Plant Breeding

I

Introduction

Plant Breeding, practical application of genetic principles to the development of improved strains of agricultural and horticultural crops. Plant breeders can adapt old crops to new areas and uses; increase yields; improve resistance to disease; enhance the nutritional value and flavour of fruits and vegetables; and develop traits that are useful for storage, shipping, and processing of foods. Improved wheat and rice varieties sparked the green revolution in the developing world during the 1960s and 1970s. In ornamental plants, breeders have developed larger and showier flowers, greater plant vigour, and myriad types, shapes, and colours.

II

History

Stone Age farmers improved crops through selection, choosing at each harvest the largest seeds from the best plants for sowing the following year. In so doing, over thousands of years, they converted favoured wild grass and legume species into such crops as maize, wheat, and soya beans. In the 18th and 19th centuries farmers attempted to speed up crop improvement. Some advances were made, partly through selection and partly through trial and error, as in the procedures used by the American horticulturist Luther Burbank. Through the work of Gregor Mendel, Hugo de Vries, and others, the development of the science of genetics at the beginning of the 20th century established a firm scientific base for plant breeding. Since that time it has continued to develop in sophistication and accomplishment, enabling professional plant breeders to achieve predictable results and uniform quality.

Plant breeders use numerous methods to develop new varieties, but their primary techniques of development are selection, hybridization, and the use of mutations.

III

Selection

Individuals within a species vary widely in a number of characteristics. Many of these traits are inheritable and can be passed on to the individuals’ progeny. In practising selection, plant breeders choose plants with desirable traits for further propagation, and discard plants that are inferior for those traits. By doing so, plant breeders can select and reselect for the trait through successive generations, shifting the population in the desired direction.

IV

Hybridization

Hybridization involves crossing plants of different strains or types to combine in the progeny the desirable traits of both parents. Undesirable traits also enter the combination, however, so hybridization is usually followed by several generations of selection. This allows breeders to discard undesirable plants, choosing for further propagation only those plants with the desired combination of traits.

Back crossing is a common variation on hybridization. This technique is often used to transfer into a desirable variety a beneficial trait from an otherwise undesirable parent. First the hybrid between the two parents is made; then the hybrid is crossed with the desirable parent. The progeny from this back crossing normally segregate widely, with individual plants showing a mixture of the characteristics of both parents. By continued back crossing and selection the plant breeder concentrates the qualities desired, and, if all goes well, in six or seven generations the variety once again breeds true but now exhibits its new trait. Back crossing is valuable for adding single-gene characteristics to crop plants, particularly for resistance to specific insects and diseases.

When desirable characteristics are fully developed in a hybrid plant, and the plant can be propagated asexually by budding, grafting, or cloning, then no further selection is necessary. A hybrid apple, for example, is propagated by grafting, so that all resultant plants are genetically identical.

Hybrids may be more vigorous than either parent. This phenomenon is called hybrid vigour (heterosis) and has been widely used by plant breeders to increase crop yields. For example, hybrid seeds have helped to double United States maize yields since the 1940s, and almost all the maize now grown in the United States and Europe is started annually from hybrid seed. Hybrid breeding has expanded in recent years, and hybrid varieties are now common in grain crops (maize, sorghum), vegetables (cabbage, tomatoes, squash), and many flower species.