Plant Physiology - Musiyenko, M. M. 2001

Physiology of Plant Reproduction
Pollination and Fertilization

The entire process associated with Fertilization can be divided into the following phases:

·pollination,

·pollen germination,

·growth of the pollen tube through the Tissues of the pistil style:

·fertilization proper, i.e., The formation of a zygote.

Pollination

After pollen grains are formed within the pollen sacs, the walls of the anthers dry out and dehisce, releasing the pollen. The transfer of pollen grains to the stigma of the pistil is called pollination. A distinction is made between self-pollination, when pollen lands on the stigma of the same flower, and cross-pollination, in which pollen from the flowers of one plant reaches the stigmas of flowers on another. The mode of pollination depends on the Structure and PHYSIOLOGICAL CHARACTERISTICS OF the flower, as well as environmental conditions.

Pollination requires pollen to be transported to the pistil stigma by insects, wind, or Water. The outer layer of the pollen grain wall (exine) contains terpenoid substances that provide protective properties against adverse environmental conditions. It also harbors Proteins that regulate the pollen-stigma incompatibility system, including both intra- and interspecific incompatibility. The inner layer of the wall (intine), In addition to Cellulose and pectic substances, contains proteins that ensure the mutual recognition of the pollen and the pistil stigma, as well as hydrolytic Enzymes (such as acid phosphatase, protease, and RNase). Upon landing on the pistil stigma, the pollen begins to swell. The stigma secretes a fluid composed of Lipids (wax derivatives), which perform a protective function, and Phenolic Compounds (anthocyanins, Flavonoids, cinnamic acids). Phenolic components regulate pollen germination, provide defense against pathogens, and play a specific role in the compatibility control system. Various types of pollen may land on the stigma, but only compatible pollen will germinate.

The incompatibility mechanism is not yet fully understood; however, it is known that during the pollen-stigma contact, Glycoproteins are released from the exine. Their interaction with an incompatible stigma triggers the secretion of callose, which isolates the pollen grain. In the case of compatibility, cutinase and other hydrolytic enzymes are activated, loosening the walls of the pistil and thereby facilitating the growth of the pollen tube.

Fertilization and Seed Development

The pollen tube penetrates the embryo sac through the micropyle. The sperm Cells are released: one of them fuses with the egg Cell Nucleus to form a zygote, while the other fuses with the secondary (diploid) Nucleus of the central cell, which gives rise to the endosperm. Thus, double fertilization occurs within the embryo sac, a phenomenon discovered by S.G. Navashin (1898) of the St. Vladimir University of Kyiv (now Taras Shevchenko National University of Kyiv). The Significance of double fertilization lies in the fact that both the zygote and the primary endosperm cell, possessing dual heredity, acquire greater viability and adaptability to environmental conditions. This explains The Role of the numerous adaptations in Flower Morphology and physiology aimed at ensuring cross-pollination.

Following fertilization, the zygote remains in a latent state for a certain period (ranging from several hours to a few days). During this time, RNA Synthesis is enhanced within it, and the zygote increases in volume. The triploid nucleus of the central cell of the embryo sac begins to divide first, forming the endosperm. Auxins and Cytokinins supplied by the nucellus are required for endosperm development. The embryo goes through a sequence of developmental phases. For most dicots, these stages are: proembryo, globular, Heart-shaped, torpedo, and maturation (Fig. 195).

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Fig. 195. Main phases of embryo development

Thus, the zygote grows through repeated mitotic divisions, transforming into a multicellular embryo consisting of a primary SHOOT or plumule, a primary ROOT, and one (in monocots) or two (in dicots) cotyledons.

The plumule consists of the stem, the first pair of true leaves, and the terminal bud. If the cotyledons serve as storage tissue, they grow at the expense of the endosperm, which may completely disappear in the process.

Note the chronological sequence of embryo differentiation into distinct specialized parts: formation of a group of embryonic initial cells for shoot parts (quadrant) → appearance of the hypophysis as the root initial cell → specification of cotyledon primordia accompanied by the inhibition of Cell Division in the region between them → appearance of procambium.

The Increasing complexity of internal correlation relationships between cells in different Regions of the embryo likely indicates the ESTABLISHMENT OF THE newly formed Organism's own hormonal system. After fertilization, the ovule is referred to as a seed, and the Ovary as a fruit. As the seeds develop, the ovary transforms into a mature fruit, and its walls are called the pericarp. The remaining PARTS OF THE flower wither, die off, and fall away. The formed seed is a typical product of Sexual reproduction in angiosperms, providing a species with advantages related to genetic Variability.

In the developing seed, the growth of the embryo and sometimes the endosperm takes place within the embryo sac. Meanwhile, the surrounding nucellus breaks down, supplying nutrients to the embryo. Subsequently, nutrient supply is maintained by the vascular bundle of the funiculus (the ovule stalk). The micropyle persists as a small pore in the seed coat, through which oxygen and water enter during subsequent seed germination.

The seed coat (testa) is a thin protective layer formed from the integuments.

During the final stages of seed maturation, its water content decreases to 10–15%. This is accompanied by a decline in metabolic activity and marks the transition of the seed into a dormant state.

It has been established for various plants that the developing seed is a center for the synthesis of auxin, gibberellin, and cytokinins, thereby becoming a sink region that draws nutrients from the leaves and shoots.

In addition to Hormones, other BIOLOGICALLY ACTIVE SUBSTANCES, particularly growth inhibitors such as salicylic, ferulic, and abscisic acids, have been detected in fruits and seeds.