PLANT PHYSIOLOGY AND BIOCHEMISTRY

Lecture Notes

12. PLANT REPRODUCTIVE PHYSIOLOGY

We have examined all stages of higher plant ontogeny, with the exception of maturity and reproduction. During the juvenile period, young plants grow and develop, accumulating significant vegetative biomass and nutrient reserves. This enables them, at a specific point in time, to form reproductive Organs that are nourished heterotrophically.

Plant reproduction is a physiological process of producing offspring, which ensures the continuity of a species and the dispersal of its representatives within the environment.

Modes of Plant Reproduction

Plants exhibit two primary modes of reproduction: sexual, where a new Organism develops As a result of the fusion of two haploid Gametes; and asexual, where a new generation arises from somatic (vegetative) Cells.

There are two forms of sexual reproduction: 1) where a new organism arises from a fertilized egg Cell (zygote); or 2) from an unfertilized egg cell via parthenogenesis.

Reproduction through The Development of a fertilized egg cell is characteristic of all higher and lower plants. An essential prerequisite for this mode of reproduction is Meiosis—a complex nuclear division that reduces the chromosome number by half. In higher plants, this occurs in the reproductive organs (flowers) and culminates in The formation of microspores and megaspores. These develop into male and female gametophytes, which give rise to male and female gametes. Species with distinct male and female individuals are called dioecious. Among flowering plants, there are monoecious and dioecious species. In monoecious plants, male and female flowers are produced on the same individual, whereas in dioecious plants, some individuals bear only male flowers while others bear only female flowers.

Asexual reproduction occurs without the formation of gametes and involves only a single organism. In asexual reproduction, a new organism develops from spores produced on the sporophyte, or from specialized or non-specialized PARTS OF THE plant body. This method produces genetically identical offspring, with random Mutations being the sole source of genetic Variability. It is characteristic of spore-bearing plants, which typically exhibit a more or less distinct alternation of two generations: the asexual

diploid (sporophyte) and the sexual haploid (gametophyte) generation.

Thus, there are two forms of asexual reproduction: 1) spore formation (sporogenesis) and 2) Vegetative Reproduction.

Sporogenesis is The process of spore formation. Sporogenesis, which occurs through Cell Division within asexual reproductive organs known as sporangia, is characteristic of lower plants. In unicellular green Algae, individual cells can transform into sporangia. In higher plants, a distinction is made between microsporogenesis (formation of microspores) and megasporogenesis (formation of megaspores), which occur as a result of two successive meiotic divisions within the sporangia. In bryophytes, the sporangium is represented by the spore capsule. The sporangia of pteridophytes and seed plants develop on sporophylls or in their axils. Most higher plants produce microsporangia and megasporangia, in which microspores and megaspores develop, respectively. In seed plants, the megasporangium is commonly referred to as the nucellus, and the microsporangium as the pollen sac.

Vegetative reproduction is a form of asexual reproduction in which new individuals, identical in their hereditary traits, are formed from parts of the parent organism. In other words, it is the regeneration of a whole organism from its vegetative parts.

According to V. Skrypchynskyi (1970-90), the evolution of plant reproductive strategies proceeded as follows: from forms that reproduced by simple cell division, spore-forming organisms emerged, followed later by organisms capable of sexual reproduction.

The integration of Selection/8.html">Asexual and sexual reproduction led to the appearance of species with Morphology/12.html">ALTERNATION OF GENERATIONS. Over time, one generation became dominant (the gametophyte in lower plants, and the sporophyte in higher plants).

While the Anatomical and morphological aspects of reproductive processes in most plant groups have been studied in considerable detail, their physiological basis remains insufficiently understood. The transition of flowering plants from vegetative growth to the generative phase is a complex, multi-stage process.

Sexual Reproduction in Flowering Plants

The Relationship Between Plant Life Cycles and Flowering Processes

The lifespan of plants varies significantly. Some plants live for a short period: a few weeks, months, or a year. Conversely, There are many herbaceous and woody plants with long lifespans (lasting several hundred, or in the case of the giant sequoia, even several thousand years). However, despite such differences in longevity, they share a common property related to their development: the formation of vegetative organs—leaves, stems, and roots—followed by the development of reproductive organs. Growth periods vary among different plants, just as the formation of reproductive organs occurs in different ways. Furthermore, the capacity for repeated flowering is not inherent to all plant species. In this regard, they are classified into monocarpic and polycarpic plants.

The first group consists of plants that, regardless of their lifespan, flower only once and die immediately after flowering and fruiting. This group is highly diverse and includes both annual and perennial species. It comprises:

ephemerals (their GROWTH AND DEVELOPMENT period lasts only a few weeks) - such as whitlow-grass (Draba verna), holosteum (Holosteum umbellatum), and spring draba (Erophila verna);

annual plants (within a single growing season, they form vegetative and reproductive organs, fruit, and mature their seeds) - such as potatoes, cucumbers, flax, spring cereals, and legumes;

biennial plants (their life cycle spans two growing seasons) - such as onions, garlic, beets, carrots, celery, cabbage, and others;

perennial plants (characterized by a long growth period—from 8 to 50 years—flowering once in their lifetime before dying) - such as certain agaves, bamboos, palms, ferulas, etc.

The second group consists of plants that flower multiple times throughout their lifespan; some may even bloom two or three times a year if mowed, as is characteristic of certain grasses. This category includes plants that reach flowering in their first year (timothy grass, alfalfa, meadow foxtail); in their second year (perennial lupine); in their third year (berry bushes); 8–12 years after grafting, such as pears, apples, and quinces; and finally, those that bloom only after 25–30 years of growth (maple, linden, oak, etc.).

Fruit-bearing capacity depends on the rootstock used. On dwarfing rootstocks, all varieties tend to bloom and fruit earlier. Pears and apples generally bloom later than stone fruits. The second group of plants is characterized by the fact that their vegetative growth renews annually.

It should be noted that these plant groups do not encompass the full spectrum of diversity in growth and development inherent in the plant kingdom.

For instance, in agricultural production, it is common to classify plants by their development type into spring crops, which yield a harvest when sown in the spring; winter crops, which are sown in the autumn; and facultative (or alternative) crops, capable of fruiting whether sown in the autumn or spring.

In crop production, it is standard to distinguish phenological phases during which specific morphological structures are formed. For example, in cereal grains, one distinguishes germination, tillering, jointing, heading, flowering, and the stages of grain development — milk, dough, and full maturity.

In annual dicotyledonous plants, the typical stages identified are germination, the appearance of the first pair of true leaves, stem branching (stem elongation), inflorescence formation, budding, flowering, fruit and seed development, and their subsequent stages of maturity.

In the ontogeny of monocarpic plants, distinct periods of vegetative and generative (reproductive) development are recognized, the latter encompassing the formation of inflorescences, flowers, seeds, and fruits, as well as their maturation phases.

Upon reaching maturity, vegetative buds transform into floral buds. Within these, the primordia of floral organs are established, followed by their growth and differentiation; subsequently, after Pollination and Fertilization, seeds and fruits are produced. The onset of these transformations is known as flowering initiation.

Flowering initiation

Flowering initiation consists of two phases: induction and evocation.

Induction. This phase occurs under the Influence of Environmental factors: Temperature (vernalization) and the alternation of day and night (Photoperiodism); or endogenous factors determined by the plant's age (T.D. Lysenko's theory of phasic Development of Plants).

Vernalization is a process occurring in winter forms of annual and biennial plants under METABOLISM/18.html">The Influence of low positive temperatures for a specific duration, which promotes the accelerated development of these plants.

Plants that require vernalization are called winter plants, while those that develop without it are known as spring plants.

Winter species will not flower without vernalization (a qualitative response); in some, low temperatures merely shorten the time required to transition to flowering (quantitative response). In plants with a quantitative response, vernalization can occur in germinating seeds. Plants with a qualitative response must reach a certain size (a specific developmental stage) before the process can begin.

Vernalization typically lasts 1–3 months. Temperatures between 0 and +7° C are most effective; for thermophilic plants, the range is +10–12° C.

For some species (rye, cabbage), periodic interruption of low-temperature exposure reduces or even negates (eliminates) The Effect of vernalization.

A necessary condition for vernalization is the presence of dividing cells. This process takes place in the embryo or in the apical Meristems of the stem and young leaves. Vernalization-induced changes are transmitted only through cell division. The PHYSIOLOGICAL AND BIOCHEMICAL essence of vernalization is not yet fully understood.

Following vernalization, most plants require long-day photoperiods. In other plants, vernalization reduces the requirement for photoperiodic exposure. For certain plants, continuous illumination facilitates the transition from winter behavior to a typical spring-like growth pattern.

Photoperiodism is the response of plants to the daily rhythm of light (a specific ratio of day to night duration), which manifests as changes in growth and development processes. One of the primary expressions of this reaction is photoperiodic induction (or inhibition) of flowering.

Photoperiodism was studied by American plant physiologists W. Garner and H. Allard between 1920 and 1925. Depending on the plant's response to day length, which accelerates flowering, they are classified as:

- long-day plants (LDP — such as cereals, crucifers, dill; common primarily in temperate and subpolar latitudes);

- short-day plants (SDP — rice, soy, hemp; subtropics);

- long-short-day plants (LSDP) and

- short-long-day plants (SLDP) — requiring an alternation of different photoperiods;

- day-neutral plants (DNP — buckwheat, peas, etc.).

Classifying plants into these groups does not depend on a specific optimal day length; rather, it provides insight into whether flowering is accelerated by increasing or decreasing the duration of light exposure within each photoperiod.

There are plants with qualitative and quantitative types of photoperiodic responses. Light intensity, temperature, and other factors can modify The Nature of a plant's photoperiodic expression.

For short-day plants (SDP) with a qualitative response, the duration of the dark period is the primary factor. Interrupting this period (even for 1 minute) inhibits the transition to flowering. Interrupting the light period with darkness does not affect flowering timing. Long-day plants (LDP) do not require a dark period and will flower under continuous illumination. These factors (temperature and photoperiod) can act sequentially, as is the case, for example, in winter cereals.

The photoperiodic effect is perceived primarily by the leaves, rather than the SHOOT apices. Leaves that have just completed their growth exhibit maximum sensitivity to the photoperiod. This process is mediated mainly by Phytochrome. It is believed that its dark conversion may serve as a time-measuring mechanism, functioning like an hourglass. A flash of red light resets the phase of the endogenous rhythm.

The requirement for a large leaf surface area and sufficient light intensity for the transition to flowering in many plants is explained by the need of growing meristems for assimilates. In day-neutral plants (DNP), the initiation of flowering is driven by age-related changes, governed by endogenous regulation.

Temperature and photoperiodic regulation serve as adaptations to environmental conditions, as they ensure favorable timing for flowering. During photoperiodic induction, a flowering stimulus is produced in the leaves and transported to the vegetative shoot buds, where it triggers the second phase of initiation: evocation.

Evocation (from Latin, 'to call forth') is the final phase of the transition to flowering, during which the processes necessary for the formation of floral primordia occur within the apex.

Upon the arrival of the floral stimulus, a series of changes occurs in the apical meristems: the content of soluble sugars and invertase activity increase sequentially, the number of Mitochondria and the intensity of Respiration rise; mitosis is activated, cell division synchronizes, the synthesis of RNA and Proteins intensifies, and their qualitative composition changes.

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

It is difficult to pinpoint the exact start of evocation, as Changes in the Cells of the shoot apex may be observed even before or simultaneously with the onset of inductive factors. In the case of photoperiodic influence on leaves, a rapid response of the apical meristem may be triggered by electrical signals resulting from changes in the electrotonic potential difference between the leaf and the apex, or through the generation of action potentials. Such rapid electrical signaling may 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 stage, intensive molecular, histological, and morphogenetic changes occur in the apex. These lead to the initiation and formation of all floral components within the meristem in a strictly defined order, followed by micro- and macrosporogenesis, and ultimately, the formation of male and female gametophytes.

All of this demonstrates that The Essence of evocation lies in switching the genetic program from the development of vegetative buds to the initiation and formation of flowers.

The Nature of the Floral Stimulus

In 1903, the German botanist G. Klebs proposed a theory stating that sexual reproduction in all plant groups depends on nutritional conditions (specifically, The ratio of CARBOHYDRATES to nitrogen compounds - C/N). A shift in these conditions toward those unfavorable for vegetative growth triggers the transition to sexual reproduction. To a certain extent, this holds true.

However, The Study of photoperiodism, The Role of phytochrome, and phytohormones in growth and development processes has confirmed that The regulation of flowering initiation is a complex phenomenon. It was established that the photoperiodic effect is perceived by the leaves, after which the floral stimulus is transmitted to the shoot apex. These facts allowed M. Chailakhyan (1937) to formulate the hormonal theory of plant development. According to this theory, under a favorable photoperiod, a flowering hormone—florigen—is produced in the leaves.

In 1958, Chailakhyan proposed a hypothesis regarding the bicomponent nature of florigen, suggesting that the flowering hormone consists of Gibberellins and anthesins. In this model, gibberellins promote the development and growth of flower stems, while anthesins induce the formation of flowers.

According to this hypothesis, LDP contain primarily anthesins. Under a favorable photoperiod, a significant amount of gibberellins is synthesized in the leaves of LDP. The presence of both components of florigen—gibberellin and anthesin—induces flowering in plants. SDP, which have sufficient gibberellins, synthesize anthesin only under short-day conditions and subsequently flower. In DNP, the content of gibberellins and anthesins increases with age until a critical threshold is reached, which is necessary for the initiation of flowering.

This hypothesis assumes the involvement of other phytohormones in the regulation of flower initiation and formation, and it provides a fairly convincing explanation for The Mechanism of the transition to the generative state. However, the nature of anthesin remains unknown. J. Bernier, J. Kinet, and R. Sachs (1985) proposed their own hypothesis of flowering induction and evocation. In their view, evocation is controlled not by a single special morphogen, but by a complex system of several factors, each of which triggers its own chain of evocational processes. The interaction of these processes leads to the formation of flowers.

Factors involved in the regulation of evocation are produced in various parts of the plant organism (in apices, buds, stems, and even roots), and they are not necessarily identical across all species. Under an unfavorable photoperiod, flowering inhibitors appear in the leaves of plants.

Sex Determination

Sex determination in plants refers to the formation of cells, organs, or individuals of a specific sex, depending on genetic factors localized in Chromosomes (genetic sex determination) and on internal and external environmental conditions (phenotypic sex determination).

Flowers, as organs of sexual reproduction, can be bisexual or unisexual. They form on the same plant (monoecious—cucumber, corn, oak, hazel, birch) or on different plants (dioecious). There are few dioecious plants (hemp, hops, poplar, willow, sea buckthorn, mulberry, aspen). In some dioecious species, sex chromosomes of the XX or XY type have been identified in the cells (some mosses, Elodea, Dreja).

However, genes that determine sex are also localized in autosomes. This is especially true for plants that lack specialized sex chromosomes. Genes that determine sex play an important role by allowing cells to respond differently to internal and external environmental factors. Unlike in animals, the process of sex formation during plant ontogenesis can change significantly depending on conditions, even to the point of one sex transforming into another. The expression of sex in plants depends on environmental factors such as day length, light intensity and spectral composition, temperature, atmospheric composition, and mineral Nutrition.

For example, good nitrogen nutrition leads to more pronounced female sexualization in dioecious plants, while potassium promotes the development of male specimens. High soil and atmospheric humidity favor the differentiation of female flowers and the formation of female plants in dioecious species. The action of Ethylene and carbon monoxide leads to a significant increase in the number of pistillate flowers. Low temperatures favor the appearance of the female sex, while high temperatures have the opposite effect.

Light conditions significantly influence the sexualization of plants, which is partly linked to photoperiodic phenomena. In SDP, a short day or a decrease in light intensity triggers rapid reproductive development and a more pronounced expression of the female sex. For example, corn grown in a greenhouse during the autumn-winter period shows 100% development of female inflorescences in place of male tassels. The spectral composition of light also influences sex differentiation. Short-wavelength light enhances the expression of female traits, while long-wavelength light inhibits this process.

Sex determination in dioecious plants is governed by the phytochrome system and is linked to the quantity and activity of phytohormones. Studies on the effects of phytohormones and growth retardants on sexualization lead to the Conclusion that gibberellins act as Hormones promoting masculinization (male sexualization), while Cytokinins, as well as Auxins and ethylene, function as phytohormones that enhance feminization in plants.

In studying the role of phytohormones in sex determination, it is essential to emphasize Structure/19.html">The Importance of the organs that synthesize them. It has been proven that roots influence plant sexualization as sites of cytokinin synthesis, while leaves act as sites of gibberellin synthesis. This explains why removing roots in monoecious and dioecious plants promotes male sexualization, and why male poplar specimens, when subjected to annual crown pruning (reducing leaf mass), transform into trees with female sexualization (producing large amounts of poplar fluff).

The mechanism by which phytohormones affect sexualization is linked to changes in The activity of the genetic apparatus, as evidenced by experiments studying the influence of nucleic acid and Protein Synthesis Inhibitors on sex expression in plants.

Thus, sex determination in angiosperms is The Emergence of a state of readiness for the formation of floral organs, or even individuals of a specific sex. This process is regulated by genetic and hormonal systems, as well as environmental conditions.

Flower development

The transition from the vegetative to the generative state is accompanied by the formation of a flower. A flower is a shortened, unbranched shoot with limited growth, whose leaves are metamorphosed for the purpose of sexual reproduction, which leads to seed formation.

A flower is a complex organ system that ensures seed reproduction in angiosperms (flowering plants). Within a bisexual flower, micro- and megasporogenesis, micro- and megagametogenesis, pollination, fertilization, embryo development, and fruit and seed formation take place. It is a shortened, modified, and growth-limited shoot bearing the perianth, stamens, and pistils. The appearance of the flower in the course of evolution was an aromorphosis that ensured the widespread dispersal of angiosperms on Earth.

The flower terminates the main or lateral stems. The leafless part of the stem below the flower is called the pedicel. In sessile flowers, the pedicel is shortened or absent. The pedicel transitions into the shortened axis of the flower, its stem portion—the receptacle. The shape of the receptacle can vary; it bears all other parts of the flower: sepals forming the calyx, petals forming the corolla, stamens, and the pistil.

The perianth consists of the calyx and the corolla. The perianth can be double (buttercup, rose hip) or simple (nettle, hemp). Flowers without a perianth are called naked (sedge, willow).

Photosynthesis occurs in green sepals that contain chlorophyll. In some plants (tulips, anemones), they become petaloid and perform the Functions of petals; they may also serve to protect ripening fruits or assist in their dispersal. Petals function to attract pollinators and facilitate pollination. THE ORIGIN OF petals is twofold: in some plants, they are modified stamens (buttercups, poppies, etc.); another group of plants has petals that, like sepals, are of foliar origin (peonies, magnolias).

Stamens are the Organs of the flower in which pollen grains are formed. The collective term for the stamens in a flower is the androecium. It is homologous to the microsporophyll of gymnosperms. A stamen consists of a filament and an anther, which has two halves connected by a connective. Each anther half contains two pollen sacs (microsporangia), where a significant number of microspores are produced, from which pollen (pollen grains) is formed. Pollen grains range from 10-200 μm in size. They are covered by two layers: the outer (exine) and the inner (intine). The exine is tough and, due to the presence of sporopollenin, is highly resistant: it does not dissolve in acids or alkalis, withstands temperatures up to 300° C, and remains preserved for millions of years in geological deposits. The surface of the exine usually features sculptural thickenings and apertures—small, thin-walled areas that allow the pollen grain to change volume and through which the pollen tube emerges during germination. Pollen grains are distinguished by their sticky or non-sticky surfaces. The intine consists mainly of pectin and Cellulose. A pollen grain contains a variety of reserve substances, Vitamins, and growth activators and inhibitors. Mature pollen contains approximately 10-15 % Water.

The pistil is located inside the flower. It is formed from one or more carpels (megasporophylls). The collective term for the carpels is the gynoecium. Each mature pistil consists of a lower, expanded part—the Ovary, a middle cylindrical part—the style, and an upper, slightly expanded part—the stigma. If the ovary is attached to the receptacle only by its base and the rest of its part is free, it is called superior (potato, tomato). If the ovary is embedded in the receptacle and fused with it, it is called inferior (cucumber, pumpkin).

Inside the ovary of the pistil, There is a cavity called a locule. Ovaries are distinguished as unilocular or multilocular. Multilocular ovaries are formed as a result of the fusion of several carpels. The number of locules corresponds to the number of fused carpels. In each locule, ovules form on the ovary walls. Their number can range from one (plum, cherry) to several thousand (poppy, orchids). Anatomical study of an ovule reveals the following components: the funiculus, nucellus, integuments, micropyle, and embryo sac. The ovule originated in the phylogeny of spore-bearing organs. It first appeared in seed ferns. In gymnosperms, ovules are exposed on carpels, while in angiosperms, they are enclosed within the ovary. The pistil emerged during plant evolution as an organ where fertilization occurs and which, as a whole, participates in fruit formation.

Flowers can be unisexual or bisexual. Unisexual flowers bear either only carpels or only stamens, as, for example, in oak, yew, and poplar. In bisexual (hermaphroditic) flowers, male and FEMALE REPRODUCTIVE ORGANS develop within the same flower (snowdrop, pea). Accordingly, there are monoecious and dioecious plants. In monoecious plants, male and female flowers grow separately on the same plant (oak, hazel, beech). In dioecious plants, male and female flowers are found on different plants, meaning each individual plant is either female or male (yew, poplar, holly, sea buckthorn).

As we can see, the process of flower formation is complex and multi-staged. It begins with induction, The first phase of flowering initiation. The transition to flowering is triggered by day length or photoperiod and cold (vernalization). In only a few species, in the absence of inductive conditions, flower formation is not observed at all (complete environmental control of flowering). In many species, inductive conditions accelerate this process (quantitative control) or have no effect at all (autonomous flower formation).

After the first phase of flowering initiation is complete, the second, final phase (evocation) occurs, during which processes take place in the apex that lead to the appearance of the flower's primordia. If the perception of the photoperiod (induction) can be called the foliar phase of flowering initiation, then evocation is the phase realized in the stem apex. During evocation, under the influence of a floral stimulus, the expression of genes responsible for the morphogenetic program of flower formation is induced in the shoot apical meristem. Cell division increases, and the shape of the apex changes.

Initially, the histone content in The Cell nuclei of the apical meristem decreases, then the content of RNA and proteins increases, and the composition of nitrogenous bases in the RNA changes. Immediately following this, DNA Synthesis AND the frequency of mitosis increase sharply. Physiologically and morphologically, groups of initial cells are distinguished, the specific divisions of which determine the next stage of morphogenesis for each part of the flower. The sepals are the first to form, followed by the appearance of petal primordia, and then the stamens and carpels are sequentially initiated.

The primordia of the carpels gradually acquire a horseshoe shape, elongate, and fuse at the edges. This is how the pistil is formed. On the inner side of each carpel, an ovule primordium is initiated. The ovule expands, forming the Components of the seed bud, from which the seed develops after fertilization. The ovule is initiated on the Placenta and usually has a spherical-elongated shape. In the center, the nucellus forms, which in most plants becomes surrounded by integuments (one or two), which do not fuse at the apex, leaving an opening—the micropyle. The part of the ovule from which the funiculus arises is called the chalaza. The funiculus connects the formed ovule to the placenta and contains a vascular bundle.

The nucellus functions as a megasporangium. One of its cells becomes the archesporial cell, which divides by reduction (meiosis) to form four haploid megaspores. Three of them degenerate, and one gives rise to the embryo sac (female gametophyte). As a result of three subsequent successive mitoses, 8 haploid nuclei appear in the embryo sac; after the First Division, two daughter nuclei migrate to opposite poles of the polarized gametophyte and divide twice more there. Three nuclei near each pole organize into cells. One of the cells near the micropyle becomes the egg cell (female gamete), and the other two become synergids. The other three nuclei at the opposite pole form antipodal cells. The remaining two nuclei migrate to the center and fuse, becoming the secondary diploid Nucleus of the central cell, which gives rise to the endosperm. The egg cell and the female gametophyte are then ready for 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 sperm into the space between the egg cell and the central cell. It is likely that the synergids function as haustoria, absorbing nutrients from somatic cells for the needs of the egg cell and the embryo sac. The role of the antipodals is to provide the embryo sac with nutrients.

The stamen filament is formed through apical and subsequently intercalary cell division and growth of the primordium. In the distal part, the initial cells give rise to the Tissues of the anther.

In the anther, cells of the sporogenous tissue divide sequentially by meiosis. As a result, 4 haploid microspores are formed from the mother cell. During microspore formation, the sporogenous mother cells lose most of their cytoplasmic RNA and proteins. From each such microspore, a pollen grain (male gametophyte) is formed. It increases in size and becomes covered by a double wall. The Nucleus of the microspore divides mitotically and asymmetrically, resulting in the appearance of a small generative cell with its own generative nucleus inside the pollen grain alongside the vegetative nucleus. The generative cell divides to form two sperm (male gametes). In many species, this occurs already within the pollen tube. The vegetative cell ensures the viability of the pollen grain and the growth of the pollen tube.

Thus, in the process of pollen grain development via meiosis and subsequent mitosis, a highly reduced male gametophyte is formed. It consists of one (angiosperms) or several (gymnosperms) vegetative cells and one small generative cell with its own generative nucleus, which usually divides into two sperm (male gametes) that participate in fertilization.

The central moment of the reproductive development of microspores is the induction of meiosis. If anthers are excised before or during the mitosis of pollen grains (after meiosis) and cultured in a simple mineral medium, most microspores become non-viable, while in others, instead of normal gametogenesis, cell division begins, leading to the formation of haploid seedlings (pollen embryoids). This is used in microspore culture to obtain haploid plants.

Carotene and carotenoids perform an important physiological function in the reduction division during microsporogenesis. A reduced content of these in the anthers leads to a disruption of microsporogenesis and the formation of abortive pollen.

During mega- and microsporogenesis, callose (β-1,3-glucan) is deposited in the cell walls during early developmental phases, providing temporary isolation for cells transitioning to the generative pathway. As the generative cell of the pollen grain forms, it is also temporarily enveloped by a callose wall, which is essential for its isolation from the vegetative Cell Cytoplasm. Subsequently, the callose disappears, and the generative cell remains surrounded by a cellulose and pectin wall. Thus, temporary cell isolation is a critical condition for the transition to the generative phase of development.

In conclusion, evocation encompasses processes at the molecular and intracellular levels, the completion of which establishes the conditions necessary 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 time interval between the opening of the first and last flowers. 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 difficult for pollinators to locate. Conversely, when there are many flowers or when they produce abundant pollen, their flowering period is brief. The flowering period also depends on pollination; if pollination occurs earlier, the flowers wither sooner. The periodicity of flower opening and closing is determined by the plant's response to heat and light.

Pollination and Fertilization

Fertilization is one of the most important processes in sexual reproduction. It can be divided into several phases: pollination, pollen germination, pollen tube growth through the pistil tissues, and the actual fertilization, i.e., the formation of a zygote.

These processes have been anatomically studied in many angiosperms. The physiological and biochemical aspects of fertilization are currently being intensively researched.

After pollen grains are formed in the pollen sacs, the walls of the anthers dry out and crack, leading to the release of pollen. Next, the pollen must be transported to the stigma of the pistil by insects, wind, or water. The transfer of pollen grains to the pistil stigma is called pollination.

A distinction is made between self-pollination, where pollen lands on the stigma of the same flower, and cross-pollination, where pollen from the flowers of one plant reaches the stigmas of another. The method of pollination depends on the structure and PHYSIOLOGICAL CHARACTERISTICS OF the flower, environmental conditions, and other factors.

Following pollination, the pollen grains swell and germinate, a process accompanied by multiple compatibility checks. The outer layer of the pollen grain wall (exine) contains terpenoid substances that provide protective properties against unfavorable environmental conditions. Its cavities also house proteins that control the compatibility system between the pollen and the stigma, including intraspecific and interspecific incompatibility. The inner part of the wall (intine), In addition to cellulose and pectin substances, contains proteins that ensure mutual recognition between the pollen and the pistil stigma, as well as hydrolytic Enzymes (acid phosphatase, protease, RNA-ase, etc.). These concentrate near the germination pores and are activated upon contact with the stigma tissues. Upon landing on the pistil stigma, the pollen begins to swell. Subsequently, a pollen tube forms, with walls composed of intine. It emerges through the Pores in the exine. The Formation of the pollen tube is stimulated by substances secreted by the stigma in response to compatible pollen.

The recognition mechanism is not yet fully understood, but it is known that upon contact between exine Glycoproteins and an incompatible stigma, callose formation begins immediately, isolating the pollen grain. The germination of incompatible pollen may also be halted within the tissues of the stigma or style. The stigma secretes S-factors (glycoproteins) that interact with components of the incompatible pollen tube Cell wall to inhibit its growth. The glandular tissue of the stigma consists of subepidermal layers and epidermal cells in the form of papillae. It secretes a lipid-based substance (wax derivatives) that serves a protective function, along with Phenolic Compounds (anthocyanins, Flavonoids, cinnamic acids). Phenolic components regulate pollen germination, protect against infection, and play a role in the compatibility control system. Various types of pollen may land on the pistil stigma, but only compatible pollen germinates.

In the case of compatibility, cutinase and other hydrolytic Enzymes of the intine are activated, softening the pistil cell walls and facilitating pollen tube growth. The Initial Stages of pollen tube penetration are determined by its positive hydrotropic response, while the final stages are guided by a positive chemotropic response to substances released by the tissues of the micropyle, synergids, and the egg cell (these include sucrose, Mineral Substances, including Ca²⁺, Amino Acids, and gibberellin).

Within the style of the pistil, the pollen tube moves between cells or, in some species, along the pectin layers of the cell walls. The conducting tissue of the style supplies the pollen tube with nutrients and growth activators. Pollen tubes secrete enzymes that loosen the stigma's cuticle and the pectin substances of the middle lamellae of the cell walls, as well as auxin, which participates in initiating fruit development. However, the increase in auxin content is linked to more than just the processes of pollen germination.

Immediately after pollination, biochemical processes in the ovary change: an influx of phosphorus is observed, along with a redistribution of proteins and carbohydrates. Electrophysiological phenomena play a significant role in regulating these processes. Upon flower pollination, an Action Potential appears in the stigmas and propagates toward the ovaries. The action potential likely serves a signaling function, as the ovary responds to the electrophysiological signal with metabolic changes that initiate the preparation of the FEMALE Reproductive System for fertilization.

After pollination, the process of fertilization occurs — the fusion of the male gamete with the female gamete. The pollen tube quickly passes through the loose tissue of the style and penetrates the embryo sac through the micropyle. The growth of the pollen tube is activated by the vegetative cell located at its tip. Following the vegetative cell, the generative cell enters the pollen tube, where it undergoes mitotic division to form two sperm cells. In some cases, sperm cells may form while still in the anther.

The pollen tube penetrates the embryo sac through the micropyle. At the point of contact between the tube and the embryo sac, the walls of the latter become mucilaginous, allowing the pollen tube to enter. Upon reaching the egg cell, the pollen tube ruptures, releasing the two sperm cells, while the vegetative cell of the tube disintegrates. The sperm cells are released; one 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 in the embryo sac, a process discovered by Professor S.G. Navashin (1898) of St. Vladimir Kiev University (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 numerous adaptations in Flower Morphology and physiology aimed at ensuring cross-pollination.

Multiple pollen tubes may penetrate the embryo sac. However, the sperm cells from these tubes do not participate in fertilization and degenerate. The fusion of sperm cells with the nuclei of the female gametophyte is preceded by Chromatin decondensation in the nuclei. When there are several ovules in the ovary, the process described above occurs in each of them.

Development of Seeds and Fruits

Formation and Maturation of Seeds

After fertilization, the zygote remains in a latent state for a certain period (from a few hours to several days), during which it increases in volume and enhances the synthesis of RNA and new proteins. During this time, intensive division of the secondary (triploid) nucleus occurs, and the endosperm tissue rapidly fills the embryo sac. Initially, A large number of nuclei are formed, followed by the development of cell walls between them, creating cells that continue to divide.

In some plants, the endosperm is completely consumed during embryo formation (legumes, cucurbits), while in others, it persists in the mature seed (cereals). Auxins and cytokinins, supplied by the nucellus, are necessary for endosperm development. After a sufficient amount of nutrients has accumulated in the endosperm, the zygote begins its development, giving rise to the seed embryo.

The embryo passes through several successive developmental phases. For most dicots, these are: proembryo, globular, Heart-shaped, torpedo, and maturation.

Embryo formation begins with the division of the zygote perpendicular to the longitudinal axis of the embryo sac. The upper cell, located closer to the micropyle, forms the suspensor, which pushes the lower cell into the endosperm. In some plant species, the suspensor remains unicellular, while in others, it divides transversely to become multicellular, pushing the lower cell further into the endosperm. The lower cell grows into the proembryo, which has a spherical shape. It divides into four cells by two perpendicular walls, and then each of these divides into two more. Initially, the cells are more or less homogeneous. As division continues, the cells differentiate into the embryonic ROOT, embryonic stem, embryonic leaves (cotyledons), and the embryonic bud. By this time, the ovule transforms into a seed, and its integuments and remnants of the endosperm form the seed coat.

Thus, the zygote grows through repeated mitotic division, transforming into a multicellular embryo consisting of a primary shoot (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 a terminal bud. If the cotyledons serve as storage tissue, they grow at the expense of the endosperm, which may disappear entirely in the process.

In the developing seed, the growth of the embryo and sometimes the endosperm occurs within the embryo sac. In the process, the surrounding nucellus is destroyed, supplying the embryo with nutrients. Sometimes nutrients (starch, less frequently fats) are deposited in the perisperm. The perisperm is similar to the endosperm but is formed from the nucellus and has a diploid set of chromosomes. The perisperm is characteristic of families such as Caryophyllaceae, Nymphaeaceae, Zingiberaceae, Piperaceae, etc., and among gymnosperms, it is found in Gnetales and Taxaceae. In the seeds of some plants, both perisperm and endosperm develop. Subsequently, nutrient supply is ensured by the vascular bundle of the funiculus—the stalk of the ovule, which leaves a scar called the hilum On the surface of the mature seed. The micropyle persists as a small pore in the seed coat, through which oxygen and water enter during future seed germination. The seed coat (testa) is a thin protective layer formed from the integuments.

Following fertilization, the ovule is referred to as a seed, and the ovary as a fruit. As the seed develops, the ovary transforms into a mature fruit, with its walls becoming the pericarp. The remaining parts of the flower wither, die, and fall off. A fully formed seed is the quintessential product of sexual reproduction in angiosperms. This mode of reproduction provides specific advantages to a species, primarily linked to genetic variability.

It has been established for various plants that developing seeds serve as a hub for the synthesis of auxins, gibberellins, and cytokinins, effectively turning them into an acceptor zone that draws nutrients from leaves and shoots. Beyond these hormones, other BIOLOGICALLY ACTIVE SUBSTANCES have been identified in fruits and seeds, including growth inhibitors such as salicylic, ferulic, p-coumaric, and abscisic acids, the levels of which rise as the seed matures.

The process of ripening is characterized by both external morphological changes and profound internal physiological and biochemical transformations within the plant organism. Histochemical and biochemical studies indicate that pollination and fertilization trigger a significant increase in the physiological activity of various plant parts, accelerating the influx of essential plastic and physiologically active substances into the reproductive organs.

For instance, during the maturation of cereals, the water content in their seeds gradually decreases while the dry matter content increases. The ripening process of cereal grains encompasses three distinct phases: milk, dough, and maturity, with corresponding water contents of 50-65%, 25-40%, and 13-15%, respectively. Throughout ripening, plastic substances flow from the stems and leaves, leading to an increase in carbohydrates, proteins, and the total dry weight of the grain. In winter wheat, for example, from the moment of stem elongation, a continuous decline in organic nitrogen content is observed in vegetative parts, with a corresponding flow toward the actively developing ear. As the ear ripens, the absolute content of cellulose, hemicellulose, and Lignin in the leaves and stems decreases, as these are utilized alongside photoassimilates to synthesize the starch and hemicellulose of the grain.

Parallel to the redistribution and restructuring of organic substances between different parts of the plant, morphological changes also occur: the green coloration gradually shifts, first taking on a grayish hue, then turning yellow, and finally transitioning to a golden-yellow.

In the seeds of monocots and dicots, the absolute and relative content of proteins, Polysaccharides, and storage fats increases continuously within the endosperm and cotyledons; the loss of water leads to a decrease in the hydrophilicity of colloids, and the absolute weight of mature grains increases by 2.5–3 times compared to the dough stage.

Thus, synthetic processes and gradual dehydration prevail during seed maturation. In the final stages of ripening, the seed's water content drops to 10-15%, while the concentration of growth inhibitors rises. This is accompanied by a decline in metabolic activity, marking the transition of the seed into a dormant state.

Mature seeds generally cannot germinate immediately. Over a period of one to two months—and in some plants, up to 5-6 months—the seeds undergo physiological (post-harvest) ripening. This occurs during the state of physiological dormancy.

It should be noted that, occasionally, seeds may germinate while still on the parent plant. This phenomenon is known as vivipary (from the Latin viviparos — live-bearing); it is characteristic of mangrove trees (Rhizophora, Avicennia).

In summary, the embryo of the seed is formed from the fertilized diploid egg cell, while the nutritive tissue (endosperm) develops from the secondary triploid cell; the integuments of the ovule transform into the seed coat, and the ovary wall expands to form the pericarp.

Formation and Ripening of Fruits

A FRUIT is an organ of angiosperm plants formed after fertilization from the pistil and, in most cases, other parts of the flower (receptacle, perianth, pedicel, floral bracts) as a result of their expansion and modification; it serves to protect and disperse seeds. It consists of an outer part—the pericarp—and the seed(s) developing within. Sometimes, a fruit is formed without fertilization (parthenocarpy). The pericarp consists of 3 layers: the outer exocarp, the middle mesocarp, and the inner endocarp. The exocarp is usually thin and may sometimes form various appendages (e.g., in maples). Typically—especially in fleshy fruits—the mesocarp is the most developed, often being succulent and rich in sugars (in cherries, peaches) or oils (in olives, avocados). The endocarp is thin; it often modifies into a stony tissue known as the pit or stone (in cherries, apricots). Depending on the structure and consistency of the pericarp, fruits are classified as fleshy or dry.

In simple fleshy fruits, the entire pericarp or a portion of it is succulent and fleshy, containing either one seed (drupe) or many (berry, pome, pepo). Aggregate fleshy fruits are formed either from several separate flowers (multiple fruit in mulberries) or from several pistils of a single flower (aggregate drupe in raspberries). Dry fruits have a dry, leathery, or woody pericarp; these include dehiscent fruits (legume, silique, capsule), indehiscent fruits (nut, nutlet, caryopsis, achene, samara), lomentaceous fruits, which break transversely into individual segments containing seeds (in wild radish, camelthorn), and schizocarpic fruits, which split longitudinally into single-seeded elements (in mallows, maples, umbellifers).

Thus, the fruit is formed from the ovary of the pistil after fertilization and is a hallmark of flowering plants. It is precisely because of the presence of fruit that flowering plants are called angiosperms. The tissues of the fruit are determined during flower formation and originate from the ovary, though other floral parts often participate in this process.

Fruit development can be divided into 4 phases:

1) formation of the ovary before pollination;

2) growth via cell division immediately after pollination and fertilization;

3) growth via cell expansion;

4) ripening.

A sharp increase in cell division within the ovary is observed after pollination. This is followed by the cell expansion phase. The pattern of growth is closely dependent on the fruit type. A sigmoidal growth curve is characteristic of tomatoes, apples, pears, avocados, and others. In this case, cells divide almost exclusively during the formation of the flower bud. After pollination, cell division continues for a short time. Further size increase occurs through expansion.

In drupe-type fruits (apricot, plum, cherry, etc.), growth is characterized by a double sigmoidal curve. The first rapid growth period is driven by the expansion of the ovary, nucellus, and seed integuments, while the embryo and endosperm remain largely undeveloped. When embryo development begins In the second phase, the ovary grows very slowly. At the same time, the hardening (sclerification) of the pit occurs. Once the embryo reaches full maturity, a second accelerated phase of fruit development begins, continuing until full ripening. Thus, there is a clear interrelation between seed development and fruit growth. The fertilized egg, endosperm, and developing seeds exert significant control over fruit growth. For instance, underdeveloped seeds, due to factors like insect pests, often cause premature fruit drop. Uneven seed development leads to fruit deformation.

The regulation of fruit development is linked to pollination. Even the mere placement of foreign pollen on the stigma, without fertilization, induces the expansion of the ovary walls without seed development. This effect can be replicated using dead pollen or extracts derived from it. The active factors are substances of a hormonal nature, specifically auxins and gibberellin-like compounds.

Ovules and seeds also regulate fruit growth through hormones. Developing seeds are sites of synthesis for auxins, gibberellins, and cytokinins. These substances are what make the immature fruit a center of nutrient attraction. Consequently, fruit development is associated with a noticeable cessation of vegetative growth, and in annuals, with the senescence of the entire plant.

In addition to the hormones mentioned above, other biologically active substances that regulate development have been identified in fruits and seeds. These include various cytokinins, growth inhibitors such as salicylic, p-coumaric, and ferulic acids, and the seed germination inhibitor, Abscisic acid.

The level and ratio of hormones change significantly from the time of fertilization until fruit ripening. For example, immediately after fertilization, a maximum in gibberellin activity is observed. Shortly thereafter, auxins reach their peak activity. The increase in auxin levels coincides with the transition of the endosperm to cellular division, and the attainment of the maximum coincides with active embryo growth. A subsequent peak in auxins is associated with the intensification of cell division at the periphery of the endosperm in already formed seeds.

Cytokinin levels are highest in very young fruits and seeds. Fruit ripening is accompanied by The production of ethylene, which dramatically accelerates the final phase of their development.

The size of the meristems, which determines the final size of the fruit, is highly dependent on nutrient supply and, more importantly, on the overall metabolism of the plant.

Significant metabolic changes occur during the development of fruits and seeds. For instance, fruit respiration, which is very intense during the early Stages of development, decreases as the fruit grows and increases slightly in succulent fruits during the ripening period. Respiration consumes sugars and organic acids, the content of which changes significantly throughout fruit development. Assimilates are primarily supplied to the fruit from photosynthesizing leaves, although some may be synthesized within the Chloroplasts of unripe fruits. Among organic acids, tricarboxylic acids (such as malic, citric, tartaric, etc.) predominate in fruits. Aromatic acids also accumulate in significant quantities. Their content and ratios shift during growth. The accumulation of Organic compounds depends on both climatic factors and the plant species.

Ripening processes begin once the fruit has completed its growth. In the initial stages of ripening, anabolic processes predominate. Subsequently, the ripening of succulent fruits is characterized by a decrease in the acid-to-sugar ratio, the formation of Aromatic Compounds, the degradation of chlorophyll and Tannins, the accumulation of anthocyanins and other vacuolar pigments, and a reduction in tissue firmness and elasticity due to the Hydrolysis of cell wall Pectins. During this period, there is a sharp increase in pericarp tissue respiration (the Climacteric rise), accompanied by enhanced ethylene synthesis. Exposure to exogenous ethylene accelerates ripening, confirming its role as a phytohormone that stimulates fruit maturation.

Interestingly, the dynamics of sugar accumulation during ripening vary among different plant species. For example, in tomatoes, sugar content increases due to Monosaccharides, while sucrose levels remain nearly constant. In melons, conversely, sucrose content rises, while the amount of monosaccharides and starch remains relatively stable. The fruit becomes soft and fragrant. Fruit firmness also depends on the presence of pectin and tannins. In green tomatoes, the insoluble pectin fraction accounts for 2/3 of the total pectin content; in ripe fruits, this drops to 4%. This indicates that pectin hydrolysis products are utilized in metabolic pathways during ripening. As the pericarp ripens, the activity of many enzymes changes, and shifts in pigment composition are observed. For instance, lycopene appears in tomatoes, while carotene appears in apricots. During ripening, most fruits develop a characteristic aroma due to the presence of esters, the synthesis of which requires significant amounts of oxygen. Vitamin C also accumulates, a process that likewise requires oxygen.

The completion of ripening is accompanied by the formation of an abscission layer in the pedicel, leading to fruit drop. The formation of this layer is induced by a decrease in auxin levels and high levels of ethylene. After the ripening period, the fruit enters the senescence phase.

In summary, angiosperms are the most highly organized vascular plants. They are distinguished from other plants by the presence of a flower adapted for cross-pollination, a pistil that protects the ovules, double fertilization, and the formation of fruit containing seeds. In flowering plants, there is a further reduction of the gametophytes. As a result of double fertilization, they produce endosperm, which provides nutrition for the embryo during its development, thereby increasing the viability of the new plant.