PLANT BREEDING

application of genetic principles to produce plants that are more useful to humans. This is accomplished by selecting plants found to be economically or aesthetically desirable, first by controlling the mating of selected individuals, and then by selecting certain individuals among the progeny. Such processes, repeated over many generations, can change the hereditary makeup and value of a plant population far beyond the natural limits of previously existing populations. This article emphasizes the application of genetic principles to the improvement of plants; the biological factors underlying plant breeding are dealt with in the article heredity. Plant breeding is an ancient activity, dating to the very beginnings of agriculture. Probably soon after the earliest domestications of cereal grains, humans began to recognize degrees of excellence among the plants in their fields and saved seed from the best for planting new crops. Such tentative selective methods were the forerunners of early plant-breeding procedures. The results of early plant-breeding procedures were conspicuous. Most present-day varieties are so modified from their wild progenitors that they are unable to survive in nature. Indeed, in some cases, the cultivated forms are so strikingly different from existing wild relatives that it is difficult even to identify their ancestors. These remarkable transformations were accomplished by early plant breeders in a very short time from an evolutionary point of view, and the rate of change was probably greater than for any other evolutionary event. Scientific plant breeding dates back hardly more than 50 years. The role of pollination and fertilization in the process of reproduction was not widely appreciated even 100 years ago, and it was not until the early part of the 20th century that the laws of genetic inheritance were recognized and a beginning was made toward applying them to the improvement of plants. One of the major facts that has emerged during the short history of scientific breeding is that an enormous wealth of genetic variability exists in the plants of the world and that only a start has been made in tapping its potential. the practical application of genetic principles in agriculture and horticulture to produce plants more useful to humans. Although scientific plant breeding is essentially a 20th-century development, selective cultivation by ancient agriculturists effected a major and rapid evolutionary change in many plants from the wild to the domesticated state. Not until the mid-18th century was the sexual nature of plants incontrovertibly demonstrated, and the term plant breeding did not become widely used until after 1900. Thereafter the intentional breeding of superior plants spread quickly; by the mid-20th century the breeding of field and horticultural crops was so standardized that it was largely in the hands of experts, much of it centred in a few large agronomic and horticultural institutions operated mostly at government expense. For a variety of reasons the breeding of new and superior ornamental plants, though carried on in part by scientists and by tradesmen, was predominantly in the hands of gifted amateurs. As the result of their efforts, thousands of new varieties of dahlia, rose, African violet, peony, narcissus, and other plants came into being. Any successful plant-breeding program must be based upon a sound understanding of the exact mode of reproduction of that plant. Plants reproduce in two ways: vegetatively (that is, asexually), through the growth and development of a small portion (bulbs, tubers, etc.) of the original plant; and sexually, through the flower and seed. Even for those plants in which vegetative reproduction is not highly developed under natural conditions, the artificial rooting of cuttings and the grafting of roots and of stems make it possible to disseminate pieces of a superior individual by vegetative means. Different kinds of plants vary not only in the extent to which they may be propagated sexually but also in the degree to which they are naturally inbred or outbred. Among the higher plants, both extremes are found. Crops such as wheat, which are naturally very inbred, are almost completely self-pollinated. Others such as corn are mostly cross-pollinated under field conditions, though self-pollination is made possible by artificial means. Still others (many plums for instance) are naturally self-sterile and cannot be self-pollinated, though any particular flower may function effectively as either a male or a female in crosses with other plants. Mass selection, in which each successive generation of a crop is grown from mixed seeds of the most desirable plants of the preceding generation, is perhaps the simplest and least expensive plant-breeding technique. Great success has been achieved with naturally self-fertilized species using pedigree or pure-line selection. Most of the older varieties of wheat, oats, barley, cotton, beans, and tomatoes have so arisen. However, as the ceiling of remaining variation is reached by selection, hybridization of pure lines is resorted to for further improvement. Naturally cross-fertilized species are more difficult to handle, because any inbreeding necessary to fix desirable characters often leads to a decline in vigour. One may accordingly use continuous mass selection broadly, avoiding inbreeding. One may, on the other hand, use a combination of inbreeding and controlled hybridization to utilize the phenomenon of hybrid vigour as was done in hybrid corn improvement. See also hybrid. Additional reading Overviews are presented by R.H.M. Langer and G.D. Hill, Agricultural Plants, 2nd ed. (1991), a description of the major varieties and their products; Jules Janick et al., Plant Science: An Introduction to World Crops, 3rd ed. (1981); and J.W. Purseglove, Tropical Crops: Dicotyledons, 2 vol. (1968, reissued in 1 vol., 1974), and Tropical Crops: Monocotyledons, 2 vol. (1972, reprinted in 1 vol., 1988). Works focusing on plant breeding include John Milton Poehlman, Breeding Field Crops, 3rd ed. (1987); Neal F. Jensen, Plant Breeding Methodology (1988); and Abraham Blum, Plant Breeding for Stress Environments (1988). The Editors of the Encyclopdia Britannica