Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 215

The plant breeder should know the lifecycle of the plant to be manipulated. The strategies for breeding are influenced by the duration of the plant growth cycle. Angiosperms (flowering plants) may be classified into four categories based on the duration of their growth cycle as follows (Figure 5.2):

1 AnnualAnnual plants (or annuals) complete their lifecycle in one growing season. Examples of such plants include corn, wheat, and sorghum. Annuals may be further categorized into winter annuals or summer annuals. Winter annuals (e.g. wheat) utilize parts of two seasons. They are planted in fall and undergo a critical physiological inductive change called vernalization that is required for flowering and fruiting in spring. In cultivation, certain non‐annuals (e.g. cotton) are produced as though they were annuals.

2 BiennialA biennial completes its lifecycle in two growing seasons. In the first season, it produces basal roots and leaves; then it grows a stem, produces flowers and fruits, and dies in the second season. The plant usually requires a special environmental condition or treatment (e.g. vernalization) to be induced to enter the reproductive phase. For example, sugar beet grows vegetatively in the first season. In winter, it becomes vernalized and starts reproductive growth in spring.

3 PerennialPerennials are plants that have the ability to repeat their lifecycles indefinitely by circumventing the death stage. They may be herbaceous, as in species with underground vegetative structures called rhizomes (e.g. indiangrass), or aboveground structures called stolons (e.g. buffalograss). They may also be woody as in shrubs, vines (grape), and trees (orange).

4 MonocarpMonocarps are annuals or biennials, but some persist in vegetative development for very long periods of time (e.g. the so‐called “century plant”) before they flower and set seed (e.g. bamboo and agave). Once flowering occurs, the plant dies. That is, monocarps are plants that flower only once. Other examples are bromeliads. The top part dies, so that new plants arise from the root system of the old plant.

Figure 5.2 Flowering plants have one of four lifecycles: annual, biennial, perennial, and monocarp. Variations occur within each of these categories, partly because of the work of plant breeders.

It should be pointed out that certain plants that may be natural biennials or perennials are cultivated by producers as annuals. For example, sugar beet, a biennial, is commercially produced as an annual for its roots. For breeding purposes, it is allowed to bolt to produce flowers for crossing, and subsequently to produce seed.