Contents

Abbreviations ix

Introduction xi

Discovery of photoperiodism xii

Impact of photoperiodism xiv

Aims of this book xv

PART I: PHOTOPERIODIC CONTROL OF FLOWER INITIATION 1

1 Some General Principles 3

Photoperiodic response groups 3

Assessment of flowering response 4

Where is daylength perceived? 7

What is perceived? 10

The roles of light 18

Conclusions 26

2 Photoperiodic Timekeeping 29

Characteristics of circadian rhythms 30

Circadian rhythms and photoperiodic responses 37

General models for circadian timekeeping in photoperiodism 42

Circadian timekeeping in short-day plants 44

Semidian rhythms 58

Overt circadian rhythms and the photoperiodic clock 59

Conclusions 61

3 Photoperiodic Photoreceptors 63

Discovery of phytochrome and its function in photoperiodism 63

The phytochrome family of molecules 67

Photochemical properties of phytochromes 69

Synthesis and destruction 72

Reversion 74

Phytochrome biogenesis 74

Molecular structure 75

Phytochrome-mediated responses 77

Mechanism of phytochrome action 78

Biological roles of different phytochromes 82

Blue light effects 82

Conclusions 83

4 Daylength Perception in Short-Day Plants 85

The night-break reaction 85

Phase setting 92

The dusk signal 97

The Pfr-requiring reaction 103

Interaction of light effects 107

Do different phytochromes control flowering in SDP? 111

Conclusions 116

5 Daylength Perception in Long-Day Plants 118

Introduction 118

Night-breaks 119

Light quality in the photoperiod 119

Responses to light quantity 123

Action spectra for LDP 125

Response to the timing of light treatments 126

End-of-day FR effects 131

Circadian rhythms 132

Possible mechanisms 134

Conclusions 140

6 The Physiology of Photoperiodic Floral Induction 143

The nature of photoperiodic floral induction 143

Effects of age 151

Juvenility 154

Interactions with temperature 155

A transmissible floral stimulus 163

The inhibition of floral initiation in non-inductive daylengths 171

Conclusions 179

7 The Nature and Identity of Photoperiodic Signals 180

Attempts to extract a floral stimulus 181

Assays for florigenic activity 188

Attempts to extract floral inhibitors 193

Growth regulators and flowering 194

Gibberellins 194

Cytokinins 208

Auxin 213

Ethylene 215

Abscisic acid and related inhibitors 217

Does florigen exist? 218

8 Biochemical and Molecular Aspects of Photoperiodism 224

Biological timekeeping 224

Photoperiodic floral induction 228

Changes at the apex 236

Conclusions 242

9 Genetic Approaches to Photoperiodism 243

Introduction 243

Flowering-time mutants 244

Photoreceptor mutants 252

Rhythm mutants 254

Growth substance mutants 255

Homeotic mutants 256

Conclusions 256

PART II: PHOTOPERIODIC CONTROL OF DEVELOPMENT 259

10 Floral development 260

Reversion to vegetative growth 260

Sex expression 261

Conclusions 277

11 Bud Dormancy 279

Effects of daylength 280

Daylength perception 294

The identity of dormancy-controlling stimuli 303

Molecular mechanisms 314

Conclusions 315

12 Vegetative Storage and Propagation 317

Tubers and corms 317

Bulb formation 324

Runner formation 328 The nature of the photoperiodic stimulus for the formation of storage organs 329

Conclusions 334

13 Other Effects of Daylength 336

Seed germination 336

Stem elongation 341

Leaf growth 344

Assimilate partitioning 349

Crassulacean acid metabolism 352

Secondary metabolism 353

viii CONTENTS

Appendix I Photoperiodic Classification of Plants 355

Appendix II Effects of Daylength on the Content of Endogenous

Growth Substances 366

References 369

Index 415

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