Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 214

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.