Seed Biology and Yield of Grain Crops

Dennis B Egli

• Publisher: CAB International
• Year: 2017
• Print ISBN: 9781780647708
• eBook ISBN: 9781780647722
• Edition: 2
• Topic: Biological Sciences
• Subtopic: Biology
• Index: Biological Sciences

This new edition of an established title examines the determination of grain crop yield from a unique perspective, by concentrating on the influence of the seed itself. As the food supply for an expanding world population is based on grain crops harvested for their seeds, understanding the process of seed growth and its regulation is crucial to our efforts to increase production and meet the needs of that population. Yield of grain crops is determined by their assimilatory processes such as photosynthesis and the biosynthetic processes in the seed, which are partly regulated within the seed itself. Providing a timely update in this field and highlighting the impact of the seed on grain crop yields, this book: · Describes all aspects of seed growth and development, including environmental and genetic effects on growth rate and length of the filling period.· Discusses the role of the seed in determining the two main yield components: individual seed weight and number of seeds per unit area.· Uses the concepts and models that have been developed to understand crop management and yield improvement. Substantially updated with new research and further developments of the practical applications of the concepts explored, this book is essential reading for those concerned with seed science and crop yield, including agronomists, crop physiologists, plant breeders, and extension workers. It is also a valuable source of information for lecturers and graduate students of agronomy and plant physiology.

Introduction

1

Seeds as a Food Source

Humans have always relied on the green plant to produce the calories needed for their sustenance, either directly or indirectly 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 seed is also the biological unit used to reproduce most crops; there would be little food production without adequate supplies of viable, vigorous planting seed. The slogan of the American Seed Trade Association – ‘First the Seed’ – makes it clear that our existence depends on seeds that can germinate to produce the next crop. Thus, seed has a dual function of being consumed as food or feed and providing the means to reproduce the crop. These attributes have made the seed the foundation of agriculture since ancient times.

Many plant species have been used as sources of food, feed or fibre. Harlan (1992) compiled a ‘short list’ of cultivated plants that contained 352 species from 55 families. Vaughan and Geissler (1997) listed approximately 300 plant species used for food. The database of agricultural statistics (FAOSTAT) of the Food and Agriculture Organization (FAO) of the United Nations lists some 130 species in their crops category including grains, vegetables, fruits, nuts, fibre crops, spices and stimulants (coffee, tea and tobacco), but seeds are harvested from only about 35 species (FAOSTAT, 2014) and only 22 of these species are produced in substantial amounts (Table 1.1).

Table 1.1. World production and seed characteristics of crops where the mature seed is harvested for food or feed.

These 22 species represent only a few families, with 18 of them from the Poaceae (grasses) (nine) and the Fabaceae (legumes) (nine). Three of the species (maize, rice and wheat) dominate the world grain (seed) production, accounting for 76% of the 2011–2014 average production of the species in Table 1.1. If soybean, the fourth major crop, is included, the total increases to 84%. These crops account for roughly half of the calories available per capita for consumption from plant sources in 2009–2011. This proportion would increase if the seeds fed to livestock were included. It is clear that humans are fed by a very small sample of the plant species that could be used to produce food. Relying on so few crop species would seem to make our food supply vulnerable to insect or disease epidemics, but the use of multiple varieties of each crop reduces the chances of widespread crop failure (Denison, 2012, p. 3) as does the worldwide distribution of each crop. The importance of maize, rice and wheat is not a recent phenomena; Heiser (1973) pointed out that most important early civilizations were based on seeds of these crops. Truly, crops harvested for their mature seeds have served us well.

There is continuing interest in increasing the number of plant species providing our food supply. Examples of new crop species under consideration include grain amaranth (Amaranthus spp.) (Gelinas and Seguin, 2008), chia (Salvia hispanica L.) (Jamboonsri et al., 2012), quinoa (Chenopodium quinoa), hemp seed (Cannabis sativa L. (Pszczola, 2012), vernonia (Vernonia galamensis) (Shimelis et al., 2008), and ­potato bean (Apios americana sp.), a legume that produces edible tubers (Belamkar et al., 2015). Attempts are also being made to develop perennial grains from conventional annual crops and exotic species.

Perennial grain crops are expected to conserve soil resources by providing continuous ground cover and perhaps produce higher yield as a result of a longer life cycle (Glover et al., 2010).

New crops are often touted on the basis of their superior nutritive characteristics and/or their ability to be productive on infertile or droughty soils. If these new species are, in fact, ‘super crops’, why were they not selected in the long domestication processes that produced the few crops that feed the world? Are the species currently used those best suited for domestication (Sinclair and Sinclair, 2010, pp. 15–23), or were they domesticated first and then simply maintained by humans’ unwillingness to start over (Warren, 2015, pp. 164–167)? The relatively poor track record of new crop development schemes in recent times suggests that there may not be ‘better’ species waiting to be discovered. Nearly 100 years of intensive plant breeding produced the high-yielding cultivars of today’s common crops; the need for a time investment of this magnitude in a new crop is a serious impediment to its successful deployment.

The harvested seed is a caryopsis in nine of the 22 species in Table 1.1, including the major crops maize, rice and wheat. Nine of the 22 species produce non-endospermic seeds; prominent crops in this group include soybean, groundnut and bean.

Composition of the seeds of these species varies widely (Table 1.1). Nine species, the cereals, produce seeds that are high in starch (>600 g kg^–1) and low in protein (≤ 131 g kg^–1). Seeds of the traditional pulse or legume crops (seven species – bean, chickpea, dry pea, cowpea, lentil, broadbean and pigeon pea) have relatively high concentrations of protein (≥230 g kg^–1), high to intermediate carbohydrate levels, and very low oil concentrations. Four species (rapeseed (canola), sunflower, sesame and safflower) are classified as oil crops, with high concentrations of oil (290–540 g kg^–1) and relatively high protein levels, with safflower a conspicuous exception (Table 1.1). Soybean and groundnut fall into a class by themselves, with seeds that contain exceptionally high protein (310–370 g kg^–1) concentrations and moderate (170 g kg^–1, soybean) to high (480 g kg^–1, groundnut) oil concentrations.

The seeds that sustain humankind were selected over the millennia from an enormous number of potential crop species. The grass seeds, the staff of life, are major sources of carbohydrates for much of the world and are complemented by the pulses (legumes) with their relatively high protein levels (poor man’s meat) (Heiser, 1973, p. 116). These crops have fed humankind for centuries and it seems likely that we will continue to rely on them for the foreseeable future. Fortunately, the productivity of these crops has increased in step with the expanding world population.

Increasing Food Supplies: Historical Trends in Seed Yield

World population has increased by approximately 1000 times since the beginning of agriculture (Cohen, 1995, p. 30). The world population was roughly one billion (Cohen, 1995, p. 400) at the turn of the 19th century, when Thomas Malthus made his apocalyptic prediction (1798) that the power of population to increase is indefinitely greater than the power of the earth to provide food. The world population reached 7.3 billion in 2015, accompanied by food supplies that are, overall, more than adequate, as indicated by low grain prices in many countries, record low levels of undernourished people and rising concerns of an obesity epidemic in developed countries (FAOSTAT, 2014). Food supplies have increased since Malthus’s day more or less in step with population.

There are only six basic avenues by which food production can be increased (Evans, 1998, p. 197).

1. Increase the land area under cultivation

2. Increase the crop yield per unit area

3. Increase the number of crops per unit area per year (multiple cropping)

4. Replace lower yielding crops with higher yielding crops

5. Reduction of post-harvest losses

6. Reduced use as feed for animals.

The first four options deal with the quantity of food produced by crops, our interest in this book, but the last two would also increase the amount of food available for consumption by the world’s population. Shortening the food chain by utilizing more plant and fewer animal products, and reducing waste in harvest, storage and utilization of food and feedstuffs could make significant contributions, as could reducing the land area devoted to non-food production (i.e. crops fed to cats, dogs, horses and other pets; fibre, industrial, and especially biofuel crops). All of these last options would contribute to a larger food supply without increasing the land used for crop production, yield per unit area or the inputs required to increase yield. We will come back to these non-production options in Chapter 6, but they all involve complicated economic and social issues that are mostly beyond the purview of crop physiologists and this book.

Historical increases in food production were often associated with cultivation of more land. For example, wheat and maize production in the US increased by 3.5- to fivefold from 1866 to 1920 as a result of a three- to fourfold increase in harvested area as production moved west onto new lands in the Corn Belt and Great Plains states (NASS, 2016). The shift from the use of animal power (primarily horses and mules) to mechanical power (cars, tractors, trucks) fuelled by petroleum products in the early years of the 20th century reduced the need for feed production and made more land available for food production. Increases in yield, however, played a much larger role in more recent times as the supply of unused land declined.

Yield from eras closer to the beginning of agriculture 10,000 years ago provide an interesting perspective on current discussions of yield and the potential for yield improvement. Estimated maize yields in Mexico in 3000 BC were approximately 100 kg ha^–1, while brown rice yields in Japan in 800 AD were 1000 kg ha^–1 (Evans, 1993, pp. 276–279). Wheat yield in England increased from roughly 500 kg ha^–1 in 1200–1400 AD to approximately 1100 kg ha^–1 in the 1700s and nearly 2000 kg ha^–1 in the 1800s (Stanhill, 1976). Wheat yields in New York averaged 1077 kg ha^–1 for the period from 1865–1875 (Jensen, 1978). Modern yields (2011–2014 averages) for comparison are 7593 and 4182 kg ha^–1 for wheat in England and New York, respectively; 6707 kg ha^–1 for rice in Japan; and 3146 and 9391 kg ha^–1 for maize in Mexico and the USA (FAOSTAT, 2014; NASS, 2016). Clearly yields have increased along with the world’s population.

Documentation of changes in crop yield over a shorter time frame in the USA is shown in Fig. 1.1 for two cereals (maize and wheat) and a legume (soybean). There was relatively little change in yield of maize and wheat from 1866 to ~1940, when the advent of high-input agriculture (chemical fertilizers, herbicides and ­pesticides) combined with the use of hybridization to produce improved cultivars (hybrids in maize, but not wheat) started a steady increase in yield that has continued to the present time. Soybean yield in the USA also increased steadily from 1924; the first year that yield data were available. The three- to sixfold increases in yield of these crops in the 75 years after 1940 is truly astounding when compared with the previous 74 years, when there was no change. The agricultural systems in place for that 74-year period were low...

Table of contents

• Cover
• Half Title
• Dedication
• Title
• Copyright
• Contents
• Preface
• Acknowledgements to First Edition
• Acknowledgements to Second Edition
• 1 Introduction
• 2 Seed Growth and Development
• 3 Seed Growth Rate and Seed-fill Duration: Variation and Regulation
• 4 Yield Components – Regulation by the Seed
• 5 The Seed, Crop Management and Yield
• 6 The Way Forward
• References
• Index
• Back Cover