Publisher Summary This chapter focuses on evolutionary significance of phenotypic plasticity in plants. The expression of an individual genotype is modified by its environment. The amount by which it can be modified is termed its plasticity. This plasticity can be either morphological or physiological; these are interrelated. The plasticity of a character is related to the general pattern of its development, and apart from this, that plasticity is a general property of the whole genotype. Plasticity of a character appears to be specific for that character, specific in relation to particular environmental influences, specific in direction, under genetic control not necessarily related to heterozygosity, and able to be radically altered by selection. Because plants are static organisms, plasticity is of marked adaptive value in a great number of situations. Examples of all these situations in plant species are discussed. They indicate that adaptation by plasticity is a widespread and important phenomenon in plants and has evolved differently in different species. The mechanisms involved in plasticity are varied. At one extreme, the character shows a continuous range of modification dependent on the intensity of the environmental stimulus. At the other, the character shows only two discrete modifications. The stimulus causing these modifications may be direct or indirect. The mechanisms found can be related to the particular environmental situation involved.

Evolutionary Significance of Phenotypic Plasticity in Plants

Most plant breeding is based on “defect elimination” or “selection for yield”. A valuable additional approach is available through the breeding of crop ideotypes, plants with model characteristics known to influence photosynthesis, growth and (in cereals) grain production. Some instances of the successful use of model characters of this kind are quoted.

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The breeding of crop ideotypes

Nitrogen is the most limiting nutrient for crop production in many of the world's agricultural areas and its efficient use is important for the economic sustainability of cropping systems Furthermore, the dynamic nature of N and its propensity for loss from soil‐plant systems creates a unique and challenging environment for its efficient management Crop response to applied N and use efficiency are important criteria for evaluating crop N requirements for maximum economic yield Recovery of N in crop plants is usually less than 50% worldwide Low recovery of N in annual crop is associated with its loss by volatilization, leaching, surface runoff, denitrification, and plant canopy Low recovery of N is not only responsible for higher cost of crop production, but also for environmental pollution Hence, improving N use efficiency (NUE) is desirable to improve crop yields, reducing cost of production, and maintaining environmental quality To improve N efficiency in agriculture, integrated N management strategies that take into consideration improved fertilizer along with soil and crop management practices are necessary Including livestock production with cropping offers one of the best opportunities to improve NUE Synchrony of N supply with crop demand is essential in order to ensure adequate quantity of uptake and utilization and optimum yield This paper discusses N dynamics in soil‐plant systems, and outlines management options for enhancing N use by annual crops

Enhancing nitrogen use efficiency in crop plants

Maize (Zea mays L.) yields have risen continually wherever hybrid maize has been adopted, starting in the U.S. corn belt in the early 1930s. Plant breeding and improved management practices have produced this gain jointly. On average, about 50% of the increase is due to management and 50% to breeding. The two tools interact so closely that neither of them could have produced such progress alone. However, genetic gains may have to bear a larger share of the load in future years. Hybrid traits have changed over the years. Trait changes that increase resistance to a wide variety of biotic and abiotic stresses (e.g., drought tolerance) are the most numerous, but morphological and physiological changes that promote efficiency in growth, development, and partitioning (e.g., smaller tassels) are also recorded. Some traits have not changed over the years because breeders have intended to hold them constant (e.g., grain maturity date in U.S. corn belt). In other instances, they have not changed, despite breeders' intention to change them (e.g., harvest index). Although breeders have always selected for high yield, the need to select simultaneously for overall dependability has been a driving force in the selection of hybrids with increasingly greater stress tolerance over the years. Newer hybrids yield more than their predecessors in unfavorable as well as favorable growing conditions. Improvement in the ability of the maize plant to overcome both large and small stress bottlenecks, rather than improvement in primary productivity, has been the primary driving force of higher yielding ability of newer hybrid.

The Contribution of Breeding to Yield Advances in maize (Zea mays L.)

Criteria for evaluating different intercropping situations are suggested, and the Land Equivalent Ratio (LER) concept is considered for situations where intercropping must be compared with growing each crop sole. The need to use different standardizing sole crop yields in forming LERs is discussed, and a method of calculating an ‘effective LER’ is proposed to evaluate situations where the yield proportions achieved in intercropping are different from those that might be required by a farmer. The possible importance of effective LERs in indicating the proportions of crops likely to give biggest yield advantages is discussed.

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The Concept of a ‘Land Equivalent Ratio’ and Advantages in Yields from Intercropping

Publisher Summary In the early days of agriculture, man must have learned of the competition among individual plants within a crop or intraspecific competition, even though his knowledge was purely in empirical terms. He must have learned by experience that if the sowing rate were sparse, his harvest would be lean, and conversely that if the seed rate were increased beyond a certain value, the plants would be spindly and poorly grown. The fuller understanding of competition among plants requires a greater knowledge of the response of plants to their environment, especially of the response to the environmental stresses created by neighbors. Plant physiologists have studied the single plant, and agronomists have looked at the whole crop, but the plant within the community has scarcely been investigated. This is a field which promises both scientific depth and great potential reward in terms of crop production.

Competition Among Crop and Pasture Plants

Publisher Summary This chapter considers that the yield potential of annual crop species increase at a faster rate than with empirical selection for yield if suitable ideotypes are identified. A considerable list of common features and practices that influence yield in all annual seed crops is identified, and it is possible to design a basic ideotype for all these crops, involving principles of crop physiology and associated agronomic practices equally applicable to any annual seed crop. A sharp distinction is drawn between the ecology of annual field crops grown for their seed and that of most horticultural crops. Horticultural plants are cultivated for their fruits, unripe seeds, roots, stems, or leaves. The chapter recognizes several categories within Darwin's general processes of selection by man and natural selection. These are discussed in relation to annual seed crops. It is evident that all crops have been subject to many similar selective and evolutionary processes during domestication. They have become adapted to both the natural environment of the region and the manmade environment of local cultural methods. Crop yield is a man-made concept. It does not necessarily relate to natural selection or to crop evolution, and it is expressed by the nonbiological criterion of weight of product per unit area. The harvest in some crops is a vegetative part whereas in others it is a reproductive organ.

The Convergent Evolution of Annual Seed Crops in Agriculture

The grain yield of F5 lines from a barley cross, grown in field plots, showed no correlation with F3 single plant grain yield. F5 grain yield did however show a significant inverse relationship with plant height and leaf length in the F3, especially at high levels of nitrogen. Shorter plants with shorter leaves in the F3 tended to produce lines of similar habit in the F5, and these characters were associated with lower F3 grain yield and higher F5 grain yield.

C. M. Donald

Papers

The breeding of crop ideotypes

Competition Among Crop and Pasture Plants

The Convergent Evolution of Annual Seed Crops in Agriculture

The relationships between plant form, competitive ability and grain yield in a barley cross