Plant Physiology - Musiyenko, M. M. 2001

Physiology of Plant Reproduction
Flower Formation

The transition from the vegetative to the generative state is accompanied by flower formation. A flower is a shortened, unbranched SHOOT of determinate growth whose leaves are metamorphosed in connection with sexual reproduction, which leads to seed formation. We have already discussed in detail in the previous chapter the induction of this process by day length, or photoperiod, and cold (vernalization). In only a few species is flower formation completely absent in the absence of inductive conditions—a case of strict environmental control of flowering. In many species, inductive conditions accelerate this process (quantitative control) or have no effect at all (autonomous flower formation).

Evocation is the final phase of flowering initiation, during which processes occur in the apex that lead to the appearance of floral primordia. While the perception of the photoperiod can be called the foliar phase of flowering initiation, evocation is the phase realized in the stem apex. During evocation, under METABOLISM/18.html">The Influence of a floral stimulus, the expression of genes responsible for the morphogenetic program of flower development is induced in the shoot apical meristem.

To perceive the flowering stimulus, the shoot apex must be competent to do so. In most woody plants during the Juvenile Stage, apical Meristems are unable to respond to the floral stimulus—for example, when grafted onto fruit-bearing trees—because they lack the appropriate competence. Competence for the floral stimulus may be associated with the appearance of receptors in the Cells of the corresponding Zones of the apex. These are likely gibberellin receptors (in the subapical meristem) and receptors for a hypothetical anthesin in another meristematic zone. Interestingly, in herbaceous plants, all shoot apices—both young and old—are capable of perceiving the floral stimulus when it arrives from the leaves. Competence for the floral stimulus in the shoot apex is determined by specific genes.

It is difficult to precisely determine the onset of evocation, since Changes in the cells of the shoot apex may be observed even before the action of inductive factors begins or simultaneously with it. In the case of photoperiodic effects on the leaves, a rather rapid response of the apical meristem may be driven by electrical signals, through changes in the electrotonic potential difference between the leaf and the apex, or via the generation of action potentials. Such rapid electrical signaling can prepare the stem apex to perceive the chemical floral stimulus, which is transported from the leaves to the apices much more slowly. The state in which the transition of the apical meristem to flower formation becomes irreversible is called floral determination. At this time, intensive molecular, histological, and morphogenetic changes take place in the apex.

Initially, the content of Histones in The Cell nuclei of the apical dome decreases, followed by an increase in RNA and Protein content, and changes in the composition of RNA nitrogenous bases. Immediately thereafter, DNA Synthesis AND The rate of mitoses sharply increase. Groups of initial cells become physiologically and morphologically distinct, and the specific patterns of their division determine the subsequent morphogenesis of each flower part. The sepals and petal primordia are laid down first, followed sequentially by the stamens and carpels, which fuse to form the pistil and to which ovules are typically attached. Externally, the ovule possesses one or two integuments, with an opening between them called the micropyle. Beneath the integument lies the nucellus, which is homologous to a megasporangium. One of its cells becomes the archesporial cell, which undergoes reduction division (Meiosis) to form four haploid macrospores. Three of them degenerate, while one gives rise to the embryo sac (female gametophyte). As a result of three successive mitotic divisions, 8 haploid nuclei appear within the embryo sac: after the First Division, two daughter nuclei migrate to opposite poles of the polarized gametophyte and divide twice more. Three nuclei at each pole differentiate into cells. One of the cells near the micropyle becomes the egg cell (female gamete), while the other two become synergids. The remaining three nuclei at the opposite pole form the antipodal cells. The two remaining nuclei migrate to the center and fuse, forming the secondary diploid Nucleus of the central cell, which gives rise to the endosperm. The egg cell and female gametophyte are thus ready for Fertilization.

In the anthers of seed plants, the pollen grain (microspore) develops, covered by a thin inner wall (intine) and an outer wall (exine). During The Development of the pollen grain via meiosis followed by mitosis, a highly reduced male gametophyte is formed within it. It consists of one (in angiosperms) or several (in gymnosperms) vegetative cells and a small generative cell with its own generative nucleus, which mostly divides into two sperms (male Gametes) participating in fertilization. The main role of the synergids is to attract the pollen tube to the embryo sac by secreting chemotropic substances and to guide the sperms into the space between the egg cell and the central cell. The Role of the antipodes is to supply the embryo sac with nutrients; in many species, this already happens within the pollen tube. The vegetative cell ensures the viability of the pollen grain and the growth of the pollen tube.

According to S.I. Lebedev, carotene and carotenoids perform an important physiological function in reduction division during microsporogenesis. Their reduced content in anthers leads to impaired microsporogenesis and The formation of abortive pollen.

During macro- and microsporogenesis, callose is deposited in the cell walls at early Selection/3.html">Stages of development, providing temporary isolation for cells that have entered the generative developmental pathway.

Thus, evocation encompasses processes at the molecular and intracellular levels, the completion of which creates the conditions for floral morphogenesis. The flowering duration of an individual flower ranges from 2–3 hours (Hibiscus) to 80 days (the tropical orchid Odontoglossum). The duration of flowering depends on The amount of pollen produced by the flower, the number of flowers, and the flowering period between the first and last flower. In plants with a single flower (Moneses uniflora) or flowers with a single stamen (orchids), the flowering duration is quite significant, as such solitary flowers are rather difficult for pollinating insects to find. Conversely, when There are many flowers or when they produce abundant pollen, the lifespan of an individual flower is short. The flowering period also depends on pollination: if pollination occurs earlier, the flowers wither sooner. The periodicity of flower closure and opening is determined by the action of heat and light on the plant.