III.

Plant Breeding

Plant breeding is similar in principle to animal breeding: Various forms of selection are combined with inbreeding and outbreeding. However, there are certain differences due to the unique aspects of plant reproduction. Unlike animals, many plants have the ability to self-fertilize or self-pollinate—that is, the pollen of one flower fertilizes the eggs within the same flower or the eggs of other flowers on the same plant. Other plants are cross-pollinators—the pollen from their flowers will not fertilize eggs from the same flower or plant. Due to persistent inbreeding, self-pollinating plants are genetically more uniform than cross-pollinated plants, which harbor more genetic variability. About half of the major agricultural plants—including wheat, rice, barley, beans, peas, and tomatoes—are self-fertilizing. The other half—including carrots, date palms, asparagus, hops, white clover, and cabbage—are cross-pollinating. Self-pollinating plants can reproduce without human intervention. Cross-pollination is more labor intensive because the breeder must prevent stray pollen from entering the flowers. To avoid stray pollination, the breeder must carefully remove the stamens (pollen-bearing organs) from a flower, dust the pistils (egg-producing organs) with the selected pollen, and then cover the flower with a small bag. The process of controlling pollination is somewhat easier in cross-pollinating forms such as maize (corn), in which the male and female organs are contained in separate flowers (the tassels bear the stamens, and the ear bears the pistils). In this case the breeder simply removes the tassels from those plants that are not to be used as pollinators. In modern breeding, the need for manual detasseling has been eliminated as a result of a genetic innovation known as cytoplasmic male sterility (CMS) in certain varieties of plants, including maize. CMS strains produce sterile male flowers, allowing breeders to place the plants in alternating rows of normal and CMS strains, thus ensuring maximum cross-pollination without high labor input.

Plant breeders supplement the methods of mass selection with other procedures, such as pure-line selection and hybridization. Pure-line selection involves continued inbreeding and selection for one or more desirable traits to produce a relatively homozygous (genetically homogenous) population. Hybridization involves crossing distinct strains or even species to produce hybrids (forms) that contain a combination of traits that is superior to that found in either parent. Because plants can form viable hybrids much more readily than animals, hybridization has long been a mainstay of plant breeding. The outstanding example of hybridization in the 20th century has been hybrid corn. Hybrid varieties have many superior qualities, especially in yield and vigor, but, because they do not breed true, farmers must buy new seeds each season.