty, R. A. and Barnes, W. C., Observations on flower bud and pod development in okra, Proc. Am. Six: Honk: Sci. 29, 509—513, 1932. 24. Mehrotra, O. N., Garg, R. C., and Singh, I., Effect of CCC (2-chloroethyl trimethyl ammonium chloride) on growth and yield of okra (Abelmoschus esculentus L. Moench.), Indian J. PI. Physiol.. 13, 173—179, 1970. 25. Njoku, E., The photoperiodic response of some Nigerian plants, J. W. Afr. Sci. Assoc., 4, 99—111. 1958. 26. Nwoke, F. I. O., Effect of number of photoperiodic cycles on flowering and fruiting in early and late varieties of okra (Abelmoschus esculentus (L.) Moench.), J. Exp. Bot., 31, 1657—1664, 1980. 27. Nwoke, F. I. O., unpublished data, 1980. 28. Omran, R. G. and Powell, R. D., Chilling injury in okra (Hibiscus esculenlus L.) in relation to plant development and nitrogen metabolism. Plant and Soil, 35, 357—369, 1971. 29. Oyolu, C., Variability in photoperiodic response in okra (Hibiscus esculentus L.), Acta Hortic., 53, 207— 215, 1977. 30. Padda, D. S., Saimbhi, M. S., and Singh, D., Genetic evaluation and correlation studies in okra, Indian J. Hortic., 27, 39—41, 1970. 31. Pal, N., Chauhan, K. S., and Pundrik, K. C., Effect of gibberellic acid, indole-3-acetic acid and betanaphthoxy-acetic acid as a presowing seed treatment on germination, vegetative growth and yield of okra. Punjab Hortic. J., 10, 155—160, 1970. 32. Patil, J. A., Polyploidy in vegetable crops — okra (Abelmoschus esculentus), Poona Agric. Coll. Mag., 54, 20—25, 1963. 33. Partap, P. S., Dhankhar, B. S., and Pandita, M. L., Genetics of yield and its components in okra. Indian J. Agric: Sci., 50, 320—323, 1980. 34. Pereira, A. A., Couto, F. A. A., and Maestri, M., Influencia do fotoperiodo na floracao do quiabo (Hibiscus esculentus L.) Rev. Ceres, 18, 131—138, 1971. 35. Purseglove, J. W., Tropical Crops: Dicotyledons, Vol. 1 and 2 combined, Longman, London, 1974, 368—370. 36. Raynard, G. B. and Porter, D. R., Emerald okra, Seed World, 67, 38, 1950. 37. Sankhla, N., Effect of morphactin on flower sex expression in Nicotiana paniculata, Z. Pflanzenphysiol., 61, 350—352, 1969. 38. Schweers, V. H. and Sims, W. L., Okra Production, University of California Cooperative Extension Publication, USDA, AXT-419, 1974, 1—5. 39. Sharma, B. R. and Mahajan, Y. P., Line X tester analysis of combining-ability and heterosis for some economic characters in okra, Sci. Hortic:, 9, 111—118, 1978. 40. Singh, H., Ever growing popular — Pusa Sawani bhindi, Indian Fmg., 13, 14—23, 1963. 41. Singh, S. P. and Singh, H. N., Line x tester analysis in okra (Abelmoschus esculentus), Indian J. Agric. Sci., 49, 500—504, 1979. 42. Singh, S. P. and Singh, H. N., Hybrid vigour for yield and its components in okra (Abelmoschus esculentus), Indian J. Agric. Sci., 49, 596—601, 1979. 43. Singh, K. and Upadhyay, S. K., Effect of soil and foliar application of naphthalene acetic acid (NAA) on the vegetative growth and yield of okra (Abelmoschus esculentus (L.) Moench.), Indian J. Agron., 12, 42^*5, 1967. 44. Spivey, C. D., Woodward, O. J., and Woodward, W. D., The production of okra in south Georgia, Bull. Ga Agric. Exp. Stn., 44, 1—39, 1957. 45. Srivastava, V. K., Studies on floral biology of Abelmoschus esculentus (L.) Moench., Indian J. Hortic., 21, 165—169, 1964. 46. Swamy Rao, T. and Sathyavathi, G. P., Genetic and environmental variability in okra, Indian J. Agric. Sci., 47, 80—81, 1976. 47. Tomoda, M., Shimada, K., Saito, Y., and Sugi, M., Plant mucilages: 26. Isolation and structural features of a mucilage, okra-mucilage F, from the immature fruits of Abelmoschus esculentus, Chem. Pharm. Bull. (Tokyo), 28, 2933—2940, 1980. 48. Venkataramani, K. S., A preliminary study of some intervarietal crosses and hybrid vigour in Hibiscus esculentus L., 7. Madras Univ., Sect. B, 22, 183—200, 1952.

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49. Venkataramani, K. S., Some observations on blossom biology and fruit formation in Hibiscus cxcitleiitux, J. Madras Univ., Sect. B, 23, 1 — 14, 1953. 50. Verma, R. B. and Singh, G. N., Studies on chemical induction of male sterility in Bhindi (Abelmoschus exculentux ( L . ) Moeneh.). Indian J. AKric. Rex., 12, 32—34, 1978. 51. Woodroof, J. G., Okra, Georgia Exp. Stn. Bull.. 145, 164—185. 1927.

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HORDEUM VULGARE En. Barley; Fr. Orge; Ge. Gerste; Sp. Cebada Gerald F. Deitzer

INTRODUCTION Barley is one of the oldest known crops to be cultivated by man. The earliest definitive evidence for fully domesticated barley was found at Jarmo in Iraq-Kurdistan 54 w> and is about 9000 years old. This appears to have been a 2-rowed subspecies similar to the modern wild subspecies Hordeum spontaneum C. Koch. A recent report84 has provided evidence that barley may have been cultivated much earlier. The evidence consists of carbonized seeds and glumes of barley associated with a late paleolithic site at Wadi Kubbaniya, near Aswan in southern Egypt. This site has been radiocarbon dated at between 17,000 to 18,300 years of age. While it is not possible to unequivocally establish, from the limited sample of barley found, whether or not it was domesticated, it is clear that it flourished there through the intervention of man. The most likely progenitor of modern cultivated barley (Hordeum vulgare L.) appears to be H. spontaneum C. Koch, an annual herbaceous plant with a 2-rowed spike that grows in much of the Middle East.' 2 A 6-rowed wild subspecies, known as H. agriocrithon Aberg, was thought to have also been a candidate.65 However, it is now considered that this subspecies also arose from a 2-rowed progenitor, since the 6-rowed allele is recessive and is more likely to have arisen by mutation than the reverse. 36 - 70 Takahashi 72 has suggested that there may have been a diphyletic origin for cultivated barley in the Orient. In any case, the differences between the wild and cultivated forms are quite minor, and they are now generally accepted as constituting a single species with a common cytogenetic stock. 56 They all have the same chromosome number (2n = 14) and karyotype, produce fertile hybrids, and for the most part exhibit only single gene differences. All forms are characterized by three spikelets at each node of the spike and when the lateral spikelets are much reduced, male or neuter, it is a 2-rowed form, while fertile lateral spikelets produce the 6-rowed forms. The central spikelet in both is female fertile and usually bears a caryopsis. While cultivated barley is adapted to a wide range of ecological and climatic conditions, the wild forms are very much more restricted. For example, H. spontaneum is limited to the Near East and western North Africa.-15-16-72 it is found along roadsides, around the edges of both abandoned and cultivated fields, 88 - m and is well adapted for this climate because of its brittle rachis and its spear-shaped, one-seeded dispersal unit. Commercial cvs of H. vulgare, on the other hand, are grown as far north as 65° latitude in the U.S.S.R., 6 at elevations from 4000 m in the Himalaya Mountains to 350 m below sea level at the Dead Sea, as well as at equatorial latitudes. 16 This review will be restricted, almost exclusively, to the commercial cvs of//, vulgare, the only exception to this being a description of work done with the wild barley H. bulbosum. This wild form, unlike other wild subspecies like H. spontaneum may constitute a separate species since it is used commercially by breeders to produce male sterile hybrids.

FLORAL MORPHOLOGY Floral development of the main stem in H. vulgare. cv. Wintex is depicted in Figure 1 but most of the description and Figure 2 were taken from Bonnett9 for both 2-rowed and 6rowed cvs. The first change indicative of the transition to flowering is a noticable elongation

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FIGURH 1. Development of the floral apex in Hordcum vulgar? L. cv Wintex. (A) Floral stage 1. single ridges, sp = spikelet primordia. (B) Floral stage 2, double ridges ( D R ) . (C) Floral stage 3. 2 rows of spikelet initials (SI). (D) Floral stage 4, 6 rows of spikelet initials. 4 lateral spikelets (LS) and 2 central spikelets (CS). (E) Floral stage 5. three anther ( A N ) primordia and awn (AWN) initials visible on each developing spikelet. (F) Floral stage 7, elongating rachis internodes with awns now covering lemmas, but anther primordia ( A N ) arc still visible-above the awns (AWN).



r^ 3= r}

S: a ?7-

o ^ r^

y^

FIGURE I E

FIGURE I F

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FIGURE: 2. Floret development of 2- and 6-rowed barley. (A) Spike of a 2-rowed barley stem showing the rachis and a side view of anthers ( a n ) , awns (a) and side spikelets, ( s ' . l (B) Spike of a 6-rowed barley stem showing awn (a) development and comparative development of side Is') and eentral (s) spikelets, as well as the undifferentiated. meristematic tip ( t ) . (C) Spike of a 2-rowed barley stem showing the differentiation of the tip ( t ) , anthers (an), empty glumes (e) of the central spikelet and an empty glume (e') of a side spikelet, and awns ( a ) . (D) Portion of a spike showing the collar (c) at the base of the spike and the second node of the rachis ( x ) . (E) Spikelet of a 2-rowed barley showing the pedicel (pe). side spikelet (s'), style (st), empty glumes (e), anthers (an), and awn (a) development. (F) Spikelet of a 6-rowcd barley showing the side spikelets (s') with empty glumes (e) and the central spikelet with anthers (an), style (st), and awns ( a ) . (G) Spike of a 2-rowed barley showing the side spikelets (s') and their anthers (an). (H) Spikelet prior to pollination with ranchis ( n ) , lodicules (lo), ovary (o), empty glumes (e), stigma (st). anther (an), and palea (p). (I) Spikelet after pollination with the same structures as in H. (From Bonnett, O. T., J. Agric. Res., 51, 451—457, 1935. With permission.)

of the apex (Figure 1A). The vegetative apex is small and hemispherical, and measures only 0.1 to 0.3 mm in height. Even before spikelet primordia are seen as double ridges (Figure IB) the apex has about doubled in size (0.4 to 0.6 mm). This is not, however, a reliable characteristic since some environmental conditions (e.g., 12-hr photoperiods in fluorescent light) can produce vegetative apices as large as about 0.4 mm. This elongated apex has been termed floral stage 1, since it contains single ridges, but it must be regarded as still vegetative until such time as clear double ridges are formed (floral stage 2). The double ridges are initially about the same size, but the upper ridge grows more rapidly and will give rise to the spikelet initials (Figure 1C). The lower ridge probably becomes the internode of the rachis since apparently all of the spikelet structures arise from the upper pair of ridges. The

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FIGURE 2 (continued)

internode, however, does not elongate until the spikelets have almost completed their differentiation. The unequal growth of the upper and lower portions of the double ridges result in a separation of the spikelet primordia into two rows that are laterally opposed. This is referred to as floral stage 3 (Figure 1C). The next distinguishable morphological change occurs when two slight depressions appear in the transverse meristematic ridge. Growth occurs on both sides and between the two furrows in preparation for the differentiation of the spikelet parts. Floral stage 4 (Figure ID) is defined by the growth of the lateral spikelets, giving rise to the 6-rowed character of the apex; two-rowed species also develop lateral primorida at this stage, but later development is restricted and functional anthers usually do not develop. The first structure of the spikelet to develop is the lemma. It appears as a distinct ridge across the spikelet primordia (no photograph) first on the central spikelets and, usually, somewhat later on the lateral spikelets. Differentiation of the palea follows that of the lemma but it is hidden by other spikelet parts and can be seen only in section. Soon 3 little papillae appear on the central spikelets above the lemma. These will become the anther primordia seen in floral stage 5 (Figure IE). The stamen remains sessile and consists almost exclusively of autheridial tissue throughout development and will, therefore, be referred to hereafter simply as anther or anther primordia. The pistil is formed from that portion of the meristem located between the anthers (Figure 2, E to F), but its differentiation lags considerably behind that of the anthers. The ovary develops first, followed by the styles and finally the stigma (Figure 2, H to I). At this point the internodes of the rachis begin to elongate and the degree and speed of elongation will ultimately determine the density of the mature spike. From this point onward, the various cvs of barley begin to diverge since spike density is one of the characters used in their classification. Floral stage 7 (Figure IF) shows the start of this elongation as seen from side view, looking at the lateral spikelets. Awn development begins at floral stage 6 (best seen in the basal portion of Figure IE) and by stage 7 has generally covered the lemmas, obscuring much of the further development from view in gross dissection (Figure 2, B-C). Also following this stage the glumes begin to grow, finally enclosing the developing anthers and pistil (Figure 2, E-F). The remaining stages (8 to 10) differ markedly from one cv to another, but generally involve the elongation of the awns and internodes.

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The awn begins its development as an outgrowth from the lemma (Figure 2A). The awns and anthers grow quite rapidly and, together with the empty glumes, are soon the most conspicuous of the spikelet structures (Figure 2, B-C). The awn grows much more rapidly than the lemma and palea, which remain short with the anthers extending above them. About the time that the last internode of the stem begins to elongate the glumes begin their growth and finally enclose the anthers and pistil. It should also be mentioned that, in barbed cvs of barley, the barbs can be seen on the awn at an early stage. At the base of the spike is a structure called the collar (Figure 2D). This structure is a distinct ridge of tissue circling the stem at the first node of the spike. A similar but less prominent ridge of tissue is found at the first node above the collar (Figure 2D). The collar and the ridge at the node above are formed by 2 leaf initials that are just beginning to differentiate but are not far enough along to continue their development as leaves at the time spikelet differentiation begins. They are arrested in their development and form the structures mentioned. While spikelets develop at the collar, they are late and usually rudimentary and sterile. A spike of a 2-rowed barley that has almost completed its development is seen in Figure 2G. The awns on the central spikelets are well developed, but not fully grown and the anthers protrude well beyond the glumes. The side spikelets are small, without awns on the lemma, and rudimentary anthers extend slightly beyond the glumes. What are very prominent are the empty glumes upon which the barbs can be seen. Barbs can also be seen on the awns. A spikelet of a 2-rowed (Figure 2E) and a 6-rowed barley (Figure 2F) are shown for comparison. Each spikelet contains a single floret. The athers and stigmas in the spikelets in the 6-rowed and in the central spikelet of the 2-rowed barley, extend above the flowering glumes. However, just before the head emerges from the boot, the lemma and palea, which up to this time have grown very slowly, begin to grow rapidly and soon enclose the anthers and stigmas. The anthers in the side spikelets of the 2-rowed barley have been enclosed by the flowering glumes. The side spikelets of the 2-rowed barley are much smaller than the central spikelet, are without awns on the lemmas, and are pedicellate. The side spikelets of the 6-rowed barley are nearly as large as the central spikelet, have awns on the lemmas, and are sessile. The side spikelets, of the 6-rowed barley are fertile, while those of the 2rowed barley are sterile. The stigmas in neither have branched (Figure 2E to F). Mature flowers before and after pollination are shown in Figure 2H and 21, respectively. The lemmas have been removed to show the flower parts. Before pollination the anther is not dehisced, the stigmas are erect, branched and feathery, the lodicules swollen and turgid, and the ovary is small. After pollination the anthers are dehisced, the stigmas are collapsed, the lodicules are shrunken, and the ovary has increased in size. The fertilized ovary, after a period of growth and differentiation, becomes the barley kernel. Kirby42" studied the growth of the shoot apex and apical dome of spring barley from the initiation of collar primordia until the maximum number of primordia were formed. During the time that floral primordia were being initiated, the relative rate of elongation of the apex was slow. This rate increased about twofold after the maximum number of primordia were formed. The relative growth rate, measured as the increase in volume, as well as fresh and dry weight, was also slow during initiation and increased after reaching the maximum primordium number. The rates of growth and the size at initiation of the floral primorida was affected by their position on the floral shoot apex. The relative rate of increase in volume increased acropetally from the first initiated (collar) primordium. The collar was the smallest and each subsequently initiated primordium increased in length. The diameter of the newly initiated primordium also increased until more than half the primordia had been initiated and then it declined. The apical dome increased in both length and diameter and both were maximal at the double ridge stage and then both measurements declined to a

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minimum when the maximum primordium number was reached. Subsequently there was an additional increase in the length of the dome, after which both dome and some of the last formed, distal primordia died (Figure 2, B and C). Thus, barley (unlike wheat, rye, and oat) has an indeterminate inflorescence, since the tip of the spike never develops a terminal spikelet. The period of spikelet initiation, therefore, is a stage during which the relative growth rate of the floral shoot apex is low, and changes in the size of the dome and the primordia show an acropetal progression of increasing growth rates. These changes produce an inflorescence in which the most advanced spikelets are in the lower midportion of the spike. Changes in the size of the apical dome, prior to reaching the maximal primordium number, may be related to the subsequent death of the terminal spikelets and thus, also to grain number in the mature inflorescence. Also, since maximal primordium number is correlated with the time of appearance of anther primordia, 55 which in turn appears to be regulated by the production of some hormone by the inflorescence in response to daylength, 51 it is possible that photoperiod may regulate the number of grains produced and ultimately the yield.

PHOTOPERIOD Barley is a facultative LDP1 that is quantitatively stimulated to flower by increasing day lengths. Garner and Allard28 first reported that long photoperiods hastened heading in barley, as well as in wheat, oats, and rye. After testing spinach, barley, rape, henbane, beet, and poppy, Borthwick et al.'° found Wintex barley (//. vulgare L. cv Wintex, C. I. 6127) best suited for experiments designed to determine the spectral sensitivity for the promotion of flowering in LDP. Earlier work with Biloxi soybean6' had demonstrated that flowering in SDP could be prevented by interrupting an otherwise inductive dark period. Both R and B light were found to be maximally effective in causing this inhibition. The work with Wintex barley showed that this same light was effective in promoting flowering in LDP, suggesting that the same photoreceptor was involved in both responses. This was the first action spectrum for the promotion of flowering in LDP. A number of other interesting observations were made during the course of this 10 and an earlier study15 that relate to the photoperiodic responsiveness in barley. Bonnett9 had reported that 4 true leaves were differentiated in the ungerminated seed, and that about 12 leaves were produced before spikelet primordia were initiated. In controlled environment chambers with 16-hr photoperiods at 18°C, however, flowering took place at the seventh to eighth node and floral initiation (FI) could be detected as early as 10 to 12 days after sowing. Differentiation of the main axis was complete after only 17 days. 15 Plants grown under 12hr photoperiods also flowered, but failed to produce fertile florets. With 16- to 24-hr photoperiods plants produced much less dry weight, but were all fertile. The 16-hr condition was found to be optimal for yield in terms of both number and weight of seed produced. Most importantly, all plants had received 12 hr of natural daylight and the 16- to 24-hr photoperiods were achieved by supplementing this period with very low irradiance (400 Ix) incandescent light. Thus, the response was not thought to be photosynthetically mediated. Based on Flint and McAlister's 25 finding that the germination of lettuce seeds was promoted by R light and inhibited by FR light, Borthwick and colleagues12 made the important discovery that both the promotion and inhibition were mediated by a single pigment. This then led to the identification, isolation, and partial purification of the photoreversible pigment 16 that has come to be known as phytochrome. Downs 23 confirmed that the promotion of flowering in barley by R light could be reversed by FR light and was, therefore, mediated by phytochrome. In fact, it is possible that the idea that the germination of lettuce seeds could be reversed by FR light came from an action spectrum for the inhibition of stem elongation in barley" that previously had been shown 10 to be correlated with floral development. Since

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this inhibition was the opposite of the promotion by R light, and reciprocity relationships suggested that both were mediated by a single photoreaction, the logical extension may have been made that they were both mediated by the same pigment. Phytochrome has since been isolated from green barley46 and some 20 other green plants. Most were reported to have about the same specific activity that was, however, much lower than that in etiolated plants. Unfortunately, no correlation could be found between phytochrome kinetics and the photoperiodic response types investigated.' 8 The rate of decrease of Pfr in the dark was about the same in the LD barley as it was in SD or DN Zea mays. Thus, it was concluded that the phytochrome control of flowering was not simply related to the time required for the decay of the active form (Pfr) in the dark. Subsequent experiments45 reported that the promotion of flowering in barley, and a number of other LDP, by low irradiance daylength extensions was most effective when the light used contained a mixture of R and FR wavelengths. This confirmed earlier reports4-14-24-26-71-77-80 for a wide variety of plants that such mixtures were more effective than R light alone. This is in marked contrast to the response to brief interruptions of the dark period and suggests that either there are two distinct photoreactions involved, or that the spectral characteristics of the same photoreaction changes with time. The action spectrum for this response66 has a single, rather sharp peak at about 710 nm and is, therefore, intermediate between the maximal absorbance of Pr (667 nm) and pfr (730 nm). Such responses have been termed "high irradiance responses" (HIR, 13 to distinguish them from the typical low irradiance, R/FR reversible responses), and are assumed to be a complex function of the photostationary state between Pr and Pfr. 31 It has also been suggested that this type of response is mediated by a combination of phytochrome with its immediate reaction partner,11 perhaps requiring the maintenance of a stable complex over a long period of time. 64 We have recently confirmed20^22 that FR enhances flowering in barley as previously described,4-45 and demonstrated that the sensitivity to FR energy is regulated by an endogenous circadian rhythm. The timing mechanism itself was also found to be influenced by FR, being reset by the same light that produces enhancement. Clearly, this is a complex, multicomponent system that is mediated neither by simple phytochrome photoconversion,37 nor by the original HIR described by Mohr.49 Elucidation of this mechanism is central to an understanding of photoperiodic control, especially as such control is exerted in the natural environment. Paleg and Aspinall60" found that while a NB was effective in promoting FI in barley, it also led to a reduction in the yield. When LD were given prior to stem elongation, yield was also reduced in two cvs that differed in photoperiodic sensitivity. 24b The effects of photoperiod on the relation between development and yield were recently tested on ten different cvs of spring barley.4211 It was found that the duration of ear initiation in all cvs was decreased by daylengths increasing from 11 to 20 hr, but the duration of ear growth was unaffected. Fewer primordia were initiated in LD, but they initiated at a faster rate, producing fewer leaves and tillers. In all cvs tested, the longer the photoperiod, the shorter was the period to ear emergence. The maximum number of primordia was greatest in 11hr photoperiods and least in 20-hr photoperiods, but the rate of primordia formation was fastest in 20-hr photoperiods. The greatest grain yield occurred in 11- to 13-hr photoperiods in all but one cv. Since tiller production ceases at the time when the maximum number of primordia are reached,2"-24a-42d and since there is a competition between the main shoot and the tillers for nutrients, 2 " 42c photoperiod alone cannot predict grain yield. Single recessive genes have been reported to condition photoperiod insensitivity in bar]£y 22h.62a,74 H owever eacn of these genes caused debilitating effects that limit their use. Other reports indicated that several genes may condition inheritance of the photoperiod response. Fischer24c reported that loci on at least four chromosomes determined the flowering response under SD in two cvs of barley. Tew76a also concluded that the inheritance of the

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photoperiod response was quantitative in crosses of sensitive and insensitive barleys. Barham and Rasmusson 7 ' 1 examined the photoperiod response in the F,, F2, F,, and backcross generations of crosses between an insensitive cv and four sensitive genotypes grown in a SD environment. The progeny distributions were continuous in all generations indicating that quantitative inheritance is important in determining the photoperiod response in barley. Additive effects predominated, although dominance appeared important in one cross and epistatic effects were determined in three of the four crosses. Although major genes were not detected, the high heritability values and the selection results indicated that breeders should have little difficulty when selecting for photoperiod response in these and similar barley populations.

TEMPERATURE While daylength is the most critical environmental factor determining floral induction and the timing from seedling emergence to spikelet formation, temperature is critical both between sowing and seedling emergence and between spikelet formation and ripeness. The former may be separated into two phases. One is an obligate or facultative requirement for a period of prolonged low temperature that is essential for, or promotes, the subsequent response to photoperiod. This response is known as vernalization and is a general characteristic of winter cvs of most cereals, including barley. The other is primarily the effect of soil temperature on imbibition and growth rate of the embryo. While this is of major agronomic interest, the mechanism of its control is reasonably well understood and it will not be considered further here. Also, while temperature is important during maturation of the spike, it is secondary to the quantity of solar energy available for photosynthesis during this period. Therefore, this section will focus primarily on vernalization and the direct interaction of temperature with the photoperiodic response in barley. A great deal of what is known about vernalization is derived from work done with the winter strain of Petkus rye (Secale cereale cv Petkus) and the biennial strain of Hyoscyamus niger (see those chapters in Volumes IV and V. respectively). Both species have an obligate requirement for a cold pretreatment in order to respond to subsequent LD and this requirement arises from a single gene difference from strains that do not require vernalization. Also, while extreme winter hardiness and a high vernalization requirement are commonly associated, they are not the same response and should not be confused. For example, a number of very frost-resistant cvs have little or no vernalization requirement. 27 - 41 Genetically, the difference between spring and winter strains of barley was found to be the result of three major genes, one recessive (shs) and two dominant (Sh 2 and Sh 3).74 The shs factor determines spring habit when plants are under 24-hr photoperiods. Any one of the dominant Sh alleles can cause plants to have spring habit. Winter habit, on the other hand, is determined by the dominant Shs allele or by the homozygous recessive condition of either Sh 2 or Sh 3. While relatively little work has been done on the physiology of the response to low temperature during vernalization in barley, its behavior does not appear to differ markedly from that in wheat (see chapter on Triticum in Volume 4). The site of vernalization appears to be in the meristematic tissue. Low temperature leads to the development of a localized vernalized state in the cells that are at a certain stage of development when they are subjected to chilling. This state is a property of all daughter cells resulting from divisions of vernalized cells, although some plants can be devernalized by high temperature or short photoperiods. The nature of the changes that take place during the transition to the vernalized state is unknown, but may involve the unblocking or derepression of genetic information. 78 Increases of RNA in the embryos of seeds of winter cereals during the course of a cold treatment have been reported,44 but such increases also occur in spring strains. New protein patterns

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appeared in vernalized wheat embryos that were inhibited by azaguanine, but not by bromouracil, suggesting an effect on the transcription of DNA. 7 6 SD treatment may replace the requirement for low temperature in some, 17 but not all 7 '' winter forms of barley. This SD effect was found to be most pronounced at the 1-leaf stage and decreased progressively as the plants aged. Under constant temperatures, the effect decreased with increasing temperature up to 18 to 22°C, although some effect could be demonstrated with temperatures as high as 30°C. The SD response could also be reversed by intercalated LD and was especially pronounced at higher temperatures. It was concluded, however, that the low temperature and SD responses were mediated by separate mechanisms. 1 7 A similar conclusion was also reached earlier from work with Campanula medium and Silene armeria (see chapters in Volumes II and IV). S '- S Although daylength is the primary determining factor for floral induction, temperature may interact directly with photoperiod to alter the photoperiodic sensitivity of the plant. Since photoperiodic floral induction, unlike vernalization, takes place in the leaf, and presumably a floral stimulus must be synthesized and transported to the apex, a temperature optimum for such chemical processes would be expected. However, there may also be influences of temperature on the dark reactions of phytochrome (reversion or destruction) that could alter the photoperiodic response. In barley, an increase of temperature from 13 to 24°C accelerated plant development and caused a reduction in plant size, but the influence of temperature decreased with increasing photoperiods. 31 Aspinall 4 found that there was little differential effect of temperature in the range between 10 and 20°C and that delays of flower formation at low temperature were explicable purely in terms of general metabolic rates. However, at higher temperature there was a marked interaction between the effects of temperature and both photoperiod and the spectral quality of the light source. Temperatures of 30°C delayed flowering more in short than in long photoperiods and more in low irradiance FR than high. Further development of the inflorescence was abnormal, especially at high temperature and short photoperiods, suggesting an interference with hormone metabolism at the apex.55 Takahashi and Yasuda73 found that temperature had a strong influence on the rate of floral development in 15 different cvs growing in the field, but the effect was similar for all cvs irrespective of photoperiodic sensitivity. Thus, the photoperiod was the primary determining factor for early flowering while the mean temperature during the growing period merely increased or decreased the rate to the same extent in all cvs tested. A ' 'biophotothermal time scale" was reported for barley85-86 that suggested that there was a threshold temperature of 4.3°C and an optimum of 19 to 20°C from planting to ear emergence. The applicability of such models appears, however, to be very restricted and of little general interest.

OTHER ENVIRONMENTAL INFLUENCES Chief among other environmental influences on flowering in barley are water stress and nutrient supply. However, these will be considered only as they relate to floral induction and development and not necessarily to crop yield. Both the initiation and differentiation of vegetative and reproductive primordia in the apical meristem, and the enlargement of the cells thus differentiated, are very sensitive to water stress.69 This stress leads to a suspension of primorida production without impairing the potential for subsequent development. Both cell division and enlargement can be affected, even at relatively slight water deficits. As a consequence of reduced cell enlargement, leaf area will be decreased, resulting in decreased total photosynthetic carbon fixation and, hence, reduced yield. As mentioned in the section on floral morphology, new floral primordia production ceases in barley at the time of the appearance of stamen initials. 55 Thus, the number of spikelets per inflorescence is determined by a balance between the rate of primordial initiation relative

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to that of spikelet development. Even slight water stress can reduce the rate of appearance of floral primordia and, therefore, reduce the potential number of grains per ear. Husain and AspinalF' found that the formation of new primorida on the apical meristem of barley was inhibited at levels of soil water potential that had little or no effect on growth. This water stress also suppressed the response of the apical meristem to an increase in the photoperiod. This appears to be due to a decrease in the supply of essential substances to the apex, possibly by preventing the production in the leaf, or transport to the apex, of some substance or substances essential for flowering. Since the effects of mild water stress could, in some cases, be reversed by subsequent watering, and since the response to increased light intensity was not inhibited by water stress, it is likely that these substances are hormonal in nature. From the stage of spikelet initiation to the fertilization of the ovules, a number of other processes associated with development of the inflorescence are sensitive to water deficits and thus cause a reduction in the number of grains per ear, or even the number of fertile ears.5-68-83 This is presumably due to a specific interference with the sexual development of the spikelets, especially at meiosis/'7-68 For later effects, during grain filling, the reader is referred to Aspinall 3 since it is beyond the scope of this paper. It should be noted that, since barley is capable of producing tillers that effectively prolong the period of flowering, the effects of water stress under certain conditions may be less severe than in other species. Some compensation occurs if stress early in the vegetative period, which may interfere with spikelet development of the main stem, serves to promote tiller development. Although tillers may not have as many spikelets as a nonstressed main stem, the total number of grains per plant may be affected relatively little by a stress that severely reduces their number on the main stem.5 However, under most field conditions, severe water stress during the early growth period will reduce both the number of tillers and the number of spikelets formed. Although it has been suggested that nutrient supply, especially the availability of nitrogen, may be important in the determination of whether a primordium will develop as a leaf or give rise to a spikelet, 18 19 the best known regulatory role of nutrients is on tillering patterns. 2 Gregory29 first stressed the importance of mineral nutrients as a primary factor in tillering. The mechanism appears to be controlled by apical dominance47 whereby auxins regulate the distribution of nutrients to axillary buds. 30 In the initial phase of tillering, both the rate and number are influenced by nutrient supply and tillering ceased earlier when the nutrient supply was low. 2 At the end of this initial phase lack of nutrient leads to senescence of the apices of developing tillers, but not of the axillary buds. These buds remain suppressed for a considerable period, but the suppression can be relieved at any time by supplying nutrients.

GROWTH REGULATORS Application of GA causes an acceleration of cell division in the shoot apices of a number of LDP. 74048 - 63 However, Paleg and Aspinall60 found that weekly applications of large amounts of GA inhibited the growth and development of the barley spike. Even a single treatment, that did not lead to an inhibition of growth, resulted in abnormal development and sterile spikes.42 Nicholls51 found that the growth of the apex in barley was regulated by the meristematic activity of 3 distinct regions. These were (1) the meristematic region of the apex that was concerned with the elongation of the apical dome above the youngest visible primoridum, (2) the meristematic regions of the simple and double ridge primordia that arose on the flanks of the shoot apex, and (3) the internodal meristems of the main axis of the young inflorescence which were involved in its elongation. 55 Marked changes in endogenous GA content were found to occur at various stages of floral development, but the extractable GA

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content did not correlate with the rate of development." Application of exogenous GA enhanced the rates of growth of the apex and the subjacent leaf primordia, but for only a relatively short period. The initial response was an increase in the length of the apical dome, followed by increased rates of growth and development of leaf and basal spikelet primordia. At the end of this period, however, the growth rates of the apices returned to rates comparable to those in untreated plants. 52 Nicholls 52 also suggested that rapid rachis internode elongation required the simultaneous action of two factors, one an unknown hormone released by the inflorescence at the initiation of floret development and the other a GA. Growth in LD resulted in a separation in time between the induction of double ridges (the initial response) and subsequent rapid rachis internode elongation. If plants treated with GA at the double ridge stage were returned to SD, a group of abnormal spikelets were formed on the lower six nodes of the inflorescence. When plants grown in SD were treated with both ABA and GA, primordia formation ceased abruptly at the time of appearance of stamen initials. 54 This is typical of the LD response and, thus, the response to long days is assumed to require both ABA and GA at specific times during floral development in order to produce normal florets. Such a complex sequence may explain the complex time courses reported for barley (Figure 3) given a series of inductive LD treatments before return to SD. 21 A single LD had a marked effect on the stimulation of double ridge production, but did not result in further floral development. Additional LD treatments resulted in little further effect on this initial rapid phase of development was saturated by three inductive cycles. This agrees well with the ABA and GA results of Nicholls 54 if it is assumed that one LD is insufficient to stimulate ABA production but is sufficient to produce GA. Relatively little work has been done with other growth regulators in barley. As was mentioned earlier in conjunction with the effects of nutrients on flowering, Leopold47 found that auxin increased grain yield in barley in terms of the number of flowers produced, but only when the plants were grown under continuous inductive conditions. This was interpreted as an effect related to apical dominance and resulted from increased tillering. Reductions in yield as a result of 2,4-D application have been reported in barley, but most of these result from effects on vegetative growth at the seedling stage of development, and thus, are not directly related to floral induction. Derscheid,223 however, found that the susceptibility to 2,4-D was dependent on the time of application during development. Plants were most susceptible before the 5-leaf stage, causing an inhibition of the differentiation of tiller buds thereby reducing the number of tillers, the number of spikes, and leading to greatly depressed yields. Plants were tolerant to 2,4-D between the 5-leaf stage and the early boot stage, suggesting that there was no direct influence on flowering, although heavy 2,4-D doses did inhibit FI and decreased the number of seeds per spike. They became susceptible again between preheading and late heading and were most resistant during the postheading period. Growth retardants such as CCC, diethylmorpholinium chloride (DMC), and ethephon have also been shown to increase tillering.8-50-87 However, the results are contradictory and Wiinsche87 found that daylength did not influence the effect of CCC on tillering.

HORDEUM BULBOSUM H. bulbosum is a perennial pasture grass that has major economic importance in semiarid regions with a Mediterranean climate. It is a 2-rowed, wild form that differs from other subspecies of Hordeum by producing a dormant "bulb" which consists of 1 to 2 swollen internodes at the base of flowering culms. 41 Sprouts appear in early winter and the plants grow vegetatively throughout the winter, flowering in early April. The heads mature and the plant then dries out and remains dormant. Endogenous dormancy of the buds is alleviated gradually during the dry summer months.

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100 -

CONTROL

I I LD

7 LD 5 LD

•7

3 LD



2 LD

5 6.0 -

4.0 -

2 0 SD

0.0

10

20

CONTROL

25

30

35

DAYS F R O M START OF LONG-DAY TREATMENT FIGURE 3. Floral stage of wintex barley as a function of time following treatment with varying numbers of 24-hr LD photoperiods supplemented with FR light prior to return to 12-hrSD photopcriods without supplemental FR light. Control, continuous 12-hr SD (V). I LD ( + ). 2 LD (•). 3 LD (V), 5 LD (^). 7 LD ( X ) . 1 1 LD ( A ) , and continuous 24-hr LD (O). (From Deitzer, G. F., Hayes. R.. and Jabben. M.. Plum Ph\siol., 64, 1015—1021, 1979. With permission.)

Both the development of dormancy and flowering are strongly accelerated by LD following seed and bulb vernalization or by SD41-58 and both processes could be initiated in vernalized plants with only a few LD.59 The onset of dormancy and the onset of flowering were found to be closely associated, but they require different levels of environmental stimuli and, thus, could be separated.5" GA was capable of enhancing bulb initiation only in vernalized plants exposed to LD, even when the number of inductive cycles was less than that required for floral or bulb development. Thus, some factor other than GA appears to be essential for bulb initiation. 57

SUMMARY Hordeum vulgare L. is a single species that consists of a large number of agronomically important cvs and wild subspecies. There are both 2- and 6-rowed forms that differ only in the fertility of the lateral florets. The apex develops as an indeterminate inflorescence with no terminal flower. Spikelet primordia cease to be produced when anther initials are formed. The timing of this event appears to be hormonally regulated and possibly involves the production of abscisic acid by the inflorescence. All cvs may be divided into two general classes referred to as winter and spring strains. The division is based largely on the requirement of the former group for a period of vernalization, although this requirement is not absolute in all winter types. Photoperiodically all are classed as quantitative LDP and floral induction is promoted, in all cvs that have

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been tested, by FR light. This promotion is probably mediated by phytochrome in a way that is still very poorly understood. However, while floral promotion leads to more rapid flowering it does not necessarily lead to increased yield. The yield components vary among different cvs in response to increasing daylengths, but are generally greater with relatively short(11- to 13-hr) photoperiods. The response to photoperiod appears to be inherited quantitatively and may result from the action of several genes located on several different chromosomes. It is possible that temperature may also directly influence the phytochrome response to change photoperiodic sensitivity. One or more of the GAs may be involved in the response to photoperiod and FR light, but it remains unclear whether GA production is involved in floral induction or is a later consequence of such induction. Other environmental factors, such as water stress and nutrient deficiency, appear to act primarily on floral development and are especially important to tillering.

REFERENCES 1 . Altman, P. L. andDittmer, D. S., Biology Data Book, Federation of the American Society for Experimental Biol., Bethesda, Md, 1964. 2. Aspinall, D., The control of tillering in the barley plant. I. The pattern of tillering and its relation to nutrient supply, Aust. J. Biol. Sci.,14, 493—505, 1961. 2a. Aspinall, D., The control of tillering in the barley plant. II. The control of tiller bud growth during ear development, Aust. J. Biol. Sci., 16, 285—304, 1963. 3. Aspinall, D., The effects of soil moisture stress on the growth of barley. II. Grain growth, Aust. J. Agric. Res., 16, 265—275, 1965. 4. Aspinall, D.,The effects of daylength and light intensity on the growth of barley. VI. Interaction between the effects of temperature, photoperiod and the spectral composition of the light source, Aust. J. Biol. Sci., 22, 53—67, 1969. 5. Aspinall, D., Nicholls, P. B., and May, L. H., The effects of soil moisture stress on growth of barley. 1. Vegetative development and grain yield, J. Agric. Res., 15, 729—745, 1964. 6. Bakhteyev, F. Kh., Sketches on the History and Geography of the Important Cultivated Plants (Rs), Educational Pedagogical Publishing House, Ministry of Culture RSFSR, Moscow. 1960. 7. Baldev, B. and Lang, A., Control of flower formation by growth retardants and gibberellin in Samo/us parviflorus, a long day plant, Am. J. Hot. 52, 408—417, 1965. 7a. Barham, R. W. and Rasmusson, D. C., Inheritance of photoperiod response in barley, Crop Sci., 21, 454—456, 1981. 8. Bokhari, U. G. and Youngner, V. B., Effects of CCC on tillering and flowering of uniculm barley, Crop Sci., 1 1 , 711—713, 1971. 9. Bonnett, O. T., The development of the barley spike, J. Agric. Res., 51, 451—457, 1935. 10. Borthwick, H. A., Hendricks, S. B. and Parker, M. W., Action spectrum for photoperiodic control of floral initiation of a long-day plant, Wintex barley (Hordeum vulgare L., Hot. Gaz., 110, 103—118, 1948. 1 1 . Borthwick, H. A., Hendricks, S. B., and Parker, M. W., Action spectrum for inhibition of stem growth in dark grown seedlings of albino and non-albino barley Hordeum vu/gare L., Bot. Gaz., 113, 95—105, 1951. 12. Borthwick, H. A., Hendricks, S. B., Parker, M. W., Toole, E. H., and Toole, V. K., A reversible photoreaction controlling seed germination, Proc. Natl. Acad. Sci. U.S.A., 38, 662—666, 1952. 13. Borthwick, H. A., Hendricks, S. B., Schneider, M. J., Taylorson, R. B., and Tooke, V. K., The high energy light action controlling plant responses and development, Proc. Natl. Acad. Sci. U.S.A., 64, 479^*86, 1969. 14. Borthwick, H. A. and Parker, M. W., Light in relation to flowering and vegetative development, Proc. 13th Int. Hortic. Congr., London, 801—810, 1952. 15. Borthwick, H. A., Parker, M. W., and Heinze, P. H., Effect of photoperiod and temperature on the development of barley, Bot. Gaz., 103, 326—341, 1941. 16. Butler, W. L., Norris, K. H., Siegelman, H. W., and Hendricks, S. B., Detection, assay and preliminary purification of the plant pigment controlling photoresponsive development of plants, Proc. Natl. Acad. Sci. U.S.A., 25, 1703—1708, 1959.

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17. Chugo, H., Studies on vernalization in wheat and barley plants, especially on effects of factors on the vernalization of growing plants, Bull. Univ. Osaka Pref. Scr. B.. 27, 253—310, 1975. 18. Dale, J. E. and Wilson, R. G., A comparison of leaf and ear development in barley cultivars as affected by nitrogen supply. J. Agric. Sci., 90, 503—508, 1978. 19. Dale, J. E. and Wilson, R. G., The effect of photoperiod and mineral nutrient supply on growth and primordia production at the stem apex of barley seedlings, Ann. Bot.. 44, 537—546, 1979. 20. Deitzer, G. F., Effect of far red energy on the photoperiodic control of flowering in Wintex barley (Hordeum vulgare L.), in Strategies of Plant Reproduction, Meudt, W., Ed., Allanheld, Osmum, Totowa, N.J., 1982, 99—115. 21. Deitzer, G. F., Hayes, R., and Jabben, M., Kinetics and time dependence of the effect of far red light on the photoperiodic induction of flowering in Wintex barley, Plant Physio!., 64, 1015—1021, 1979. 22. Deitzer, G. F., Hayes, R., and Jabben, M., Phase shift in the circadian rhythm of floral promotion by far red energy in Hordeum vulgare L., Plant Physiol., 69, 597—601, 1982. 22a. Derscheid, L. A., Physiological and morphological responses of barley to 2,4-dichlorophenoxyacetic acid, Plant Physiol., 27, 121—134, 1952. 22b. Dormling, I. and Gustafsson, A., Phytotron cultivation of early barley mutant, Theor. Appl. Genet., 39, 51—61, 1969. 23. Downs, R. J., Photoreversibility of flower initiation. Plant Physiol., 31, 279—284, 1956. 24. Downs, R. J., Piringer, A. A., and Wiebe, G. A., Effects of photoperiod and kind of supplemental light on growth and reproduction of several varieties of wheat and barley, Bot. Gaz., 120, 170—177, 1959. 24a. Fairey, D. T., Hunt, L. A., and Stoshopf, N. C., Daylength influence on reproductive development and tillering in 'Fergus' barley, Can. J. Bot., 53, 2770—2775, 1975. 24b. Faris, D. G. and Guitard, A. A., Yield and primary culm yield components of two spring barley cultivars as influenced by temperature, daylength and growth stage. Can. J. Plant Sci., 49, 701—713, 1969. 24c. Fischer, V., Untersuchungen uber die Vererbung des Sommer-Winter-Typus bei der Gerste unter Beruchsichtigung des Bluhtermins und der Winterfestigkeit, Z. Pflanzenzuecht., 71, 69—84, 1974. 25. Flint, L. H. and McAlister, E. D., Wavelengths of radiation in the visible spectrum inhibiting the germination of light-sensitive lettuce seeds, Smithsonian Misc. Coll., 94, 1—11, 1935. 26. Friend, D. J. C., Helson, V. A., and Fisher, J. E., The influence of the ratio of incandescent to fluorescent light on the flowering response of Marquis wheat grown under controlled conditions, Can. J. Plant Sci., 41, 418^27, 1961. 27. Froier, K., Hoffmann, W., Sandegren, E., and Thunaeus, H., Gerste. Resistenzzuchtung: Winterfestigkeit, Handburch fur Pflanzenziichtung, Vol. 2, Getreidearten, Paul Parey, Berlin, 1959, 338—349. 28. Garner, W. W. and Allard, H. A., Further studies in photoperiodism: the response of the plant to relative length of day and night, J. Agric. Res., 23, 871—920, 1923. 29. Gregory, F. G., Mineral nutrition of plants, Annu. Rev. Biochem., 6, 357—358, 1937. 30. Gregory, F. G. and Veale, J. A., A reassessment of the problem of apical dominance, in The Biological Action of Growth Substances, Porter, H. K., Ed., Cambridge University Press, London, 1957, 1—20. 31. Guitard, A. A., The influence of variety, temperature and stage of growth on the response of spring barley to photoperiod, Can. J. Plant Sci., 40, 65—80, 1960. 32. Harlan, J. R. and Zohary, D., Distribution of wild wheats and barley, Science. 153, 1074—1080, 1966. 33. Hartmann, K. M., A general hypothesis to interpret "high energy phenomena" of photomorphogenesis on the basis of phytochrome, Photochem. Photobiol., 5, 349—366, 1966. 34. Helbaeck, H., Archaeology and agricultural botany. Ann. Rep. Inst. Archaeol. Univ. London, 9, 44—59, 1953. 35. Helbaeck, H., Die Palaoethnobotanik des Nahen Ostens und Europas, in Opuscula Ethnologica Memoriae, Bodrogi, R. T. and Boglar, L., Eds., Ludovici Biro Sacra, Budapest, 1959, 265—289. 36. Helbaeck, H., Domestication of food plants in the old world, Science, 130, 365—372, 1959. 37. Hendricks, S. B. and Borthwick, H. A., Control of plant growth by light, in Environmental Control of Plant Growth, Evans, L. T., Ed., Academic Press, New York, 1963, 233—263. 38. Hopkins, W. G. and Hillman, W. S., Phytochrome changes in tissues of dark grown seedlings representing various photoperiodic classes, Plant Phvsiol., 41, 593—598, 1965. 39. Husain, I. and Aspinall, D., Water stress and apical morphogenesis in barley, Ann. Bot., 34, 393—407, 1970. 40. Jacqmard, A., Early effects of gibberellic acid on mitotic activity and DNA synthesis in the apical bud of Rudbeckia bicolor, Physiol. Veg., 6, 409^16, 1968. 41. Jenkins, G., Breeding for cold resistance in winter cereals, Eur. Assoc. Res. Plant Breeding, Cereals Section Proc. Sect. Cereals Physiol., Eucarpia, Dijon, 1971, 163—172. 42. Kirby, E. J. M., Abnormalities induced in barley ears by gibberellic acid, J. Exp. Bot., 22, 411—419, 1971

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42a. Kirby, E. J. M., The growth of the shoot apex and the apical dome of barley during ear initiation. Ann. Bin., 41, 1297—1308, 1977. 42b. Kirby, E. J. M. and Appleyard, M., Effects of photoperiod on the relation between development and yield per plant of a range of spring barley varieties, Z. Pflanzenzuechl., 85, 226—239. 1980. 42c. Kirby, E. J. M. and Jones, H. G., The relations between the main shoot and tillers in barley plants, J. AKric. Sci., 88, 381—389, 1977. 42d. Kirby, E. J. M. and Riggs, T. J., Developmental consequences of two-row and six-row ear type in spring barley. II. Shoot apex, leaf and tiller development, J. Agric. Sci., 91. 207—216, 1978. 43. Roller. D. and Highkin, H. R., Environmental control of reproductive development in Hordeum bulbosum, a perennial pasture grass. Am. J. Hot., 47, 843—847, 1960. 44. Konarev, V. G., The influence of vernalization on the behavior of nucleoproteins and nucleic acids in grass embryos (Rs), Biokhimiya, 19, 131—136, 1954. 45. Lane, H. C., Cathey, H. M., and Evans, L. T., The dependence of flowering in several long-day plants on the spectral composition of light extending the dark period, Am. J. Bot., 52. 1006—1014, 1965. 46. Lane, H. C., Siegelman, H. W., Butler, W. L., and Firer, E. M., Detection of phytochrome in green plants, Plant Physiol., 38, 414-^16, 1963. 47. Leopold, A. C., The control of tillering in grasses by auxin. Am. J. Bot., 36, 437—440, 1949. 48. Liu, P. B. W. and Loy, J. B., Action of gibberellic acid on cell proliferation in the sub-apical shoot meristem of watermelon seedlings, Am. J. Bar., 63, 700—704, 1976. 49. Mohr, H., Primary effects of light on growth, Annu. Rev. Plant Physio/.. 13, 465—488, 1962. 50. Morgan, D. G., Plant growth substances and flower development. Pest. Sci., 8, 230—235. 1977. 51. Nicholls, P. B., Interrelationship between meristematic regions of developing inflorescences of four cereal species, Ann. Bot., 38, 827—837, 1974. 52. Nicholls, P. B., The effect of daylength on the development of the barley inflorescence and the endogenous gibberellin concentration, R. Soc. N.Z. Bull., 12, 305—309, 1974. 53. Nicholls, P. B., Response of barley shoot apices to application of gibberellic acid: initial response pattern, Aust. J. Plant Physiol., 5, 311—319, 1978. 54. Nicholls, P. B. Response of barley shoot apices to application of gibberellic acid: dependence on tissue sensitivity, Aust. J. Plant Physio/., 5, 581—588, 1978. 55. Nicholls, P. B. and May, L. H., Studies on the growth of the barley apex. I. Interrelationships between primordium formation, apex length, and spikelet development, Aust. J. Biol. Sci., 16, 561—571, 1963. 56. Nilan, R. A., The Cytology and Genetics of Barley 1951—7962. Monographic Suppl. 3, Research Studies, Washington State University Press, Pullman, 1964. 57. Ofir, M., Interaction of gibberellin with photoinduction in the initiation of the dormant phase in vernalized Hordeum bulbosum L., Aust. J. Plant Physiol., 3, 827—832, 1976. 58. Ofir, M. and Koller, D., A kinetic analysis of the relationships between flowering and the initiation of the dormant state in Hordeum bulbosum L., a perennial grass, Isr. J. Rot., 21, 21—34, 1972. 59. Ofir, M. and Koller, D., Relationship between thermoinduction and photoinduction of flowering and dormancy in Hordeum bulbosum L., a perennial grass, Aust. J. Plant Physiol., 1, 259—270, 1974. 60. Paleg, L. G. and Aspinall, D., Inhibition of the barley spike by gibberellic acid, Nature (London), 181, 1743_1744, 1958. 60a. Paleg, L. G. and Aspinall D., Effects of daylength and light intensity on the growth of barley. V. Response by plants in the field to night interruption, Aust. J. Biol. Sci., 19, 719—734, 1966. 61. Parker, M. W., Hendricks, S. B., Borthwick, H. A., and Skully, N. J., Action spectrum for the photoperiodic control of floral initiation of short-day plants, Bot. Gaz., 108, 1—26, 1946. 62. Piringer, A. A. and Cathey, H. M., Effects of photoperiod, kind of supplemental light, and temperature on growth and flowering of petunia plants, Proc. Am. Soc. Hortic. Sci., 76, 649—660, 1960. 62a. Ramage, R. T. and Suneson, C. A., A gene marker for the g chromosome of barley, Agron. J., 50, 114. 1958. 63. Sachs, R. H., Bretz, C., and Lang, A., Cell division and gibberellic acid, Exp. Cell Res., 18, 230— 244, 1959. 64. Schafer, E., The 'high irradiance reaction', in Light and Plant Development, Smith, H., Ed., Butterworths, London, 1975, 45—59. 65. Schiemann, E., New results on the history of cultivated cereals, Heredity, 5, 305—320, 1951. 66. Schneider, M. J., Borthwick, H. A., and Hendricks, S. B., Effects of radiation on flowering of Hyoscyamus niger. Am. J. Bot., 54, 1241—1249, 1967. 67. Shazkin, F. D. and Leiman, R. I., Effects of deficiency of soil water on vernalized and non-vernalized cereals in different periods of their development, C. R. Acad. Sci. If.S.S.R., 84, 627—630, 1952. 68. Shazkin, F. D. and Zavadskaya, I. G., The effect of soil moisture deficiency and nitrogen nutrition on microsporogenesis in barley plants, Dokl. Akad. Nauk. S.S.S.R., 117, 150—152, 1957.

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69. Slaytyer, R. O., Physiological significance of internal water relations to crop yield, in Physiological Aspects of Crop Yield, Eastin, J. D., Haskings, F. A., Sullivan, C. Y., and Van Bavel, C. H. M, Eds., University of Wisconsin Press, Madison, 1969, 53—88. 70. Staudt, G., The origin of cultivated barleys: a discussion, Econ. Bot., 15, 205—212. 1961. 71. Stolwijk, J. A. J. and Zeevaart, J. A. D., Wavelength-dependence of different light reactions governing flowering in H\osc\amus niger, Proc. Kon. Ned. Akad. Wetensch. Ser. C., 58, 386—396, 1955. 72. Takahashi, R., The origin and evolution of cultivated barley. Adv. Genet., 7, 227—266, 1955. 73. Takahashi, R. and Yasuda, S., Varietal differences in responses to photoperiod and temperature in barley, Ber. Oharalnst. Landwirtsch Biol., 11, 365—381, 1960. 74. Takahashi, R. and Yasuda, S., Genetics of earliness and growth habit in barley, Barley Genet., 2, 388— 408, 1971. 75. Takimoto, A., Photoperiodic induction in Silene armeria as influenced by various light sources, Bot. Mag. (Tokyo), 70, 312—321, 1957. 76. Teroaka, H., Proteins of wheat embryos in the period of vernalization, Plant Cell Physiol., 8, 87—95, 1972. 76a. Tew, T. L., Inheritance of Photoperiod Response in Barley, Ph.D. thesis, University of Minnesota, St. Paul, 1977. 77. Vince, D., Blake, J., and Spencer, R., Some effects of wavelength of the supplementary light on the photoperiodic beavior of the long-day plants carnation and lettuce, Physiol. Plant., 47, 119—125, 1964. 78. Vince-Prue, D., Photoperiodism in Plants, McGraw-Hill, London, 1975. 79. Von Denffer, D., Uber die Zussammenwirkung von Keimstimmung und taglicher Belichtungsdaur auf die Etwicklung von Sinapis und Hordeum, Jahrb. Wiss. Bot., 88, 759—813, 1939. 80. Wassink, E. C., Stolwijk, J. A. J., and Beemster, A. B. R., Dependence of formative and photoperiodic reactions in Brassica rapa var. Cosmos and Lactuca on wavelength and time of irradiation, Proc. Kon. Ned. Akad. Wetensch. Ser. C., 54, 421^32, 1951. 81. Wellensiek, S. J., Stem elongation and flower initiation, Proc. Kon. Ned. Akad. Wetensch. Ser. C., 63, 159—166, 1960. 82. Wellensiak, S. J., Recent developments in vernalization, Acta Bot. Neeri, 14, 308—314, 1965. 83. Wells, S. A. and Dubetz, S., Reaction of barley varieties to soil water stress, Can. J. Plant Sci., 46, 507—512, 1966. 84. Wendorf, F., Schild, R., El Hadidi, N., Close, A. E., Kobusiewicz, M., Wieckowska, H., Issawi, B., and Haas, H., Use of barley in the Egyptian late paleolithic, Science, 205, 1341—1347, 1979. 85. Williams. G. D. V., Deriving a biophotothermal timescale for barley, Int. J. Biometeor., 18, 57—69, 1974. 86. Williams, G. D. V., A critical evaluation of biophotothermal time scale for barley, Int. J. Biometeor., 18, 259—271, 1974. 87. Wiinsche, U., Influence of growth retarding substances on cereals. I. Influence of CCC, DMC and CEPA on components of yield in greenhouse and growth chamber experiments with barley and spring wheat, Z. Acker. Pflanzenbau., 136, 331—341, 1972. 88. Zohary, D., Is Hordeum agriocrithon the ancestor of six-rowed cultivated barley?, Evolution, 8, 279— 280, 1959. 89. Zohary, D., Studies on the origin of cultivated barley, Bull. Res. Coun. Isr. 9, 21—42, 1960.

Volume HI HUMULUS

167

LUPULUS

En. Hop; Fr. Houblon; Ge. Hopfen; Sp. Lupulo Graham G. Thomas and Walter W. Schwabe

INTRODUCTION The hop, Humulus lupulus L., has been described as "a dioecious, anemophilous, dextrorse twining, herbaceous vine, indigenous in north temperature areas."8 It is a member of the family Cannabaceae (formerly Cannabinaceae), other species being the annual Japanese hop, H. japonicus and a Chinese species, only recently recognized, H. yunnanensis.* The only species of commercial interest is H. lupulus, the resin contained in glands on the bracts, bracteoles, and perianth of the ripe female inflorescence ("cones") being used in the brewing industry as a bittering and preservative agent and to provide flavor in beer. It is a climbing perennial with a woody rootstock. The stems, which can grow 7.5 to 9.0 m in a season, die down in the autumn, the below-ground bases thickening to form an extension to the overwintering rootstock. Fleshy buds on the swollen portion of the stem and the old rootstock grow out the following spring to form the new shoots. Male flowers of H. lupulus bear very few resin glands, whereas up to 25% of the dry weight of the female inflorescence may be resinous material, especially in some of the newer cultivars, bred for maximum alpha-acid production.

FLORAL MORPHOLOGY AND SEX EXPRESSION The male inflorescence is a much-branched cymose panicle arising from the axils of the leaves of the lateral branches. The individual flowers consist of a simple 5-lobed perianth, at the base of each lobe there being a short filament bearing a long anther at its end. Female inflorescences develop into the "cones" which are harvested, in the Northern Hemisphere, around September. The central axis, or "strig", of the inflorescence is divided into "nodes," each of which bears 4 flowers. The nodal cluster is considered by Hamaguchi4 to be cymose in form with all median flowers aborted. The four flowers are carried on short branches in the axils of small stipular bracts. Each flower, enfolded within a small scalelike bracteole, consists of a cup-shaped perianth enclosing an ovary bearing long papillated stigmas. The ovary, if pollinated, develops into an achene enclosed by a persistent perianth. Occasionally male and female flowers may be found on the same plant and hermaphrodite flowers have also been observed. Hermaphrodite or monoecious plants are usually cytologically female, with XX chromosomes. Such plants do not produce viable pollen the production of which appears to depend on the presence of a Y chromosome although male-determining genes are located principally on the autosomes.7 No environmental treatment has been recorded as altering the sex expression of hop plants but Weston 15 showed that various hormone materials brought about the production of some male flowers on laterals otherwise bearing female cones. Results with hormone preparations have been variable and inconsistent.

DORMANCY The overwintering hop rootstock requires a period of chilling for the growth of shoots from the fleshy buds to begin in the spring. About 5 weeks at temperatures below 4°C are needed, depending on the variety, 9 -' 6 but dormancy may be extended by low temperatures

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until conditions are favorable for growth. 2 However, this is not a prerequisite for flowering; other treatments which break dormancy, such as GA applied to the buds, if causing growth, will result, eventually, in flowering if other conditions are favorable. Entry into dormancy is induced by short daylengths, the critical daylength, beyond which growth is unchecked, differing with variety.

EFFECT OF PHOTOPERIOD Although Garner and Allard 3 were the first to gain general recognition for the discovery that the length of the light period in the 24-hr cycle (the photoperiod) was of importance in determining the reproductive behavior of certain plants, it was Tournois 12 in 1912 who first demonstrated that under SD of 6 hr light H. japonicus can be induced to flower, whereas under LD it remains vegetative. This was the first recorded evidence that daylength can influence the flowering behavior of plants. Because most varieties of H. lupulus flower soon after the longest day of the year it was assumed that it must be a LD-requiring plant. 1 4 Brooks' also concluded from some preliminary experiments that H. lupulus required LD in order to initiate flowers. Thomas and Schwabe'" showed that H. lupulus is, in fact, a SDP, flowering being prevented by continuous light. The critical daylength beyond which flowering will not occur differs with variety but for most it is between 15V, and 16V 2 hr. The number of flowers initiated varies with the length of the induction photoperiod, 10 the optimum daylength being just below the critical daylength. With shorter photoperiods the number of flowers initiated is progressively decreased and eventually only a terminal flower is formed. FI is accelerated with shorter days (Figure 1) which also promote premature entry into dormancy. The number of inductive cycles also affects the number of flowers initiated. With a single inductive cycle occasional flowers may be induced on a small percentage of plants. With an increasing number of promotive cycles the proportion of plants induced to flower increases, as also does the number of flowers per plant. These effects were explored in more detail; thus a LB of 30 min duration in the middle of the dark period, given nightly during a 3-week induction period of either 10- or 15-hr SD, resulted in either flowering or completely vegetative plants, respectively. 10 In the first instance the effect was as if the daylength had been increased to be long enough to prevent dormancy but short enough to bring about FI. In the second instance the effect was as if the daylength had been increased beyond the critical length and the plants remained vegetative. This was further proof that the hop is a SDP. Supplementary light to increase the length of day for 5 weeks during the period of FI in the field was shown by Umeda et al. 11 to increase cone production dramatically under Japanese conditions. This is not an unexpected result since the relatively short days of that latitude would induce very rapid flowering but with relatively few cones, while extension of the daylength, which would delay flowering, results in a greater number of cones. Kubo et al.6 also studied the effects of illumination on the flowering of the hop and showed the advantage of extending the daylength to increase yield. The hop plant responds to light of low intensity so that the commercial use of lights, either as a LB or extending the photoperiod, in hop gardens at latitudes where daylengths are otherwise too short for normal hop growth, may be economically feasible.

JUVENILITY The reason that flowering is often delayed until after the longest day of the year is that the hop goes through a pseudo-juvenile stage, the new shoots emerging from the overwintering rootstock needing to attain a certain size or age before flowers can be induced in

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:ar, Nuuka, 6(8), 3—8, 1969. 54. Woodworth, R. E., Meiosis of microporogenesis in the Juglandaceae, Am. J. Hot., 17, 863—869, 1930.

Volume III KALANCHOE

217

BLOSSFELDIANA

W. W. Schwabe

INTRODUCTION The genus Kalanchoe is a member of the Crassulaceae, and numerous species of this genus are in cultivation, many of them for their interesting succulent growth habits. K. blossfeldiana, a native of Madagascar, itself produces large numbers of attractive flowers and is widely grown. Cultivated under glasshouse conditions it tends to flower in the late autumn and continues to do so for many weeks. Several cvs have been selected for commercial production and are popular pot plants for winter and early spring flowering. These include the following: Tom Thumb, Feuerbliite, Pixie, Norisfeuer, Eldorado, Rotglut, Vulcan, Mace, Thor, Telstar, Tetra Vulcan (cf. References 1, 2, and 3). Much of the early experimental work was done by Harder and his school, 4 who used an unnamed variety, sometimes referred to as var. 'Gottingen'. Varietal differences in response to daylength, temperature, light intensity, etc., are summarized in a useful account by Riinger.4" Practical information on Kalanchoe culture is given by Love.4'1 Kalanchoe seedlings raised in SD conditions can flower when less than 2.5 cm in height, but the plant is normally grown 25 to 30 cm tall and will, when given adequate inductive treatment, produce numerous branches with many flowers which can exceed as many as 1000 per plant.

GROWTH HABIT AND PROPAGATION K. blossfeldiana has an upright growth habit with the leaves arranged in an opposite-anddecussate phyllotaxy (occasionally, specimens with 3-membered whorls occur). With adequate illumination and full mineral nutrient supply, Kalanchoe tends to branch freely and becomes bushy. For experimental purposes, however, a single axis is preferred and the lateral branches have often been removed as was initially done by Harder and his school. Kalanchoe may be propagated from seeds, which are very small, but germinate readily in the light. Eldabh et al. 5 and Fredericq et al.'; however, found that in cv Feuerbliite FR was inhibitory to germination but with prolonged irradiation there was an escape from inhibition. The testa transmits much more red than FR and the ratio of R/FR transmitted can be as high as 65:1. GA treatment of seeds gives very high germination percentages. However, because of the slow growth of Kalanchoe, the raising of plant material from seeds requires a considerable time, and vegetative propagation from cuttings ensures both the maintenance of cvs and a more rigid production of larger plants suitable for commercial production and experimentation. Generally, lateral shoots are detached from the plant and rooted for this purpose. Under suitable conditions of temperature (about 25°C), root initiation is usually complete after a week or 10 days, and the plant becomes fully independent within a couple of weeks. Wilson and Schwabe7 found that the plastochron is approximately 10 days per leaf pair, while the leaf requires some 90 days before reaching its final size. Daylength, as first described by Harder and his school,11 has great effects on leaf succulence; under SD condititions Kalanchoe leaves become much thicker and more succulent and rigid than leaves produced under LD with much higher water contents (2400 as against 900% of dry matter), while the latter may be about 1.2 mm thick on the plant, SD leaves may attain double this thickness. Such increased succulence in SD enables water loss to be substantially reduced from the leaves during the day." Detached leaves which can easily be rooted under both LD and SD conditions will then increase in area, weight, and succulence for a prolonged

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period, reaching up to twice the normal area of 8 to 10 cnr they would have attained if left on the parent plant. Moreover, they also respond to daylength and rooted leaves transferred to SD become twice or more as succulent as those held in LD, increasing from a mean thickness of 1.7 mm to as much as 4.4 mm, all this without any cell divisions occurring. Such enlargement is entirely due to cell extension and where this leads to succulence it is by elongation of cells normal to the surface of the leaf. On the other hand, if the epidermis of the leaves is injured in any way, palisade or mesophyll cells of Kalanchoe leaves are quite capable of becoming meristematic again, producing a cork cambium, which will repair such injury, and on stripping the epidermis from leaves, 6 or 7 cell layers may be added in a very short time by the operation of such a cambium.

FLORAL MORPHOLOGY The structure of the inflorescence of Kalanchoe is a dichasial cyme which, however, terminates in cincinni, 1 0 i.e., a change in branching habit occurring late in very large inflorescences. The transition to flowering in some cvs of A', blossfeldiana can be detected after 8 to 9 days upon dissection of the terminal growing point, when a change of shape from a fairly flat apex to a raised dome-shape can be detected. 10 -" The particular advantage of using Kalanchoe for photoperiod experiments has been the fact that the number of flowers produced bears a direct relationship to the degree of flower induction. 10 The very minimum expression of photoperiodically inductive treatment in Kalanchoe is seen in a marked degree of precocious development of lateral shoots in the axils of very young leaf primordia. A detailed study of the mutual relations of main axis and lateral axes was carried out by Biinsow. l2 The next stage is represented by the disappearance or suppression of the terminal growing point itself (it seems by abortion of a single flower at its earliest stage of differentiation), and the equal development of the uppermost two lateral shoots giving rise to dichotomous branching. The first flowering stage as such, is represented by a single terminal flower occupying the place of the original terminal meristem. Flower numbers then increase, usually in the following steps: 1, 3, 7, 15, 31 ..., in conformity with the degree of branching of the inflorescence, and if it were not for late cincinni formation and loss of the regularity in relation to the branching system, the number of flowers could be expressed by the formula 2"' + " — 1, n being the degree of branching.

MINIMUM LEAF NUMBER — JUVENILE PERIOD In Kalanchoe grown from seed and kept in continuous SD, flowering does not take place immediately, but there appears to be a minimum leaf number of approximately 8 leaf pairs. However, in view of the small seed size, even with 8 leaf pairs a flowering plant of Kalanchoe can still be very small (Figure 1). The normal number of unexpanded leaf pairs in the vegetative terminal bud of Kalanchoe is about three and, together with partly expanded leaves, there are about eight pairs by the time the first leaf pair is fully expanded. There is no experimental evidence as to the age when the individual leaf becomes photoperiodically sensitive. However, the entire plant remains highly sensitive to daylength induction from this very early stage onwards. Harder and von Witsch 4 B have noted that fewer SD are required with increasing plant age. They have also claimed that relatively old plants of Kalanchoe tend to become day neutral, flowering even in noninductive conditions. This claim has later been withdrawn, 14 the probable explanation being that old plants may have had some very slight SD exposure. However, in old stock plants, kept for many months in continuous light, occasionally individuals may be found which have initiated a single or even several terminal flowers.

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IK FIGURE 1. K. blnssfeldiami left to right: mature plant after prolonged SD exposure ((lowering); seedling grown in SD throughout (flowering); mature plant grown in continuous light (vegetative). (From Schwahe. W. W.. in The Induction of Flowering, Evans L. T., Ed., Macmillan. Melbourne. 1969. chap. 9. With permission.)

PHOTOPERIOD RESPONSE The first recorded photoperiodic investigation with this species is by the Dutch investigator Roodenburg, 15 who noted that SD accelerated flowering. K. blossfeldiana is a SDP, the minimum number of SD required for induction being two or more. With increasing numbers of SD cycles, flower numbers increase exponentially and over a range from one to several hundred flowers, the logarithm of the flower numbers is linearly related to the number of inducing SD cycles. 16 (Figure 2). When the number of inductive cycles exceeds approximately 14 SD, the rate of increase in flower number drops, yielding a sigmoid type of curve. Photoperiods effective for induction in 24-hr cycles range from about 11 hr of light to as little as 20 min, with hardly any reduction in flower numbers. With light periods in excess of 11 or 11'/ 2 hr in 24, flowering falls off rapidly, and 12 hr light represents the critical photoperiod. 17 These authors also investigated further reductions in the daily light period (10, 4, 3, 2, 1 min and 50, 40, 30, 20, 10, 5, 3 sec) per day and still obtained flowering. The shortest period they used yielded the remarkable result that as little as one second of bright light per day was enough to induce flowering. This result was confirmed by Schwabe ls using not daylight, but a bright incandescent light source. According to Oltmans' 1 ' the effect is temperature dependent, and at 15°C flowering can also be induced in continuous darkness. Fredericq2" also confirmed the Harder and Glimmer effect at slightly higher temperatures (>18°C) using light of different wavelengths (see below).

220

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FIGURE 2. Effect of number of SD at induction on flower number. (Note log scale from Schwabe, W. W., Ann. Bot. N.S., 20, 1—14, 1956. With permission.)

A large number of experiments with cycles of total duration other than 24 hr have been carried out by numerous investigators. A particularly interesting series by Harder and Glimmer21 also has some bearing on the endogenous rhythms theory of Biinning.22 The majority of these were summarized by Schmitz 23 in a useful diagram, an extended version of which is given in Figure 3. Although the experiments included in this figure were obtained under a variety of conditions, it is clear that mKalanchoe flowering is invariably suppressed when the dark period in any one cycle is substantially below the critical (approximately 12 hr). However, another aspect revealed by the study of these collected data is perhaps equally important: there appears to be an upper limit to the light period which can be inductive and, however long the dark period which may follow it, flowering is no longer possible when the light period exceeds 16 to 17 hr. Kalanchoe is as sensitive to a LB in the dark period24 as other SDP,25 this response not being affected appreciably by temperature.26 LBs have since been used extensively, particularly in the investigation of endogenous rhythms in this species. Relatively low light intensities (about 1 klx) saturate the response, for periods from some minutes to half an hour.24 Very brief durations at high intensity (e.g., 1/40 of a second with a Vacu-Blitz lamp) were also effective. 27 Over a narrow range of intensities the product of intensity x duration was related to the degree of flowering. Below about 20 Ix flowering was not appreciably affected, and at still lower intensities induction occurred in continuous illumination. 24 Another important result emerged from these experiments; the LBs were given at different times during the dark period and the results obtained revealed striking differences in their effectiveness. The most effective time was slightly after the middle of the dark period, breaks close to the start or end of the dark being very much less effective; 21 cycles with 1-min LBs given after different durations of darkness from the start ( 1 , 3 , 5 , 7 , 9 , 11, and 13 hr) within a fixed 15-hr dark period, gave the following flower numbers: 400, 386, 79, 9, 1.3, 312, 410.

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• 1

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FIGURE 3. Combinations of light and dark periods tested in photoperiodic cycles of varying total lengths. Solid circles indicate flowering, open circles vegetative growth (redrawn and amplified after Schmitz 21 ).

In spite of the fact that K. blossfeldiana is able to flower with as little as 1 sec of light per day, this species nevertheless shows a high degree of sensitivity to light intensity during the much longer photoperiods which normally precede the inductive dark period. This can be seen from a comparison of flower numbers produced in winter or in summer under otherwise identical conditions and may imply a direct effect of carbohydrate level. This would agree with the fall in flower numbers seen upon shortening the daily photoperiod from, say, 8 hr to 20 min 17 and from the need for CO, in the light. However, this explanation is not supported by results of an investigation into the sensitivity to light intensity in relation to the critical daylength. 28 The experiments were designed to exclude effects of photosynthetic activity by changes in the duration of high and low intensity light periods, using either daylight or incandescent light sources. The use of suitable daylight comparisons was possible because the experiments were conducted during midsummer at the Abisko Research Station of the Swedish Royal Academy, north of the Arctic Circle. The experiments established that a period of low intensity light immediately before the dark period was detrimental to flowering and that this effect could be compensated by lengthening the total dark period. For instance, plants held in an 11-hr day of full light produced 26.3 flowers per plant; if the photoperiod included 5 hr of low intensity light, flower numbers fell to 11.6; but if the dark period was increased to 14 hr by cutting the full light part of the photoperiod, flower numbers were restored to 24.2, although photosynthesis was reduced even further. Since the treatment which restored flower numbers differed from the reduced light treatment merely in having 1 hr less of full light, gross lack of carbohydrate must be excluded from any explanation, particularly also as far as carbohydrate level during the light period preceding the inductive dark is concerned; this agrees well with Harder and Glimmer's results with very brief daily photoperiods. It also seems unlikely that merely reducing the intensity of illumination in daylight caused any changes in the equilibrium of the two forms of phytochrome and thus an explanation based on a change of Pr/Pfr balance may be ruled out. A possible explanation is that increased and inhibitory auxin levels produced under low light conditions are involved. 2 "

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SPECTRAL DEPENDENCE — PHYTOCHROME The effectiveness of light of different spectral composition needs to be tested in photoperiodically sensitive plants both during the main light period and also during the LB inserted in order to disturb the dark reactions. Fredericq2" investigated the effect of light from different parts of the spectrum during the minimum main light period required by Kalanchoe. His evidence points clearly to the fact that phytochrome is involved, R being by far the most effective in promoting flowering, while subsequent irradiation with FR diminishes flowering. The inhibition by FR following short main light periods was further confirmed by Fredericq,29 who also noted that the effect depended on light intensity; on the whole these observations agree well with data for other SDP. The LB effect in Kalanchoe is almost certainly mediated by phytochrome. The earliest tests on the effective spectrum were carried out by Wallrabe30 and demonstrated that light from the red part of the spectrum was much more effective than other wavebands. Biinning and Engelmann's experiments 31 also confirmed the high efficiency of R, suggesting the probable involvement of phytochrome in the photoperiodic responses of Kalanchoe. (See also Karve et al. 32 on flower movements.)

ENDOGENOUS RHYTHMS K. blossfeldiana has been subjected to particularly extensive investigations into rhythmic changes in photoperiodic sensitivity, metabolic function, and flower movement. Studies involving rhythmic changes which have been recorded for Kalanchoe are concerned, among others, with the following characteristics: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

Sensitivity to photoperiod (numerous authors c.f. below) Floral movements32 37 though this is not affected by ABA application38 Fresh weight of flowers' 9 Guttation rates40 Organic acid content 39 Respiration rates23 Phosphatase activity 41 Resistance to thermal death42 Stomatal opening in relation to photoperiod43 Rhythmic CCX output under the influence of phytochrome in K. fedchenkoi44 Change of malic enzyme activity synchronizing day/night functioning of CAM and stimulated by a dawn signal 45 Spontaneous changes of PEP carboxylase and malate dehydrogenase enzyme activities in a circadian pattern observed in extracts held in constant conditions46

In the present context, we are particularly concerned with rhythmic changes in the sensitivity to flower-inducing conditions. These have been investigated in two main types of experiment: 1. 2.

Involving combinations of light and dark periods differing more or less widely from the normal 24 hr to which endogenous circadian rhythms are tied Interrupting dark periods of up to 96 hr by a single light interruption given at different times

In both types of experiment, light given either as a full light period or as a LB should have favorable effects at times which coincide with the normal light period of a normal

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H"

15"

Time of light

W"

23

break (1hr)

27°'

30"

34"

3»"

42"

44

after start of 72 hr dark period

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50

FIGURE 4. Effect on flower numbers of LB given at different times in a 72-hr dark period. (After Melchers. G., Z. Naturforsch., Bl I , 544—548, 1956. With permission.)

cycle, say 11 hr light, 13 hr dark, if Blinning's theory of a close relation between endogenous rhythms and phtoperiodism is applicable. At other times, light should inhibit flowering. The first type of experiment has not produced much support for this theory-1 (cf. Figure 3). The second kind of experimentation, with LBs in very long dark periods, was first applied to Kalanchoe in a 72-hr cycle by Carr.47 Single LBs given during a 60-hr dark period promoted FI when falling into the period in which the plant would normally have received light in a 24-hr cycle; given at times when the plant would normally have been in the dark, they inhibited FI. This experiment was repeated by Schwabe48 and by Melchers;49 both confirmed the overall pattern of Carr's results. However, they also indicated that LB given near the beginning or end of very long dark periods, i.e., during the time of the dark periods of the first and last 24-hr photoperiodic cycle, were more inhibitory than LB near the middle of the 60-hr dark period. Thus, the data lent considerable support to the suggestion by Claes and Lang50 that the plant responds to two light periods, given with some dark interval of a few hours, as if there had been a period of continuous illumination. Melchers' data are shown in Figure 4. Similar data were obtained by Bunning and Engelmann. 31 - 51 Extensive studies on the rhythmic opening and closing of the flowers in Kalanchoe, particularly by Biinsow," have shown that if subjected to a variety of cycles of light and dark, the flower opening rhythm would adapt itself to a considerable number of these, (e.g., phase shifts studied by Johnsson et al.52). However, when rhythms of light and dark periods (of equal duration) become shorter than 6 + 6 hr, the capacity to adapt begins to fail and the flower movements become disturbed or regain their previous periodicity. Under constant light or dark, the movements are much reduced in magnitude and usually come to a halt after some time, entering a state of light or dark-rigor, but the onset of this may depend on light intensity and the carbohydrate status of the plant.33 The adaptability of such rhythms represents one of the difficulties of making valid assessments of the regulatory action of the endogenous rhythmic system. In Kalanchoe for instance, the flower movement is clearly more adaptable than the rhythm which may be involved in flower induction, where even a homophasic 8 + 8 rhythm is quite ineffective and a 12 + 12 rhythm tends to represent the critical level.

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More detailed attempts to interpret phase shifts in the floral movements of Kalanchoe have been made,"-54 but a detailed discussion of these would go beyond the scope of this chapter. It is still impossible to assess the degree of control exerted by endogenous rhythmic changes over the photoperiodic responses of Kalanchoe. However, there is little doubt that circadian rhythms are somehow involved in the control of flowering, but the system may operate decisively only under conditions of very long dark periods and in interaction with phytochrome setting.

THE EFFECTS OF TEMPERATURE The photoperiodic response of Kalanchoe is affected very much by temperature, as assessed quantitatively, using the number of flowers produced per plant as a measure of the inductive effects. In the first such investigation, on the night temperature/daylength interaction, Harder et al.55 noted a reduction of flowering as the temperature was reduced from 24 to 8°C. At the lowest temperature flowering was entirely suppressed. Schwabe18 showed that reducing the night temperature from 25 to 10°C caused flower numbers to decline by 94%, while 12°C in the photoperiod and 25°C in the dark caused a 51% reduction from a constant 25°C treatment. Reduction of the temperature in either light or dark slows down the promotive effects of flower induction. Flowering is also reduced at high temperatures, 30°C or above.56-57 The effect of a LB, on the other hand, is independent of temperature. 26 As expected, progress to flowering is also accelerated by higher temperatures (e.g., see References 58 and 59).

CO2 REQUIREMENT FOR PHOTOPERIODIC INDUCTION AND PHOTOSYNTHESIS Harder et al.60 found that CO2 was needed by Kalanchoe plants during the main light period preceding the inductive dark periods for normal induction subsequently to take place. Induced leaves kept in a CO2-free atmosphere during the main light period failed to promote initiation. In contrast, no external CO2 supply was needed to make long photoperiods inhibitory, a result later confirmed by Spear61 and Fredericq.62 Subsequently, Gregory et al."3 showed that Kalanchoe develops a mechanism for dark-fixation of CO, when subjected to SD conditions, or to other photoperiodic cycles which are capable of inducing flowering.64 The dark-fixation is usually associated with an initial loss of CO2 on return to light. Relatively high temperature has a similar effect and temperature-labile and photo-labile fixation products have been postulated.65 Although this mechanism develops relatively slowly, it was initially suspected that this net fixation of CO, in the dark might be an integral part of the biochemical reaction chain involved in photoperiodic induction. However, subsequent work66"61* with varieties 'Feuerbltite' and 'Tom Thumb' has shown that this somewhat optimistic suggestion does not hold true. Thus, plants given 8-hr SD treatment while grown in a dilute culture solution failed to develop a dark-fixation mechanism, and yet flowered normally. Net darkfixation of CO2 could then be initiated by increasing the nutrient concentration or treatment of plants with dwarfing compounds. 70 - 71 It seems more likely that the phenomena of flowering and dark-fixation of CO2 may be parallel results of inductive treatment, rather than causally related to each other. Leaf succulence also appears to be independent of flowering and darkfixation of CO2, but may be related to the organic acid level. Nevertheless, the CO, requirement for induction needs to be explained. The functional role of the leaf epidermis is of considerable interest in this context. 72 When stomata on the upper or lower surface of a leaf are blocked by Vaselining during induction, flowering is reduced by 59 and 72% compared to the controls, and flower induction is totally prevented by occluding both sets of stomata.

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However, stripping the epidermis (upper or lower) which opens the palisade and mesophyll to free access of CO, is also detrimental to flower induction, reducing flower numbers by about 90% even when wound repair over the whole surface has occurred. In K. daigremonfiaiid this has since been found to increase CO, fixation in the light, while largely inhibiting deacidificalion in the light. 7 -' There are several possible explanations, involving the need for stomatal functioning and/or specific epidermal effects, and the role of the epidermis needs further clarification. While in many plants the high light intensity reaction has been identified very largely with a requirement for photosynthate, in Kalanchoe the situation is somewhat more complicated because of its crassulacean acid metabolism and also because of the results of Harder and Glimmer 17 and Schwabe 18 that as little as 1 sec of light per day is sufficient to cause floral induction, which would a priori seem to exclude a major quantitative photosynthetic requirement. Nevertheless, it is known that the Kalanchoe leaves induced on the plant cannot produce the photoperiodic stimulus when held in CO, free air. This has been studied in considerable detail by Ireland and Schwabe, 7 4 - 7 5 who have attempted to replace the end products of photosynthesis, or other metabolites, by injection into the induced leaf in CO2free air. The experimental set-up was such that the youngest mature leaf was the only leaf that was induced by SD, all younger leaves, apart from unexpanded ones near the apex, having been removed; while all other expanded leaves on the experimental plant were in fact held in continuous light in normal air. The induced leaf itself then was injected at regular intervals (usually on two occasions during a 14 day induction period) with solutions of the metabolites to be tested. These may be listed as follows: glucose succinate isocitrate 3-phosphoglycerate ADP NADPH, 4-amino-«-butyric acid tryptophan

sucrose citrate a-ketogluterate oxaloacetate ATP serine cystein proline

malate fumarate glycollate AMP NADP glutamine cystine

However, it was found that in no case was it possible to substitute for the presence of CO2 in the leaf under these circumstances. Another series of experiments was carried out attempting to test the effect of photosynthetic inhibitors on the capacity of leaves to become photoperiodically inductive when held in SD and normal air. Again single leaf induction with the rest of the foliage in continuing light in ordinary air being employed. Here again the results are quite unequivocal and 10 s DCMU urea substantially reduces floral induction. Salicylaldehyde (5 x 10~ 4 M) and antimycin A also diminished flower induction by SD in ordinary air. The negative outcome of all these experiments has led to the suggestion that a product of the photosynthetic reaction, which may be involved in the photoperiodic stimulation, may be a minor byproduct of photosynthesis, but may be crucial in the reaction system leading to the production of a flowering promoter in the induced leaf itself.

TRANSLOCATION AND GRAFT TRANSMISSION Early work on translocation of the SD stimulus concentrated on the movement of the hypothetical succulence-inducing factor "metaplasin," 76 which appears to move in the same orthostichy as the induced leaf, but with some lateral spread. The same pathway is likely

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to be involved in the movement of the flowering stimulus. Harder and Gall 77 succeeded in separating these two effects by exposing plants to 0.005% chloroform during induction, when succulence increases occurred in the absence of floral stimulation. The mode of action of the effect remains unknown. It is an observational fact that induction of the terminal apex always appears to precede that of lateral branches, even if the induced leaf subtends an axillary shoot, though this will generally become induced earlier than other axillaries. Harder10 failed in all attempts to get graft transmission of the flowering stimulus in Kalanchoe, but in 1954, Carr and Melchers7* succeeded in transmitting the flowering stimulus from induced plants acting as donors to noninduced receptors. In these experiments the donors were placed as wedge grafts into the split, decapitated stem of the receptor and merely tied with raffia. Flower induction subsequently occurred in lateral, completely defoliated branches produced below the donor. Zeevaart79 also included Kalanchoe in his extensive studies of translocation of the flowering stimulus in several genera and species. The flowering response of Kalanchoe receptors tended to increase when the Kalanchoe donors comprised some stem and more than one leaf. He also reported positive transmission of the flowering stimulus from K. blossfeldiana to Sedum spec-labile and S. ellacombianum and from both these LDP to Kalanchoe. Similar results were obtained in cross-grafting experiments with Echeveria harmsii.*0 Attempts by Schwabe81 to use isolated leaves which had been induced when separate from the plant as donors, grafted into the stem some distance below the growing points of noninduced receptors, failed to give any induction. Although the leaf grafts took with little difficulty, no transmission of the stimulus was found, regardless of whether the receptors were defoliated or not above the graft. However, in view of Carr's and Zeevaart's results, the failure in obtaining graft transmission from such leaves may well be due to some difference of technique only. FAILURE TO FLOWER IN LD AND SPECIFIC INHIBITORS OF FLOWERING Investigations of fractional induction in Kalanchoe have led to studies of the effects of noninductive photoperiodic cycles. It has been known for some time that such cycles are inhibitory, 21 and the inhibitory effect was interpreted in relation to that of leaves in unfavorable day lengths when interposed between the induced leaves and the apical meristem. 82 The inhibitory effect itself was shown to be restricted to the same orthostichy as the treated leaf, much as had been shown for the morphogenetic effects of a hypothetical SD substance (metaplasin). Later experiments by Harder and Biinsow1" showed that inhibitory effects of LD, given prior to SD induction, could be also detected. Thus effects on translocation of the inducing stimulus could not be involved and the authors stated that the LD effect acted as "Bliihhemmstoff". If mature Kalanchoe plants are given 12 to 14 SD under optimum conditions each plant may produce several hundred flowers. Interruption by a single LD in the middle of an induction period of 12 SD reduces flower numbers by approximately 50%. Insertion of single LD after every 4 SD (out of 12) cuts flower numbers by 90% and insertion of a LD after every 3 SD by 98%. Alternation of LD and SD, until a total of 12 SD has been given, prevents flowering altogether. 16 It is clear, therefore, that even when all treatments receive the same number of inductive photocycles, the symmetrically intercalated LD have a very strong inhibitory effect. This can be assessed quantitatively by calculating how many of a fixed number of 12 SD are "annulled" by each intercalated LD, using a calibration curve of log flower numbers vs. days of induction. This calculation shows that each intercalated LD can destroy the effect of approximately 2 SD. A similar calculation carried out with another parameter for assessing the degree of induction, the reciprocal of the number of the

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days from induction to the first appearance of the inflorescence, yields an almost identical result. A SD with a LB in the middle of the dark period has exactly the same effect as a LD. In an experiment in which a total of 14 SD were given with LBs of 30-sec duration in the middle of the fifth and tenth nights (i.e., a total of 1 min extra light in 14 days), flower numbers were reduced to only 10% of the controls."' These responses raised a number of questions. If the inhibitory effect of LD were cumulative, maintaining a plant in LD conditions should lead to the accumulation of so much inhibitor that any subsequent induction by SD would be difficult. Moreover, the minimum induction period should then be proportional to the previous LD period. This argument against flower inhibition was used by Borthwick. 84 Nevertheless, however long the previous LD treatment, under optimal conditions the variety of Kalanchoe used shows the first signs of flower induction with a minimum of 2 or 3 SD. Also, when plants were exposed to 12 SD in two groups of 6 separated by increasing numbers of LD (up to 16), it was found that the inhibitory effect hardly increased as more LD were given, at least up to 12 LD, by which time the morphogenetic changes due to the first 6 SD had already begun to take place. 16 One may conclude from this that there is an upper limit to the amount of inhibition which can be accumulated and that this is approached after 1 LD. Once it is recognized that there is such a limit, the argument against counter-induction can be dismissed. Further experiments were undertaken to discover whether the inhibitory effect acts on the inductive stimulus produced during the SD preceding the interrupting LD, or whether the inhibitory effect prevents succeeding SD from becoming effective. The method used was to separate the inductive SD from the intercalated LD by a period which would allow either the SD effect to become consolidated before exposure to the LD, or the inhibitory effect to disperse before exposure to the next SD. Ideally, this neutral period should be neither promoting nor inhibitory, i.e., have neither SD nor LD effects, since it is known (cf. Figure 3) that a 16-hr light period followed by a 32-hr dark period does not induce flowering by itself, a 24-hr dark period was chosen as a neutral period to separate the LD and SD. Added to a LD this yielded 16 hr light/32 hr dark; added to a SD it gave 8 hr light/40 hr dark. Since it was known that alternating SD and LD prevent flowering, 1685 this set of cycles was chosen for the test. With a constant total of 12 SD, the 24-hr dark period was inserted, either after the SD or after the LD. An alternation of SD and 24-hr dark periods, and an alternation of LD and SD, days, served as controls. The results shown in Table 1 are very clear-cut indeed and suggest strongly that insertion of the 24-hr dark period immediately after the LD allowed dispersal of the inhibition to take place, thereby protecting the succeeding SD from its effect. By contrast the 24-hr dark period inserted immediately after the SD does nothing to consolidate the SD induction. This result is easier to understand in terms of an inhibition produced in LD which reaches a maximum and which will prevent succeeding SD from having an effect, rather than a mechanism by which a light period of say, 8 hr, is harmless to a previously manufactured promoting substance, but which tends to destroy the latter if the light period is extended to. say, 16 hr. If a single LD is inserted at various times among 12 SD, its effect is somewhat more inhibitory in the middle of the SD period than near the beginning or the end. Although this is somewhat reminiscent of LB effects in a single dark period, the mechanism is unlikely to be similar. In explanation of these phenomena Schwabe' 6 1X put forward an hypothesis according to which photoperiodic induction leads to the production of an adaptively formed enzyme which in turn catalyses production of the flower hormone. The effect of LD, LBs, and other inhibitory conditions would operate through the production of an inhibitor which interferes with the formation of the enzyme. The level to which the inhibitor can accumulate is limited

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CRC Handbook of Flowering Table 1 EFFECT OF 24-HR DARK PERIODS (24 D) PRECEDING OR FOLLOWING LD, THEMSELVES ALTERNATING WITH SD, ON THE FLOWERING OF KALANCHOE16 Order of cycle

No. plants budded

Mean no. flowers per plant

2 5 12 12

0.8 2.5 158.1 174.6

SD:LD:SD: SD:24D:LD:SD: SD:LD:24D:SD SD;24D:SD: Note: Total of 12 replicates.

and the maximum may be attained after little more than 1 LD. Inhibition and promotion can then be quantitatively related, and as more of the enzyme is produced inhibition becomes less complete. It could be predicted, therefore, that such changes should lead to a change in the point of balance, i.e., in the critical daylength. Two experiments designed to test this hypothesis have borne out the prediction:86 if induction periods (with 8-hr SD) which comprised too few cycles to give full flowering were followed by cycles with light periods longer than the critical, these nevertheless promoted flowering, but they did not do so in controls without partial induction, or if presented in reverse order. In a similar experiment with a prolonged period of 12V 2 -hr days following varying amounts of induction, flowering was again promoted compared with controls. Hence the suggested adaptive formation of the hypothetical flower hormone-producing enzyme was supported as well as the inhibitor hypothesis. Harder's study on the effect of daylength following SD induction is very relevant here, and his results agree with those above.87 Unfortunately, the promotive daylength of 12 hr which he used is still slightly inductive even on control plants and thus could not be regarded as evidence for the shift in the critical daylength. In the interpretation of the experiments described earlier, in which the photoperiodic effect of LD, intercalated among SD, was interpreted as the production of an inhibitory substance in the induced leaf itself which prevented the inductive SD treatment from becoming effective for at least 2 SD cycles, attempts have been made to determine whether such inhibitory substances can be demonstrated as such. Extensive experiments using the injection technique into leaves have demonstrated that sap expressed from LD leaves, without any further concentration, itself contains a potent inhibitor of flower induction compared with sap extracted from SD leaves which served as control. The results of some 20 experiments of this nature have been tabulated in Table 2. Attempts are now being made to isolate and identify, if possible, the substances involved in this inhibition, which in many ways is now based on firmer evidence than any of the flower-promoting (florigen) types of hypothesis. Preliminary tests carried out so far suggest that the substances involved are, in fact, phenolic in nature, and may represent relatively small molecules, which are being produced under these circumstances. However, more work will need to be done before positive identification can be achieved. Schwabe and Wimble88 have incorporated these results in a model of possible reactions which incorporates all the known results and which is also applicable to LD-requiring species and other response types. However, a full account of this schema cannot be given here for lack of space.

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Table 2 EFFECT OF INJECTING CRUDE AND PARTLY PURIFIED INHIBITOR "SAP" INTO LEAVES DURING INDUCTION Crude sap

Expt. no. 1

7

3 4 5 6 7 8 9 10 Mean

Centrifuged sap

Flowering as % of that of plants injected with SD sap

Expt. no.

Flowering as % of that of plants injected with SD sap

63 58 54 29 26 13 43 73 23 38 42.0

II 12 13 14 15 16 17 18 19 20 Mean

54 10 23 6 66 107 52 0 41 65 42.4

Note: The "sap" consisted of expressed material from LD-treated leaves after centrifugation, but without concentration. Injections were made on two occasions during a 14-SD-induction period (on 5th and 10th days). Similar plants injected with "sap" from SD-treated leaves served as controls.

EFFECTS OF GROWTH REGULATORS Harder and van Senden89-90 showed that IAA applied during induction was detrimental or inhibitory to flowering in K. blossfeldiana. In these experiments leaves above those which were being induced were immersed in solutions of IAA ranging in concentration from 0.00005 to 0.05%. IAA was also inhibitory if supplied to the leaf below the induced one, causing increased leafiness of the inflorescence and reduced flower numbers. TIBA, sometimes regarded as having anti-auxin activity, also caused flower inhibition, as well as malformaion of stems and leaves. 91 - 92 Exposure for 2 to 3 days to ethylene vapor (1 |x£/€ air) caused leaf abscission and closure of flowers, which was permanent except in previously unopened buds.93 Harder and Biinsow94"96 investigated the effects of GA on flower induction in Kalanchoe and again only inhibitory effect on flowering were noted. Concentrations used ranged from 0.1 to 50 ppm and in all experiments these led to excessive elongation of internodes and leaves. When previously induced plants were treated, excessive elongation of peduncles took place (cf. Schmalz97). Tests with ABA and xanthoxin98 injected into induced leaves at 50 and 100 ppm, respectively, also prevented induction in SD. It is worthy of note too that the extracts made from leaves of SD-induced plants (used as controls in the bioassay of LD-produced inhibitors) have on no occasion shown flowerpromoting activity. Hence, evidence for flower-promoting effects rests on the promotion found in grafting experiments. K. blossfeldiana in its growth habit responds readily to dwarfing compounds such as ancymidol (e.g., see Reference 99).

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CHEMICAL, HISTOCHEMICAL, AND OTHER CHANGES CORRELATED WITH PHOTOPERIOD TREATMENT AND FLOWER INDUCTION Generally, analytical studies of such nature can only establish degrees of correlation with flowering, but will need to be supported by other more cogent evidence before causality can be established. Chromosome numbers in the leaves of Kalanchoe revealed a degree of polyploidy which varied in relation to the daylength condition to which the leaves were exposed.100-"" LD leaves showed a high incidence of octoploidy, while the highly vacuolated cells in the SD leaves were commonly 32-ploid. Schwabe and Wilson 102 investigated the apparent viscosity of the cytoplasm of mesophyll cells in the leaves of Kalanchoe plants subjected to a variety of photoperiodic conditions. This was done by a centrifugation technique. 7 They found considerable differences in this physical character associated with daylength and LB treatments. The effectiveness of R LB suggests that these changes are subject to phytochrome control. Immediate effects of light and darkness can also be demonstrated, but these are superimposed on diurnal variations. Viscosity increased progressively with increasing exposure to SD, particularly with prolonged SD treatment. Changes in the amounts of nucleic acids 5 to 7 days after the start of inductive treatment have been recorded in the leaves (especially younger ones) using dry matter as a basis for expressing the results. 101 An initial rapid rise of RNA is followed in the tip region of the plant by a steep fall to the original level and ultimately a slow decline. The DNA content changes follow exactly the opposite pattern. Older leaves show generally similar changes though of lesser magnitude. Growth hormone contents and their diurnal variations showed no clear relation to the inductive processes.39 Leaf anthocyanin content, however, is clearly related to daylength treatment.104 In LD or LB conditions only traces of anthocyanins are found, while considerable amounts are formed in SD, the major component in the leaves being chrysanthemin. The first products of photosynthetic carbon dioxide fixation in Kalanchoe exposed to LD and SD conditions were investigated by Norris and Calvin, 105 but the quantitative differences found did not suggest any fundamental differences in pathway or products which might be related to induction, e.g., increased C 14 fixation in aspartic acid under LD when expressed on a fresh weight basis. A great deal of work on the underlying mechanisms of the crassulacean acid metabolism and dark-fixation products has been carried out. 1 " 6 -" 17 Kunitake et a j ]*tf"^^>'

'^&3S^ FIGURE 4. Microtome section (in parallel to the frond surface) through a frond pocket of L. gibbu. Visible are some tissue of the mother frond (at the margin), the well-developed first daughter frond (the whole central part), and a young primordium of a further daughter frond (in the middle, above). The first daughter frond possesses two pockets: the + pocket on the left-hand side with primordia of a granddaughter and a great-granddaughter, and the - pocket on the right-hand side with a granddaughter primordium (in the basal angle of the — pocket) and an inflorescence primordium with dark-stained primordia of the 2 stamens (in the upper part of the —pocket). (Magnification X240.) (Photograph courtesy of R. Wolf, University of Wurzburg, West Germany.)

EFFECTS OF PLANT AGE Aging of plants includes accelerated senescence in certain organs. This process can have some back-effects on the morphogenetic potentialities of meristems. One such effect may be the more-or-less irreversible transition from juvenility to maturity: the state, in which new produced organs are sensitive to flower-inducing factors. InLemna the stage of juvenility is rather short: most seedlings of L. aequinoctialis 6746 show a low but distinct flowering even 2 weeks after germination;67 5 weeks later the flowering of seedling descendents reaches 70 to 92% compared with the vegetatively cultivated parent strain, if the plant material is cultivated in 1/2-strength Hutner's medium. A slower transition from juvenility to maturity has been found in experiments with M medium (Hoagland-type) and a 1/2-strength KNO3 modification of Hutner's medium. Another effect of frond senescence on developmental processes at the meristem is the reduction of frond size and longevity of daughter fronds, which can be reverted in the following frond generations (see Figure 1). Simultaneously flowering of daughter fronds is also influenced. In the LDP L. gibba Gl only the first members of a sister-frond series produce flowers under LD conditions. In the SDP L. aequinoctialis 6746, on the other hand, the last members of a sister-frond series come into flower even under noninductive (LD) conditions. 101 Therefore senescence of the mother frond has a flower-inhibiting effect in the LDP, but a flower-promoting effect in the SDP under LD. Induction of flowering by frond senescence under LD has been observed also in the day-neutral S. polyrrhiza.^24

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VERNALIZATION A positive effect of long-lasting low temperature on flowering has been found occasionally in Wolffia arrhiza, but this result could not be confirmed in further experiments." 7

PHOTOSYNTHESIS In L. aequinoctialis 6746 flowering can proceed in complete darkness, if sucrose is added to the nutrient medium. 50 - 202 Also, heterotrophically cultured pale frond colonies of this strain flower under inductive skeleton photoperiods (see Endogenous Rhythms). 5 ' 1 From these results it may be concluded that the only effect of photosynthesis is the supply of sufficient amount of photosynthate. Experiments under more "normal" conditions have shown, however, that photosynthesis is not only a prerequisite for flowering in Lemna, but can also have effects — depending on mineral nutrition and photoperiodic reaction type of the species — which encounter other flower-regulating factors. In Hoagland-type media the flower-inhibiting effect of LD or NB can be counteracted by increase of light intensity in L. aequinoctialis 6746 (SDP). 10 - 196 Sucrose feeding supports but not replaces high intensity light. Photosynthesis seems to be responsible because 2 x 10~ 7 M DCMU blocks the effect of high intensity light (with only a very small effect on growth rate). l97 The opposite results are obtained, however, when a modified Hutner medium is used. High intensity light in this case is inhibiting to flower formation under LD conditions and DCMU repromotes flowering. 186 A photosynthetic mutant, strain 1073, produced from wild type 6746 by X-irradiation, which has a block between plastoquinone and cytochrome f, shows similar behavior to the DCMU-treated wild type material. In 1/10 Hutner medium sucrose inhibited flowering even under inductive SD in L. aequinoctialis 6746 (see Sugars)."" This effect too is reversible by DCMU. In conclusion, photosynthesis can act as a flower-inducing or a flower-inhibiting agent, opposing other factors depending on composition of the nutrient medium. The flowerinducing effect of high intensity light depends on the presence or availability of copper in the nutrient medium.55 l96 Addition of the chelating agent EDTA cancels the LD flowering in L. aequinoctialis 6746. 1052 The flower-inhibiting effect of sucrose under SD, on the other hand, is restricted to a medium with low calcium and phosphate content.180 Only little is known, about the way by which photosynthesis may act on flowering besides the supply of assimilates. Posner et al. V) IS6 argue on grounds of their experimental results that photosynthetically produced reductants but not ATP may be important for FI in L. aequinoctialis. In the LDP L. gibba G l , on the other hand, photosynthesis seems to act on flowering also by ATP production. The flower-promoting effect of DCMU in 1/10 PirsonSeidel medium can be imitated by application of ADP in this species.94 Experiments with uncouplers of photophosphorylation (arsenate, atebrin) have led to the conclusion that DCMU acts on flowering in L. gibba through enhancement of cyclic photophosphorylation. 92102

PHOTOPERIODIC RESPONSE Most of the different photoperiodic reaction types have been described up to now within the ten species which have been brought to flowering under controlled conditions up to now. Already in L. aequinoctialis qualitative SD, DN, and quantitative LD responses have been found in the diverse clones.225 In Table 1 all the strains are listed and provided with additional statements on critical daylength (CDL) and minimal number of inductive light-dark cycles (MNIC) as far as it is known. The well-known strain 6746 of L. aequinoctialis (formerly named "L. perpusilla" or "L. paucicostata") has been classified as a qualitative SDP, but it should be mentioned that this

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Table 1 PHOTOPERIODIC REACTION TYPES IN LEMNACEAE Reaction type Qualitative long-day plants Lemna gibha L.

Lemna minor L. Lemna turionifera Landolt ( = "L. minor 1") Quantitative long-day plants Spirodela punctata (G. F. W. Meyer) Thompson (= S. oligorrhiza (Kurz) Hegelm.) Lemna aequinoctialis Welwitsch ( = L. paucicostata Hegelm.) Qualitative short-day plants Lemna aequinoctialis Welwitsch ( = L. paucicostata Hegelm. = "L. perpusilla" of several authors)

Lemna perpusilla Torrey Wolffm microscopica (Griff.) Kurz Quantitative short-day plants Lemna aequinoctialis Welwitsch ( = L. paucicostata Hegelm.) Wolffia microscopica (Griff.) Kurz

Strains Gl G3

CDL: MNIC:

13 hr87 i^ 16 - 141 - 158 - 159 6 (mature flowers) 1 " ll-Shr'-159 10.5 hr 14 ll.Shr'4 9.5 hr 5

CDL: G3-C CDL: G3-K CDL: 7007 CDL: Strain Odranci120 Strain Petanjci, strain Obrov 122 6566, 6583, 6729, 6745, 6751 125 6573 CDL: 10.5 hr5 Unnamed strain,' strain Maribor120 Strain Barje, strain Doklezovje 122 6573, 6601, 6619, 6727, 6734, 6736 6739, 68531-5

05'"5 S type strains: 331, 325, 315, 261, 241 7 - 225

6746

MNIC: CDL:

[50

14.5 hr5" 13.5 hr' 8K T-101 CDL: 13 hr' 51 Strain Sohna MNIC: I 41 CDL: 13—14hr" N-l type strains (MNIC: 1—2): 441 CDL: 14.5 (14)" hr 14.5 (14.5) 432 14.75 (14.5) 421 14.5 411 (14.0) 14.5 (14.0) 401 14.0 (13.5) 392 14.75 (14.25) 371 365 14.75 (13.5) (13.75) 14.0 361 13.5 (13.25) 352 14.0 (12,75) 354 12.75 (12.0) 341 (12.5) 13.0 345 335 12.75 (11.75) 13.25 (11.75) 321 N-2 type strains (MNIC: 3—5):7-225 391 12.75 (10.5) 381 13.5 (11,75) P1468' Strain Sohna (in Hoagland medium) MNIC: I2'7 CDL: 16 hr 217 151222 Strain Sohna (in Bonner-Devirian medium)217

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CRC Handbook of Flowering Table 1 (continued) PHOTOPERIODIC REACTION TYPES IN LEMNACEAE Strains

Reaction type Wolffia hrasiliensis Weddell (= W. papulifera Thompson) Long-short-day plants Wolffia arrhiza (L.) Horkel ex Wimmer Day-neutral plants Spirodela polyrrhiza (L.) Schleid. Lemna aequinoctialis Welwitsch ( = L. paucicostata Hegelm.) Photoperiodic reaction type unknown Lemna aequinoctialis Welwitsch ( = L. paucicostata Hegelm. = "L. perpusilla" of several authors)

Unnamed strain" 1 Strain Velica Polana 122 Strain Odranci,120 Petanjci, Zvirce, Velica Polana, Lenart, Dobrnifi' 22 K type strain 351 225 6609, 6612, 6748, 700112S

Note: Strains of the diverse species are listed with their collection numbers. CDL and MNIC are noted in addition as far as possible. Values of critical dark periods7-225 were converted to CDL; values without brackets determined under 25°C (light)/23°C (darkness); values within brackets determined under constant temperature of 25°C.

is true only under certain environmental conditions (see Spectral Dependence, and Effect of Mineral Nutrition). In contrast to several other strains of the same species,which show a qualitative SD response in a strict sense,225 strain 6746 should be denoted as a "conditional SDP". 149 SPECTRAL DEPENDENCE Main Light Period Photoperiodic responses depend strongly on the spectral composition of light sources. Particularly changes in the balance of B, R, and FR spectral regions during the main light period can alter the effect of daylength and of additional light pulses given during the dark period. In the LDP L. gibba, strains Gl and G3, flowering under LD is enhanced, if a FRcontaining white light is used instead of white light without FR. 878893 In addition, the effectiveness of a NB with 10 min R as well as of 10 min FR terminating the main light period increases in strain Gl, if the radiation during the preceding SD contains FR.93 Furthermore, L. gibba Gl cultivated in an "aged" medium and irradiated with a FR containing light source initiates flowers even under SD.S9 Irradiation with only R or FR light has a strong flower-inhibiting effect in comparison to white or B light in L. gibba G3. 1 7 3 1 As little as 1 hr white light per day given within the otherwise continuous R light enhances the flowering response distinctly. 17 In L. aequinoctialis 6746 the effect of white light has been compared with pure R, B, and FR light. B or FR given instead of white or R during the whole light period cancels both the flower-inhibiting effect of LD 31 - 58 - 149 - 2 " 2 and the effectiveness of a short end-of-day FR light. 84 A R NB is effective more or less independently of the spectral composition of radiation during the preceding SD in strain 6746, 6184 but the R/FR reversibility of NB is restricted to experiments with B or FR main-light periods. 59 - 84 The optimal FR or B/R ratio for flower induction has been tested under continuous light conditions. 31 The values are FR/R = 1.0 and B/R = 4.9 in L. gibba G3 and FR/R = 14.8 and B/R = 13.5 in L. aequinoctialis 6746. In both plants, the optimal ratio of FR/R or B/ R for flowering is always greater than that for frond production (these amount to FR/R = 0.25 and B/R = 0.31 in strain G3 and FR/R = 0.95 and B/R = 0.77 in strain 6746).

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Experiments with continuous monochromatic irradiation of 3.5 to 4.3 kergs crrr 2 sec ' have given the following results. 85 In L. gibba G3 flowering proceeds under wavelengths near 400 nm and from about 530 to 655 nm (with a peak at 543 nm) and in L. aequinoctialis 6746 under wavelengths shorter than 550 nm and longer than 700 nm. With a very low level of light intensity (20 erg cm 2 sec" 1 ), however, flowering has been achieved even under continuous white or R light in L. aequinoctialis 6746.202 At this point it should be emphasized again that strain 6746 cannot be claimed as a typical representative of the SD strains of L. aequinoctialis. Strains 335,421 (N-l type) and 391 (N:2 type) show no flowering response under continuous B or FR light as well as under continuous low intensity light (10 lx). 225 Also strain T-101 cannot be induced to flowering by pure B or FR light (with daily light periods from 8 up to 16 hrs). 151 Unequivocally many of the referred results can be interpreted on the basis of phytochrome actions and this has been done by most of the authors. 58 - 61 - 85 - 89 - 151 - 202 Nevertheless, some of the data give a hint that blue-absorbing pigments and/or the so-called "high-energy reaction" and/or the Emerson effect of photosynthesis may also be involved. l7 - 31 - 84 - 89 - 91 This question should be investigated further. End-of-Day Treatment Irradiation with narrow-band spectral regions has been tested also after termination of the main light period. 3I - 60 - 82 - 83 Most interesting in this connection are experiments with relatively short light pulses, in which a phytochrome action can be demonstrated. In L. gibba G l , 10 min of FR following a noninductive SD induce flowering. 88 - 89 This FR effect is abolished completely by 1 min R light given after FR.89 The later FR is applied in the course of long dark period, the longer FR irradiation is needed to cause a flower-inducing effect. 88 An hourglass mechanism seems to be responsible for the change of sensitivity to this phytochrome effect as in Pharbitis.201 In the SDPs L. aequinoctialis 6746 and L. perpusilla P146 endof-day FR is inhibiting to FI. 8 '- 188 In this case, too, R/FR reversibility and the gradual decrease of FR effectiveness during the dark period have been shown. 138 - 188 As already mentioned (in Main Light Period) end-of-day FR is flower inhibiting in strain 6746 after a main light period of R, but not of B light. 84 Further modifying factors of the end-of-day FR effect in strain 6746 are light intensity and duration of the foregoing SD and the copper content of the nutrient medium. 196 Action spectra for the light inhibition of flowering at the beginning of an inductive 72-hr dark period and its reversal have been determined in the SDP L. aequinoctialis T-101. 153 For flower inhibition a symmetrical action maximum is seen at 730 nm and a minor peak at 412 nm with a shoulder near 447 nm. The action spectrum for reversal of the FR inhibition of flowering shows a broad action maximum around 612 nm, with a half-band width of 125 nm. The authors concluded that both the investigated light pulses act exclusively through phytochrome. This could be substantiated by experiments with etiolated plants, in which the screening effect of photosynthetic pigments is lacking. 152 The action spectrum for reversal then shows the typical phytochrome peak near 660 nm and, in addition, a peak in the near-UV (380 nm). 154Light sensitivity in the R reversal of FR inhibition of flowering in etiolated plants was 10- (612 nm) to 33-fold (660 nm) higher than that of green plants. NB Treatment In L. aequinoctialis 6746 a brief interruption with R light given at various times during a long dark period has a maximal flower-inhibiting effect, when applied 7 or 9 hr after the beginning of darkness.50-188-202 The involvement of a physiological clock in this phenomenon is discussed in the section Endogenous Rhythms. At this point it should be stated only that phytochrome seems to be the sensor pigment. R/FR reversibility can be demonstrated under conditions which abolish the effectiveness of the end-of-day FR (see Main Light Period,

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and End-of-Day Treatment). 59 - 84 It", on the other hand, the two phytochrome effects — the oscillator-depending R effect and the hourglass-depending FR effect — are active concurrently and in the same direction, then both effects seem to be additive and therefore no R/ FR reversibility can be seen. 50 - 18 " Other SDPs, in which a NB with R light is effective, are Wolffia microscopica2" and W. brasiliensis.1" A combined action of the rhythmic and nonrhythmic phytochrome effect presumably provides the explanation for several experimental results obtained with diverse colored light schedules in L. gibba G3, too.82-83 In strain G3 threshold conditions for flowering are needed for NB effectiveness. I7 - I05J06J43 In comparison to strain G3, L. gibba Gl shows a higher sensitivity to a R light pulse given 10 hr after beginning of the daily long dark period.93 Possibly the primary steps of the two phytochrome effects are located at different sites in plant cells and/or organs, 81 and the respective phytochrome populations have different properties (rate of dark reversion, for example). Therefore, the so-called null response method used by Kato, i.e., application of various mixtures of R and FR for determination of the phytochrome state during darkness, may give only limited information on the real phytochrome situation. Kato found a rhythmical change in the duration of high Pfr level, which is maintained during darkness after photoperiods of different length. 105 •"*'

ENDOGENOUS RHYTHMS In L. gibba G3 several metabolic activities show circadian rhythmicity under continuous light and constant temperature (for review see Reference 170): oxygen uptake, 134 •'" I4CO2output from glucose-1- I4 C (through pentosephosphate pathway),'-" activity of glyceraldehyde-3-phosphate dehydrogenases3(>38-135 and acid phosphatase,135 RNA synthesis, 144 - 145 activity of nuclear RNA polymerase I, 146 potassium uptake," 5 " 8 - 213 magnesium uptake," 5 and electrolyte efflux." 3 No circadian oscillations have been found, however, in strain G3 for titratable acidity of crude extracts, content of some organic acids and the total pentoses, the respiratory quotient (RQ),135 ATP level," 4 energy charge," 4 RNA polymerase II,' 46 and calcium uptake." 5 Similar investigations have been done with L. aequinoctialis 6746. Under continuous light (but not darkness) a circadian rhythm of phenylalanine ammonia-lyase and tyrosine ammonialyase activities was demonstrated.35 In contrast toL. gibba, diurnal oscillations of potassium uptake are displayed only in the dark and not in the light.' 15 The same is true for CO2 output in strain 6746. In darkness following either continuous dim R light or entrainment to a 12 (12) light (dark) schedule, the rate of CO2 output oscillates with a circadian periodicity, before damping after 2 days. In continuous R light, the rate is linear. 62 The CO2 output rhythm in darkness is prolonged if entrained by a daily short R light (acting through phytochrome).64-66 O2 uptake in strain 6746 is essentially parallel to CO2 output in all conditions tested.71 Therefore, the gas exchange oscillations reflect a circadian rhythm of respiratory metabolism. (Distinctness and the pattern of CO2 output oscillations are modified strongly by different sources and amounts of nitrogen in the nutrient medium.63-65-66-68-70-72-73 Furthermore, the picture becomes even more complicated by a variety of reaction types, if the results from other strains of L. aequinoctialis are considered.72) In another strain of L. aequinoctialis and in Wolffia microscopica diurnal fluctuations of nitrate reductase activity were observed in both continuous light and continuous darkness, at least for 1 or 2 days. 3 - 4 - 22 The cited rhythms can be taken as a proof that in Lemnaceae, as in other plants, physiological clocks (oscillators) are regulating several metabolic pathways. The significance of such oscillators for the process of FI maybe manifold, but until now only the use of oscillators for time measurement in photoperiodism has been worked out in greater detail. As in Kalanchoe, Pharbitis, (see those chapters), and other plants, also in the Lemnaceae a

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rhythmic change of sensitivity to flower-inhibiting or -inducing light pulses during long dark periods has been elaborated (for SDP L. aequinoctialis 6747 see References 50, 57, 137, and 150; for LDP L. gibba G3 see References 141. 142, and 143). In his investigations, Hillman used heterotrophic cultures of strain 6746. which received "skeleton photoperiods" and later on "filled skeleton photoperiods". that is a series of 2,4. or 6 short light pulses equally distributed during the time of regular photoperiod/ 6 ''* w In this way he demonstrated the influence of photoperiod duration on the time of maximal light sensitivity during dark period (compare also the work of Oda I M ) ). A 6-hr increase in length of the light period delays the time of maximal sensitivity by 3.6 hr (measured from the start of each light period). These results are important, because Hillman could show that the circadian changes of CO, output in strain 6746 are entrained by skeleton photoperiods in the same way. If the plants are cultivated on a nutrient medium with nitrate, the time of maximal CO, output depends on the length of daily photoperiod. Also in this case a delay of about 3 hr is caused by a 6-hr increase in photoperiod. So it seems reasonable to conclude that both rhythms, light sensitivity of flowering and respiration are dependent on the same timer. Nevertheless, both processes can be uncoupled: replacement of nitrate by aspartate in the medium modifies the entrainment of CO2 output but not the photoperiodie control of flowering (at least as estimated by critical daylength). From Hillman's results it can be deduced that the "dawn" as well as the "dusk" signal of a filled skeleton photoperiod participates in entrainment of a circadian oscillator, which determines the position of the photoindueible phase during the diurnal cycle. With a great number of experiments using special light-dark schedules, Oota and group have analyzed the connections between endogenous circadian rhythms and flowering in L. gibba G3.' 6 I J 6 3 J 6 6 - I 6 8 J 7 0 J 7 2 A nutrient medium without chelating agents has been used throughout the work and the results are valid especially for these conditions. l f > 1 The following conclusions can be drawn from the results. Entrainment and phase shifting of the circadian oscillator(s) are accomplished by both dawn and dusk of complete or skeleton photoperiods. A light pulse of at least 0.5-hr duration given during the first half of nyctiperiod is registered after two transient cycles as delayed dusk ("false dusk"). The main light period and night interruption act together as an asymmetric skeleton photoperiod. A 0.5-hr light pulse during the second half of nyctiperiod is perceived as advanced dawn ("false dawn"). Also in this case two cycles with NB are enough for completion of entrainment. Flower induction needs at least two cycles, in which (1) dawn and dusk signals are kept apart 12 hr or more and (2) a photoindueible phase (presumably identical with so-called "L2-phase" 161 ) appearing 12 to 15 hr after dawn is lightened. Therefore, NB during the first half of nyctiperiod has a dual role: to function as a delayed dusk signal and to operate on the photoindueible phase. The two light actions are distinguishable by different requirements. False dusk needs 30 min of strong R or white light (reversible with FR) and is effective on flower induction not before two transient cycles. The photoindueible phase needs only 5 min of low intensity B or FR irradiation (not reversible with R) and is sensitive immediately. (False dusk with R light seems to accomplish the light requirement of photoindueible phase on account of a contamination with FR.) A NB during the second half of nyctiperiod (false dawn) shifts the circadian rhythm within two cycles as mentioned above. Then the main light period fulfills the requirements of photoindueible phase and dusk.

PHOTOPERIODIC INHIBITION If flowering plants of L. gibba Gl are transferred to SD, initiation of flower primordia in the youngest frond generation is stopped immediately. In most cases, flower primordia and young organs are dying and shrinking to an ivory "mummy" (see Inflorescence Differentiation). These facts can be determined quantitatively by use of single-plant cultures. 226 Consecutive frond generations are examined for flowering stages after outgrowth. In the

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generation which is the last but one at the beginning of SD treatment, shrunken primordia of stage II (equivalent to Figure 3C) dominate. In the following generation, only dried stage I primordia (equivalent to Figure 3B) or no flower residues can be seen. Therefore, no persistence of the flower-inducing LD effect is demonstrable. The lack of such an aftereffect of LD seems to be a hint for the flower-inhibiting action of noninductive day lengths. A rapid reversibility of flower-inducing processes has been shown also under conditions in which a certain number of inductive light-dark cycles were inserted in otherwise noninductive cycles. In L. gibba G3 the percentage of flowering fronds increases after SD after pretreatment with 5 LD to a maximum on the 6th day and then declines to zero on the 12 day. l 6 In L. aequinoctialis 6746, flowering reaches maximum 5 days after one uninterrupted long night and then falls off. On day 6 after induction with 1 SD, flower primordia can be counted no longer in the young developing fronds.50

EFFECTS OF TEMPERATURE In contrast to what is known from other plants, lowering of night temperature reduces flowering in the LDP L. gibba G3, but enhances flowering in SDPs L. aequinoctialis 6746 and Wolffia tnicroscopica. In L. gibba G3 chilling at 21°C during the second half of a LD cycle (with 16-hr light and 8-hr darkness) inhibits flowering in comparison to controls at 26°C.159 Furthermore, temperatures below 26°C increase the minimal number of LD cycles, which are necessary for flower induction in this plant. l59 Under continuous light the sensitivity of flowering to chilling shows diurnal rhythmicity in L. gibba G3, if acetylcholine, eserine, or IAA is added to the culture medium. 77167 In many strains of L. aequinoctialis the critical light period is prolonged, if a night temperature of 23 instead of 25°C is used (see Table I). 7 Further, strain 6746 of L. aequinoctialis grown under flower-inducing skeleton photoperiods reaches maximal flowering at 23°C. At temperatures above and below 23°C a marked decline in percent flowering is seen.23 In full SD Hillman found an inhibition of flowering above 30°C in strain 6746." Night temperature seems to be decisive: a 4-hr-period of 33°C inhibit flowering when given during the dark period but not during the light period. With a Hoagland-type medium, which allows strain 6746 to flower also under LD conditions, temperatures above 27°C are flower inhibiting under LD. 51 In Wolffia tnicroscopica, a quantitative SDP, temperature has to be lowered from 27 to 22°C for an optimal induction of flowering with SD.189

EFFECT OF MINERAL NUTRITION Macro Nutrients Major components of nutrient media have been tested for their flower-modifying effectiveness in L. aequinoctialis 6746 only and under very special conditions. Diluted Hutner medium (1/10 cone.) in combination with sucrose, glucose, or fructose has a flower-inhibiting effect in strain 6746 and this inhibition can be prevented partially by raising Ca +2 or PO 4 ~ 3 concentration but not by addition of K + , Mg+ 2 , NH 4 + , NCV or SCV2.180 Similarly, the inhibition of flowering — induced in 1/2 Hutner-sucrose medium by a daily transfer of plants to water for a short period of time — is reversed partially by supplementing the water with a calcium salt.41 Phosphate is effective only at pH 7.3 but not at pH 6.4. Mg + 2 , NH 4 + , and SO 4 ~ 2 , on the other hand, increase the effect of water treatment. The effect of nitrate and ammonia on flowering has been analyzed in some detail in the various photoperiodic reaction types. In general it can be said that nitrate concentrations, which are optimal for vegetative growth, as well as additions of ammonia to the medium, are inhibitory in all investigated species. Especially under conditions of high intensity light and/or sucrose feeding L. gibba G3 shows good flowering in continuous light only at nitrate

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concentrations which are lower or higher than the concentration for optimal vegetative growth. 32 In L. aequinoctialis 6746 the optimal nitrate concentration for flowering in SD lies above, but for flowering in continuous light, below the optimal value for vegetative growth. The DNP Spirodela polyrrhiza reacts in all day lengths like L. aequinoctialis 6746 in continuous light. 12 Ammonium ions completely inhibit flowering in L. gibba Gl and G3 without a simultaneous effect on growth and multiplication of fronds. 95 In strain G3 a concentration of 10~ 4 M NH 4 + is effective. In L. aequinoctialis 6746 a flower-inhibiting effect of ammonia is seen under SD conditions only in the case that nutrient medium is diluted (1/10 cone.) and complemented with 1% sucrose.74 Under LD NH 4 + cancels the flower-inducing effect of some SH inhibitors. 209 Flowering in the DNP L. aequinoctialis 351 is prevented by about 10-3 M NH4C1.225 The acidification of medium, which occurs in connection with uptake of NH 4 + by plants, does not seem to be the cause for the flower-inhibiting effect of ammonium ions, at least in L. gibba95 but the acidification of free space within the plant (apoplast) may be an important event. In L. aequinoctialis 6746 the NH 4 + effect is abolished if the medium is adjusted to pH 6.5 each day.200 In the case of SH inhibitor-induced LD flowering in strain 6746 not only ammonium ions, but also nitrite, casamino acids, glutamine, and asparagine are effective. 209 Tanaka and Takimoto conclude that a general increase in nitrogen metabolism may be the cause for this inhibition of LD flowering (compare the section, Amino Acids). For interpretation of results with L. gibba it has been hypothesized that availability of ATP may be lowered as a consequence of NH 4 + application. In fact, NH 4 + and ATP act antagonistically in L. gibba under certain conditions. 95 In L. aequinoctialis 6746, however, NH4 + leads to an increase rather than decrease of the internal ATP level.39 Micro Nutrients and Chelating Agents The mere comparison of flowering in different nutrient media leads to the conclusion that the content of media on certain chelating substances must play an important role in FI. Hutner's medium, for example, contains such a great amount (1.7 X 10"-1 M) of EDTA that not only the heavy metals (micro nutrients) are chelated but also nearly all calcium ions. In general, it can be stated that addition of EDTA, or to a higher degree, EDDHA (ethylenediamine-di-o-hydroxy-phenylacetic acid) improved flowering, if only the micro nutrients are chelated. Higher doses of chelating agents, on the contrary, can even inhibit flowering. 131 In many cases, FI is rendered possible only when EDTA or EDDHA is included in the nutrient medium: L. g/Ww, 52 - 53 - 89 - 177 L. minor,9 S. punctata,'95 L. aequinoctialis (Sohna), 41 i30 and W. microscopical In the DNPL. aequinoctialis 351 EDTA and EDDHA have a flower-promoting effect. 225 In L. gibba G3 the minimal LD requirement is reduced by EDTA to a single LD I5S and, furthermore, the light requirement of a special light-sensitive phase about 12 hr after dawn ("L2-phase") is removed. 16 ' An exception from the general statement given above seems to be the case of L. aequinoctialis 6746. EDTA introduces SD dependence in this species, which otherwise reacts as a DNP. 51 This means that EDTA inhibits flowering under LD conditions in strain 6746. In an analysis of these results, Hillman has found, however, that LD flowering occurred in the cited experiments only because the medium contained a certain amount of unchelated copper.55 Therefore, EDTA is effective in this special case as a sequestering agent to remove free copper from the medium. The action of copper is interesting in itself: Cu + 2 abolishes the effect of LD in the SDP strain 6746 as well as in the LDP L. gibba G3. Free copper ions render possible LD flowering in strain 6746 but suppress FI in strain G3 and have no effect under SD conditions. 55 - 196 Correspondingly, copper chelators as salicylaldoxime, diethyldithiocarbamate, and hydroxyquinoline enhance flowering in strain G3. 156 Takimoto and co-workers have collected

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some arguments that C u " 2 exerts its effect on flowering by blocking nitrate reductase. Tungstate, a specific inhibitor of nitrate reductase, has the same effect as Cu ' - in strains 6746 and G3. 205 - 2 ' 2 - 221 Furthermore, deletion of molybdate, an activator of nitrate reductase, from the medium also imitates the Cu * 2 effect (only strain 6746 has been tested). 205 - 212 The authors are puzzled, however, by their result that lowering of nitrate concentration in the medium does not act like inhibitors of nitrate reductase, but rather like molybdate. 2 " Modifying Takimoto's interpretation it may be speculated that inhibition of nitrate reductase by Cu + 2 or deletion of Mo in fact reduces protein metabolism and in consequence accelerates senescence, which is presumably correlated with a lowering of the GA/ABA balance (compare the sections Effects of Plant Age and Effects of Growth Substances and Growth Retardants). Presence of nitrate in the cytoplasm, on the other hand, may be necessary for current induction of cytokinin synthesis; 24 to 48 hr after omission of NO 3 ~ from the medium the level of cytokinins is lowered in L. aequinoctialis 6746.228 Besides elimination of free copper, EDTA and EDDHA obviously influence flowering by chelation of iron and thereby an improvement of iron availability. Hillman has shown that a temporary lowering of iron level during SD induction inhibits flowering in strain 6746 without a simultaneous effect on vegetative frond multiplication. 51 In the strain Sohna of L. aequinoctialis flowering is completely suppressed if iron is omitted from the EDDHAcontaining medium (whereas growth is reduced only partially). 41 Excessive doses of iron, on the other hand, lead to flowering in the Sohna strain even in the absence of EDTA and EDDHA. 41 - 13 " No such an effect has been observed with a range of concentrations of Mn + 2 , C u t 2 , or Zn + 2 in this plant material. The relatively higher flower-inducing effectivity of EDDHA may be explained by the observation, made in W. microscopica, that 59Fe uptake is increased manifold by use of EDDHA instead of EDTA.198 In L. gibba G3 some iron reagents (o-phenanthroline, ct,a'-dipyridyl, azide) inhibit flowering under LD. 156 In contrast to this result Pieterse et al. found no inhibition of FI in L. gibba G3 by deletion of iron from the EDDHA-containing medium. 177 In any case, EDDHA is flower promoting over a greater range of concentrations than Fe-EDDHA in this plant. Presumably heavy metals other than iron and copper also take part in the network of flowerinducing processes. It has been hypothesized that manganese may activate lAA-oxidase and therefore enhance flowering, if made available in the leaf tissue by a chelating agent. 195 Some promotion of flowering has been observed in the DNP S. polyrrhiza, strain Pentanjci, and the LDP L. minor, strain Barje, after addition of Zn + 2 , Cu + 2 , or Mn + 2 to the EDTAcontaining medium. 121

EFFECTS OF GAS COMPOSITION L. gibba, strains Gl and G3, provided with CO2-enriched air (3.5% CO2), show a strong reduction of flowering percentage, when held under LD condition. Under SD with additional end-of-day FR, however, flowering of strain Gl is distinctly promoted by 3.5% CO,.90 "Feeding" of sucrose (1%) through the medium has the same effects as CO2 enrichment.93 So CO2 may be effective via photosynthesis and the production of assimilates. L. aequinoctialis 6746 exposed for 5 hr/day to ozone (0.1 ppm) not only has a lower rate of frond multiplication with smaller, slightly yellow fronds, but also a complete suppression of flower production. After removal of plants from the ozone-supplemented environment flowering is resumed but with a reduced rate (for at least 10 days).34 Ethylene as a phytohormone is mentioned in the following section. EFFECTS OF GROWTH SUBSTANCES AND GROWTH RETARDANTS In Table 2 hormonal effects on flowering are compiled. Preferably, publications are cited which refer to experiments with sterile-filtered media or hormone solutions. Especially in

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Table 2 EFFECTS OF PHYTOHORMONES AND SOME CONNECTED SUBSTANCES ON FLOWERING IN LEMNACEAE Phytohormone

L. gibba G3 (LDP)

Allogibberic acid

10~ 5 M promoting ( L D ) P I 10 4M inhibiting (LD) 1 7 1 I O - 7 — 1 0 5 M without effect (LD) 1 8 10 J —10 ' M inhibiting (LD) 18 0.1—10 mg/( inhibiting (LD) 1 7 5

ABA

10~ 8 —10

Kinetin

l O ' * M promoting (LD) 171 10 " M inhibiting (LD) 1 7 1 10 " M promoting (LD) 171 10 " M mhibiting(LD)' 71 1 — 10 mg/t inhibiting (LD) 175

GA, CCC

IAA Ethrel (ethylene-releasing agent)

7

M inhibiting (LD)'" 4

L. aequinoctialis 6746 (SDP) 10' W without effect (SD) 7 " 1()- 4 M inhibiting (SD) 7 " 10 7— l O ' " M promoting (LD)"" 0.1-1 mg/t without effect (SD) 187 6-10 mg/t inhibiting (SD) 1 8 7 2 x 10'"—2 x 10 » M promoting (LD)"" Zeatin—1 mg/t promoting (LD) 4 Ben/.yladenine 10 7 —10~ 4 M inhibiting (LD) 4: 10 5 —-10~ 4 M inhibiting (SD)4-

Note: Data from experiments have been cited in which media, or at least hormone solutions, were not autoclaved but sterile-filtered. For comparison, some data from L. aequinoctialis strain Sohna are included within brackets, which were obtained with autoclaved media. In parenthesis daylengths during experiments are given.

the case of gibberellins autoclaving leads to several decomposition products, allogibberic acid for example, which have another effect on flowering than the original substances (see Table 2 and below). Summarizing all the published results, the following may be said. As in other plants, GAs have an opposite effect in LDPs and SDPs. Under LD conditions, flowering of strain G3 is promoted by GA, at 10~ 6 to 10~ 5 A/, 11 *- 155 - 171 but in strain 6746, flowering is promoted by the GA inhibitor CCC." In L. perpusilla P146, even5 x 10 ~ 7 M G A 3 cause some depression of flowering under SD.81 Higher concentrations of GA3 (10~ 4 M), however, are flower inhibiting in both photoperiodic reaction types. An explanation for the differing reactions of LDP and SDP to daylengths as well as GA or CCC applications may lie in the fact that L. aequinoctialis 6746 has a higher capacity for accumulation of GA than L. gibba. Uptake of [ 3 H]-GA, reaches a higher saturation level in strain 6746 than in strain Gl of L. gibba.2-45 Furthermore, preliminary determinations of the endogenous GA content in both species cultivated under the same conditions have shown higher values in strain 6746 than in strain Gl. 2 2 9 Finally, in vitro development of flower primordia from L. gibba Gl results in a feminization, which can be corrected by addition of GA3 to the agar medium. Corresponding explants of L. aequinoctialis 6746, on the other hand, must be treated with CCC for a normal development, otherwise yielding a masculinization. 78 - 79 GAs are possibly concerned not only with the transmission of LD signal, but may also participate in the mediation of end-of-day FR effect (see End-of-Day Treatment). Pekic and Neskovic, working with L. aequinoctialis 6746, have demonstrated a marked increase in endogenous GA after one night, if plants were treated with 10 min of FR instead of R at the end of the foregoing light period. 173 ABA in combination with CCC renders possible some flowering in L. aequinoctialis 6746 under LD." This result is in good agreement with the fact that leaf senescence promotes FI in the same strain (see Effects of Plant Age). In L. gibba ABA as well as senescence inhibit flowering under LD. 10 '- 194 Therefore, ABA belongs to the inhibitors of LD effect in both photoperiodic reaction types.

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Cytokinins generally are flower promoters in the LDP L. gibba155-111-"* and the SDPs L. aequinoctialis,V>A2 L. perpusilla,6 and W. microscopica.132-2" Of five different cytokinins tested in W. microscopica, BA has the highest effectiveness. 217 The only exception is the inhibition of flowering in L. gibba by i?10~ 6 M kinetin under LD, but under LD conditions a supra-optimal endogenous cytokinin level seems to be present in this species.1 Under SD 10~ 6 M kinetin strengthens the flower-promoting effect of GA in L. gibba G3.155 Also 1 mg/€ benzyladenine (about 5 x 10~ 6 M) in combination with EDDHA forces SD flowering inL. gibba G3. 178 A weak flower-promoting effect of IAA has been found only at the very low concentration of 10"9 M in L. gibba G3. 171 Concentrations of IAA > 10~ 7 M are progressively flowerinhibiting in L. gibba as well as in L. aequinoctialis 6746. 42 - 155 - 171 Ethrel as an ethylenereleasing agent inhibits FI in L. gibba G3. 175 In spite of this result there are some arguments that ethylene might have a flower-promoting effect in strain G3. The flower-promoting substance EDDHA enhances the production of ethylene and the flower-inhibiting agent allogibberic acid negates the ethrel effect on gibbosity of fronds in L. gibba G3. 29J75 Possibly the localization of predominant release (leaf or meristem) may be of importance for the effect of FI. EFFECTS OF OTHER CHEMICALS In addition to growth substances and growth retardants many other chemicals have been tested for their influence on FI. In most cases substances cited below act specifically on flowering, without an effect on general development. Membrane Effectors Acetylcholine (a transmitter substance which depolarizes the postsynaptic membrane in many animal nerves) and eserine (which inhibits the acetylcholine-degrading enzyme acetylcholine esterase), both at a concentration of 10~ 5 M, are flower inhibiting in L. gibba Gl. 9 8 In L. aequinoctialis 6746, on the other hand, these substances are flower inducing under LD conditions, when ascorbic acid (10~ 4 M) is given in addition.98 From experiments with L. gibba-G3, Oota concluded that acetylcholine eliminates especially the effect of photoperiodic "LI-phase", which includes the dawn signal for entrainment of circadian rhythm (see Endogenous Rhythms). 164 Furthermore, light requirement of "LI-phase" for flowering in strain G3 can be abolished by 10 |xg/€ valinomycin or gramicidin, which are K + -ionophores.165 Also the effect of 10~ 3 M lithium chloride — flower-inhibiting in the LDP L. gibba G l , flower-promoting in the SDP L. aequinoctialis 6746 under LD — may be attributable to a change in membrane transport of alkali ions.96 The period of the free-running potassium uptake rhythm in strain G3 was lengthened by Li + and shortened by ethanol." 8 Consequently, membrane effectors seem to exert their effect on flowering through an influence on circadian rhythm. Sugars Some effects of sucrose on flowering have already been mentioned in connection with photosynthesis (see Photosynthesis), mineral nutrition (see Effect of Mineral Nutrition), and CO2 enrichment (see Effects of Gas Composition). Whether these and other sugar effects (Table 3) have a common physiological basis is not clear as yet. The analysis of three flower-inhibiting sugar effects has led to different characteristics in each case. In L. gibba Gl, cultivated in Pirson-Seidel medium, only sucrose, but not glucose or fructose (3 x 10~ 2 M each), suppresses LD flowering. The sucrose effect can be cancelled by 10~ 4 M ADP or ATP, but not AMP. 102 In L. gibba G3, precultivated in M-sucrose medium and cultivated in M medium (Hoagland type), glucose as well as sucrose (1%) reduced the floral

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Table 3 EFFECTS OF SUGARS ON FI IN LEMNA Strain G3

Sucrose Glucose Sucrose Sucrose Glucose Sucrose

Gl

Sugar

Effect

1% ca. \7, 1% 1% or fructose 1% 1%

0 +

6746

Sucrose 0.5—4% Sucrose 1% Sucrose 1 % or 30 mM Glucose or fructose 30 mM Sucrose, glucose, fructose, or mannose 30 mM

+ + + 0 —

P146

Sucrose 30 mM

-

Experimental conditions LD: M medium LD; M medium • LD; Pirson-Seidel medium LD; Pirson-Seidel medium LD; Pirson-Seidel medium SD with end-of-day; FR Pirson-Seidel medium LD; Hoagland medium LD; M medium with 2 ^M CuSO4 LD; Pirson-Seidel medium LD; Pirson-Seidel medium SD; 1/10 strength Hutner's medium (preculture under LD with sucrose 30 mM) SD; Pirson-Seidel medium

Ref. 52,160 157 93 93, 102 102 93 30 196 102,197 102 179—181 81

+ , promoting; - , inhibiting; 0, small or negligible

index and supplemental addition of 10 ~ 5 M cyclic AMP or 10 ~ 7 M isoproterenol (activator of membrane-bound adenyl cyclase) removed the sugar action. ATP, ADP, or AMP (10~ 5 M each) rather increase the glucose inhibition (acid hydrolysis of nucleotides has not been excluded, however). 15716 ° In the case of L. aequinoctialis 6746 inhibition of flowering by sucrose, glucose, or fructose takes places under SD, if plants are precultivated with sucrose and a diluted Hutner medium is used for experimental cultures. The inhibition requires the presence of ammonium ions and is, at least partially, reversed by cyclic AMP, ATP, ADP, AMP, inorganic phosphate, calcium ions, certain amino acids, several respiratory intermediates, CO2-free atmosphere, and shaking of culture vessels. 179182184 Redox-State Effectors In continuous light, flowering of the SDP L. aequinoctialis 6746 is promoted by ascorbic acid (10- 4 M) and inhibited by NADH (1Q- 4 M) and NADPH (lO" 4 M). Corresponding experiments with the LDP L. gibba G1 yielded the opposite results in most cases: an inhibition of flowering by ascorbic acid and a promotion by NADH. 97 Sulfhydryl Inhibitors In a series of publications Takimoto and co-workers have demonstrated and analyzed the effect of SH inhibitors on flowering. CuSO4 (5 x 10~ 6 M), AgNO, (1Q-" M), HgCl2 (2 x 10~ 5 M), potassium ferricyanide (5 x 10"1 M) and iodoacetamide (10~ 6 M), all induced LD flowering in L. aequinoctialis 67462"1 (but not in some other SD strains of L. aequinoctialis225). The effect can be cancelled by supplemental addition of cysteine (10~ 4 M).204 In L. gibba G3, on the other hand, the same SH inhibitors (with the exception of ferricyanide) inhibit flowering under continuous illumination. 221 Ferricyanide seems to be a special case, because the action presumably comes about not only by itself, but also by release of cyanide. The effect of ferricyanide, which is flower promoting in both species, can be imitated by ferrocyanide or CN' in L. aequinoctialis 6746205 2" and L. gibba G3.206 In their later publications Takimoto et al. rejected the original hypothesis that SH inhibitors act on flowering by blocking nitrate reductase.205 2 "- 221 In connection with effects of tungstate and molybdate-deficiency (see Micro Nutrients and Chelating Agents), redox-state effectors, and amino acids this remains, however, an attractive idea.

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Amino Acids Testing of single amino acids for their flower-modifying activity has led to very differing results. Even in the same laboratory flower-inhibiting or flower-promoting effects have been obtained with one and the same amino acid, varying the experimental conditions only slightly. Alanine, glycine, and serine, for example, inhibit LD flowering caused by 0.5 |xM ferricyanide or by tungstate in L. aequinoctialis 6746.209-212 The same amino acids applied in the same concentration range to the same species, enhance, however, flowering percentages drastically, when a suboptimal concentration (0.1 to 0.2 jxM) of ferricyanide or Mo-deficient medium is used for initiation of flowering under LD conditions. 212 Such and other results could be made understandable, if it is assumed that several amino acids have a twofold effect on protein biosynthesis: a promotive effect as a direct or indirect substrate and a moreor-less inhibitory effect via repression of nitrate reductase and in consequence lowering of the general amino acid pool. Promotion of protein biosynthesis counteracts senescence and therefore influences FI (see Effects of Plant Age). Some further results with amino acids may be summarized. In strain 6746 alanine, glycine, and serine as well as aspartate, asparagine, and glutamate influence flower induction not only under LD, but also under SD conditions. The cited amino acids shorten the critical dark period by 1 to 2 hr, nullify the flower-inhibiting effect of a NB, 210 and reverse partially the flower inhibition caused by diluted Hutner-sucrose medium. l81 - 182 Furthermore, they are effective also in all other SD strains of L. aequinoctialis.2*0 Working with a lower concentration of amino acids (10~ 6 M) Maeng and Khudairi found for strain 6746 a flower-promoting action of serine, threonine, and tryptophan and a flower-inhibiting effect of cysteine, if amino acids were added to the culture medium during the last dark period of three inductive SD cycles. l29 Relatively high concentrations of several amino acids inhibit flower production in L. gibba G3. 139 Interestingly, lysine, which is flower inhibiting in itself, in strain G3 can partially or wholly reverse the floral inhibition caused by some other amino acids. 139 - 140 A promotive effect on flowering of some amino acids (10~ 6 to 2 x 10~ 6 M) has been stated for L. minor strain Barje (LDP) and S. polyrrhiza strain Petanjci (DNP). 121 Also, in an unnamed strain of S. polyrrhiza casein hydrolysate stimulates flowering. 123 Adenosine Phosphates In several cases, AMP, ADP, or ATP can counteract other flower-modifying agents. In L. gibba G1 ADP and/or ATP (10 4 M) cancel the flower-inhibiting effects of diluted PirsonSeidel medium,1'4 ammonia, 9 '' sucrose, arsenate, and Atebrin. l02 Further, in L. aequinoctialis 6746 flower inhibition under SD caused by diluted Hutner-sucrose medium is partially reversed by AMP or ADP (10 4 to 3 X 10~ 4 M). 184 10 6 M ATP, applied during the last period of three inductive SD cycles, increases the flowering percentage in strain 6746. 'm Copper-mediated LD flowering of strain 6746, on the other hand, is prevented by 10~ 4 M ADP.96 Adenosine phosphates are substrates for synthesis of nucleic acids, carriers of phosphorylation energy, and precursors for the "second messenger" cyclic AMP. All these functions may be involved in connection with the flower-influencing activity of AMP, ADP, and ATP. The importance of DNA synthesis for the process of flower induction has been shown in L. gibba G3 by the use of specific inhibitors. 5-FDU, for example, prevents flowering at a concentration of 10~ 8 M, whereas multiplication of vegetative fronds is stopped only at 10~ 7 M 5-FDU.2"' The significance of phosphorylation energy for flowering was demonstrated by the flower-inhibiting effect of uncouplers. Arsenate (10~ 4 to 5 x 10~ 4 M) blocks FI completely in L. gibba Gl and G3 with only a slight promoting effect on frond multiplication. 92 - 15 ' 1 2,4-DNP reduces flowering in strain G3156 as well as in L. aequinoctialis 6746. l09 Participation of endogenous cyclic AMP in the flower-induction process seems to be likely, because isoproterenol and other adenyl cyclase-activating catecholamines (10~*

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to 10 (> A/) enhance flowering of L. gibba G3 under conditions of sugar "feeding" or darkening of the so-called "LI-phase". 160 - 164 Correspondingly, addition of cyclic AMP (10~ 5 M) to the culture medium increases flowering percentage in strain G3 under a variety of experimental conditions. I O S - I 5 7 J 6 4 J 6 9 Salicylic Acid and Related Substances Salicylic acid (SA) was identified as the active material which is included in honeydew of aphids fed on Xanthium plants and can induce flowering in L. gibba G3 under SD conditions. I 2 J * Following up this striking result, Cleland found benzoic acid (BEA), acetylsalicylic acid (ASA), and some other closely related compounds to be active, too. 13 In continuation of this work several authors tried to analyze the effects of SA, BEA, and ASA. Flower-inducing or flower-promoting effects were ascertained also in LDPs S. punctata O5,195 L. minor M 332, M 601,86 L. aequinoctialis S type strains, 7 - 86 in SDPs L. aequinoctialis 6746, 1 "-* 6 -"" LP6,"" 151, 321, 341, 353, 361, 381, 391, 441, 7 - 86 and in DNPs S. polyrrhiza SP 20 '" and L. aequinoctialis K type strain. 7 Besides strain 315 (S type) of L. aequinoctialis as an exception, SA generally is more effective than BEA in LDPs, 13 - 86 but the contrary is true in SDPs.*6-"0-222 In L. gibba G3, SA reduces the critical daylength by 1 to 2 hr.' 4 - 1 9 Furthermore, SA reverses the flower-inhibiting effect of several agents in this strain: sucrose, ammonia, NH 4 free Hutner medium, 201 * copper,206 and darkened "L2-phase". 162 Synergistic flower-promoting effects of S A have been found in strain G3 in combination with ferricyanide (reduction of critical daylength to about 3 hr/day), 206 BA, 178 and cyclic AMP. 164 (3-Naphthol, on the other hand, nullifies the SA conditioned promotion of flowering in strain G3.' 76 In L. aequinoctialis 6746 the critical daylength is extended by SA (about 1 hr), if plants are cultivated in half-strength Hutner medium. 19 After omission of NH4+ from Hutner medium, SA renders possible flowering even under continuous light. 207 Therefore, the flowerinhibiting effect of ammonia prevails against the flower-promoting effect of SA in strain 6746. The similarities between effects of SA and EDDHA as well as some other arguments have led to the hypothesis that SA might exert its effects by chelating certain heavy meta j s iio.i62.i78.i95.208.22o interestingly, SA enhances gibbosity (formation of aerenchyma) like EDDHA in L. gibba G3,' 1 - 1 7 5 - 1 7 8 S. punctata,1^ and S. polyrrhiza SP2().m This may be a hint that SA acts on flowering through a gentle enhancement of ethylene evolution, because EDDHA promotes ethylene synthesis in Lemna,2^ and ethylene is known to induce development of aerenchyma also in some other plants. "- 2 8 - 1 1 9 Some Further Chemicals Sometimes, but not regularly, estrogens initiate flowering in L. minor.2n Chlorogenic acid (10 3 A/) 214 and benzo(a)anthracene (10"" A/)10 inhibit flower formation in L. gibba G3. Unidentified flower-promoting material leaks out from L. aequinoctialis 6746 and L. gibba G3, if plants are transferred from a sucrose-containing medium to distilled water for 4 to 12 hr during dark phase. 43 - 44169 Leakage material from Lemna,44 or extracts from flowering Xanthium plants, 75 can compensate for the effect of water treatment in strain 6746. Cyclic AMP has a corresponding effect in strain G3.' 6y

CHEMICAL AND PHYSICOCHEMICAL CHANGES AT LIGHT INDUCTION Photoperiodic induction of flowering is connected with changes of nucleic acid and protein metabolism. In the second inductive dark period a peak of RNA synthesis (determined by 3 H-uridine incorporation) occurs during the 4th to 5th hr in L. aequinoctialis 6746. NB treatment within the foregoing dark period cancels this RNA-synthesis peak.47 In accordance

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with this result, the inhibitor of RNA synthesis 5-fluorouracil has a high flower-inhibiting activity in strain 6746, when treatment occurs early in the inductive dark period. 26 Inhibitors of protein biosynthesis are most effective in reducing flowering percentage when applied 12 to 16 hr after beginning of an 8 hr light: 16 hr darkness cycle, that is near the time of highest NB sensitivity.- 6 The flower-inhibiting effect of NB, however, is reversed by 2thiouracil (inhibitor of RNA synthesis) and actinomycin D (inhibitor of gene transcription); 25 2 days after beginning of SD induction the first change in composition of soluble proteins is demonstrable in strain 6746.47 FR/R reversion of phytochrome state at the beginning of the daily dark period (see Endof-Day Treatment) modulates the active component of membrane potential within 1 min in subepidermal cells of L. aequinoctialis 6746128 and L. gibba Gl."" The active component of membrane potential is generated by H *-extrusion in these plants. 127 Consequently, one of the early effects of phytochrome consists in an alteration of the plasmalemma-located proton gradient, which is responsible for uptake of hexoses and other substances. 147 I4K Additional early effects of end-of-day FR are a lowering of starch degradation in chloroplasts 103 - 191 and a reduction of RNA synthesis. 107 After about four light-dark cycles including end-of-day FR an adaptation of plants to this light condition seems to take place. 46 x l Uptake and transport of labeled GA, then are enhanced in fronds of L. gibba Gl, 4 5 but not in L. aequinoctialis 6746.2

INFLORESCENCE DIFFERENTIATION Photoperiodism controls not only the initiation of floral primordia but also the further development of flower organs. In the LDP L. gibba as well as in the SDPs L. aequinoctialis and L. perpusilla the same daylength is needed for initiation and for differentiation of inflorescences. A minimum of 1 LD seems to be sufficient to induce the formation of flower primordia in L. gibba G3, but at least 6 LDs are required to obtain mature flowers.16 For strain 6746 of L. aequinoctialis several authors have stated time and again that LD or NB treatment inhibit the further development of floral primordia leading to a "regression" or "abortion" of these primordia. 245061 l l 2 1 9 ' 1 9 9 In proportion to an increasing number of SDs given before continuous light the floral stage reached before abortion also increases. 191 The LD inhibition of flower-bud differentiation is reversed or even overcompensated by lowering of temperature and/or light intensity. 61 l l 2 1 9 2 At each temperature there exists a certain threshold level of light intensity, which is without any inhibiting or promoting activity for flower-bud development.'" Blue light is effective in the same way as low light intensity." 2 In L. perpusilla P146 benzyladenine (0.075 ppm) makes possible further development of SD-induced flower primordia after transfer of plants to LD.6 The development of the 2 stamens and the pistil has been studied by Hugel using in vitro culture techniques.78"80 On an agar medium containing minerals, sucrose, and kinetin only, explants of L. gibba Gl preferentially develop the pistil, whereas the stamens remain relatively small. This feminization can be corrected by addition of GA3 to the agar medium. Explants of L. aequinoctialis 6746, on the other hand, show a masculinization on kinetin medium, which is reversed to a normal (in vivo-like) development by application of CCC or ethrel. Sex expression in both species seems to be dependent on an adequate balance of hormones of the GA-type and of ethylene, although the influence of other hormones should also be taken into account. GA always promotes the development of male flowers, whereas ethylene (ethrel) always enhances development of the female flower part.

CONCLUDING REMARKS Not all, but most of the more important results have been compiled in this chapter on Lemnaceae. Putting all the data into a consistent total picture is not possible at present, but

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Photosynthesis Sugars Adenosine phosphates

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Micro nutrients Chelating agents Salicylic acid

Effects of plant age Growth substances Growth retardants

Photoperiodic response Night break treatment Endogenous rhythms Membrane effectors End-of-day treatment

Flowering FIGURE 5.

Some relationships between (lower-regulating factors in Lemnaceae (see text).

several connections between the different aspects of flower induction have been envisaged in the foregoing sections. As a summary, the key words of section titles are arranged in a certain order in Figure 5. Flower-regulating factors, which are closely connected, are plotted together in one block. Then some interrelations (discussed in the text) have been marked by arrows. Without doubt, the resulting figure is incomplete and somewhat indistinct, but may give a synopsis on most of the themes which have been treated up to now in connection with FI in Lemnaceae. Presumably the potential of Lemnaceae as pilot plants to explore the mechanism of flower induction has not been exhausted and some further progress can be expected.

ACKNOWLEDGMENT Financial support from the "Fonds zur Forderung der wissenschaftlichen Forschung" of the Republic of Austria is gratefully acknowledged.

REFERENCES 1 . Al-Shalan, I. and Kandeler, R., Tagesliingenabhangigkeit der Wirkung von Abscisinsiiure und Ben/.yladenin auf das Wachstum von Lemna gibba G l , Biochem. Physiol. Pflanzen, 172, 521—529, 1978. 2. Al-Shalan, I. and Kandeler, R., Aufnahme und Verteilung von Gibberellin A, bei der Kurztagpflanze Lemna paucicostata 6746, Z, Pflanzenphvsiol., 94, 257—262, 1979. 3. Bakshi, I. S., Farooqi, A. H. A., and Maheshwari, S. C., Circadian rhythm in nitrate reductase activity in the duckweed Wo/ffia microscopica Griff., Z. Pflanzenphysiol., 90, 165—169, 1978. 4. Bakshi, I. S., Farooqi, A. H. A., and Maheshwari, S. C., Control of circadian rhythm in nitrate reductase activity in Wolffia microscopica Griff., Plant Cell Physiol., 20, 957—963, 1979. 5. Bennink, G. J. H., van den Berg, R., Kool, H. J., and Stegwee, D., Flowering in Lemna minor, Acta Bot. NeerL, 19, 385—392, 1970. 6. Bennink, G. J. H. and de Vries, J. W. A., Flower development in Chrysanthemum and Lemna under long-day conditions with the cytokinin benzyladenine. II. Symposium on Plant Growth Regulators, Bulg. Acad. Sci. Sofia Abstr., p. 66—67, 1975. 7. Beppu, T. and Takimoto, A., Further studies on the flowering of Lemna paucicostata in Japan, Bot. Mug. Tokyo, 94, 69—76, 1981. 8. Bhalla, P. R., Pieterse, A. H., and Sabharwal, P. S., Some aspects of flowering, gibbosity and turion formation in Lemnaceae, Acta Bot. NeerL, 22, 433—445, 1973.

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9. Bhalla, P. R. and Sabharwal, P. S., Induction of (lowering in Lemna minor by EDDHA. Acta Dot. Neeii.. 2 1 , 200—202. 1972. 10. Bhalla, P. R. and Sabharwal, P. S., Effects of lobacco smoke and some of its constituents on growth and flowering of Lemna gibba 03, Environ. Pollut., 6, 59—66. 1974. 1 1 . Blake, T. J. and Reid, D. M., Ethylene. water relations and tolerance to water logging of three Eucalyptus species. Ausr. J. Plant Physiol.. 8. 497—505. 19X1. 12. Cleland, C. F., Isolation of flower-inducing and flower-inhibitory factors from aphid honeyew, Plant Physiol.. 54, 899—903, 1974. 13. Cleland, C. F., The influence of salicylic acid on flowering and growth in the long-day plant Lemna gibba G3, in Mechanisms of Regulation of Plant Growth, Bull. 12, Bieleski, R. L., Ferguson, A. R., and Cresswell, M. M., Eds., Royal Society of New Zealand, Wellington. 1974, 553—557. 14. Cleland, C. F., Comparison of the flowering behavior of the long-day plant Lemna gibba G3 from different laboratories, Plant Cell Physiol.. 20. 1263—1271. 1979. 15. Cleland, C. F. and Ajami, A., Identification of the flower-inducing factor isolated from aphid honeydew as being salicylic acid, Plant Physiol., 54, 904—906, 1974. 16. Cleland, C. F. and Briggs, W. R., Flowering responses of the long-day plant Lemna gibba G3, Plant Physiol.. 42, 1553—1561, 1967. 17. Cleland, C. F. and Briggs, W. R., Effect of low-intensity red and far-red light and high intensity white light on the flowering response of the long-day plant Lemna gibba G3, Plant Physiol., 43, 157—162, 1968. 18. Cleland, C. F. and Briggs, W. S., Gibberellin and CCC effects on flowering and growth in the long-day plant Lemna gibba G3, Plant Physiol., 44, 503—507, 1969. 19. Cleland, C. F. and Tanaka, O., Effect of daylength on the ability of salicylic acid to induce flowering in the long-day plant Lemna gibba G3 and the short-day plant Lemna paucicostata 6746, Plant Physiol., 64, 421—424, 1979. 20. Czygan, F. C., Bliitenbildung bei Lemna minor nach Zusatz von Oestrogenen, Naturwissenschaften, 49, 285—286, 1962. 21. Datko, A. H., Mudd, S. H., and Giovanelli, J., Lemna paucicostata Hegelm. 6746. Development of standardized growth conditions suitable for biochemical experimentation, Plant Physiol., 65, 906—912, 1980. 22. Devi, S. L. and Maheshwari, S. C., Diurnal fluctuations in the activity of the enzyme nitrate reductase in Lemna paucicostata, Physiol. Plant., 45. 467—469, 1979. 23. Doss, R. P., Influence of temperature on the flowering of Lemna perpusil/a 6746 grown under skeleton photoperiods, Plant Physiol., 55, 108—109, 1975. 24. Doss, R. P., Influence of timing and number of consecutive inductive photoperiodic cycles on the flowering of Lemna, Plant Physio/., 55, 110—111, 1975. 25. Doss, R. P., Reversal of the effects of a night interruption in Lemna by inhibitors of ribonucleic acid synthesis. Plant Physiol., 55, 112—113, 1975. 26. Doss, R. P., Influence of short term inhibitor treatment on the flowering of Lemna perpusilla 6746, Plant Physiol., 56, 360—363, 1975. 27. Doss, R. P., Handedness in duckweed: double flowering fronds produce right- and left-handed lineages, Science, 199, 1465—1466, 1978. 28. Drew, M. C., Jackson, M. B., Giffard, S. C., and Campbell, R., Inhibition by silver ions of gas space (aerenchyma) formation in adventitious roots of Zea mays L. subjected to exogenous ethylene or to oxygen deficiency, Planta, 153, 217—224. 1981. 29. Elzenga, J. T. M., de Lange, L., and Pieterse, A. H., Further indications that ethylene is the gibbosity regulator of the Lemna gibba/Lemna minor complex in natural waters, Acta Bot. Neerl., 29, 225—229, 1980. 30. Esashi, Y. and Oda, Y., Effects of light intensity and sucrose on the flowering of Lemna perpusilla, Plant Cell Physiol., 5, 513—516, 1964. 3 1 . Esashi, Y. and Oda, Y., Two light reactions in the photoperiodic control of flowering of Lemna perpusilla andL. gibba, Plant Cell Physio/., 7, 59—74, 1966. 32. Esashi, Y,, Shibasaki, T., and Saito, K., Flowering responses of Lemna perpusilla and L. gibba in relation to nitrate concentration in the culture medium. Plant Cell Physiol., 13, 623—631, 1972. 33. Evans, L. T., The Induction of Flowering. Some Case Histories, Macmillan South Melbourne, Australia, 1969. 34. Feder, W. A. and Sullivan, F., Ozone: depression of frond multiplication and floral production in duckweed, Science, 165, 1373, 1969. 35. Gordon, W. R. and Koukkari, W. L., Circadian rhythmicity in the activities of phenylalanine ammonialyase from Lemna perpusilla and Spirodela polyrhi:a. Plant Physiol., 62, 612—615, 1978. 36. Goto, K., Mutually inverse rhythmic and sigmoidal changes in activity of cytoplasmic and chloroplast glyceraldehyde 3-phosphate dehydrogenases in Lemna gibba G3, Plant Cell Physiol., 19, 749—758, 1978.

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37. Goto, K., Modes of control by the circadian oscillator and the hourglass mechanism of the activities of cytoplasmic and chloroplast glyceraldehyde 3-phosphate dehydrogenases in Lemna gibba G3, Plant Cell Physiol.. 20, 513—521, 1979. 38. Goto, K., Mechanism of control by a circadian oscillator of chloroplast NADP-linked glyceraldehyde 3phosphate dehydrogenase in Lemna gibba G3, Plant Cell Physiol., 20, 523—532, 1979. 39. Gower, R. A. and Posner, H. B., Effects of light and 3-(3.4-dichlorophenyl)-l,l-dimethylurea on levels of ATP in Lemna paucicostata 6746 and a photosynthetic mutant with abnormal flowering responses, Plant Physio/.. 63, 548—551, 1979. 40. Gupta, S. and Maheshwari, S. C., Induction of flowering by cytokinins in a short-day plant, Lemna paucicostata. Plant Cell Physio!., 10,231—233. 1969. 41. Gupta, S. and Maheshwari, S. C., Growth and flowering of Lemna paucicostata. I. General aspects and role of chelating agents in flowering, Plant Cell Physiol., 11, 83—95, 1970. 42. Gupta, S. and Maheshwari, S. C., Growth and flowering of Lemna paucicostata II. Role of growth regulators, Plant Cell Physio/.. 1 1 , 97—106, 1970. 43. Halaban, R. and Hillman, W. S., Response of Lemna perpusilla to periodic transfer to destilled water, Plant Physiol., 46, 641—644, 1970. 44. Halaban, R. and Hillman, W. S., Factors affecting the water-sensitive phase of flowering in the short day plant Lemna perpusilla, Plant Physiol., 48. 760—764, 1971. 45. Hartung, W. and Kandeler, R., Die Wirkung abendlicher Dunkelrotbestrahlung auf die Aufnahme und Verteilung markierter Phytohormone in Kurztagkultivierten Lemna gibba Gl-Pflanzen, Z. Pflanzenphysiol., 79, 360—367, 1976. 46. Heldwein, R. and Kandeler, R., Adaptation to end-of-day far red and malate accumulation in Lemna, Z. Pflanzenphysiol., 102, 141—146, 1981. 47. Hemleben, V., Untersuchungen zur Biosynthese und Funktion von Nucleinsauren in hoheren Pflanzen, thesis for Habilitation, University Tubingen, Germany, 1972. 48. Hicks, L. E., Flower production in the Lemnaceae, Ohio J. Sci., 32, 115—131, 1932. 49. Hillman, W. S., Photoperiodic control of flowering in Lemna perpusilla. Nature (London), 181, 1275, 1958. 50. Hillman, W. S., Experimental control of flowering in Lemna I. General methods. Photoperiodism in L. perpusilla 6746, Am. J. Bot., 46, 466—473, 1959. 51. Hillman, W. S., Experimental control of flowering in Lemna. II. Some effects of medium composition, chelating agents and high temperatures on flowering in L. perpusilla 6746, Am. J. Bot., 46, 489—495, 1959. 52. Hillman, W. S., Experimental control of flowering in Lemna. III. A relationship between medium composition and the opposite photoperiodic responses of L. perpusilla 6746 and L. gibba G3, Am. J. Bot., 48, 413^19, 1961. 53. Hillman, W. S., Photoperiodism, chelating agents, and flowering of Lemna perpusilla and L. gibba in aseptic culture, in A Symposium on Light and Life, McElroy, W. D. and Glass, B., Eds., Johns Hopkins Press, Baltimore. 1961, 673—686. 54. Hillman, W. S., The Lemnaceae or duckweeds, Bot. Rev., 27, 221—287, 1961. 55. Hillman, W. S., Experimental control of flowering in Lemna. IV. Inhibition of photoperiodic sensitivity by copper, Am. J. Bot., 49, 892—897, 1962. 56. Hillman, W. S., Photoperiodism: an effect of darkness during the light period on critical night length, Science. 140, 1397—1398, 1963. 57. Hillman, W. S., Endogenous circadian rhythms and the response of Lemna perpusilla to skeleton photoperiods, Am. Nat., 98, 323—328, 1964. 58. Hillman, W. S., Red light, blue light, and copper ion in the photoperiodic control of flowering in Lemna perpusilla 6746, Plant Cell Physiol., 6, 499—506, 1965. 59. Hillman, W. S., Photoperiodism in Lemna: reversal of night-interruption depends on color of the main photoperiod, Science, 154. 1360—1362. 1966. 60. Hillman, W. S., Blue light, phytochrome and the flowering of Lemna perpusilla 6746, Plant Cell Physiol., g, 467-^473, 1967. 61. Hillman, W. S., Lemna perpusilla Torr.. Strain 6746, in The Induction of Flowering, Evans, L. T., Ed., Macmillan. South Melbourne, Australia, 1969, 186—204. 62. Hillman, W. S., Carbon dioxide output as an index of circadian timing in Lemna photoperiodism, Plant Physiol., 45, 273—279, 1970. 63. Hillman, W. S., Nitrate and the course of Lemna perpusilla carbon dioxide output under photoperiodic cycles. Plant Physiol., 47, 431—434, 1971. 64. Hillman, W. S., Entrainment of Lemna CO, output through phytochrome. Plant Physiol., 48, 770—774, 1971. 65. Hillman, W. S., Photoperiodic entrainment patterns in the CO, output of Lemna perpusilla 6746 and of several other Lemnaceae, Plant Ph\siol.. 49. 907—911. 1972.

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66. Hillman, W. S., Effects of inorganic nitrogen on the response of Lemnu carbon dioxide output to light quality and timing, Photochem. PhotobioL, 21, 39—47, 1975. 67. Hillman, W. S., Photoperiodism in seedling strains of Lemnu perpusilla: juvenility without obvious morphological correlates?. Am. J. Hot., 62, 537—540, 1975. 68. Hillman, W. S., A metabolic indicator of photoperiodic timing, Proc. Nail. Acad. Sci. U.S.A.. 73, 501 — 504, 1976. 69. Hillman, W. S., Calibrating duckweeds: light, clocks, metabolism, (lowering, Science, 193, 453—458, 1976. 70. Hillman, W. S., Light/timer interactions in photoperiodism and carbon dioxide output patterns: towards a real-time analysis of photoperiodism, in Light ami Plant Development, Smith, H., Ed., Butterworth, London, 1976, 383—397. 71. Hillman, W. S., Control of plant respiration through non-photosynthetic light action. Nature (London), 266, 833—835, 1977. 72. Hillman, W. S., Temporal compartmentation in Lemna paucicostata: Photoperiodism, respiration, nitrogen nutrition and heterotrophic growth of different strains, Am. J. Bar., 66. 1021 —1028, 1979. 73. Hillman, W. S., Temperature sensitivity of daily respiratory patterns entrained through phytochrome action in Lemna paucicostata strain 6746, Physiol. Plant., 47, 56—60, 1979. 74. Hillman, W. S. and Posner, H. B., Ammonium ion and the flowering of Lemna perpusilla. Plant Physiol., 47, 586—587, 1971. 75. Hodson, H. K. and Hamner, K. C., Floral inducing extract from Xanthium, Science, 167, 384—385, 1970. 76. Hodson, H. K. and Hammer, K. C., A comparison of the effects of autoclaved and nonautoclaved gibberellic acid on Lemna perpusilla 6746, Plant Physiol., 47, 726—728, 1971. 77. Hoshino, T., Simulation of acetylcholin action by fi-indole acetic acid in inducing diurnal change of floral response to chilling under continuous light in Lemna gibba G3, Plant Cell Physiol., 20, 43—50, 1979. 78. Hiigel, B., Gegensatzliche Geschlechtsauspragung von Bliitenstandsanlagen der Langtagpflanze Lemna gibba und der Kurztagpflanze Lemna paucicostata in vitro, Z. Pflanzenphysiol., 77, 395—405, 1976. 79. Hiigel, B., Wirkung von Gibberellin-A,, CCC, Ethrel und Indolessigsaure auf die Geschlechtsauspragung isolierter Bliitenstandsanlagen von Lemnaceen, Z. Pflanzenphysiol., 80, 283—297, 1976. 80. Hiigel, B., Wirkung von Kinetin und Abscisinsaure auf die Entwicklung von Lemnaceen-Blutenstandsanlagen in vitro, Z. Pflanzenphysiol., 80, 298—305, 1976. 81. Hiigel, B., Rottenburg, Th., and Kandeler, R., Phytocromsteuerung der Turionenbildung und anderer Entwicklungsprozesee bei Lemna perpusilla P146, Biochem. Physiol. Pflanzen, 174, 761—771, 1979. 82. Ishiguri, Y. and Oda, Y., The relationship between red and far-red light on flowering of the long-day plant, Lemna gibba, Plant Cell Physiol., 13, 131 — 138, 1972. 83. Ishiguri, Y. and Oda, Y., Flowering of the long-day plant, Lemna gibba, under short-day schedules composed of red and far-red light. Plant Cell Physiol., 15, 287—293, 1974. 84. Ishiguri, Y. and Oda, Y., Photoreversibility of flower initiation in Lemna perpusilla as affected by the light quality of the main light period, Plant Cell Physiol., 17, 255-264, 1976. 85. Ishiguri, Y., Oda, Y., and Inada, K., Spectral dependences of flowering in Lemna perpusilla and L. gibba, Plant Cell Physiol., 16, 521—523, 1975. 86. Kaihara, S., Watanabe, K., and Takimoto, A., Flower-inducing effect of benzoic and salicylic acids in various strains of Lemna paucicostata and L. minor, Plant Cell Physiol., 22, 819—825, 1981. 87. Kandeler, R., Uber die Bliitenbildung bei Lemna gibba L. I. Kulturbedingungen und Tageslangenabhangigkeit, Z. Bot., 43, 61—71, 1955. 88. Kandeler, R., Uber die Blutenbildung bei Lemna gibba L. II. Das Wirkungsspektrum von bluhforderndem Schwachlicht, Z. Bot., 44, 153—174, 1956. 89. Kandeler, R., Die Aufhebung der photoperiodischen Steuerung bei Lemna gibba, Ber. Dtsch. Bot. Ges., 75, 43i_442, 1962. 90. Kandeler, R., Wirkungen des Kohlendioxyds auf die Blutenbildung von Lemna gibba, Naturwissenschaften, 51, 561—562, 1964. 9 1 . Kandeler, R., Trennung zweier Dunkelrotwirkungen bei der Lichtsteuerung der SproBvermehrung vonLemna gibba, Z. Pflanzenphysiol., 54, 161—173, 1966. 92. Kandeler, R., The role of photophosphorylation in flower initiation of the long-day plant Lemna gibba, European PhotobioL Symp., September 19th to 22nd, 1967 Hvar, Yugoslavia, Book of Abstracts, 45, 1967. 93. Kandeler, R., Bluhinduktion bei Lemnaceen, Bio/. Rundsch., 6, 49—57, 1968. 94. Kandeler, R., Forderung der Blutenbildung von Lemna gibba durch DCMU und ADP, Z. Pflanzenphysiol., 61, 20—28, 1969. 95. Kandeler, R., Hemmung der Blutenbildung von Lemna gibba durch Ammonium, Planta, 84, 279—291, 1969.

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96. Kandeler, R., Die Wirkung von Lithium und ADP auf die Phytochromsteuerung dcr Bliitenbildung, Planta. 90, 203—207, 1970. 97. Kandeler, R., Die Wirkung von Ascorbinsiiure, NADH and NADPH auf" die Bliitenbildung von Lemnu perpusilla 6746 im Dauerlicht. Z. Pflanzenphvsiol., 64, 278—280, 1971. 98. Kandeler, R., Die Wirkung von Acetylcholin auf die photoperiodische Steuerung der Bliitenbildung bei Lemnacecn, Z. Pjlanzenphysiol., 67, 86—92, 1972. 99. Kandeler, R. und Hiigel, B., Bliitenbildung bei Lemnu paucicostata 6746 durch kombinierte Anwendung von Abscisinsaure und CCC, Plant Cell Physiol., 14, 515—520, 1973. 100. Kandeler, R. und Hiigel, B., Wiederentdeckung der echten Lemna perpusilla TORR. und Vergleieh mil L. paucicostata HEGELM, Plant Syst. Evol., 123, 83—96, 1974. 1 0 1 . Kandeler, R., Hiigel, B., und Rottenburg, Th., Gegensatzliche Wirkung der SproSalterung auf die Blutenbildung bei Lemna paucicostata und Lemna gibba, Biochem. Physiol. Pflanzen, 165, 331—336, 1974. 102. Kandeler, R., Hiigel, B., and Rottenburg, Th., Relations between photosynthesis and flowering in Lemnaceae, in Environmental and Biological Control of Photosynthesis, Marcelle, R., Ed., Dr. W. Junk, The Hague, 1975, 161—169. 103. Kandeler, R., Loppert, H., Rottenburg, Th., and Scharfetter, E., Early effects of phytochrome in Lemna, in Photoreceptors and Plant Development, de Greef, J., Ed., Antwerpen University Press, Antwerpen, 1980, 485^92. 104. Kasinov, V. B., Rechts- oder Linkstendenz bei Lemnaceen. Uber die Determinierung der rechts oder links betonten Entwicklungsrichtung bei Lemna-K.\onen und ihre experimentell induzierte Umkehrung, Beitr. Biol. Pflanzen, 49, 321—337, 1973. 105. Kato, A., Effect of interruption of the nyctiperiod with an R/FR-mixture of various ratios of red and farred light on flowering in Lemna gibba G3, Plant Cell Physiol.. 20, 1273—1283, 1979. 106. Kato, A., Maintenance of high Pfr-level in the dark period in relation to flowering in Lemna gibba G3, Plant Cell Physiol., 20, 1285—1293, 1979. 107. Kato, A. and Nakashima, H., The effects on RNA synthesis in a long-day duckweed, Lemna gibba G3, of irradiation with different ratios of red and far red light during the prolonged dark period. Z. Pflanzenphysiol., 91, 109—117, 1979. 108. Kessler, B. and Steinberg, N., Cyclic mononudeotide-gibberellin interactions in the flowering and proliferation of the long-day plant Lemna gibba G3, Physiol. Plant.. 28, 548—553, 1973. 109. Khudairi, A. K. and Maeng, J., Studies on the flowering mechanism in Lemna. II. The dark reaction of the short-day plant Lemna perpusilla, Physiol. Plant., 28, 271—277, 1973. 110. Khurana, J. P. and Maheshwari, S. C., Induction of flowering in Lemna paucicostata by salicylic acid, Plant Sci. Lett., 12, 127—131, 1978. 1 1 1 . Khurana, J. P. and Maheshwari, S. C., Some effects of salicylic acid on growth and flowering in Spirodela polyrrhiza SP,0, Plant Cell Physio/., 21, 923—927, 1980. 1 1 2 . Kirkland, L. and Posner, H. B., The role of light in the photoperiodic inhibition of flower development in Lemna perpusilla 6746, Plant Physio/., 53, Suppl. 3, 1974. 1 1 3 . Kondo, T., Diurnal change in leakage of electrolytes from a long-day duckweed. Lemna gibba G3, under osmotic stress induced by water treatment, Plant Cell Physiol., 19, 985—995, 1978. 114. Kondo, T. and Nakashima, H., Content of adenosine phosphate compounds in a long-day duckweed, Lemna gibba G3, under different light and nutritional conditions, Physio/. Plant., 45, 357—362, 1979. 1 1 5 . Kondo, T. and Tsudzuki, T., Rhythm in potassium uptake by a duckweed, Lemna gibba G3, Plant Cell Physiol., 19, 1465—1473, 1979. 1 1 6 . Kondo, T. and Tsudzuki, T., Phase progress under low temperature treatment of the potassium uptake rhythm in a duckweed, Lemna gibba G3, Plant Cell Physiol., 21, 95—103, 1980. 117. Kondo, T. and Tsudzuki, T., Energy supply for potassium uptake rhythm in a duckweed, Lemna gibba G3, Plant Cell Physiol., 21, 433-^43, 1980. 118. Kondo, T. and Tsudzuki, T., Participation of a membrane system in the potassium uptake rhythm in a duckweed, Lemna gibba G3, Plant Cell Physio/., 21, 627—635, 1980. 119. Konings, H., Ethylene-promoted formation of aerenchyma in seedling roots of Zea mays L. under aerated and non-aerated conditions, Physio/. Plant., 54, 119—124, 1982. 120. Krajncic, B., Photoperiodic responses of Lemnaceae from Northeastern Slovenia, Acta Bot. Croat., 33, 81—88, 1974. 121. Krajncic, B., The Mechanisms of Floral Induction in Lemnaceae from Slovenia, thesis, Zagreb, 1976. 122. Krajncic, B. and Devide, Z., Report on photoperiodic responses in Lemnaceae from Slovenia, Ber. Geobot. inst. Eidg. Tech. Hochsch. Stift. Riibel, Zurich, 47, 75—86, 1980. 123. Lacor, M. A. M., Flowering of Spirode/a polyrhiza (L.) Schleiden, Acta Bot. Neer/., 17,357—359, 1968. 124. Lacor, M. A. M., Some physiological and morphogenetic aspects of flowering of Spirodela polyrhiza (L.) Schleiden, Acta Bot. Need., 19, 53—60, 1970.

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125. Landolt, E., Physiologische und okologische Untersuchungen an Lemnaceen, Rer. Schweiz. Bot. Ges., 67, 271-^HO, 1957. 126. Landolt, E., Ed., Biosystematic Investigations in the Family of Duckweeds (Lemnticeae), Vol. 1, Veriiff. Geobol. Inst. Eidg. Tech. Hochsch Siii't. Riibel, Zurich. Heft 70. 1980. 127. Loppert, H. G., Evidence for electrogenic proton extrusion by subepidermal cells of Lemna pane-icostata 6746, Planta. 144. 311—315. 1979. 128. Loppert, H. G., Kronberger, W., and Kandeler, R., Phytochrome-mediated changes in the membrane potential of subepidermal cells of Lemna paucicoslata 6746, Planta, 138. 133—136. 1978. 129. Maeng, J. and Khudairi, A. K., Studies on the (lowering mechanism in Lemna. I. Amino acid changes during flower induction. Physio/. Plant.. 28, 264—270, 1973. 130. Maheshwari, S. C. and Gupta, S., Induction of flowering in Lemna paucicostata, a short-day plant, by chelating agents and iron, Planta, 77, 95—98, 1967. 131. Maheshwari, S. C. and Seth, P. N., Photoperiodic control of flowering in Wolffia papulifera. Plant Cell Physio/., 1, 163—165, 1966. 132. Maheshwari, S. C. and Venkataraman, R., Induction of flowering in a duckweed — Woljjui microscopica — by a new kinin, Zeatin, Planta, 70. 304—306. 1966. 133. Mason, H. L., A Flora of the Marshes of California, University of California Press. Berkeley. 1957. 134. Miyata, H., Endogenous light-on rhythm in respiration of a long-day duckweed. Lemna gibbet G3. Plant Cell Physio/.. 11,293—301. 1970. 135. Miyata, H. and Yamamoto, Y., Rhythms in respiratory metabolism of Lemna gibba G3 under continuous illumination. Plant Ce//Physio/.. 10,875—889. 1969. 136. Mori, H., Kinetics of induction and production of flowers in a short-day duckweed, Lemna paucicostata 6746, in darkness, Plant Cell Physio/.. 20, 615—621. 1979. 137. Mori, H., Effect of red light pulse on induction and production of flowers in a short-day duckweed, Lemna paucicostata 6746, in darkness, Plant Cell Physio/.. 20, 623—630, 1979. 138. Mori, H., Effect of far-red light pulse on induction and production of flowers in Lemna paucicostata 6746 in darkness, Plant Cell Physio/., 20, 639—647, 1979. 139. Nakashima, H., Effects of exogenous amino acids on the flower and frond production in duckweed, Lemna gibba G3. Plant Cell Physiol., 5, 217—225, 1964. 140. Nakashima, H., Further studies on the action of free amino acids on flowering of duckweed, Lemna gibba G3, Plant Cell Physiol., 6, 441-^52. 1965. 141. Nakashima, H., The rhythmical change in sensitivity of a long-day duckweed, Lemna gibba G3. to darkbreak, Plant Cell Physio/., 7, 11—24, 1966. 142. Nakashima, H., Change in sensitivity to dark break and its relation to floral induction in long-day duckweed. Lemna gibba G3, Plant Cell Physiol., 8, 637—645, 1967. 143. Nakashima, H., On the rhythm of sensitivity to light interruption in a long-day duckweed, Lemna gibba G3, Plant Cell Physiol., 9, 247—257, 1968. 144. Nakashima, H., Diurnal rhythm of uridine incorporation into RNA regulated by two light-perceiving systems in a long-day duckweed, Lemna gibba G3, Plant Cell Physiol., 17, 209—217, 1976. 145. Nakashima, H., Further studies on the diurnal rhythm of uridine incorporation into RNA in the long-day duckweed, Lemna gibba G3, Plant Cell Physiol., 19. 375—383, 1978. 146. Nakashima, H., Diurnal rhythm of nuclear RNA polymerase 1 activity in a duckweed, Lemna gibba G3, under continuous light conditions, Plant Cell Physiol., 20, 165—176, 1979. 147. Novacky, A., Fischer, E., Ullrich-Eberius, C. I., Liittge, II., and Ullrich, W. R., Membrane potential changes during transport of glycine as a neutral amino acid and nitrate in Lemna gibba G1, FEBS Lett., 88, 264—267, 1978. 148. Novacki, A., Ullrich-Eberius, C. I., and Liittge, U., Membrane potential changes during transport of hexoses in Lemna gibba G l , Planta, 138, 263—270, 1978. 149. Oda, Y., Effect of light quality on flowering of Lemna perpusilla 6746, Plant Cell Phvsiol., 3, 415—417, 1962. 150. Oda, Y., The action of skeleton photoperiods on flowering in Lemna perpusilla. Plant Cell Physiol., 10, 399—^09, 1969. 151. Ohtani, T. and Ishiguri, Y., Inhibitory action of blue and far-red light in the flowering of Lemna paucicostata, Physiol. Plant., 47, 255-259, 1979. 152. Ohtani, T. and Kumagai, T., Spectral sensitivity of the flowering response in green and etiolated Lemna paucicostata T-101, Plant Cell Physiol., 21, 1335—1338, 1980. 153. Ohtani, T. and Kumagai, T., Action spectra for the light inhibition of flowering and its reversal in Lemna paucicostata T-101, Planta, 149, 332—335, 1980. 154. Ohtani, T. and Kumagai, T., Phytochrome-mediated effects of near-ultraviolet radiation in the induction of flowering in etiolated Lemna paucicostata T-101, a short-day plant, Planta, 153, 543—546, 1981. 155. Oota, Y., Effects of growth substances on frond and flower production in Lemna gibba G3, Plant Cell Physiol., 6, 547—559, 1965.

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156. Oota, Y., Frond and flower production in Lemna gibbu G3 in presence of respiratory inhibitors. Plant Cell Physiol.. 10.621—633, 1969. 157. Oota, Y., A possible mechanism for sugar inhibition of duckweed flowering. Plant Cell Ph\siol., 13, 195—199. 1972. 158. Oota, Y., The response of Lemna gibbet G3 to a single long day in the presence of EDTA. Plant Cell Physiol., 13.575—580, 1972. 159. Oota, Y., The length of the nduction period vs. the minimum number of long-day cycles needed for floral induction in Lemna gibba G3, Plant Cell Physiol., 14, 307—317. 1973. 160. Oota, Y., Removal of the sugar inhibition of flowering in Lemna gibba G3 by catecholamines. Plant Cell Physiol.. 15, 63—68, 1974. 161. Oota, Y., Photoperiodic requirements for flowering of the long-day duckweed. Lemna gibba G3. Plant Celt Physiol.. 16, 885—894, 1975. 162. Oota, Y., Short-day flowering of Lemna gibba G3 induced by salicylic acid, Plant Cell Physiol., 16, 1131—1135, 1975. 163. Oota, Y., Time measurement in photoperiodic floral induction in duckweeds, Curr. Adv. Plant Sci., 6, 665—674. 1975. 164. Oota, Y., Removal by chemicals of photoperiodic light requirements of Lemna gibba G3, Plant Cell Physiol., 18. 95—105, 1977. 165. Oota, Y., Replacement of ionophores of the photoperiodic light requirement in Lemna gibba G3, Plant Cell Physiol., 18, 1363—1367, 1977. 166. Oota, Y., Bimodal floral response of Lemna gibba G3 to night interruption: photoperiodic time measurement, Plant Cell Physiol, 22, 99—113, 1981. 167. Oota, Y. and Hoshino, T., Diurnal change in temperature sensitivity of Lemna gibba G3 induced by acetylcholine in continuous light, Plant Cell Physiol., 15, 1063—1072, 1974. 168. Oota, Y. and Hoshino, T., Spectral dependence of the critical photoperiod in the long-day duckweed, Lemna gibba G3. Plant Cell Physio/., 20, 1531 — 1536, 1979. 169. Oota, Y. and Kondo, T., Removal by cyclic AMP of the inhibition of duckweed flowering due to ammonium and water-treatment, Plant Cell Physiol., 15, 403—411, 1974. 170. Oota, Y. and Nakashima, H., Photoperiodic flowering in Lemna gibba G3: time measurement, Hot. Mag. Tokyo, Special Issue 1, 177—198. 1978. 171. Oota, Y. and Tsudzuki, T., Resemblance of growth substances to metal chelators with respect to their actions on duckweed growth. Plant Cell Physiol., 12, 619—631, 1971. 172. Oota, Y. and Tsudzuki, T., Evidence against involvement of circadian floral rhythm in the critical daylength measurement in Lemna gibba G3, Plant Cell Physiol., 20, 725—732, 1979. 173. Pekic, S. and Neskovic, M., Influence of phytochrome on the content of endogenous hormones in Lemna aeauinoctialis during the long night period, Bull. Inst. Jard. Hot. Univ. Beograd, in press. 174. Pieterse, A. H., Gibberellin-EDDHA interaction in flowering and gibbosity of Lemna gibba G3, Plant Cell Physiol., 15, 1125—1127, 1974. 175. Pieterse, A. H., Specific interactions in the physiology of flowering and gibbosity of Lemna gibba G3, Plant Cell Physiol., 17, 713—720, 1976. 176. Pieterse, A. H., Interaction of naphthol with EDDHA/salicylic acid in flowering and gibbosity of Lemna gibba G3, Plant Cell Physiol., 19, 1307—1310, 1978. 177. Pieterse, A. H., Bhalla, P. R., and Sabharwal, P. S., Investigations on the effects of metal ions and chelating agents on growth and flowering of Lemna gibba G3, Plant Cell Physiol., 11, 879—889, 1970. 178. Pieterse, A. H. and Miiller, L. J., Induction of flowering in Lemna gibba G3 under short-day conditions, Plant Cell Physiol, 18,45—53, 1977. 179. Posner, H. B., Inhibitory effect of sucrose on flowering in Lemna perpusilla 6746 and mutant strain 1073, Plant Cell Physiol, 8, 535—539, 1967. 180. Posner, H. B., Inhibitory effect of carbohydrate on flowering in Lemna perpusilla. I. Interaction of sucrose with calcium and phosphate ions, Plant Physiol., 44, 562—566, 1969. 181. Posner, H. B., Inhibitory effect of carbohydrate on flowering in Lemna perpusilla. II. Reversal by glycine and L-aspartate. Correlation with reduced levels of (3-carotene and chlorophyll. Plant Physiol., 45, 687— 690. 1970. 182. Posner, H. B., Inhibitory effect of carbohydrate on flowering in Lemna perpusilla. I I I . Effects of respiratory intermediates, amino acids, and CO,. Glucose 6-phosphate dehydrogenase activity. Plant Physiol., 48, 361—365, 1971. 183. Posner, H. B., Lack of flowering responses to DCMU in a mutant of Lemna perpusilla 6746, Plant Cell Physiol., 14, 1031—1033, 1973. 184. Posner, H. B., Reversal of sucrose inhibition of Lemna flowering by adenine derivatives, Plant Cell Physiol., 14, 1199—1200, 1973. 185. Posner, H. B., and Hillman, W. S., Effects of irradiation on Lemna perpusilla. Am.. J. Bot., 47, 506511, 1960.

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186. Posner, H. B., Posner, R. S., and Gower R. A., Effects of DCMU on long day flowering of Lemna perpusilla 6746 and photosynthetic mutant strain 1073, Plant Cell Physio/., 18, 1301 — 1307, 1977. 187. Pryce, R. J., Allogibberic acid: an inhibitor of flowering in Lemna perpusilla, Phytochemistry, 12, 1745— 1754, 1973. 188. Purves, W. K., Dark reactions in the flowering of Lemna perpusilla 6746, Plants, 56, 684—690, 1961. 189. Rimon, D. and Galun, E., Morphogenesis of Wolffia microscopica: frond and flower development, Phytomorphology, 18, 364—372. 1968. 190. Rimon, D. and Galun, E., Morphogenesis of Spirodela oligorrhiza: ontogenesis of fronds. Dot. Gaz., 129. 138—144, 1968. 191. Sato, T. and Oda, Y., Significance of timing and number of short-day cycles for initiation and subsequent development of floral buds in Lemna perpusilla 6746, Plant Cell Physio/., 18, 1041—1046, 1977. 192. Sato, T. and Oda, Y., Effects of culture conditions on development in continuous light of floral buds in Lemna perpusilla 6746, Plant Cell Physio/., 19, 537—543, 1978. 193. Scharfetter, E., Back, E., and Kandeler, R., End-of-day far-red effect on short-period oscillations of starch content in Lemna, Poster at the European Symposium "Light Mediated Plant Development", Bischofsmais, Germany, April 5th to l l t h , 1981. 194. Scharfetter, E. and Kandeler, R., unpublished results, 1982. 195. Scharfetter, E., Rottenburg, Th., and Kandeler, R., Die Wirkung von EDDHA und Salicylsaure auf Bltitenbildung und vegetative Entwicklung von Spirodela punctata, Z. PflanzenphysioL, 87, 445—454, 1978. 196. Schuster, M., Die Bedeutung von Starklicht und Kupfer fur die phytochromgesteuerte Morphogenese von Lemna perpusilla. Doctoral thesis. University of Wiirzburg, Germany, 1968. 197. Schuster M. und Kandeler, R., Die Bedeutung der Photosynthese fur die Langtag-Bliite der Kurztagpflanze Lemna perpusilla 6746, Z. PflanzenphysioL, 63, 308—315, 1970. 198. Seth, P. N., Venkataraman, R., and Maheshwari, S. C., Studies on the growth and flowering of a short-day plant. Wolffia microscopica. II. Role of metal ions and chelates, Planta, 90, 349—359, 1970. 199. Shibata, O. and Takimoto, A., Flowering response of Lemna perpusilla 6746 to a single dark period, Plant Cell Physiol., 16.513—519, 1975. 200. Soulen, T. K. and Koukkari, W. L., Ammonium ion inhibition of flowering in Lemna: a pH effect?, Plant Physiol., 59 (Suppl.). 48. 1977. 201. Takimoto, A., The spectral dependence of photoperiodic responses, in Physiology of Flowering in Pharbitis nil, Imamura. S., Ed., Japan Society of Plant Physiology, Tokyo, 1967, 73—93. 202. Takimoto, A., Flower initiation of Lemna perpusilla under continuous low-intensity light. Plant Cell Physiol., 14. 1217—1219, 1973. 203. Takimoto, A. and Tanaka, O., Effects of some SH-inhibitors and EDTA on flowering in Lemna perpusilla 6746, Plant Cell Physiol., 14. 1133—1141, 1973. 204. Takimoto, A. and Tanaka, O., Effects of some sulfhydryl inhibitors on floral initiation under various light conditions in Lemna perpusilla, in Plant Growth Substances 1973, Proc. 8th Int. Conf. Plant Growth Substances. Hirokawa Publishing, Tokyo, 1974, 953—959. 205. Takimoto, A. and Tanaka, O., Long-day flowering of Lemna perpusilla 6746 in Mo-deficient medium, Plant Cell Physiol., 17,299-303, 1976. 206. Tanaka, O. and Cleland C. F., Comparison of the ability of salicyclic acid and ferricyanide to induce flowering in the long-day plant, Lemna gibba G3, Plant Physiol., 65, 1058—1061, 1980. 207. Tanaka, O. and Cleland, C. F., Influence of ammonium on the ability of salicylic acid to induce flowering in the short-day plant Lemna paucicostata 6746, Plant Cell Physiol., 22, 597—602. 1981. 208. Tanaka, O., Cleland, C. F., and Hillman, W. S., Inhibition of flowering in the long-day plant Lemna gibba G3 by Hutner's medium and its reversal by salicyclic acid. Plant Cell Physiol., 20, 839—846, 1979. 209. Tanaka, O. and Takimoto, A., Suppression of long-day flowering by nitrogenous compounds in Lemna perpusilla 6746, Plant Cell Physiol., 16, 603—610, 1975. 210. Tanaka, O. and Takimoto, A., Flower-promoting effect of some amino acids and amides in Lemna paucicostata 6746. Plant Cell Physiol., 18, 27—34, 1977. 2 1 1 . Tanaka, G. and Takimoto, A., Effect of nitrate concentration in the medium on the flowering of Lemna paucicostata 6746. Plant Cell Physiol., 19, 701—704. 1978. 212. Tanaka, O., Takimoto, A., and Cleland, C. F., Enhancement of long-day flowering by Mo-deficiency and application of some amino acids and asparagine in the short-day plant Lemna paucicostata, 6746, Plant Cell Physiol., 20. 267—270. 1979. 213. Tsudzuki, T. and Kondo, T., Further studies on potassium uptake rhythm in the long-day duckweed Lemna gibba G3 with special reference to vegetative growth. Plant Cell Physiol., 20. 1079—1086, 1979. 214. Umemoto, T., Effect of chlorogenic acid on flower production in long-day duckweed, Lemna gibba G3, Plant Cell Plnsiol.. 12. 165—169. 1971.

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215. Umemura, K., Inokuchi, H., and Oota, Y., Flowering in Lemna gibha G3, Plant Cell Pli\sioL. 4. 289— 292. 1963. 216. Umemura, K. and Oota, Y., Effects of nucleic acid- and protein-antimetabolites on frond and flower production in Lemna gihhti G3. Plant Cell Pli\siol., 6, 73—86, 1965. 2 1 7 . Venkataraman, R., Seth, P. N., and Maheshwari, S. C., Studies on the growth and (lowering of a short day plant. Wolf/hi microscopica. I. General aspects and induction of flowering by cytokinins, Z. Pflanzenpln-siol., 62. 316—327. 1970. 218. Wangermann, E., Longevity and ageing in plants and plant organs, in Encyclopedia of Plant Ph\sioli>gy, Vol. 15. part 2. Ruhland. W., Ed.. Springer-Verlag. New York. 1965. 1026—1057. 219. Wangermann, E. and Lacey, H. J., Studies on the morphogenesis of leaves. IX. Experiments on Lemna minor with adenine. trijodobenzoic acid and ultra-violet radiation, N. Phylol., 52. 298—311, 1953. 220. Watanabe, K., Fujita, T., and Takimoto, A., Relationship between structure and flower-inducing activity of ben/.oic acid derivates in Lemna paucicostata 151, Plant Cell Physiol., 22, 1469—1479. 1981. 2 2 1 . Watanabe, K. and Takimoto, A., Effects of some metabolic inhibitors on (lowering of Lemna gibba G3, a long-day duckweed. Plant Cell Physiol., 18, 1369—1372, 1977. 222. Watanabe, K., and Takimoto, A., Flower-inducing effects of benzoic acid and some related compounds in Lemna paucicoslatu 151, Plant Cell Physio!., 20. 847—850. 1979. 223. Witztum, A., Morphogenesis of asymmetry and symmetry in Lemna perpusilla Torr., Ann. Hot., 43. 423—HO. 1979. 224. Yoshimura, F., The significance of molybdenum for the growth of Lemnaceue plants. Rot. Maf>. (Tokyo), 57, 371—386, 1943. 225. Yukawa, I. and Takimoto, A., Flowering response of Lemna paucicostata in Japan, Bot. Mag. (Tokyo), 89. 241—250, 1976. 226. Kandeler, R., unpublished results. 1954. 227. Krajncic, B., personal communication. 1981. 228 Loppert, H. G., Gruntzel, S., and Kandeler, R., unpublished results. 1982. 229. Ladenburger, K., Bauer, J., and Kandeler, R., unpublished results. 1983.

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LEPTOSPERMUM SCOPARIUM En. Tea-tree, Manuka Naftaly Zieslin

INTRODUCTION The genus Leptospermum (Myrtaceae), with about 50 species, is native to the Malayan Archipelago, Australia, and New Zealand. One of those species, L. scoparium Forst., is to be found in Australia and New Zealand. Plants of this species have white flowers and vary in form from prostrate shrubs to small trees up to 6 m in height. 1 7 9 " This species began to attract attention when flowers with pink and red colors and double flowers were introduced. Due to the hybridization and breeding of Lammerts in California and others in Australia and New Zealand, 78 -" various cvs were developed and cultivated as a florist's crop in California and Israel and as ornamental shrubs in areas of Europe having mild winters. 2 - 3

FLOWERING Morphology The flowers are axillary at the second upper-most leaf of short lateral branchlets or brachyblasts 1 to 3 cm long. The solitary or 2 to 3 flowers are sessile with 4 to 5 petals in the wild forms and multipetal in the existing cvs. Natural Flowering Late spring and summer were reported as the usual flowering season for most of the cvs. However, in some of the cvs or clones winter flowering is often recorded, while intermittent flowering may also occur in autumn. 6 " In plants of L. scoparium with winter flowering introduced to Israel, flower buds become visible in the beginning of October. Thus, it may be assumed that flower differentiation took place earlier in the fall. 1 3 Flowering under Controlled Conditions Experimental results show 13 (Table 1) that under LD longer than 16 hr, the appearance of flowering buds was completely prevented. This was obtained under two temperature regimes examined, while under SD of 8 hr, flowering was observed at both temperatures. Under the natural daylength of the Israeli summer (13 to 14.5 hr) there was a pronounced flowering at the lower temperature tested. A significant enhancement of the vegetative growth, demonstrated by shoot elongation and increased lateral breaking, was obtained under the LD conditions. No flower bud formation was obtained under SD at temperature below 8°C.12 Flower Development Experiments with transferring L. scoparium plants to LD conditions following various periods of SD treatment showed13 that flower development proceeds normally after 8 weeks of SD of 8 hr although the number of flowers per shoot decreased as compared to plants held constantly in SD. The transfer to LD after a shorter SD period resulted in flower bud abscission.

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THE EFFECT OF TEMPERATURE ON THE PHOTOPERIODIC RESPONSE AND THE INFLUENCE OF THE PHOTOPERIOD ON THE VEGETATIVE GROWTH OF PLANTS OF L. SCOPARIUM

Photoperiodic treatment LD—16 h r SD—8 hr Natural day (13—14 hr) Natural day (10—11 hr)

Flower bud presence ' Mild temp High temp 0 + +

Flower bud formation buds per 5 cm of shoot length —•„— Mild temp High temp

0 + 0

Vegetative growth Laterals per shoot

Shoot length increment (cm) 50 "5

0 3.1 6.0 1

7

Note: The photoperiod was applied as day extension with light from incandescent lamps of 1640 mW/m.- + Flower buds formed, - no flower buds present. Mild temperature IO°C N and 20°C D. high temperature 20°C N and 26°C D.

Table 2 THE TIME REQUIREMENTS (DAYS) FOR TWO DIFFERENT STAGES OF DEVELOPMENT IN TWO CLONES OF L. SCOPARIUM Clone Stages of development

White

Red

From color visible to fully open petals From fully open petals to petal abscission Total

16

9

13

18

29

27

Eight weeks of SD were required until color was visible in the flower buds, with an additional 4 weeks necessary until complete anthesis. 11 During the second stage, the various clones differed in the time-period required for different stages of flower development, while the total period remained similar in the various clones tested5 (Table 2). CONCLUSIONS According to the experimental data available it seems that clones of L. scoparium examined are SDP with a critical daylength between 14 and 16 hr. At the daylength shorter than 8 hr flower differention is independent of the temperature in the range of 10 to 26°C. No flower formation takes place under daylength longer than 16 hr at these temperatures or at temperature lower than 8°C, either at the SD or LD conditions. It is of interest to mention that cvs of L. scoparium flower during mild winters of England. 6 Different periods of flowering of the various cvs 12 may represent different photoperiodic and other environmental factors for FI or development.

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REFERENCES 1 . Allen, H. H., Flora of New-Zealand, Vol. I , Government Printer, Wellington, New Zealand, 1961, 320— 359. 2. Auman, C. W., Leoplospermiim scoparium, in Introduction to Floriculture, Larson. R. A., Ed., Academic Press, New York, 1980. 3. Bean, W. J., Trees ami Shrubs Hardy in the British hies. Vol. 2, 8th ed., 1973, 548—553. 4. Gottesman, V., Hebrew University, Rehovot, unpublished. 1982. 5. Gottesman, V., Post Harvest Responses of Excised Shoots of Leplospermum scoparium. M.Sc. Agric. thesis, Hebrew University, of Jerusalem. Rehovot, 1982. 6. Halliwell, B., Leplospermum scoparium and its cultivar, Plantsman, 3, 175—177, 1981. 7. Harrison, R. E., Handbook of Trees and Shrubs for the Southern Hemispere, A. W. Reed Publishing, Wellington, 1974. 8. Lammerts, W. E, Flowering of double flowers, Leplospermum, J. Calif. Hortic. Soc., 6, 250—257, 1975. 9. Lament, B., Leplospermum, Aust. Plants, 10, 74—78, 1979. 10. McNairn, R. B., Leplospermum Final project report, California State University, Chico, unpublished, 1975. 1 1 . Metcalf, L. J., The cultivation of New-Zealand Trees and Shrubs, A. H. and A. W. Reed Publishing, Wellington, 1972, 152—159. 12. Zieslin, N., Hebrew University, Rehovot, unpublished, 1982. 13. Zieslin, N. and Eagel, S., Leptospermum — a new crop for floriculture, Hassadeh, 61, 608—611, 1981 (Hebrew).

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LEUCOSPERMUM En. Pincushion, Sunburst protea Gerard Jacobs INTRODUCTION The genus Leucospermum (Proteaceae) with its 47 species is confined to southern Africa. They are evergreen woody perennial plants and most species are erect shrubs or small trees. Eleven species (L. spathulatum, L. profugum, L. cordatum, L. hypophyllocarpodendron, L. royenifolium, L. heterophyllum, L. prostratum, L. pendunculatum, L. secundifolium, L. gracile, and L. saxatile) have a prostrate growth habit. Seven species (L. saxosum, L. cuneiforme, L. gerrardii, L. innovans, L. hypophyllocarpodendron, L. tomentosum, and L. prostratrum) have a woody, subterranean rootstock, while the other species usually have a single main stem.6 Cut flowers are harvested mainly from L. cordifolium, L. patersonii, L. lineare, L. conocarpodendron and L. vestitum growing under cultivation or in the wild. However, many other species such as L. bolusii, L. catherinae, L. cuneiforme, L. erubescens, L. fulgens, L. glabrum, L. oleifolium, L. pluridens, L. reflexum, and L. tottum are also exploited. An increasing number of interspecies hybrids are grown commercially (such as cv Red Sunset, which is a natural hybrid between L. cordifolium and L. lineare) as clonal selections propagated from cuttings. The vegetative and reproductive growth of Leucospermum follow in sequence. The plants grow vegetatively during spring and summer. Individual shoots have strong apical dominance. Reproductive development commences in autumn after shoot extension growth has terminated and the flower heads develop during winter. The flowers open in early spring. FLORAL MORPHOLOGY The inflorescence is a capitulum arising from axillary buds situated distally on a shoot. The inflorescences are borne solitary (L. cordifolium, L. lineare, L. vestitum) or in groups of up to 10 (L. oleifolium, L. bolusii). Before opening, individual flowers consist of a perianth formed by 4 fused perianth segments. When the flower opens one of the perianth segments separates from the other 3 and the entire perianth curls up, displaying a prominent white, pink, yellow, orange, or crimson colored style with a prominent pollen presenter.6 Most often the styles are the main attractive feature of the inflorescence. In some species (L. conocarpodendron, L. glabrum, L. pluridens) the brightly colored and curled up perianths or the involucral bracts (L. oleifolium) contribute to the beauty of the inflorescence. The number of flowers per capitulum varies greatly among species. FLOWER INITIATION AND DEVELOPMENT Axillary buds on actively growing shoots of Leucospermum can be forced to grow out as shoots by removal of the growing point (pinching). Even after shoot growth has stopped (December to February) pinching results in shoot growth from the upper axillary buds, indicating that the plants have not yet entered the induced state for FI.'- 2 Decapitating shoots by 5 cm in April results in an inflorescence developing from the uppermost axillary bud. This condition is maintained for about 2 months, followed by a gradual loss in the induced state until the plants return to the noninduced vegetative state by September to October in the Southern Hemisphere (Table I ) . ' - 2 Photoperiod does not appear to control FI because

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Table 1 FI ON SHOOTS OF LEUCOSPERMUM CV RED SUNSET HEADED BACK BY 5 CM AT DIFFERENT DATES IN STELLENBOSCH, SOUTH AFRICA" % Shoots initiating an inflorescence1' after headed back on Shoot length (cm)

Feb. 1

Feb. 28

30—45 20—25

0 0

0 0

11 h

March 28 0 16

April 25 96 100

May 28 100 52

June 20 100 28

July 30

Aug. 22

Sept. 22

Oct. 22

28 0

28 0

12 0

0 0

Values expressed as percentage of 25 shoots per treatment. Shoot growth occurred on shoots failing to initiate an inflorescence.

From Jacobs, G., J. Am. Soc. Hortic. Sri., 108, 32—35, 1983. With permission.

this does not occur during LD between December and March, and the induced state is lost during the SD between July and August. 2 The developing inflorescence correlatively inhibits other axillary buds from developing. When the developing primary inflorescence is removed, one or more of the proximal buds develop into an inflorescence. 1 - 3 The degree of inhibition of the axillary buds depends on their position on the shoot. The first 6 to 10 axillary buds immediately below the developing inflorescence are only partly inhibited since they develop to a size of 5 mm in diameter. They are thus referred to a secondary "flower buds". Buds lower down on the flowering shoot show no signs of development and are apparently completely inhibited (Figure 1). Two factors within the plant have been found to regulate FI: (1) FI can only take place after cessation of shoot growth. (2) Axillary buds can only initiate flowers if they are released from correlative inhibition during the stage when plants are in the induced state. At least three factors appear to be involved in the cessation of shoot growth. Old plants with many branches stop shoot growth relatively early (December) as compared to young plants where shoot growth extends until the end of February or even later. Early cessation of shoot growth in old plants is possibly related to shoot/root ratio becoming unfavorable for shoot growth. Those factors which culminate in the plant's readiness for reproductive development are possibly the cause for cessation of shoot growth in young plants. Thirdly, early cessation of shoot growth may also be induced by water stress for plants growing under dry land conditions. Secondary "flower buds" will develop into an inflorescence when the correlative inhibition of the primary inflorescence is removed as late as October. By contrast, all or most of the axillary buds lower on the stem will develop into vegetative shoots when the correlative inhibition is removed by cutting shoots back at this time of the year.' If inhibition of secondary flower buds is not released, they abscise during November and December. Leaf removal and shading prevent FI in Leucospermum cv Red Sunset. 1 Heavy shading applied from December reduced the number of stems forming an inflorescence (Table 2). Long shoots are less responsive to inhibition of flowering at low light intensities. 2 Shading applied from February did not affect FI. Apparently Leucospermum is a day-neutral plant and FI is evoked in response to high light intensity in conjunction with intraplant factors such as cessation of shoot growth and release of axillary buds from correlative inhibition. No information is available on factors controlling loss of the induced state in Leucospermum. Secondary "flower buds" will flower later than the primary inflorescence depending on the date when the primary inflorescence is removed.3 The flowering time of Leucospermum cv Golden Star was delayed by 2 months when the primary inflorescence was removed during the first week of October.3 For this cv about 925 units (°C days) calculated from a base temperature of 6°C are required to mature 90% of the secondary inflorescences.4 Shading

Volume HI £f^

primary

inflorescence

secondary

completely

flower bud

inhibited bud

X3nleaf

FIGURE 1. Schematic presentation of Leucospermum cv Red Sunset shoot. The developmental stage represented is after April. Shoot growth has stopped, the primary inflorescence is developing, and secondary "flower buds" are visible. (From Jacobs, G., J. Am. Soc. Hortic. Sci., 108. 32— 35, 1983. With permission.)

Table 2 EFFECT OF SHADING AND SHOOT LENGTH ON FI OF LEUCOSPERMUM CV RED SUNSET (IN % FLOWERING) Shoot length (cm) Shade 0 30 60 80

Note:

30—40

40—50

50—60

60

Mean'

100 94 0 1

100 88 19 7

100 89 59 36

100

lOOa 88a 34b 9c



86 —

Values are mean percentage from 4 single plant replicates.

a, b, c — Mean separation by Duncan's multiple range test. 5,•*;

^:W ' •£. >A IV

ffe *:

r

h-

1

tf..*

V^Vtrn. .••

,|t^ ,i>

.E 11. Median longitudinal sections of the flanks of shoot apices of L. temulentum on successive days after the LD, showing the progressive increase in RNA the cells of the spikelet sites from Day III to Day VI, and the loss of RNA (as stained with methyl green-pyronin) from the leaf primordia above and below the spil From Knox, R. B. and Evans, L. T.,Aust. J. Biol. Sci, 19, 223-245, 1966. With permission.)

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development begins on Day V greatly increase its subsequent rate, which suggests that floral evocation is a quantitative rather than merely qualitative phenomenon. This is why the rate of inflorescence development is used as a measure of photoperiodic induction in L. temnlentum. On the other hand, additional LD given after Day V also increase the rate of inflorescence development, which suggests that the LD stimulus to floral evocation also stimulates later stages in the development of spikelets and florets, i.e., it is not specific to evocation per se. The precocious release of the spikelet sites from inhibition is closely analogous to later steps in development, such as the inception of the floret and stamen primordia. Spikelets, florets, and stamens are all of "cauline" origin, 3 and what stimulates one may stimulate the others. Inflorescence initiation in L. temulentum may be quite analogous to FI in other plants, although this point has been queried by Arzee et al. 1 and Lyndon. 40 The summit of the Lolium shoot apex is certainly different from many vegetative dicotyledonous shoot apices in showing no sign of a "meristeme d'attente", all the cells being high in RNA, 34 but the many potential spikelet sites on the two flanks of a vegetative apex can be viewed as equivalents of dicotyledonous meristemes d'attente, waiting to be evoked, low in RNA, and with more active leaf primordia on each side of them.

RESPONSE TO APPLIED COMPOUNDS In considering the specificity of effects of applied chemicals on flowering it is important to know where and when the chemical acts. Is it for example, influencing flower induction in the leaf or floral evocation or differentiation at the apex? Even with clear answers to this question we also need to know whether the action is indirect and pharmacological or directly influences the flowering processes in the plant. If a physiological role is suggested, what is the target tissue, what is the concentration there, and what metabolic processes are regulated? Chemicals Controlling Specific Metabolic Processes The effect on flowering of the time of application of DCMU to the leaf of Lolium highlights the problems of establishing specificity. DCMU acts at the leaf and is ineffective at the apex. 23 - 29 It inhibits photosynthesis rapidly (within 2 hr) and its inhibitory effect on photoperiodic induction in L. temulentum closely parallels that on photosynthesis (Figure 4). DCMU is inhibitory even when applied after the photoperiodic processes in the leaf have passed their threshold. The reduction of inhibition of flowering by later DCMU treatment does, however, correlate with the export from the leaf of the LD stimulus to flowering (Figure 8). Thus, photosynthesis is essential for the expression of the photoperiodic processes in the leaves of Lolium, possibly for the export of the LD stimulus. Even after that has taken place, however, DCMU remains somewhat inhibitory, probably reflecting the need for a continuing supply of photosynthate by the developing inflorescence. Another chemical, SK & F 7997 A 3 (tris-[2-dethylaminoethyl] phosphate trihydrochloride), may also be leaf-active since it is inhibitory to flowering only when applied to leaves before photoperiodic induction has occurred. Its inhibitory action at the shoot apex is much less marked and is not time dependent. Chemicals which inhibit RNA and protein synthesis might be expected to act at the shoot apex on the processes of floral evocation and development. For D,L-ethionine and chloramphenicol, action is specific to the apex but there is no sharp time dependence, and applications to the apex as late as Day III inhibit flowering.17-24 Time specificity for action at the apex is clearest for applications of Actinomycin D, an inhibitor of RNA synthesis, and for cycloheximide and chlorpropham among the various inhibitors of protein synthesis that have been used 17 - 24 - 26 (Figure 9). The precise timing of sensitivity to these inhibitors suggests that RNA and protein synthesis are essential to floral evocation in L. temulentum.

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Plant Growth Regulators

Auxins and Ethvlene Earlier studies on the effects of IAA and NAA on flowering in Lolium have not been extended. The earlier findings were summarized by Evans. 20 Auxin inhibits flowering in plants exposed to 1 LD but is promotive in SD given with a 2-hr light break during one night." 1 The dose responses are quite distinct in these experiments: greatest promotion in SD was found with a concentration of 5.7 X 10 -s M, whereas in LD inhibition was apparent only with concentrations in excess of 3 x 10 4 M. It is now known that high doses of auxin can trigger ethylene production and this could inhibit flowering as it does in other species.53 However, at a concentration of 100 ppm ethylene neither inhibited flowering in LD conditions nor promoted it in SD.24 Gibberellins and Growth Retardants Gibberellins (GA) have long been known to induce flowering in many species of LDPs held in noninductive conditions and this is true for L. temulentum.I6J9-20 The response occurs whether the GA is applied to leaf or apex. Since GAs are readily mobile, it is not possible to determine a site of action and the fluctuations seen in the response to the timing of GA application do not help to clarify this matter. Of the eight gibberellins tested to date only four are effective for flowering, namely GA,, GA,. GA S , and GA 7 . It would be of interest to examine the effects of GAs characteristic of the various biosynthetic pathways, to establish which GAs occur naturally in L. temulentum, and whether their levels in leaf or apex change with photoperiodic treatment. It is not known whether LDs affect the endogenous level of GAs. If GA levels increased in LD, a higher dose of GA should be required for the induction of flowering in SD than for the promotion of flowering in LD, which is not the case. 1 ' 1 Treatment with growth retardants should prevent any increase of GA in LD and, hence, block flowering, while application of GA should reverse this action. However, one such retardant, CCC, while it interacts with GA, on stem length as expected, is not only noninhibitory to flowering in LD but actually promotes the flowering response to applied GA,. 1 9 possibly due to reduced competition between inflorescence growth and that of the stem. Another retardant, B9 (A'.A'-dimethylamino succinamic acid), does inhibit flowering and its effect is reversed by GA, but there is also considerable damage to the plant. Evidence is accumulating to show that these retardants not only inhibit specific steps in GA biosynthesis but also have other effects unrelated to it.-'" LD-induced flowering is not inhibited by DNA synthesis inhibitors such as 5-FDU, yet the promotive effect of GA in LD is blocked by 5FDU. 19 Abscisic Acid and Xanthoxin Abscisic acid (ABA) and xanthoxin. a related compound, have been considered principally as transmissible inhibitors moving from the leaves to the apex. Application of ABA to the apex or leaves of L. temulentum inhibits flowering.'"•'*•" However, the endogenous content of ABA and xanthoxin in the leaf and of ABA in the apex of L. temulentum is not altered by photoperiod." Whereas ABA levels in L. temulentum are not responsive to photoperiod. rapid and dramatic increases are seen with the onset of water stress, 4- to 30-fold within 8 hr. 12 The increase is first seen in the leaf and later (4 to 24 hr) in the shoot apex, implying its transport to the apex from the leaf. With termination of stress there is a rapid drop in the level of ABA. The effect of brief (8 hr) periods of water stress on flowering is also marked, and the timing of sensitivity to stress suggests that floral evocation is inhibited by endogenous ABA as it is by injected ABA. Earlier or later stresses induced an increase in ABA but, as with stress given on the day of induction, the increase in ABA was only short lived. The similar timing in the sensitivity of flowering response to water stress and to injected ABA

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points to ABA as a mediator of the effect of stress on flowering. In an earlier study Aspinall and Husain 2 argued that water stress inhibited transport of the LD stimulus to flowering. ABA increase may not be the only component of the effects of stress on flowering, and inhibition of transmission of the LD stimulus may well occur. Cvtokininx Cytokinins (benzyladcnine and SD 8339) were consistently inhibitory to LD induction in L. temulentum when injected near the shoot apex, the more so the earlier the application, and did not promote flower induction in plants held in SD. 24

CONCLUSION Several independent approaches suggest that there is a dual system of daylength control over floral evocation in L. temulentum. In plants in photoperiods of less than 12 hr, there is a daily arrival at the shoot apex of an inhibitor from the SD leaves which may prevent the potential spikelet sites from being activated, unless the leaves are in anaerobic conditions or the shoot apex is excised. The identity of the inhibitor is unknown. It might be abscisic acid, since both exogenous ABA and endogenous ABA generated by water stress are inhibitory to evocation. The finding that daylength does not appear to control ABA production in L. temulentum does not rule it out, because it remains to be established whether the transmissible inhibitor is exported only from leaves in SD. The second component of daylength control in Lolium is a stimulus to inflorescence initiation and development which is exported by leaves in photoperiods longer than 14 hr, at temperatures above 10°C, regardless of oxygen supply. Whether this LD stimulus moves with the assimilate stream is not certain, but is not ruled out. The nature of the stimulus is unclear, but it could be a compound which shares synthetic pathways with the GAs. The stimulus appears to have a quantitative effect on floral evocation, promoting the influx of nitrogenous and other precursors into the shoot apex as well as their incorporation into RNA and protein. At what stage gene activation first occurs remains to be shown, but experiments with Actinomycin D and 5-fluorouracil suggest this may be during early evocation. Whether it is in response to the arrival of the LD stimulus or to the nonarrival of the SD inhibitor is yet to be established, as is the mechanism by which the LD stimulus continues to accelerate inflorescence development long after floral evocation has taken place.

REFERENCES 1. Arzee, T., Zilberstein, A., and Gressel, J., Immediate intraplumular distribution of macromolecular syntheses following floral induction in Pharbitis nil, Plant Cell Physio!., 16, 505—511. 1975. 2. Aspinall, D. and Husain, I., The inhibition of flowering by water stress, Aust. J. Biol. Sci., 23, 925— 936, 1970. 3. Barnard, C., Histogenesis of the inflorescence and flower of Triticum aestivum L., Aust. J. Bot., 3, 1— 20, 1955. 4. Blondon, F. and Jacques, R., Action de la lumiere sur 1'initiation florale du Lolium temulentum L. spectre d'action et role du phytochrome, C. R. Acad. Sci. Paris, 270, 947—950, 1970. 5. Cooper, J. P., Studies on growth and development in Lolium. II. Pattern of bud development of the shoot apex and its ecological significance, J. Ecol., 39, 228—270, 1951. 6. Cooper, J. P., Studies on growth and development in Lolium. III. Influence of season and latitude on ear emergence, J. Ecol., 40, 352—379.1952.

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1. Cooper, J. P., Studies on growth and development in Lolium. IV. Genetic control of heading responses in local populations. ./. Ecol, 42. 521—556. 1954. 8. Cooper, J. P., Developmental analysis of populations in the cereals and herbage grasses. I. Methods and techniques, J. Agric. Sci. Ctimbr., 47, 262—279, 1956. 9. Cooper, J. P., Short day and low temperature induction in Lolium. Ann. Rot. N.S.. 24, 232—246. 1960. 10. El Antably, H. M. ML, Wareing, P. F., and Hillman, J., Some physiological responses to D, L. Abscisin (Dormin), Planta, 73. 74—90. 1967. 1 1 . Evans, L. T., Lolium temulentum L.. a long day plant requiring only one inductive photocycle. Nature (London). 182, 197—198. 1958. 12. Evans, L. T., The influence of environmental conditions on inflorescence development in some long day grasses. New Phytol., 59. 163—174, 1960. 13. Evans, L. T., Inflorescence initiation in Lolium temulentum L. I. Effect of plant age and leaf area on sensitivity to photoperiodic induction. Aust. J. Biol. Sci.. 13, 123—131. 1960. 14. Evans, L. T., Inflorescence initiation in Lolium temulcntum L. II. Evidence for inhibitory and promotive photoperiodic processes involving transmissible products. Aust. J. Biol. Sci., 13, 429—440, 1960. 15. Evans, L. T., Inflorescence initiation in Lolium temulentum L. III. The effect of anaerobic conditions during photoperiodic induction. Aust. J. Bio/. Sci., 15, 281—290, 1962. 16. Evans, L. T., Inflorescence initiation in Lolium temulentum L. V. The role of auxins and gibberellins, Aust. J. Biol. Sci., 17, 10—23, 1964. 17. Evans, L. T., Inflorescence initiation in Lolium temulentum L. VI. Effects of some inhibitors of nucleic acid, protein, and steroid biosynthesis, Aust. J. Biol. Sci., 17. 24—35. 1964. 18. Evans, L. T., Abscisin. II. Inhibitory effect on flower induction in a long day plant, Science, 151, 107— 108, 1966. 19. Evans, L. T., Inflorescence initiation in Lolium temulentum L. XIII. The role of gibberellins, Aust. J. Biol. Sci., 22, 773—786. 1969. 20. Evans, L. T., Lolium temulentum L.. in The Induction of Flowering, Evans, L. T., Ed., Macmillan, London, 1969, 328—349. 21. Evans, L. T., Flower induction and the llorigen concept, Anna. Rev. Plant Physiol.. 22, 365—394, 1971. 22. Evans, L. T., Inflorescence initiation in Lolium temulentum L. XIV. The role of phytochrome in long day induction, Aust. J. Plant Physiol., 3, 207—217. 1976. 23. Evans, L. T., Inhibition of (lowering in Lolium temulentum by the photosynthetic inhibitor 3(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU) in relation to assimilate supply to the shoot apex, in Eludes de Biologic Vegetate, Homage au Professeur P. Chouard, Jacques. R.. Ed., Paris, 1976, 265—275. 24. Evans, L. T., unpublished data. 25. Evans, L. T., Borthwick, H. A., and Hendricks, S. B., Inflorescence initiation in Lolium temulentum L. V I I . The spectral dependence of induction, Aust. J. Biol. Sci., 18, 745—762. 1965. 26. Evans, L. T., Knox, R. B., and Rijven, A. H. G. C., The nature and localization of early events in the shoot apex of Lolium temulentum during floral induction, in Cellular and Molecular Aspects of Floral Induction, Bernier, G., Ed.. Longman. London, 1970, 192—206. 27. Evans, L. T. and Rijven, A. H. G. C., Inflorescence initiation in Lolium temulentum L. XL Early increases in the incorporation of -'-P and "S by shoot apices during induction, Aust. J. Biol. Sci.. 20, 1033—1042, 1968. 28. Evans, L. T. and Wardlaw, I. F., Inflorescence initiation in Lolium temulentum L. IV. Translocation of the floral stimulus in relation to that of assimilates, Aust. J. Biol. Sci.. 17, 1—9, 1964. 29. Evans, L. T. and Wardlaw, I. F., Independent translocation of 14 C-labelled assimilates and of the floral stimulus. Planta. 68. 310—326, 1966. 30. Graebe, J. E. and Ropers, H. J., Gibberellins, in Plntohormones anil Related Compounds — a Comprehensive Treatise, Vol. 1, Letham. D. S.. Goodwin. P. B., and Higgins, T. J. V., Ed.. Elsevier, Amsterdam, 1978. 107—204. 31. Holland, R. W. K. and Vince, D., Floral initiation in Lolium temulentum L.: The role of phytochrome in the responses to red and far-red light. Planta, 98. 232—243, 1971. 32. King, R. W. and Evans, L. T., Inhibition of flowering in Lolium temulentum L. by water stress: a role for abscisic acid, Aust. ,/. Plant Physiol., 4, 225—233, 1977. 33. King, R- W., Evans, L. T., and Firn, R. D., Abscisic acid and xanthoxin contents in the long day plant Lolium temulentum L. in relation to daylength. Aust. J. Plant Plnsiol.. 4. 217—223, 1977. 34. Knox, R. B. and Evans, L. T., Inflorescence initiation in Lolium temulentum L. VIII. Histochemical changes at the shoot apex during induction. Aust. J. Biol. Sci.. 19, 233—245. 1966. 35. Knox, R. B. and Evans, L. T., Inflorescence initiation in Lolium temulentum L. X I I . An autoradiographic study of evocation of the shoot apex. Aust. J. Biol. Sci.. 21, 1083—1094. 1968. 36. Lane, H. C., Cathey, H. M., and Evans, L. T., The dependence of flowering in several long day plants on the spectral composition of light extending the photoperiod. Am. J. Bot.. 52. 1006—1014, 1965.

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37. Lang, A., Physiology of flower initiation, in Handbuch tier Pflan:enph\siologie, Vol. 15, Part 1. Ruhland, W.. Ed.. Springer, Berlin, 1965. 1380—1536. 38. Lang, A., Inhibition of flowering in long-day plants, in Plunl Growth Substances, Skoog, F., Ed., Springer, Berlin. 1980. 310—322. 39. Lush, W. M. and Evans, L. T., Longitudinal translocation of u C-labelled assimilates in leaf blades of Lolium tfiniik'nluin. Aust. ./. Plant Physiol.. 1. 433—443. 1974. 40. Lyndon, R. F., Interacting processes in vegetative development and in the transition to flowering at the shoot apex, in Integration of Activity in the Higher Plant, Jennings, D. H., Ed., S\mp. Six. Exp. Biol.. 31, 221—250. 1977. 4 1 . Peterson, M. L., Cooper, J. P., and Bendixen, L. E., Thermal and photoperiodie induction of flowering in darnel (Lolium tcmulentum). Crop Sci., I . 17—20. 1961. 42. Rijven, A. H. G. C. and Evans, L. T., Inflorescence initiation in Lolium tcmulentum L. IX. Some chemical changes in the shoot apex at induction. Aust. J. Biol. Sci., 20, 1 —12, 1966. 43. Rijven, A. H. G. C. and Evans, L. T., Inflorescence initiation in Lolium temulentum L. X. Changes in I: P incorporation into nucleic acids of the shoot apex at induction. Aust. J. Biol. Sci., 20, 13—24, 1966. 44. Ryle,G. .1. A., Distribution patterns of assimilated I4 C in vegetative and reproductive shoots of Lolium perenne and L. temulentum, Ann. Appl. Biol., 66. 155—167. 1970. 45. Ryle, G. J. A., A quantitative analysis of the uptake of carbon and of the supply of l4 C-labelled assimilates to areas of meristematic growth in Lolium temulentum, Ann. Bot., 36. 497—512. 1972. 46. Ryle, G. J. A. and Powell, C. E., The export and distribution of l4 C-labelled assimilates from each leaf on the shoot of Lolium temulentum during reproductive and vegetative growth, Ann. Bot., 36, 363—375, 1972. 47. Vince, D., The promoting effect of far-red light on flowering in the long day plant Lolium temulentum, Physiol. Plantar., 18, 474—482, 1965. 48. Vince, D., An interpretation of the promoting effect of far-red light on the flowering of long-day plants, Photochem. Photobiol., 5. 449—450. 1966. 49. Vince-Prue, D., Photoperiodism in Plants. McGraw-Hill. London, 1975, 174 and 183. 50. Vince-Prue, D. and Cockshull, K. E., Photoperiodism and crop production, in Phvsiological Processes Limiting Plant Productivity, Johnson, C. B., Ed., Butterworths, London. 1981, 175. 51. Zeevaart, J. A. D., Physiology of flowering. Science, 137. 723—731, 1962. 52. Zeevaart, J. A. D., Climatic control of reproductive development in Environmental Control of Plant Growth, Evans, L. T., Ed.. Academic Press. New York, 1963, 289—310. 53. Zeevaart, J. A. D., Phytohormones and flower formation, in Phytohormones and Related Compounds — a Comprehensive Treatise, Vol. 2, Letham, D. S., Goodwin, P. B., and Higgins, T. J. V., Eds.. Elsevier, Amsterdam, 1978, 291—327.

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LUNARIA ANNUA En. Dollar Plant, Honesty, Money plant; Fr. Lunaire, Medaille de Judas; Monnaie du pape; Ge. Mondviole, Stumpfes Silberblatt S. J. Wellensiek INTRODUCTION In regions with a low winter temperature, L. annua L. (Cruciferae) is a frequently met garden plant, but it is especially known for the decorative value of its silvery middle-fruit lamellae in gardens and in dried bouquets. At present it is being studied as a prospective new crop for the production of long-chain industrial oils.' L. annua is especially interesting for research on flower formation by its high regenerative capacity. Unfortunately, the legitimate Latin name does not express the fact that the commonly used genotype is absolutely cold-requiring for flower formation and is a typical biennial. The preferable name L. biennis Monch. is illegitimate. The confusion was increased when a non-cold-requiring annual genotype was produced, details of which will be discussed in a later section. Therefore, "biennial L. annua" and "annual L. annua" will be used to indicate the 2 genotypes, which deserve separate discussions.

BIENNIAL LUNARIA ANNUA Vernalization of Whole Plants As early as 1932, Hagemann 2 recognized L. annua as an absolute cold-requiring biennial. Several particulars have since become known. Juvenility From seed germination the plant passes through a juvenile phase, during which it does not react to low temperature with complete flower formation. Juvenility lasts roughly at least 6 weeks and is completed at the age of 10 weeks, with a transitory stage in between.1 Wellensiek and Higazy 4 demonstrated that the juvenile phase is shortened by high light intensity. Higazy 5 studied several other factors and found that mineral nutrition and water do not have clear effects, while GA3 has a tendency to maintain juvenility with a vernalization of limited duration. By means of vernalization of regenerated isolated plant parts, Pierik6 concluded that juvenility is not distributed regularly over the whole plant, but is located in the terminal and lateral meristems. Cotyledon cuttings 5 weeks old could already be vernalized successfully. Duration of Vernalization The necessary duration of vernalization for complete flowering at ambient greenhouse temperatures after the cold-treatment depends on the age of the adult plant when the vernalization starts:3 the older the plants, the shorter the necessary duration of the cold-treatment, and the more rapid the ultimate flower formation. Plants 10 weeks old are successfully vernalized by 10 weeks at 5°C. Even during a prolonged cold-treatment (6 to 8°C) with a photoperiod of 12 hr, complete flowering took place, after the very long lapse of time of 150 to 196 days. 3 The temperature during vernalization can be expected to influence the duration, but has not been studied systematically. Higazys observed that after a marginal vernalization regarding age and duration, resulting in less than 100% flowering, lateral flowering predominated, while in optimally vernalized

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plants both lateral and terminal flowering occurred side-by-side. This has been explained by Pierik 7 as unevenly distributed juvenility among the plant: lateral meristems change from juvenile into adult earlier than terminal ones. Influence of Seed Vernalization The existence of juvenility would exclude an effect of seed vernalization. This turned out to be only partially true, however. Wellensiek 3 applied plant vernalization at 5°C during 5 periods on plants of nine ages, in one series without preceding seed vernalization, and in a parallel series after seed vernalization at 2°C during 6 weeks. By comparing the percentages of flowering plants and the numbers of days to visible flower formation during the aftertreatment under greenhouse circumstances, it turned out that seed vernalization had promoted the effect of the following plant vernalization in relatively young plants which received relatively short plant vernalizations. Hence seed vernalization evidently exerts a partial induction. Miscellaneous Pierik6 did not find a clear difference between the effects of natural vernalization with intermittent temperatures as compared to artificial vernalization under constant temperature, nor did he find evidence of devernalization and of prenatal vernalization. Vernalization of Isolated Organs Regeneration Organs of L. annua — roots, cotyledons, stems, leaves + petioles — have a high regenerative capacity, and this made it possible to study some specific problems regarding vernalization. Already Hagemann 2 has demonstrated that both roots and leaves could regenerate and after vernalization developed into flowering plants. Pierik 6 * studied regeneration in details, but it would be beyond the present subject to enter into a discussion of them. Pierik6-9 succeeded, however, in obtaining flower formation in vitro, starting from regenerating petioles or main axes. This means flower formation occurs under completely controlled conditions (Figure 1). Dividing Cells By studying root vernalization and leaf vernalization, Wellensiek 1 " l 2 arrived at the concept that dividing cells are a prerequisite for vernalization. Convincing proof consisted of starting from leaf cuttings (lamina + piece of petiole) from plants of 5 ages, which were vernalized during 5 periods. Flowering in the after-treatment under ambient greenhouse conditions increased as the mother plants were older and as the vernalization lasted longer. However, when, in a parallel series immediately after the vernalization, 0.5 cm of the basal end of the petioles was removed, no flowering took place whatsoever, although the cuttings reregenerated rather well. Evidently vernalization took place in the lowest parts of the petioles only and this is where the mitoses take place. Furthermore, leaves on intact plants could not be vernalized when full-grown at the start of the cold-treatment, while such a vernalization was possible with young leaves with dividing cells. An attractive modification of "dividing cells" is "cells prepared to divide"," implying that the pre-mitotic stage, where DNA is supposed to multiply, is essential. Photoperiod Length of daily illumination is not an inducing factor, but LD (16 hr) after optimum vernalization resulted in a more rapid realization than SD (8 hr) 3 . No systematic experiments, but incidental observations, suggest that high light intensity is rather important for rapid flowering, but photoperiodically the biennial Lunaria is day-neutral, or — at most — quantitatively reacting.

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FIGURE 1. In vitro-formed generative sprouts in an explant derived from the main axis ol' a Lunariu plant in full bloom. (From Pienk. R. L. M . . Mi'di-d. Lundbinmho^cxch. Wai>cningen. 67(6). 1 — 7 1 . 1967. With permission.)

High Temperature Because of the floral inducing effect of high temperature in other plants, such as Sumolus parvijlorus and Silene cirmeria, the possible effect of partially applied 45°C was preliminarily studied, but was found completely ineffective. (See Reference 14. details unpublished.) Gibberellins The effect of GA, on flower formation has not been studied in great detail. Wellensiek 15 reported that GA, does not replace the cold-requirement, but that pretreatment with 150 jo,g GA, twice a week during 10'/ 2 weeks, followed by vernalization, speeded up realization by 29 days. Pierik" found that low concentrations of GA3 on cotyledon cuttings promoted flowering, but that higher concentrations hampered it. Regarding stem elongation, relatively high doses of GA, are necessary to obtain an effect. 1 Brian et al. 1 6 tested 8 gibberellins and found GA 7 specifically effective. Results with GA3 might have to be explained by contaminations with GA 7 , but this was not confirmed by Zeevaaart, 17 who studied the native gibberellins of L. annua. Relatively small quantities were found and no evidence was obtained for an accumulation of gibberellins during vernalization.

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Floral Hormone Evidence for the existence of a floral hormone was presented by Wellensiek. 12 In a first experiment no effect was obtained of grafting R(eceptor) on D(onor) = R/D, but the reciprocal combination D/R resulted in flowering Rs. Defoliation, defloweration, or GA3 had no effect. In a following large-scale experiment, 298 D/R-combinations were made, and 158 survived, of which 95 = 61% resulted in flowering Rs, control grafts R/R remaining completely vegetative. This proves the existence of a floral hormone. Mechanism of Flower Formation Wellensiek 12 put up a tentative scheme for the whole mechanism of flower formation. The direct effect of low temperature is a "vernalized condition", which only arises during mitoses and which is translocated from cell to cell by mitoses only. A floral hormone is produced by cells in the vernalized condition. In modernized terminology, "vernalized condition" could be replaced by "deblocking of a floral-hormone forming gene", while "during mitoses" has perhaps to be replaced by "during pre-mitotic stages" (see "Dividing Cells").

ANNUAL LUNARIA ANNUA Origin Pierik 6 collected seed samples of L. annua L. from 48 botanical gardens in 17 westernEuropean countries, but did not find a single annual plant among them, thus disproving the claim of Chouard (personal communication) that an annual, non-cold-requiring form exists. Pierik'1 also treated seeds of biennial L. annua with 11 concentrations of the mutagenic substance ethyl methane sulfonate. Among 291 M 2 -lines,* 1 line yielded 12 viable plants, of which 1 flowered without vernalization. This non-cold-requiring plant yielded 52 seeds, of which, in the M,,* 22 viable plants were obtained, all flowering without vernalization, the earliest one 56 days after sowing in a greenhouse. Hence an actual annual L. annua was produced by artificial mutation. Conditions for Flowering Details were studied by Wellensiek. 14 who found that the biennial character is dominant over annual with a monogenic difference. Next it turned out that annual is indeed non-coldrequiring, but nevertheless is sensitive to vernalization. After vernalization, annual flowers in any daylength, but without vernalization it is absolutely LD-requiring. Hence (perhaps contrary to expectation) the essential difference between biennial and annual is not a difference in reaction to low temperature, but to daylength. This means that the dominant gene in biennial prevents the possibility of reaction to LD as inducing factor. The mutation into its recessive allele has opened up this possibility in annual. The cold-reaction is not gene determined, but the LD-reaction is (Figure 2). Perspectives Pierik's annual genotype of L. annua may have economic possibilities. It can flower in regions without vernalizing winter temperatures, provided the daylength is long enough. More important is the possibility of turning L. annua into a field crop for the production of industrial oils. 1 Of course, the annual genotype is much more suited for this purpose than the biennial. However, this means that quite a lot of selection has to be done. Pierik's original annual line possessed the usual undesirable characteristics which occur *

M,. second generation from mutagenic treatment; M,, third generation from mutagenic treatment.

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FIGURE 2. Annual L. annua, from left to right: plant no. 1, — V, SD — not vernalized, continuously in SD; no. 2. — V, LD — not vernalized, after raising in SD transferred to LD; no. 3, + V, SD — vernalized, followed by SD; no. 4, + V, LD — vernalized, followed by LD. Series illustrates flower formation in LD without or with preceding vernalization, and in SD only after vernalization. (From Wellensiek, S. J., Neth. J. Agric. Sci., 21, 163—166. 1973. With permission.)

after artificial mutation, such as poor germination, necrotic black spots on the cotyledons, chlorophyll deficiencies, great variation in time of flowering, and partial sterility. The author's own unpublished research led to the selection of an early and a late line, flowering from sowing in LD after 20 to 51 days (average 43 days, 61 plants) and 91 to 138 days (average 109 days, 49 plants), respectively. Preliminary results showed that, with 12-hr photoperiod, high temperature of 45°C, applied during 6 hr in the middle of the dark phase, induced flower formation. This is opposite to the biennial line. GA3 had a strong stemelongating effect in SD and did not induce flower formation in SD, but speeded up the induction in LD. Without special experimental evidence, the impression was obtained that high light intensity is essential for optimum flowering. CONCLUDING REMARKS With regard to its flowering-reactions, L. annua presents as specific details: 1. 2. 3. 4.

Unevenly distributed juvenility within the plant A favorable effect of seed vernalization on subsequent plant vernalization under marginal circumstances of plant vernalization The regeneration and vernalization of isolated organs, resulting in flower formation in vitro, and in the concept of cells in the mitotic or pre-mitotic phase as prerequisite for vernalization Pierik's annual mutant

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REFERENCES 1 . Princen, L. H., New crop developments for industrial oils, ./. Am. Oil Chemist's Soc., 56(9), 845—848. 1979. 2. Hagemann, A., Unlersuchungen an Blattstecklingen, Garlenbaumssemchafl, 6. 69—195, 1932. 3. Wellensiek, S. J., Vernalization and age in Lunaria biennis, Proc. Kon. Ned. Akad. Wetenschappen Amsterdam, C61, 561—571, 1958. 4. Wellensiek, S. J. and Higazy, M. K., The juvenile phase for (lowering in Lunaria biennis, Proc. Kon. Ned. Akad. Wetenschappen Amsterdam, C64, 458—463, 1961. 5. Higazy, M. K. M. T., Shortening the juvenile phase for flowering, Meded. Landbouwhogesch. Wageningen 62(8), 1—53, 1962. 6. Pierik, R. L. M., Regeneration, vernalization and flowering in Lunaria annua, Meded. Landbouwhogesch. Wageningen, 67(6), 1—71, 1967. 7. Pierik, R. L. M., Regulation of morphogenesis by growth regulators and temperature in isolated tissues of Lunaria annua L., Proc. Kon. Ned. Akad. Wetenschappen Amsterdam, C68, 324—332, 1965. 8. Pierik, R. L. M., Adventitious root formation in isolated petiole segments of Lunaria annua L., Z. Pflanzenphysiol., 66, 343—351, 1972. 9. Pierik, R. L. M., The induction and initiation of flower buds in vitro in tissues of Lunaria annua L.. Naturwissenschaften, 53, 45—46, 1966. 10. Wellensiek, S. J., Leaf vernalization, Nature, 192. 1097—1098, 1961. 1 1 . Wellensiek, S. J., Dividing cells as the locus for vernalization, Nature, 195, 307—308, 1962. 12. Wellensiek, S. J., Dividing cells as the prerequisite for vernalization, Plant Physiol., 39, 832—835, 1964. 13. Wellensiek, S. J., The control of flower formation, Proc. 17th Int. Hortic. Congr., Vol. 2, 61—70, 1967. 14. Wellensiek, S. J., Genetics and flower formation of annual Lunaria, Neth. J. Agric. Sci., 21, 163—166, 1973. 15. Wellensiek, S. J., Sten elongation and flower initiation, Proc. Kon. Ned. Akad. Wetenschappen Amsterdam, C63, 159—166, 1960. 16. Brian, P. W., Hemming, H. G., and Lowe, D., Comparative potency of nine gibberellins, Ann. Bot., 28, 369—389. 1964. 17. Zeevaart, J. A. D., Vernalization and gibberellins in Lunaria annua L., in Biochemistry and Physiology of Plant Growth Substances, Wightman, F. and Setterfield, G., Eds., Runge Press, Ottawa, 1357—1370, 1968.

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LYCOPERS1CON

ESCULENTUM

En. Tomato; Fr. Tomate or Pomme d'amour; Ge. Tomate; Sp. Tomate A. J. F. Picken, R. G. Hurd, and D. Vince-Prue

INTRODUCTION This chapter deals mainly with work on flowering in the tomato published over the last 25 years; an earlier review on the subject appeared in 1969.' Tomato, Lycopersicon esculentum Mill, (the spelling Lycopersicum esculentum Mill, is also in common use), is a member of the Solanaceae. It is grown for its edible fruits which are produced in a series of inflorescences, otherwise termed clusters or trusses. It is grown extensively in the open or under plastic cladding, often as a single-harvest crop. It is also cultivated in greenhouses, particularly in northern temperate climates, as a continuously cropped plant. Although cultivated as an annual, it grows as a straggling perennial in its original habitat on the western coastal plain of northern South America. This is a region with less than 25 mm of rainfall annually, but with high relative humidities of 90% or more for long periods. The temperature ranges from 10 to 24°C and daylength from 1 1'A to 12'/ 2 hr. 2 The tomato nevertheless grows well in a wide range of conditions and flowers at all times of the year. It has, therefore, been used extensively as an experimental plant.

FLORAL MORPHOLOGY There are two flowering types, which are incorrectly termed "determinate" and "indeterminate". In "determinate" types, there are 6 to 11 leaves up to the first inflorescence and usually only 2 or 3 inflorescences on a stem, with one or more leaves between them. Strong axillary growth produces several shoots in addition to the main one, giving a bushy habit which is ideal for growing unsupported in the open. More inflorescences can develop on the main stem if axillary shoots are pruned, but the number is still limited.' In "indeterminate" types, after 6 to about 11 leaves have been formed, an indefinite number of inflorescences are produced at approximately 3-leaf intervals. These types are favored in greenhouses as they produce a large crop over an extended period. The lateral shoots are removed manually, giving a simple stem which is easy to train up a string. Botanically, both types are determinate since each inflorescence terminates the main axis; the lateral shoot in the axil of the last initiated leaf becomes the new apex and apears to grow along the initial axis by displacing the inflorescence into a lateral position 16 (Figure 1). The inflorescence is a monochasial cyme in which the vegetative axis terminates in the king flower. At greenhouse temperatures it can take from 15 to 40 days for the inflorescence to develop from being just visible near the apex to the stage of anthesis of the first flower.7 8 Subsequent flowers on the inflorescence develop from successive lateral buds and open at approximately daily intervals. The inflorescence may divide into 2, or rarely more, branches but the unbranched structure is dominant. 910 It commonly carries about 8 flowers with some flower buds towards the tip which usually fail to develop. If other inflorescences are removed or if vegetative growth is restricted these may eventually flower and set fruit." The ultimate vegetative bud may develop later to produce further flowers or leaves with secondary flower clusters. 12 These "running trusses" are removed in greenhouse practice as they seldom bear marketable fruit. Flower structure and ontogeny have been described in detail by Hay ward." The flowers are perfect, hypogynous, and regular and they measure about 10 to 20 mm in diameter

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FIGURE 1. Sympodial nature of flowering in the tomato. The initial axis terminates in the inflorescence. Subsequent growth, S, is initiated in the axil of the last initiated leaf, C, and it displaces the inflorescence and leaf C, giving the appearance of indeterminate internodal flowering. (After Calvert. 4 )

(Figure 2). The calyx is persistent and green, the petals and anthers are bright yellow, and at anthesis the petals are reflexed. There are usually 5 anthers on very short filaments, pressed together to form a hollow anther cone which surrounds the style. Each anther is bilobed and each locule contains several hundred pollen grains. At anthesis the anthers split longitudinally and introrsely. Microsporogenesis begins soon after floral initiation (FI). In a study in which mature anthers were 11.5 mm long, first meiosis occurred when anthers were about 4 mm; pollen tetrads were formed at 5 mm and young microspores were present at 7 mm. 1 4 The gynoecium consists of 2 or more syncarpous carpels. Bicarpellate cvs are popular in greenhouses in the U.K. and Holland, while multicarpellate cvs with larger fruit are more favored elsewhere. The style is elongated with a smooth flattened stigma which usually remains enclosed within the anther cone (Figure 2). This arrangement and the introrse pollen release normally ensure self-pollination, 15 although cross-pollination can occur, particularly if the style is exserted beyond the anther cone. Bees are the natural cross-pollinators of closely related species but in northern temperate zones their activity in pollinating cultivated tomatoes is low. 16 Generally tomatoes flower, set, and develop fruit freely but there may be unproductive flowers. Under adverse conditions, particularly in low light or high temperature, an inflorescence may fail to develop satisfactorily. In the extreme it may be seen only as a small bump on the stem, or it may grow but all its flower buds abscise. Individual flowers or buds may abscise, or persist with no fruit set or with small unmarketable fruit. 4 1 3 1 7

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Style Stamen Petal Ovary Ovule Sepal Vascular bundles FIGURE 2. Cultivated tomato flower in longitudinal section. (From Esau, K., Plam Anatomy, 2nd ed., John Wiley & Sons, New York, 1965,544. Courtesy of John Wiley & Sons. New York.)

Some cvs produce up to 20% off-types, sometimes called rogue plants, jacks, or Christmas trees. They have smaller cotyledons, shorter internodes, and precocious axillary bud growth. They have a faster than normal rate of leaf production and earlier flowering in summer, but, particularly in winter greenhouse production, many of the flowers may be sterile and abortion is frequent. 18 The production of rogue plants is increased by high temperature during germination and by low light intensity and SD after germination. These conditions are believed to favor genetic instability but the phenomenon is not well understood.

CRITERIA OF FLOWERING RESONSE Several criteria have been used to index the earliness and magnitude of flowering and confusion has arisen in the absence of clear definitions. We shall consider flowering response mainly in terms of the leaf number, excluding cotyledons, to flowering; the time from sowing to the macroscopic appearance of the inflorescence, the time to first anthesis, and the number of flowers which achieve anthesis on the first inflorescence. GENETIC VARIATION The genetic variation in modern greenhouse tomato cvs is small and there are only differences of a few days in their times to anthesis. In the U.K. these times vary from about 50 days in July sowings at 15 to 20°C to over 100 days at similar temperatures in March sowings." I 9 2 ° In a Canadian study using a wide range of cvs, times to anthesis in winter varied between 80 and 105 days, the earliest cvs producing the smallest fruits. 21

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Table 1 INFLUENCE OF IRRADIANCE AND DAYLENGTH ON FLOWERING IN TOMATO CV KING PLUS GROWN IN CONTROLLED ENVIRONMENTS UNDER ARTIFICIAL LIGHT

Irradiance (Wm- 2 )

Daylength (hr)

Integral (MJm May-')

Leaf no. to flowering

Time to 1st inflorescence appearance (days)

9 9 18 18

8 16 8 16

0.26 0.52 0.52 1.04

12.2 9.8 9.2 9.3

78.1 52.1 48.5 40.6

Time to 1st anthesis (days)

Plants flowering at 1st inflorescence (%)

66.5 53.7

0 0 62.5 100

From Kinet, J. M., Sci. Hortic., 6, 15—26. 1977. With permission.

IRRADIANCE The effect of light on flowering of tomatoes has been investigated in artificial light regimes at constant temperatures in order to avoid possible interactions. 7 2 2 2 3 It has also been studied in naturally lit greenhouses at different seasons when daylength and temperature may vary together. 19 - 24 - 2S Varying the light integral — the total daily quantity of photosynthetically active radiation — influences floral development, but within the same light integral, light intensity and daylength can be varied with little effect on flowering. As light integral increases, leaf number to flowering decreases from over 20 to a minimum, which is 6 or 7 in "indeterminate" cvs, depending on temperature. In a controlled environment study by Kinet7 (Table 1) doubling the light integral reduced the leaf number from 12 to 9. It sometimes appears that inflorescences are initiated at the normal node, but completely abort in low light at the earliest development stage. Other authors confirm this pattern22 23 except for Cooper,26 who found that in natural light the leaf number to flowering of cv Potentate decreased in winter. There was a compensating increase in the number of leaves between the first and second inflorescences so that the leaf number to the second inflorescence remained constant throughout the year. As leaf number is reduced by increasing daily light integral, so is the time to truss appearance and to first anthesis. 7 - 22 - 23 (Table 1). In greenhouses at 18°C, plants of cv Potentate sown in autumn and winter reached anthesis after a fairly constant total light integral (4800 klx hr, or approximately 70 MJmr 2 PAR received between cotyledon expansion and flowering). 19 However, in spring and summer sowings the time to first anthesis was constant and independent of light integral. There was a minimum time to anthesis which could not be reduced by the highest summer light integrals. Low light can increase the leaf number to flowering if given prior to inflorescence initiation. It may delay anthesis when given after the inflorescence is visible, and it may cause some or all of the buds to abort. This effect is associated particularly with low light intensity and LD rather than low light integral. In Kinet's 7 experiment (Table 1), all flowers aborted following days with 16 hr at 9 W/m 2 , but few aborted after days with 8 hr at 18 W/m 2 . Plants transferred from an adequate to a low light regime for 10 days, particularly when the first inflorescence was appearing, aborted the whole inflorescence. Plants transferred from low light to adequate light just before inflorescence appearance required 15 days at this light level to ensure normal first inflorescence development.7 The reduced assimilate supply from leaves in low light is probably responsible for these effects. Darkening only the inflorescence did not delay anthesis or induce abortion.27

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The number of flowers in the first inflorescence is also increased as the light integral is increased. In cv Potentate, grown in a 16-hr photoperiod with light between 2.7 and 10.8 klx (1 klx is approximately equal to 4 W/nr PAR for the lamps used here), it varied between 8 and 12, but there was less variation in cv Ailsa Craig." Starch levels near an inflorescence are positively correlated with the number of buds which open, 2 * and in low light integrals, flower buds and flowers are smaller with fewer locules and lower sugar, polysaccharide, and auxin levels.29-30 Flower structure may be impaired by low light. For example, low light can cause style exsertion, preventing self-pollination, 11 and it may result in the production of sterile pollen. 17 Adverse effects on fruit setting are outside the scope of the present chapter and are reviewed elsewhere. 17 - 32 Extremely high irradiances, such as occur in summer in Texas, may induce an apparently dormant state in tomato plants. Shading had a substantial effect on growth, flower development, and fruit set, even though air temperature was still high. 11 It was postulated that an IAA inactivating mechanism was responsible.

PHOTOPERIOD The effects of daylength on tomato are small and it has been variously classified as a DNP, a SDP, or a LDP.14 The concept that it is a LDP arose since flowering occurs sooner in the LD of summer than in winter. It is only when light intensity effects are removed that specific photoperiodic effects are clear. In Kinet's 7 experiment (Table 1) the leaf number to flowering, and the time to inflorescence appearance were reduced in SD when compared with LD at the same light integral, but the results were still complicated by the possible influence of low light intensity which may have induced abortion, independently of light integral. Two experiments have been designed to try to distinguish daylength from light intensity and integral effects. Hurd34 grew cv Minibelle at 20°C in controlled environments under SD and LD regimes. SD plants were grown at 20 W/m 2 PAR for 8 hr. To provide LD with the same light integral and with comparable light intensities he took 10% of the light from the 8-hr period and supplied it over the next 8 hr. Flowering occurred 1 or 2 nodes earlier in SD, and time to anthesis was reduced. Hurd found no effect on first inflorescence flower number. Aung 20 grew a range of cvs in a 9-hr photoperiod with or without a 3-hr low intensity incandescent daylength extension. While generally supporting Hurd's results, he showed that cvs differ in their responses to photoperiod and that there may be interactions with temperature. Cooper,24-15 using serial sowings, found that tomatoes flowered more and earlier under increasing rather than under decreasing daylength. In the natural days which he used it was not possible to distinguish effects due to daylength per se from those due to changing light integral and temperature. He suggested a specific role for changing daylength because he found a discontinuity in the time to anthesis as the days changed from increasing to decreasing in length. TEMPERATURE For most cvs, the optimum temperature range for vegetative growth is about 18 to 25°C. Lower temperatures reduce vegetative growth more than flower development whereas higher temperatures favor vegetative growth at the expense of flowering. Hussey23-36 considered that this was associated with competition for assimilates between the apex and young leaves. High temperatures enhanced the production of leaf primordia at the expense of apical growth while low temperature, or the removal of young leaves, favored apical growth and hence

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its early transition to a flowering apex. Abdul and Harris 17 postulated that temperature affected the levels of endogenous hormones. They found that low temperature reduced the levels of diffusible GAs in young leaves, and this was associated with an increase in the number of flowers in the first inflorescence. Of the flowering criteria, leaf number to flowering is most sensitive to temperature. It increases with temperature in step with the rate of leaf production. 22 - 21 When temperature is reduced before FI, a minimum number of leaves is formed, depending on cv, provided that the light integral is sufficiently high. This may be between 6 and 9 in "indeterminate" types.22-'*-'9 These appear to be largely responses to mean temperature, although there is some evidence of a differential response to day and night temperatures, particularly in mature plants, lower night temperatures being more effective in promoting flowering. 2 " 12 - 4 " 41 However, the effects of differential temperature on flowering are small compared with those on vegetative growth. The time to inflorescence appearance is little affected by temperature provided that the light integral is sufficient. 8 "-2-1 High temperature may result in FI at a higher node, but since the rate of leaf production is increased, the higher leaf number may be achieved in a similar time. At 17.2 klx, FI was slightly hastened at 25°C compared to 15°C but at 8.6 klx it was slightly delayed. 21 In very low light integrals FI and consequently inflorescence appearance may be considerably delayed by high temperature: at 2.7 klx increasing the temperature from 15 to 25°C delayed FI by about 12 days. 22 The period from FI to first anthesis is shortened by increases in temperature, 8 and consequently anthesis is generally earlier at higher temperatures, provided again that light integrals are sufficient. With respect to the time of anthesis, plants respond to the mean temperature, diurnal variation having only a slight effect. In 5 cvs grown in 3 temperature regimes in well-lit controlled environment cabinets, times to anthesis averaged 63 days at low temperatures (18°C day, 14°C night) and 43.5 days at high temperatures (26°C day, 22°C night). 20 This represents a mean temperature difference of 8°C and a Q, 0 of 1.6. Similar results have been obtained in greenhouse conditions. 42 The first inflorescence can be encouraged to branch or "split" when low temperatures are given during a critical period.2""'8-'9-43 Cv Potentate was sensitive from 9 days until about 21 days after cotyledon expansion, 18 but the timing and effect vary with cv.32-'8-'9-4-'-44 Later exposure may increase flower number on subsequent inflorescences. Indeed cold treatment of seedlings is practiced commercially: if night temperatures of 10 to 13°C are given for 2 or 3 weeks during early seedling growth, early yield is increased.' 9 Plants are sturdier or harder, and have a lower leaf number to the inflorescence and a higher first inflorescence flower number than those grown at a higher night temperature. Although it is convenient to lower only night temperature, two regimes (15.6°C day, 4.4°C night for 12 hr and 10°C constant) were equally effective. 4 ' In this work, the first inflorescence flower number could be doubled in summer but could only be increased by 30% in winter. The cold treatment also delayed anthesis by up to 10 days. There has been controversy over whether inflorescence branching is responsible for, or is a response to the increase in first inflorescence flower number. Lewis9 argued that low temperature directly increased flower number, and branching occurred to accommodate the increased number. Calvert45 thought that since flower number per branch tended to be constant, increased numbers were the result of the bifurcation, a view supported by Hurd and Cooper.43 Nevertheless, in 2 cvs in which low temperature did not apparently induce branching, there was an increase in flower number with decreasing temperature. 44 The genetics of branching are discussed by Vriesenga and Honma.'" Flowering may be affected when the roots are at a different temperature from the shoots. Cooling the roots of several cvs from above 15°C to 10 to 13°C at the seedling stage was found to decrease the leaf number to flowering, while cooling the tops by the same amount

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increased first inflorescence flower number. Grafting experiments indicated the possibility that these temperature effects were hormonally mediatedv 46 Although high temperature tends to decrease the number of flower buds in low light, it may not always decrease the number of open flowers. In winter, high temperature before inflorescence appearance resulted in a decrease in the number of flower buds, but the number which achieved anthesis sometimes increased.45 It was suggested that either early high temperature increased the leaf area which was then able to support the development of more flower buds, or that partitioning of assimilates between root and shoot was influenced in favor of the shoot. Increased temperatures after inflorescence appearance reduced the number of buds which reached anthesis. 45 Commercial practice takes full advantage of these findings. In plants sown late, truss splitting may be encouraged by appropriate low temperature treatments, but with a November-sown tomato crop in northern Europe, it is now normal practice to keep temperatures high up to the inflorescence appearance and to lower the night temperatures between then and the time of first anthesis, in order to avoid abortion of part or all of the first inflorescence. The mechanism of floral bud abscission is described by Hay ward. 13 If temperatures are not optimal, fruit setting may also be impaired. High temperatures reduce flower size, locule number, and the sizes of flower parts.29'47 The style may be exserted beyond the anther cone, reducing the possibility of self-pollination.3I-44-48-49 Low temperatures also reduce fruit set, primarily by reducing pollen production and viability. 44 The subject of fruit setting is reviewed elsewhere17-32 and pollen production is discussed in more detail in the section Pollen Production and Viability. SEED VERNALIZATION Wittwer and Teubner39 and Calvert38 could not confirm an earlier report50 which claimed that tomato seeds could be vernalized. It is important to distinguish a hastening of flowering by techniques which accelerate germination from true vernalization which results in flowering at a lower node. CARBON DIOXIDE ENRICHMENT It is widespread commercial practice to raise the carbon dioxide concentration in glasshouses to enhance photosynthesis and hence early and total yields. 51 ' 52 Enrichment is less practicable in summer because of the need to control temperature by ventilation. Anthesis was hastened by 6 or 7 days when enrichment to 1000 vpm CO2 (approximately 1.8 gm~ 3 of CO2) was given for the period from sowing (in November) to planting in the glasshouse border.52 The effect diminished in later inflorescences (unless enrichment was continued) so that the tenth inflorescence was only 2 or 3 days earlier than the controls. However, the enriched plants were grown with a higher ventilation temperature than the controls, so the results must be interpreted with caution. In controlled environments with enrichment to 1000 vpm and simulated winter light, 53 there were no effects on the leaf number to flowering, the first truss flower number, or on the flower primordia number. Enrichment can also prevent arrested first inflorescence development. In five Novembersown cvs in England, enrichment at 1300 to 1500 vpm (2.3 to 2.7 g~ 3 ) increased the percentage of plants flowering at the first inflorescence from 65 to 95%.54 Present evidence suggests that the beneficial effects of enrichment are entirely due to increased net photosynthesis. 5153 ROOT RESTRICTION AND PLANT WATER STATUS November-sown glasshouse tomatoes in northern Europe are grown with a restricted water

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supply and in small pots in the vegetative stage. Planting is delayed until the appearance of the first inflorescence or until first anthesis. This encourages flower development which may otherwise be arrested by heavy watering and unrestricted growth.24-55-56 In a multifactorial glasshouse experiment with 5 cvs sown in November, 62% of the plants flowered on the first inflorescence when they were planted in the ground as seedlings, compared with 93% when planted at inflorescence appearance, and 99% when planted at first anthesis. These differences were much smaller in December-sown plants56 First anthesis was hastened by about 5 days in plants held in pots compared to those planted out at germination. Anthesis on second inflorescences was hastened by lesser amounts, and on subsequent inflorescences it was delayed by late planting. Delay in planting also increased branching of the first inflorescence.54 The key factor in the delayed planting appears to be restriction of watering. When tomatoes were grown under three levels of irrigation, over 50% of the inflorescences aborted in moist conditions compared to 4% in the dryest regime. However, there were more buds formed in the apex in the moist regime.57 Root restriction itself is probably not involved, since root pruning had little effect on fruiting when plants were grown in unrestricted water culture.58 Similarly pot size had little effect on growth and fruiting when plants were grown in flowing nutrient solution.59 Plants grown in the nutrient film technique also require restriction of water supply to encourage flower development in winter. Such plants have an increase in stored carbohydrates.60 Salt stress may delay bud and flower formation, particularly if applied early in vegetative growth. Stress applied to older plants had only minor effects on flowering.61 In field conditions it may be necessary to improve plant water status. Wax or polythene coated paper mulches are used to increase growth and flowering of field tomatoes in Canada.52 The mulches increase soil temperature and moisture while reducing soil compaction. In hot climates they are also useful because they reduce soil temperature and conserve moisture. NUTRITION Kraus and Kraybill 63 suggested that reproductive growth and fruitfulness were promoted by the correct balance of carbohydrate and nitrogen supply. They did not clearly dissociate flowering from fruiting but in general, high carbohydrate promoted fruiting, while high nitrogen impaired it. Later studies have qualified this concept. In summer light (high carbohydrate status), high or low nitrogen levels (between twice and one tenth of usual levels) applied before FI had little effect on first truss flower number39-64-65 and leaf number to flowering, 39 although high nitrogen applications may enhance the promotion caused by chilling of inflorescence branching and flower number39 and advance the time to anthesis.64 If high nitrogen is maintained or applied after inflorescence initiation first inflorescence flower number is increased. 3964 In contrast, high nitrogen decreased the first inflorescence flower number in winter light. 65 Hewlett 31 found that fruit setting was reduced by high nitrogen, mainly because it encouraged style exsertion. Nitrogen deficiency on the other hand had little effect on flower development. 28 Increasing potassium concentrations increase the first inflorescence flower number in both winter and summer. 65 In sand culture, liquid feed concentrations below 3 mM K reduced fruit set, and those below 0.5 mM also reduced flower number. Anthesis was delayed at concentrations above 2 mM or below 0.5 mM.66 Phosphorus deficiency reduces flowering. When phosphorus was withheld from solution culture for 10 days from about the time of FI, the first inflorescence flower number was reduced from 7 to 3, and anthesis delayed by 7 days. The treatment was applied too late to affect the leaf number to flowering. Phosphorus deficiency may have reduced the supply

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from the roots of cytokinin-like chemicals which may be necessary for high flower number 67 (see below).

GROWTH SUBSTANCES There is fragmentary information about the effects of growth substances on flowering in tomato, in contrast to the many studies of their effects on fruit set and development. The mode of application, concentration, timing, and environment all alter the response, and different criteria have been used to index flowering. A summary of the effects of growth substances on leaf number to flowering, time to anthesis, and on first inflorescence flower number is given in Table 2. Auxins Auxins are frequently used to improve fruit set in tomatoes. Their ability to induce parthenocarpy and increase yield in conditions adverse for pollination obscures any specific effect on flowering. Generally, application of IAA has little effect on anthesis time or on leaf number to flowering, but may increase first inflorescence flower number. Other auxins may reduce leaf number to flowering. /V-w-tolylphthalamic acid is used as a foliar spray to increase flowering and fruit set.68 In high concentration it delayed or prevented the growth of the axillary buds which would normally continue vegetative growth by displacing the inflorescence, and this may indicate its mode of action.3 Cytokinins The primary effect of cytokinin application is to delay flowering, both in terms of leaf number and time. 69 The first inflorescence flower number may be increased by up to 100%.67 At high concentrations flowering may be prevented.69 Kinet et al. 7 0 7 1 grew plants in sufficiently poor light to cause all first inflorescences to abort. After applying BA directly to the inflorescence at its macroscopic appearance, the number and size of the flower buds increased, but they were still infertile. 70 When GA 3 , which alone had little effect, was applied with the BA, 58% of plants had at least 1 flower on the first inflorescence which opened, and of these more than one third were fertile. This combination was most effective when the BA was applied before the GA 3 . 71

GAs It seems that GA effects depend critically on the site and mode of application and on cv as is seen in Table 2. In one study, application to the soil increased leaf number to flowering, but did not alter the time to anthesis nor the first inflorescence flower number. 72 Although unaffected in the "indeterminate" cvs tested, anthesis was hastened in a dwarf and a strongly "determinate" cv when GA3 was applied to the young apex.73 When GA, was sprayed onto the foliage the first inflorescence flower number was reduced. It was suggested that GA3 might be diverting assimilates away from the inflorescence towards the points of application.37 However, first inflorescence flower number was unchanged when the inflorescence itself was dipped into the GA, 74 and when GA3 was applied to the leaf nearest the apex, flower size was increased rather than decreased.75 Kinet et al.70'71 applied GA to the inflorescences of plants grown in poor light with little effect unless the GA was preceded by BA, as described above. In this combination GA4 + 7 was more effective than GA3 in promoting flowering. Dipping inflorescences in GA3 also improved fruit set at high temperatures (35°C day; 25°C night) when fruit setting is normally impaired but was without effect at normal temperatures.74 Application of GA to some mutant tomatoes with abnormal stamen development restores

Table 2 COMPARATIVE EFFECTS OF GROWTH SUBSTANCES ON FLOWERING OF TOMATO Leaf no. to flowering How applied" IAA NPA TlBA TIBA "Duraset" "Duraset" GA, , CA, GA, GA, dwarf CV GA, determinate CV C A , indeterminate C V Kinetin Kinetin CCC CCC CCC CCC CCC Phosphon D MH B-9 B-9 PMA 8HQ

Time to anthesis

Flower no. in 1st inflorescence

Control Treated Control Treated Control Treated Ref.

Table 2 (continued) COMPARATIVE EFFECTS OF GROWTH SUBSTANCES ON FLOWERING OF TOMATO "

Substances: IAA (indole-3-acetic acid); NPA (a-2-naphthoxy phenylacetic acid); TIBA (2.3.5. tricodobenzoic acid); "Dura\et" (N-t11tolylphthalamic acid); GAS, GA,, , and CA,; Kinetin: CCC: phosphon D (2-4 dichlorobenzyltributylphospho~iiumchloride): MH: H-9: PMA (phenyl mercuric acetate) and 8HQ (8 hydroxyquinoline). How appl~ed:as a follar spray (F): a weekly loliar spray (FW): to the apex (Ap); to the inflorescence (1): or in one of three ways to the roots: by adding a measured quantity to the soil ( R ) ; by drenching the soil (RD); or plants were grown in solution culture at the stated concentration (RS). Timing: chemicals were applied very approximatcly before (B), at (at) o r after ( A ) the appearance of the firct inflorescence.

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the normal development of the stamens. 76 7S It is suggested that tomatoes require GA for stamen development. Growth Retardants CCC has been used to promote flowering and fruit set. When applied to the roots, plants had thicker stems and darker leaves, the opposite effects to those of GA V Anthesis was hastened and leaf number to flowering decreased. There were more flower buds in the first inflorescence and fewer of these aborted before achieving anthesis. 72 Abdul et al. 79 considered the possibility that CCC acts by restricting vegetative growth and so releasing assimilates for flowering, but concluded that because daminozide (another vegetative growth retardant) did not enhance flowering, the influence of CCC is specific. They suggested that CCC reduced the concentration of endogenous GAs, thereby favoring flowering, since the application of GA nullifies the influence of CCC. However, in another study CCC slightly increased the endogenous GAs.80 Commercial Applications DCIB (sodium 2,3-dichloroisobutyrate, sometimes DCB) has been used to induce male sterility for the production of F, hybrid seed. It impairs the development of microspores and tapetal cells but does not inhibit embryo sac development. 14 Growth regulators are used extensively for improving yields. Synthetic auxins are used to improve fruit set in adverse conditions. 17 - 69 - 81 - 82 Retardants can be used to increase the yield of ripe tomatoes obtainable in a single harvest. Alar, for example, increased the flower number per inflorescence if applied as a foliar spray to seedlings. Subsequent spraying wih a higher Alar concentration resulted in the loss of all unfertilized flowers and buds, and so hastened the harvest of the remaining fruits. Plants were also harder, with greater tolerance of water and heat stress.83 Partial manual deblossoming of plants with more than 4 inflorescences improves the fruit size and yield. DCIB and ethephon have been the most promising of the substances used to achieve this chemically."4 Growth retardants are also used in tomato propagation. Many tomatoes are propagated in the southern U.S. and are then transplanted further north. If flower buds are present at transplanting, subsequent recovery and growth is impaired. Manual deblossoming has been practiced but chemicals, in particular ethephon, may be more economical.85-86

GROWTH CORRELATIONS — COMPETITION BETWEEN ORGANS Factors which retard growth without retarding photosynthesis may result in a surplus of assimilates and tend to increase the structural and storage material in the plant." Reproductive growth is improved in these so-called "hard" plants as noted earlier in the section on plant water status. Since organs compete for assimilates, restriction or removal of one organ may increase the assimilate supply to another in competition with it. Removal of young leaves in tomato promoted apex enlargement and FI, accelerated the development of inflorescences and reduced bud abortion.36-70-87-89 The young leaves may produce a floral inhibitor since the increased first inflorescence flower number caused by removal of young leaves was prevented by the application of GA3 or by low temperatures. 37 In contrast, removal of mature leaves, which supply assimilates has inhibiting effects on flowering.3'1 •7()-s7-89 Other studies of inter-organ competition have been made. Floral development, halted by unfavorable environment, was resumed when the apex was removed,8 and the early removal of side shoots also promoted floral development.90 Removing the first inflorescence, removing or emasculating individual flowers, or removing three quarters of the roots, enhanced

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growth and fruiting on the axillary shoot in a "determinate" cv. 91 On the other hand, replacement of the inflorescence by a combination of IAA, GA 3 , and BA delayed axillary shoot development. The root pruning results contrast with those of Cooper,58 who pruned roots of "indeterminate" plants at weekly intervals without effect on the inflorescence. The different results may have been due to stress causing hardening of the plant. Cooper's plants were unlikely to have been stressed as they were grown in solution, and his treatments were less extreme.

POLLEN PRODUCTION AND VIABILITY Cvs differ in their pollen production. 92 93 There is evidence that modern TMV resistant cvs may have reduced pollen productivity, 94 9S although there is variation even within such cvs. Cvs may also differ in their abilities to produce viable pollen under adverse conditions.49 Screening a large number of cvs has revealed considerable variation in the amount of good quality pollen produced at 5 to 10°C night temperatures, 9293 indicating the potential for breeding for good fruit set at low temperature. High temperature may also drastically influence pollen development. The phase most sensitive to heat injury (>40°C for several hours) is at meiosis which occurred 8 to 9 days before anthesis; 96 before meiosis or after about 3 days before anthesis, this treatment did not adversely affect either pollen production or ovule development. Hand pollination with pollen produced at normal temperatures increased fruit set of flowers exposed to high temperatures 7 to 5 days before anthesis but failed to increase fruit set of flowers treated at other stages.97 These results indicate that such a heat treatment given 7 to 5 days before anthesis mainly influenced pollen development. Low fruit set at 27°C or more is partly attributable to a reduction of fertile pollen98•" but this may not be the only factor.44 Style exsertion, splitting of the antheridial cone, and poor endothecium development at high temperature may interfere with the transfer of pollen to the stigma (see Temperature)." 4 4 4 R 4 9 9 9 - 1 0 0 High temperature may also disturb pollen germination and tube growth, fertilization, and fruit initiation. 99 There is genetic variation in tolerance to high temperature as well as to low temperature. 44 - 100 - 101 The effects of low light on pollen production have been discussed in the Section, Irradiance. Various kinds of male-sterile mutants are used to remove the need for emasculation in hybrid seed production. All are determined by recessive genes. Mutants are known with varying degrees of anther modification. 1 " 2 Several of these are variable in expression but, in stamenless mutants, stamens fail to develop in most environments. 16 Male sterility may also occur with viable pollen, where structural defects prevent its release. It is interesting that fertility has been restored by GA application in several male-sterile mutants (see GAs) 7678 while it may prevent normal self-pollination by promoting stigma exsertion.' 03

REFERENCES 1 . Wittwer, S. H. and Aung, L. H., L\a>persicon escit/entum Mill., in The Induction of Flowering: Some Case Histories, Evans, L. T, Ed., Macmillan of Australia. Melbourne. 1969, chap. 11. 2. Cooper, A. J., The native habitat of the tomato, Anna. Rep. Glasshouse Crops Res. Inst. 1971, 123— 129. 1972. 3. Cordner, H. B. and Hedges, G., Determinateness in the tomato in relation to variety and to application of

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FIGURE 8. Relationship between the lime (month) of FI and the time of inflorescence emergence in the subtropics. Mean daily temperature, inflorescence quality, and frequency of inflorescence emergence are indicated. Data are for cv 'Dwarf Cavendish' (AAA) in the Jordan Valley, Israel. The frequency of inflorescence emergence ("shooting") was affected by the usual method of followers' selection. Modified from Israeli.™

apex,75 the follower's growth is considerably accelerated, probably by removing the suppressive effect of the mother plant, leaving the nutrient supply from the mother's organs and also less competition for light, water, and nutrients.

CONCLUDING REMARKS The banana is a native of the tropics where it is widely grown and much of our information about banana growth was gathered there. In the tropics suitable and uniform temperatures enable undisturbed growth and flowering all year round, while in the subtropics the low winter temperatures dominate plant growth and flowering. Therefore, extensive work was undertaken there in order to study the effects of low temperatures on every stage of plant life and to find ways to control plant development. The up-to-date knowledge of the plant physiology is used today in practice. However, more information is required on the biochemical basis of flowering and its control, and better understanding and control of other events in the plant's life which affect flowering. The subjects include sprouting of buds at a desired site and time, promotion and inhibition of sucker growth and hence of flowering, the determination of sex expression, and control over true stem growth and inflorescence emergence.

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REFERENCES 1. Alexandrowicz, L., Etude de 1'inflorescence du bananier nam. IFAC Ann., 9. 1—32, 1955. 2. Anno, A. and Lambert, C., Caracteristiques de croissance et phases de developpement chez le bananier plantain (var. 'Corne'). Fruits, 31. 678—683, 1976. 3. Aubert, B., Action du climat sur le comportement du bananier en zones tropicale et subtropicale, Fruits, 26, 175—188, 1971. 4. Barker, W. G., Response of various portions of the banana flower to growth substances, Plant. Ph\siol., 35 (Suppl.), V—VI, 1960. 5. Barker, W. G. and Dickson, D. E., Early flower initiation in the banana, Nature (London), 190, 1131— 1132, 1961. 6. Barker, W. G. and Steward, F. C., Growth and development of the banana plant. I. The growing region of the vegetative shoot. II. The transition from the vegetative to the floral shoot in Musa acuminata cv. Gros Michel, Ann. Bar., 26, 389-^23, 1962. 7. Blake, J. R. and Peacock, B. C., Some aspects of the abnormal fruit of November-flowering bananas, Q. J. Agric. Sci., 23, 449^52, 1966. 8. Cann, H. J., How cold weather affects banana growing in N.S.W., Agric. Gaz. N.S.W., 75, 1012—1019, 1964. 9. Champion, J., Indications preliminaires sur la croissance du bananier 'Poyo', Fruits, 16, 191—194, 1961. 10. Champion, J., Le Bananier, Maisonneuve et Larose, Paris, 1963, chap. 2 11. Champion, J., Les Bananiers et Leur Culture. Vol. 1, Paris. 1967, chaps. 1, 2, 3. 12. Charpentier, J. M. and Martin-Prevel, P., Culture sur milieu artificiel. Carances attenuees ou temporaires en elements majeurs. Carences en oligoelements chez le bananier, Fruits, 20, 521—557, 1965. 13. Chorin, M. and Rotem, J., The flower types of the Cavendish banana, Ktavim, 10, 165—171, 1960. 14. De Langhe, E., La phyllotaxie du bananier. Fruits, 16, 429—441. 1961. 15. De Langhe, E., Influence de la parthenocarpie sur la degenerescence florale chez le bananier, Fruits, 19, 239—257, 311—322, 1964. 16. El-Khoreiby, A. M. K., Effect of Nitrogen and Some Growth Regulators on the Physiological and Histological Characters of Banana Plants, Ph.D. thesis, Cairo University Faculty of Agriculture, 1980. 17. Fahn, A., Studies in the ecology of nectar secretion. Palest. J. Bot., 4, 207—224, 1949. 18. Fahn, A., The origin of the banana inflorescence, Kew Bull., pp. 229—306, 1953. 19. Fahn, A. and Benouaiche, P., Ultrastructure, development and secretion in the nectary of banana flowers, Ann. Bot.. 44, 85—93, 1979. 20. Fahn, A., Klarman-Kislev, N., and Ziv, D., The abnormal flower and fruit of May-flowering Dwarf Cavendish bananas. Bot. Gaz., 123, 116—125, 1961. 21. Fahn, A., and Kotler, M., The development of the flower and nectary of the Dwarf Cavendish banana, Adv. Plant Morphol.. p. 153—169, 1972. 22. Fahn, A., Stoler, S., and First, T., Vegetative shoot apex in banana and zonal changes as it becomes reproductive, Bot. Gaz., 124. 246—250. 1963. 23. Fisher, J. B., Leal-opposed buds in MUM: their development and a comparison with allied monocotyledons. Am. ./. Hot.. 65. 784—791. 1978. 24. Freebairn, H. T., Geotropic response and the movement of growth materials causing curvature of banana fruit. Plant Physio!., 36 (Suppl.), XX. 1961. 25. Ganry, J., Determination 'in situ' du stade de transition entre la phase vegetative et la phase florale chez le bananier utilisant le 'coefficient de vitesse de croissance de feuilles'. Essai d'interpretation de quelques processus de developpement durant la periode florale, Fruits, 32, 323—386, 1977. 26. Ganry, J., Note de synthese: le developpement du bananier en relation avec les facleurs du milieu: Action de la temperature et du rayonnement d'origin solaire sur la vitesse de croissance des feuilles. Etude du rhythme de developpement de la plant, Fruits, 35, 727—743, 1980. 27. Ghosh, A. K. and Chakravorty, A. K., On the nature of the inflorescence axis of banana, Bull. Bot. Soc. Bengal, 3, 119—122. 1949. 28. Gill, M. M., A note on dichotomy of the inflorescence in the plantain (Musa paradisiaca Linn.), Trap. Agric.. 45, 337—342, 1968. 29. Gottreich, M., Observations on the effect of growth-regulating substances on bananas. Prelim. Rep. Natl. Univ. Inst. Agric. Rehovot, Israel, 439, 1—23, 1964 (in Hebrew). 30. Israeli, Y., The effect of air temperature at the time of floral initiation on the number of female hands in banana in the Jordan Valley, Alon Hanoteu, 31, 682—685, 1976 (in (Hebrew). 31. Israeli, Y., A comparison trial among six Cavendish cultivars in the Jordan Valley, A Rep. Min. Agric. Ext. Serv., Bet Sheaan, Israel, 23, 92—100, 1976 (in Hebrew).

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32. Israeli, Y., Deciduous and Persistent Flowers in the Cavendish Banana, M.Sc. thesis, Hebrew University of Jerusalem, Rehovot. Israel. 1977 (in Hebrew). 33. Israeli, Y. and Blumenfeld, A., Ethylene production by banana flowers, HonScience, 15, 187—189, 1980. 34. Israeli, Y., Gazit, S., and Blumenfeld, A., Influence of relative humidity on the type of (lower in the Cavendish banana. Fruits. 35. 295—299. 1980. 35. Kotler, M., Anatomical Aspects of Development of the Flower and Fruit of Banana Correlated with the 'Black heart' Disease, M.Sc. thesis, Hebrew University of Jerusalem, Rehovot, Israel, 1964 (in Hebrew). 36. Kuhne, F. A., Seasonal variations in the development cycle of the Dwarf Cavendish banana in a subtropical climate. Crop Prod. 7. 135—137, 1978. 37. Kuhne, F. A., Kruger, J. J., and Green, G. C., Phenological studies of the banana plant. Citrus Subtrop. Fr. }., 472, 12—16, 1973. 38. Lassoudiere, A., Quelques aspects de la croissance et du developpement du bananier 'Poyo' en Cote d'lvoire. III. Le faux-tronc et le systeme foliaire, Fruits, 33, 373—412, 1978. 39. Lassoudiere, A., Qtielques aspects de la croissance du bananier 'Poyo' en Cote d'lvoire. IV. L'inflorescence. Fruits, 33, 457-^t91. 1978. 40. Lassoudiere, A., Comportement du bananier 'Poyo' au second cycle. I. Regetonnage et multiplication vegetative, Fruits, 34, 645—658, 1979. 41. Lassoudiere, A., Comportement du bananier 'Poyo' en second cycle. III. Etude d'une population; IV. Wise en evidence d'interactions entre reject et pied mere et entre rejects, Fruits. 35, 3—17, 69—93, 1980. 42. Lassoudiere, A., Matiere vegetable elaboree par le bananier 'Poyo' depuis la plantation jusqu'a la recolte du dieuxieme cycle. Fruits, 35, 405—446, 1980. 43. Lockard, R. G., The effect of growth inhibitors and promoter on the growth, flowering and fruit size of banana plants, Malaysian Agric. Res., 4, 19—29, 1975. 44. Morau, B., Croissance et developpement du bananier 'Gros Michel' en Hquateur. Fruits, 20. 201—220. 1965. 45. Nayar, G. T. and Sandararaj, D. D., Abnormalities in bananas, Indian J. Hortic., 8, 24—27, 1951. 46. Nur, N., Studies on pollination in Musaceae, Ann. Hot., 40, 167—177, 1976. 47. Pillai, O. A. A. and Shanmugavelu, K. G., Studies on the effect of number of functional leaves on the growth and development of "Poovan" banana — total leaf production, phylachron, total leaf area and leaf function hypothesis. S. Indian Hortic.. 24, 83—87, 1976. 48. Pillai, O. A. A. and Shanmugavelu, K. G., Leaf function hypothesis in banana in tropics, Curr. Sci., 47. 356. 1978. 49. Mohan Ram, H. Y., Ram Manasi, and Steward, F. C., Growth and development of the banana plant. I I I . A. The origin of the inflorescence and the development of the flower. III. B.The structure and development of the fruit, Ann. Rot.. 26, 657—672. 1962. 50. Richardson, D. L., Hamilton, K. S., and Hutchison, D. J., Notes on bananas. I. Natural edible tetraploids, Trap. Agric., 42, 125—137, 1965. 51. Robinson, J. C., Studies on the phenology and production potential of Williams banana in a subtropical climate, Subtropica, 2, 12—16, 1981. 52. Rowe, P. R. and Richardson, D. L., Breeding bananas for disease resistance, fruit quality and yield, Bull. Trap. Agric. Res. Serv. La Lima Honduras, 2, 1—41, 1975. 53. Sathiamoorthy, S. and Madhava Rao, V. N., Pollen production in relation to genome and ploidy of some banana cultivars. 1st Int. Symp. Trop. Subtrop. Fruit, Lima, Peru, Acta Hortic., P, 57, 1976. 54. Shepherd, K., Seed fertility of edible bananas, J. Hortic. Sci., 35, 6—20, 1960. 55. Simmonds, N.W., The Evolution of the Bananas, Longmans, London, 1962, chap. VII. 56. Simmonds, N. W., Bananas, 2nd ed., Longmans, London, 1966, chaps. 1, 2, 3, 6, 7. 57. Simmonds, N. W. and Shepherd, K., The taxonomy and origin of the cultivated bananas, J. Linn. Soc. Land. Bot., 55, 302—312, 1955. 58. Skutch, A. F., The nature of sword and water suckers in the banana, Res. Bull. United Fruit Co., 22, 1— 16, 1929. 59. Skutch, A. F., Anatomy of the axis of the banana, Bot. Gaz., 93, 233—258, 1932. 60. Smirin, S., Banana growing in Israel, Trop. Agric., 37, 87—95, 1960. 61. Stoler, S., The banana, in Notes and Studies, Sifriat Hassadeh, Tel Aviv, 1960, 75—165 (in Hebrew). 62. Stover, R. H., Banana, Plantain and Abaca Diseases, C. A. B., London, 1972, chap. 16. 63. Stover, R. H., Pseudostem growth, leaf production and flower initiation in the Grand Nain bananas. Bull. Trop. Agric. Res. Serv. La Lima. Honduras, No. 8, 1—37, 1979. 64. Subra, P. and Guillemot, J., Contribution a 1'etude du rhizome et des rejects du bananier, Fruits, 16, 19—23, 1961. 65. Summer-vine, W. A. T., Studies on nutrition as qualified by development in Musa Cavendishii L., Queensland J. Agric. Sci., 1, 1—127, 1944.

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66. Teisson, C., Conduction vers un bananier d'elements mineraux absorbes par son reject, Fruits, 25, 451— 454, 1970. 67. Ticho, R. J., The banana industry in Israel, Trop. Sci., 13, 289—301. 1971. 68. Tilak, V. D., On the morphology of the perianth in Musa L., Marathwada Univ. J., 7, 6—8, 1968. 69. Turner, D. W., The growth of the banana, J. Aust. Inst. Agric. Sci.. 36, 102—110, 1970. 70. Turner, D. W., Banana plant growth. I. Gross morphology. II. Dry matter production, leaf area and growth analysis, Aust. J. Exp. Agric. Anim. Hush., 12, 209—224, 1972. 71. Turner, D. W., Some factors related to yield components of bananas, in relation to sampling to assess nutrient status. Fruits, 35, 19—23, 1980. 72. Turner, D. W. and Barkus, B., Loss of mineral nutrients from banana pseudostem after harvest, Trop. Agric., 50, 224—234, 1973. 73. Wardlaw, C. W., Banana Diseases, Longmans, London, 1961, Chap. 1. 74. White, P. R., Studies on the banana, Z. Zetlforsch. Mikrosk. Anal.. 7, 673—733, 1928. 75. Ziv, D., The principles of pruning and decapitation, in Investigations on the Banana and the Rotation of Irrigated Crops, Sifriat Hassadeh, Tel-Aviv. 1962, 187—250. 76. Israeli, Y. unpublished data.

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OLEANDER

En. Oleander; Ge. Oleander; Fr. Laurier rose E. Str0mme and H. Hildrum

INTRODUCTION Oleander (Apocynaceae) is an evergreen shrub native to the Mediterranean region. It has leathery, oblong-lanceolate leaves and showy flowers, white to red or purple, often double. Cvs with leaves variegated with white or yellow and with bright red or pink flowers are popular in gardens and parks in southern Europe and the U.S. Some are also used as house plants. The production of flowering pot plants from cuttings takes about 1 year. Attempts have been made to shorten the production period.2-1

PLANT AND FLOWER MORPHOLOGY Nerium oleander L. has upright branches with leaves usually in whorls of 3, leathery, narrowly oblong-lanceolate, 4 to 12 in. long, acuminate, prominent midrib, glabrous. Flowers are borne in terminal branching cymes. 1.5 to 3 in. across, often double. Corolla arc funnel-formed. The name refers to the resemblance of the leaves to olive. 1

CONTROL OF GROWTH AND FLOWERING Oleander flowers profusely in summer. In the regions where it is grown outdoors, it is "dormant" in the winter. This dormancy is apparently not associated with daylength but brought about by low temperature. A critical temperature of about 12°C for growth and flowering is indicated.' Temperatures well above this level are necessary for rapid growth and flowering and an optimal temperature of about 18°C has been indicated. Thus plants grown at 18°C night flowered earlier than plants grown at either 15 or 21°C. 2 The number of nodes increased with increase in temperatures indicating that low temperature promotes flower induction, but slows down flower development. It is suggested that in order to obtain early flowering, plants should be kept at low temperatures until flower buds are formed and then at high temperature for rapid flower development. 2

GROWTH RETARDANTS CCC is effective in reducing both internode number and internode length and CCC is therefore the tool which makes oleander into a compact potted plant (Figure I ) . 1 Flowering is hastened, especially at high temperature and time of production might then be significantly shortened.2-3 Reducymol (Ancymidol) has given inconsistent results 3 . CCC concentration of 2 to 6 % as a soil drench has been recommended. 2 3 Foliar application of Dikegulac (0.1 to 1.9% Atrinal) inhibited shoot elongation and axillary bud break for more than 3 months. 4 It might thus be used to inhibit excess growth in outdoor plantings.

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FIGURE 1. The effect of CCC on growth and flowering of Oleander grown at 21°C night temperature. From left to right: untreated; 0.5, 1.0, and 2.0% CCC.

REFERENCES 1. Bailey, L. H., Manual uf Cultivated Plants, Macmillan, New York. 1949, 811—812. 2. Hildrum, H., Wirkung von Temperatur und Stauchemittel bei Oleander, Gartenwelt, 72(6), 121—123, 1972. 3. Kasper, J. J., The culture of Nerium oleander has good prospects, Vakbl. Bloemisterij, 34(49), 40—41, 1979 (in Dutch). 4. Sachs, R.H., Hicld. H., and DeBie, J., Digeculac, a promising new foliar-applied growth regulator for woody species, Hon. Science. 10, 367—369, 1975.

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NIGELLA En. Fennel flower. Love-in-a-mist: Fr. Nigella; Ge. Schwarzkiimmel Richard I. Greyson and K. Raman

INTRODUCTION The genus Nigella (Ranunculaceae) includes approximately 10 annual, diploid species, endemic or naturalized over a wide area of southern Europe, north Africa, and southwest Asia. 1 Horticultural interest is restricted to three species: (1) N. saliva, cultivated for its small, black, aromatic seeds which are used to flavor bread;2 (2) N. damascena ("love-ina-mist"), cultivated in flower gardens in a number of colors and forms; and (3) A', hispanica (fennel flower), grown in gardens for its blue flowers and, at maturity, its showy fruit. Considerable inconsistency exists in the application of English common names. The use of these species in research, not explicitly related to flowering, includes cytogenetics, seed dormancy, and tissue culture. Because the chromosomes are few (n = 6) and karyotypes are readily prepared, N. damascena is being used in basic and experimental cytogenetics.' 4 Another feature of A', damascena, only briefly explored, is the negative photoblasticity of its seeds.5-'' Considerable potential for experimentation with tissue culture seems evident from studies on N. saliva1'10 and embryogenesis in N. damascena." Beyond these instances, the species are used occasionally in basic morphological, taxonomic, biogeographic, 1 genetic, 12 and phytochemical 13 studies. Because Nigella has only recently become used experimentally, many aspects of its flowering biology have been incompletely or only casually studied. The following account, therefore, contains isolated, sometimes preliminary, observations and personal opinions, some of which may require reinterpretation in the light of more extensive study.

CULTURE CONDITIONS Seeds of the three species are available either through commercial seed companies or from botanical gardens. However, large quantities of genetically defined material suitable for careful physiological studies are, to our knowledge, not readily available. Where neither wild nor horticultural populations will contaminate, large plantings of open-pollinated populations will produce large seed stocks of individual species or cvs. Seed viability is limited to 3 to 5 years. Seeds germinate and grow in well-drained, fertilized soils and can be studied in garden, greenhouse, or growth chamber conditions. In our experience, seeds are susceptible to fungal contamination during germination and should be treated with anti-damp-off fungicides.

PHOTOPERIODIC INDUCTION Though there are no specific references to the photoperiodic responses of N. sativa and N. hispanica, we conclude, based upon personal observations, that both species resemble N. damascena in their response to photoperiod. N. damascena behaves as a classic LDP. 14 If maintained under a 8-hr light/16-hr dark regime, plants of N. damascena remain as vegetative rosettes for 150 days or more. If maintained on 16-hr light/8-hr dark regime, however, N. damascena bolts and flowers within 35 days. 15 As noted below, different quantitative responses of the "double" and "single" forms have been detected.

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FIGURE 1. Median longitudinal section through floral meristem of Nigella damascene!. Diameter approximately 0.5 mm.

ANATOMY AND MORPHOLOGY OF FLOWERING On induction, the terminal vegetative meristem of N. damascena is transformed into an inflorescence meristem. This involves considerable change in the size and structure of the meristem leading to the production of a variable number of leaf-like bracts after which it becomes a flower meristem. The number and extent of development of these bracts and their axillary buds is variable and the relative roles of light intensity, photoperiod, and nutrition in this variability have not been studied. Axillary buds of the earlier formed leaves as well as bracts produce flowering branches each with its terminal flower. Thus the plant, with considerable variety depending upon environmental cues (temperature, moisture, daylength), develops a complex cymose inflorescence. Depending upon its position in the plant and the general growing conditions, the flower meristem of N. damascena can be large and experimentally useful. 16 Following sepal initiation but prior to initiation of petals or stamens the dimensions of the meristem can exceed 500 |xm in width and height (Figure 1). Floral organs, in some cases in excess of 100, subsequently helical sequences on the meristem. In addition to the significant structural and morphogenetic change of the meristem associated with the onset of flowering, cells of transition and early floral meristem possess a vacuolar organelle, the nature and function of which has not been investigated. 17 Because of the variability of meristem size and organ number, related both to position on the plant and the growing conditions, care must be taken in experimental studies to make appropriate comparisons. For example, in many studies, using only the terminal flower of the main stalk is a useful procedure. FLOWER MERISTEM CULTURE Originating with speculation concerning the factors which regulate the ontogeny and morphogenesis of flower organs, a line of research began in the 1960s in which isolated

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immature flower meristems were cultured on sterile media. While the study began with Aquilegia[K some workers found that Nigella, because it is an annual which can be readily grown from seed and possesses a large, green, easily accessible meristem, offered distinct advantages for this work. Thus, Peterson' 1 '- 20 demonstrated that excised carpels of A', saliva grew and developed ovules on a defined medium which possessed enriched nitrogen as — NO, or as basic amino acids. These isolated organ primordia grew and underwent ovule initiation in the absence of added plant growth regulators (PGRs). In another study, Raman 15 2 I " compared the nutritional requirements for development of meristem from two genetically related, "double" and "single", flower forms. Doubleness in A', damascene! behaves as a single-gene recessive character21 and is expressed in the homozygous recessive condition with the proliferation of many petaloid organs (Figure 2). The "single" wild form, genetically either homozygous or heterozygous, is characterized by flowers possessing distinct sepals, nectariferous petals, stamens and an ovary consisting of up to 8 partially fused carpels (Figure 3). In contrast to the apparent autonomy of the A', saliva carpel primordia, distinct sensitivities and requirements for PGRs were recognized for the culture of the two meristems of N. damascena.21-22 These can be briefly summarized: 1. 2. 3.

"Single" meristems initiated primordia on a basic medium in the absence of added PGRs though the addition of kinetin enhanced the responses. GA, at 10 8 M, ihibited stamen and petal initiation on "single" meristems. "Double" meristems failed to initiate primordia in the absence of either GA, or kinetin. In the presence of GA, (10~ 6 M) or kinetin ( 1 0 ~ 7 M) considerable organ initiation was attained.

The full causal significance of these striking and consistent observations is not yet apparent for none of the experimental treatments has reverted the phenotype of the cultured meristems. Two independent observations suggest, however, that these two genotypes possess strikingly different physiologies, perhaps mediated by PGRs. First, anther removal from "double" flowers strongly inhibits subsequent flower development while emasculation of "single" flowers inhibits flower organ growth to only a small degree.21 Second, though grafted and cultured pairs of half-meristems of the same genotype grow and initiate organ primordia on both halves, reciprocal pairs of half-meristem of each genotype ('A, "double" and V2 "single") develop with considerable inhibition of the "single" half. 16

ENDOGENOUS HORMONAL STATUS ASSOCIATED WITH FLOWERING The different sensitivities to GAs and cytokinins in culture suggested that some differential endogenous situations might also exist. Extraction and assay for gibberellins revealed some interesting developmental correlations.24 First, "double" tissue at all stages contained higher levels of methanol-extractable GA-like material than "single" tissue. In other words, the "double" form of N. damascena can be considered a GA mutant with endogenous levels 10 to 20 times those of the alternative genotype. A second discovery is that during development, the total methanol-extractable levels of GAs in both genotypes increase. "Double" flower tissues contain 20 to 30 times the levels found in seedling tissue, whereas "single" flowers contain only 2 to 3 times the seedling levels. In addition to the positive correlation between GA levels, development, and genotype, substantial differences in the compartmentalization patterns between acidic ethyl acetatesoluble (AES) and highly water-soluble (HWS) forms were detected in the two genotypes. Whereas seedling tissues of both genotypes contained both AES and HWS forms, the GAlike materials became differentially partitioned as development proceeded. In "doubles",

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FIGURES 2 and 3. Figure 2. Mature "double" flower of Nigella iluimi.iccmi. Figure 3. Mature "single" flower of" Nine/lit duinusccmi. C. carpel; N. nectary: P, petal; Se, Sepal; St. stamen (both flowers approximately 5 cm in diameter) (From Raman, K. and Greyson. R. I.. Am. J. Hot., 65. 180—191, 1978. With permission.)

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by the young flower stage essentially all of the extractable material was found in the AES fraction, whereas in "singles", GA-like materials was in the HWS form. To date, this work represents the only study of the endogenous status of plant growth regulators in Nigella. Whether these observations will stand when further study with other isolates, perhaps in different growing conditions, remains to be seen. The possibility of a cytokinin component accompanying the physiological changes associated with flowering remains unexplored, though a preliminary study 25 suggests that one exists.

RESPONSE OF N. DAMASCENA TO PHOTOPERIOD AND EXOGENOUS APPLICATIONS OF GIBBERELLINS In view of the differential responses of the two flower forms of jV. damascena to in vitro culture and their differential endogenous GA status, some previously unpublished observations15 from studies in which responses to photoperiod combined with exogenous GA applications are instructive. In these studies, in which seedlings (5 per 15-cm clay pot; 3 pots per treatment) were germinated and grown in either LD or SD, applications of 10 \ig of either GA 3 , GA 7 , or GA9 were applied (except controls) to the tip of the plant 18 days following germination. The findings of this study include the following observations: 1.

2.

3.

Both "single" and "double" plants responded like LDP, failing to flower in SD conditions (8 hr light; 16 hr dark) within 90 days. When germinated and grown under LD (16 hr light; 8 hr dark), seedlings elongated rapidly and flower primordia appeared within 20 to 22 days from germination. Plants of either genotype maintained in SD for more than 75 days flowered in all GA treatments but "double" plants flowered sooner than "singles". After 90 days in SD conditions GA,-treated plants were shorter than those treated with GA7 and GA9. GAytreated plants of both genotypes were approximately 1V 2 times taller than their GA3treated counterparts. Plants treated with GA9 were of intermediate height. In LD conditions, the application of GA, hastened stem elongation as well as flower formation in both genotypes. However "double" plants were more responsive to GA3 than were "single" as they were taller at maturity and flowered earlier. GA9-treated "doubles" grew faster and flowered earlier than "double" seedlings treated with either GA3 and GA7 in LD conditions.

POSTINITIATION FLOWER BIOLOGY To the student of flower opening, anthesis, and pollination biology, the Nigella flower presents many intriguing developmental situations, all, for the most part, unstudied. The cell elongation component of stamen filament elongation in N. hispanica, is strongly inhibited by emasculation and stimulated by GA, 26 - 27 though the presence of GAs in the anther has not been reported. Developmental interactions between stamens and styles have been detected. 12 Although bees are common pollination vectors, style and stamen bending frequently leads to self-pollination through mutual entanglement. SUMMARY Though the flowering physiology of Nigella is virtually unstudied, enough is known to suggest that this genus is a rich source of experimental material. Because the meristem is large and easily exposed, basic studies on the siting and determination of primoridia are possible. Since these species are reasonably sensitive LDP and appear to possess-explorable GA and cytokinin metabolisms they will be useful test material with which to study the role PGRs of LDP.

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CRC Handbook of Flowering ACKNOWLEDGMENTS

We are most grateful to Dr. A. J. Karpoff, University of Louisville, and Dr. C. M. Peterson, of Auburn University for their review of the manuscript and their helpful suggestions.

REFERENCES 1 . Strid, A., Evolutionary trends in the breeding system of Nigella (Ranunculaceae), Bot. Not., 122, 380— 397, 1969. 2. Davis, P. H., Nigella L., in Flora of Turkey and of East Aegean Islands, Vol. 1, Davis, P. H., Ed., Edinburgh University Press, Edinburgh. 1965, 98. 3. Moutschen-Dahmen, J. and Moutschen-Dahmen, M., A suitable plant material for chromosome breakage studies: Nigel/a damascena L., Naturwissenschaflen. 20, 566—567. 1965. 4. Moutschen-Dahmen, J. and Moutschen-Dahmen, M., Radiation induced polyploidy in Nigella Damascena. Caryologia. 23, 501—513. 1970. 5. Malcoste, M. R., Physiologie-vegetale — I'induction de la germination des graines de Nigella damascena L. par la lumiere, C. R. Acad. Sci. Ser. D., 267, 613—616. 1968. 6. Pamukov, K. and Schneider, M. J., Light inhibition of Nigella germination: the dependence of a highirradiance reaction on 720 nm irradiance, Bot. Ga:., 139, 56—59, 1978. 7. Gupta, S., Tissue culture of Nigella saliva. I. The behavior of nucleus, Experienlia, 28, 441—443, 1972. 8. Banerjee, S. and Gupta, S., Embryogenesis and differentiation in Nigella saliva leaf callus in vitro, Physio/. Plant.. 38. 115—170, 1976. 9. Banerjee, S. and Gupta, S., Embryoid and plantlet formation from stock culture of Nigella saliva tissues, Physiol. Plant.. 34. 243—245. 1975. 10. Chand, S. and Roy, B. C., Study of callus tissues from different parts of Nigella sativa (Ranunculaceae), Experienlia. 36, 305—306. 1980. 1 1 . Raman, K. and Greyson, R. I., In vitro induction of embryoids in tissue cultures of Nigel/a damascena, Can. J. Rot.. 52, 1988—1989. 1974. 12. Reinders, D. E., Species crosses in the genus, Nigella, Genetica {the Hague), 33, 22—30, 1941. 13. Salama, R. B., Sterols in the seed oil of Nigella sativa. P/anta Med., 24, 375—377, 1973. 14. Tincker, M. A. H., Contribution from the Wisley Laboratory, No. 54. On the effect of length of daily period of illumination upon the growth of plants, J. R. Hortic. Soc., 54, 354—378, 1929. 15. Raman, K., Hormones and Flower Morphogenesis in Nigel/a damascena, Ph.D. thesis. University of Western Ontario, London, Ontario. 1976. 16. Raman, K. and Greyson, R. I., Graft unions between floral half-meristems of differing genotypes of Nigel/a damascena L., Plant Sci. Lett., 8, 367—373, 1977. 17. Greyson, R. I. and Mitchell, K. R., Light and electron microscope observation of a vacuolar structure associated with the floral apex of Nigella damascena. Can. J. Bot., 47, 597—601, 1969. 18. Tepfer, S. S., Greyson, R. I., Craig, W. R., and Hindman, J. L., In vitro culture of floral buds of Aquilegia. Am. J. Bot.. 50, 1035—1045, 1963. 19. Peterson, C. M., Nutritional requirements of ovule formation in excised pistils of Nigella, Am. J. Bot.. 60, 381—386, 1973. 20. Peterson, C. M., The effects of gibberellic acid and a growth retardant on ovule formation and growth retardant on ovule formation and growth of excised pistils of Nigella (Ranunculaceae), Am. J. Bot., 61, 693—698. 1974. 21. Greyson, R. I. and Raman, K., Differential sensitivity of "double" and "single" flowers of Nigella damascena (Ranunculaceae) to emasculation and to GA,, Am. J. Bot., 62, 531—536, 1975. 22. Raman, K. and Greyson, R. I., Further observations on the differential sensitivities to plant growth regulators by cultured "single" and "double" flower buds of Nigella damascena L. (Ranunculaceae), Am. J. Bot., 65, 180—191. 1978. 23. Toxopeus, H. J., Eiblechkeitsunter suchungen an Nigel/a damascena L., Genetica (the Hague), 9, 341— 440, 1927.

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24. Raman, K. and Greyson, R. I., Changes during development in the compartmentalization patterns of extractable gibberellin-like substances in "single" and "double" genotypes of Nigella damascena. Can. J. Bin., 55. 2115—2121. 1977. 25. Raman, K. and Greyson, R. I., Differential effects of kinetin on acidic ethyl acetate soluble and highly water soluble gibberellin fractions in "single" and "double" Nigellu damascena L.. ./. £.177. Hot., 29, 141 — 145, I97X. 26. Greyson, R.I. and Tepl'er, S. S., An analysis of stamen filament growth of Nit-ella luspanica L.. Am. ./. But., 53. 4X5—592, 1966. 27. Greyson, R. I. and Tepfer, S. S., Emasculation effects of the stamen filament of Nigella hispanica L. and their partial reversal by gibberellic acid. Am. J. Hot., 54, 971—976, 1967.

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CRC Handbook of Flowering

NOTOCACTUS En. Ball cactus H. F. Wilkins and W. Riinger

INTRODUCTION Notocactus (K. Schum.) A. Berger (Eriocactus Backeb.) species are native to southern South America. There are some 25 species, belonging to the Cactaceae. Notocactus tabularis (J. F. Cels) A. Berger and Notocactus scopa, (K. Spreng-)A. Berger, the silverball cactus, are of commercial interest and are both natives of Brazil and Uruguay. 2

MORPHOLOGY These are short plants whose globuse stem grows to be some 45 cm tall. Notocactus scopa has some 30 to 40 low, obtuse ribs, while N. tabularis has only 16 to 18 ribs; the former has 40 or more radial spines which are bristle-like, white, and central; the latter has 16 to 18 radial spines which are needle-shaped and central. Both species have yellow flowers which are 6 cm wide and long. FLOWER INDUCTION AND DEVELOPMENT Plant size interacts with temperature treatments as larger plants have a greater propensity to flower. Flower formation is not controlled by photoperiod. Temperatures from 5 to 15°C control FI, with 10°C being optimum; 5°C was more effective than 15°C (Figure 1). Plants remained vegetative at 20°C. However, flower development at 10°C, the optimal temperature for initiation, is greatly reduced or does not occur.4 The time span for the 10°C temperature treatment, which controlled flower induction and FI, is 50 days at the minimum when plants are forced afterwards at 20°C. A span of 100 to 110 days at 10°C was needed for maximum number of flowers per plant when forced at 20 to 25°C.4 For 5-cm sized N. tabularis and A', scopa plants, more flower buds were initiated as the duration of the 5 to 10°C treatment increased; flower bud development was optimal at 20°C or higher temperature regimes if plants were under LD.5 High light intensities and LD during and after the cold treatment increased the number of flowers initiated. During winter flower buds may abort. High temperatures above 20°C and LD are favorable for flower development. Flowers rarely form in the winter. 5

GROWTH REGULATORS Jansen 3 reported that GA application had a marked effect on growth and earlier flowering and Cycocel had no effect. Riinger6 reported that GA3 applications were not effective when plants were given only 50 days of cold. However, after 70 or 90 days of chilling, GA3 treatments could substitute for cold (10°C) in that 100 ppm or 1000 ppm GA3 increased the number of flowers initiated by two- to three-fold. Some flower bud abortion occurred at the 1- to 2-cm stage of development. Lower concentrations of GA3 (10 ppm) applied more frequently (7 times) were not effective. Flower anthesis was hastened and the flowers were morphologically more slender by GA3 treatments. The upper portion of the plant was more elongated with more vigorous spine growth. At 20°C, a noninductive temperature for FI, GA3 applications (7 x 1000 ppm) resulted in greater plant growth (up to 40%) without significant internodal elongation.6

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