Humans have always relied on the green plant to produce the calories needed for their sustenance, either directly or indirecdy after conversion by animals, and as a source of fuel and fibre. As a result of this reliance on green plants, the sun was essentially the only source of energy until the exploitation of fossil forms of solar energy ushered in the industrial revolution. Agricultural production systems became increasingly dependent upon these fossil forms of energy (coal, petroleum), but solar energy, diffuse but reliable, continued to be the primary source of our food supply (Hall and Kitgaard, 2012, p. 4). The green plant driven by solar energy will, for the foreseeable future, continue to feed humankind.
The plants utilized by humans are consumed in many different ways; for some, fresh fruits are harvested, in other cases stems, leaves, roots or tubers represent the economic yield. The entire above-ground plant is harvested in some vegetable or forage crops whereas immature fruits or seeds represent the economic yield of other vegetable crops. But the crop plants making the largest contribution, by far, to the world's food supply, are those harvested at maturity for their seed.
Seeds are important and useful because they are nutrient-dense packages of carbohydrates, protein and oil that are relatively easy to harvest, store and transport. Once the seed is dried, it can be stored indefinitely if it is kept dry and free of insects and other pests. Storage of seed is cheaper and the shelf-life is infinitely longer than plant parts that are consumed fresh. Its ease of transport provided the foundation of the global grain trade that has helped equalize worldwide supply and demand since the development of ocean-going ships (originally moved by solar energy in the form of wind). Seeds are an important source of animal feed to produce meat, eggs, milk and other animal products.

The world's food supply depends on crops harvested for their seeds. Roughly half of the calories available from plant sources in recent years came from just four crops harvested for their seeds — maize, rice, wheat and soybean. Seeds are harvested because they are rich in carbohydrate, protein and oil stored in the seed as reserves for germination and the beginning of the next generation. Dry seeds are easy to transport and store; characteristics that contribute to their usefulness and popularity.
The unique carbohydrates, proteins and oils in the seed result from a complex series of biochemical processes, starting with the capture of light energy and the fixation of carbon in the leaf and ending with the synthesis of storage compounds in the seed. The mother plant produces the raw materials, primarily sucrose and various amino acids that are used by the seed to synthesize the complex molecules we use as food or feed. Understanding the production of yield by a crop community requires consideration of both the assimilatory and the synthesis processes.
Crop physiologists historically focused on the assimilatory processes. Investigations of dry matter accumulation by plants and plant communities and photosynthesis and other primary assimilatory processes were considered important because these processes are fundamental to the production of yield. However, the production of dry matter by a crop community is only part of the story in a grain crop where the economic yield is the seed. Utilization by the seed of raw materials translocated from the source is an equally important part of the yield production process. That is what this book is about.
My objectives in this book are, first, to gain an understanding of the growth and development of seeds, the processes involved, the regulation of these processes and the effect of plant and environmental factors. The second objective is to use this knowledge of seed growth and development to define the role of the seed in the yield production process.
What will we gain from such considerations? By approaching the production of yield from the viewpoint of the accumulation of dry matter by the seed (the sink), we will be able to integrate the source and the sink, assimilatory and synthesis processes, into a unified description or model of yield production. This model will be better than one that considers only the assimilatory processes in the source and relegates sink activity to a black box. A unified model including the seed will help us understand many important questions in yield physiology, including the determination of seed number, the relationship between seed size and yield, partitioning and source-sink relations. We cannot hope to answer all questions about the regulation of yield in a single book, but a thorough consideration of the seed sink will contribute to that goal.

Preface vii

Acknowledgements to First Edition ix

Acknowledgements to Second Edition xi

1 Introduction 1

Seeds as a Food Source 1

Increasing Food Supplies: Historical Trends in Seed Yield 5

Crop Physiology and Yield Improvement 10

The Seed: an Integral Component of the Yield Production Process 15

2 Seed Growth and Development 18

Seed Structure, Composition and Size 18

The Three Phases of Seed Development 21

      Development of seed structures (Phase I) 26

      The linear phase of seed development (Phase II) 28

      The end of seed growth - physiological maturity (Phase III) 33

Summary 41

3 Seed Growth Rate and Seed-fill Duration: Variation

and Regulation 42

Species and Cultivar Variation 45

Seed Growth Rate (SGR) 48

      Genetic variation 48

      Environmental and physiological variation 50

      Regulation of seed growth rate 56

      Summary 62

Seed-Fill Duration (SFD) 63

      Genetic variation 63

      Environmental and physiological variation 64

      Regulation of seed-fill duration 70

Summary 76

4 Yield Components - Regulation by the Seed 77

Yield Components - Seeds per Unit Area and Seed Size 78

C3Historical use and misuse 78

Yield components and plant development 81

Yield components and yield 87

Determination of Seed Number 91

      Components of seed number 91

      Summary 96

      Environmental effects 98

      Modelling seed number and assimilate supply relationships 103

Determination of Seed Size 111

      Potential seed size 112

      Components of seed size - seed growth rate and seed-fill duration 113

Summary 117

5 The Seed, Crop Management and Yield 119

Size of the Yield Container 121

      Canopy photosynthesis 122

      Length of Murata's Stage Two 124

      Partitioning 128

      Characteristics of the seed 129

      Summary 130

Filling the Yield Container 131

      Seed growth rate (SGR) 133

      Seed-fill duration (SFD) 133

      The enigma 135

      Seed size and yield 137

Source-Sink Limitations of Yield 139

Partitioning and Harvest Index 143

Time and Yield 150

      Potential productivity 150

      Utilization of potential productivity 153

      Summary 161

Summary 161

6 The Way Forward 162

Yield Improvement 162

Food Availability for the Future 168

General Summary 178

References 181

Index 215

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