Contact: Diane Relf, Extension Specialist, Environmental Horticulture
Posted April 1997
Plants reproduce asexually and sexually. Asexual reproduction is the propagation or perpetuation of a species by vegetative means. It is also referred to as clonal propagation because in most cases the offspring are genetically identical to the parent plants.
Plants reproduce sexually by means of seeds or spores. Seed development generally involves genetic recombination and the union of gametes (sperm and egg cells), resulting in many forms or varieties of a plant species. Varieties that have been further developed through artificial selection are designated as cultivars (for CULTIvated VARiety). Humans have utilized asexual and sexual methods for plant propagation, cloning for the maintenance of desirable cultivars, and seed propagation for the development of cultivars.
Many cultivars of popular vegetable or ornamental plants are described as hybrids. Hybrid seed production uses genetic recombination in a predictable way. Parent plants used to produce hybrids are from "pure lines" developed through inbreeding. Pure lines are plants that "breed true" or produce sexual offspring that closely resemble their parents. By crossing pure lines, a uniform population of F1 (first filial generation) hybrid seed can be produced with predictable characteristics.
F1 hybrids, in contrast to their true-breeding parents, are not true breeding. They can produce seed (F2 or second filial generation) with various genetic combinations for each trait. Seeds produced by an F1 hybrid tend to develop into plants that are highly variable and generally inferior to the F1 hybrid parents. These combinations can be observed when seeds are collected from an F1 hybrid and planted. For example, a gardener planting F2 seed could observe both red- and orange-fleshed tomatoes on offspring produced by a F1 plant bearing red tomatoes. Examples of F1 hybrids that set seed easily include tomatoes, peppers, cucurbits, purple coneflowers, and black-eyed susans.
A few principles must be considered before attempting to breed plants. Some plants are tolerant of inbreeding and can be self-pollinated. These include houseplants such as asparagus ferns and spider plants, many perennials, and vegetable crops such as peppers and peas. Other plant species exhibit inbreeding depression or self-incompatibility and must be cross-pollinated for sexual reproduction. Good candidates for cross-pollination between different cultivars include African violets, geraniums, marigolds, petunias, and lilies.
Seed fertility and viability must also be considered. Some plants may set seeds or spores that have a low germination percentage or are totally sterile. For these reasons, gerbera daisies, Boston ferns, and banana cultivars should be avoided in breeding efforts. Other seeds may be fertile, but dormant and require a length of time in storage or a cold treatment to break dormancy. Many perennial plants exhibit such dormancy. The length of time that seeds retain their viability in storage depends on storage conditions and plant species. Some long-lived seeds include zinnia, petunia, and lotus. Generally, low humidity and cool temperatures extend seed viability.
Hybrid plant production involves the transfer of pollen from one plant to the stigma of a different plant of the same species. This essentially is a 6-step procedure:
In most instances, don't expect spectacular performance in flower quality or yield from seeds produced from commercial plant cultivars due to their hybrid origin. However, backyard plant breeding may be worthwhile and rewarding to gardeners who wish to expose inherent genetic variability, observe trait segregation, and grow plants with new combinations of traits.
(Adapted from "Plant Breeding for the Home Gardener," by Henry R. Owen, Jr., former Graduate Student, Genetics, in The Virginia Gardener Newsletter, Volume 5, Number 9.)
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