between the time of pollen shed and stigma receptivity.
Self‐fertilization versus cross‐fertilizationJust because a flower is successfully pollinated does not necessarily mean fertilization would occur. The mechanism of self‐incompatibility causes some species to reject pollen from their own flowers, thereby promoting outcrossing.
Sexuality versus asexualitySexually reproducing species are capable of providing seed through sexual means. Asexuality manifests in one of two ways – vegetative reproduction (in which no seed is produced), or agamospermy (in which seed is produced).
5.3 Types of reproduction
Plants are generally classified into two groups based on mode of reproduction: sexually reproducing or asexually reproducing. Sexually reproducing plants produce seed as the primary propagule. Seed is produced after sexual union (fertilization) involving the fusion of sex cells or gametes. Gametes are products of meiosis and, consequently, seeds are genetically variable. Asexual or vegetative reproduction mode entails the use of any vegetative part of the plant for propagation. Some plants produce modified parts such as creeping stems (stolons or rhizomes), bulbs, or corms, which are used for their propagation. Asexual reproduction is also applied to the condition whereby seed is produced without fusion of gametes (called apomixis). It should be pointed out that some plants could be reproduced by either the sexual or asexual mode. However, for either ease of propagation or for product quality, one mode of reproduction, often the vegetative mode, is preferred. Such is the case in flowering species such as potato (propagated by tubers or stem cuttings) and the sugarcane (propagated by stem cuttings).
5.4 Sexual reproduction
Sexual reproduction increases genetic diversity through the involvement of meiosis. Flowering plants dominate the terrestrial species. Whereas flowers are required, flowering plants may reproduce sexually or asexually.
5.4.1 Sexual lifecycle of a plant (alternation of generation)
The normal sexual lifecycle of a flowering plant may be simply described as consisting of events from first establishment to death (from seed to seed in seed‐bearing species). A flowering plant goes through two basic growth phases – vegetative and reproductive, the former preceding the latter. In the vegetative phase, the plant produces vegetative growth only (stem, branches, leaves, etc., as applicable). In the reproductive phase, flowers are produced. In some species, exposure to a certain environmental factor (e.g. temperature, photoperiod) is required to switch from vegetative to reproductive phase. The duration between phases varies among species and can be manipulated by modifying the growing environment.
In order for sexual reproduction to occur, two processes must occur in sexually reproducing species. The first process, meiosis, reduces the chromosome number of the diploid (2n) cell to the haploid (n) number. The second process, fertilization, unites the nuclei of two gametes, each with the haploid number of chromosomes, to form a diploid. In most plants, these processes divide the lifecycle of the plant into two distinct phases or generations, between which the plant alternates (called alternation of generation) (Figure 5.1). The first phase or generation, called the gametophyte generation, begins with a haploid spore produced by meiosis. Cells derived from the gametophyte by mitosis are haploid. The multicellular gametophyte produces gametes by mitosis. The sexual reproductive process unites the gametes to produce a zygote that begins the diploid sporophyte generation phase.
Figure 5.1 Schematic representation of alternation of generations in flowering plants. The sporophyte generation is diploid, and often the more conspicuous phase of the plant lifecycle. The gametophyte is haploid.
In lower plants (mosses, liverworts), the sporophyte is small and dependent upon the gametophyte. However, in higher plants (ferns, gymnosperms, angiosperms), the male gametophyte generation is reduced to a tiny pollen tube and three haploid nuclei (called the microgametophyte). The female gametophyte (called the megagametophyte) is a single multi‐nucleated cell, also called the embryo sac. The genotype of the gametophyte or sporophyte influences sexual reproduction in species with self‐incompatibility problems. This has implications in the breeding of certain plants as discussed further in this chapter.
5.4.2 Duration of plant growth cycles
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
