PLANT PHYSIOLOGY WITH FUNDAMENTALS OF BIOCHEMISTRY - Prytuliak R. M. - 2016

Lecture Notes

TOPIC No. 7. PLANT GROWTH AND DEVELOPMENT

Outline

1. Concepts of ontogeny, growth, and development.

2. Types of plant organ growth.

3. Phytohormones.

4. Seed germination.

5. Plant dormancy and its types.

6. Breaking dormancy.

7. Plant Movements.

8. Photoperiodism.

9. Correlations.

10. Theory of cyclic Aging and rejuvenation in plants.

11. Physiology of flowering.

12. Role of internal and external factors in flowering.

13. Flowering and Fertilization.

14. Physiology of seed and fruit formation.

15. Metabolic transformations during seed maturation.

1. Concepts of ontogeny, growth, and development.

The process of individual development in every plant is accompanied by a series of systematic changes characteristic of a given biological species. The totality of these physiological, biochemical, and morphological changes, driven by genetic factors and occurring within the plant Organism from its inception—whether from a zygote, spore, or specialized vegetative primordium—until its natural death under standard environmental conditions, is defined as The life cycle, or ontogeny. Based on The Nature of their life cycle, plants are classified as:

- monocarpic (bearing fruit only once). Monocarpic plants include annuals, biennials, and perennials. Their lifespan depends on growing conditions: the less favorable the conditions, the longer the plants delay flowering and death.

- polycarpic (bearing fruit multiple times). In polycarpic plants, two life cycles are distinguished: the small cycle (the Life Cycle of an individual SHOOT) and the large cycle (the life cycle of the entire plant). The lifespan of polycarpic plants varies widely and can reach 20-30 years or more.

The fundamental processes identified in ontogeny are growth, development, aging, and rejuvenation.

Growth is the irreversible increase in the linear dimensions, surface area, volume, and mass of a plant organism, as well as the de novo formation of cytoplasmic structures occurring within Cells.

The most prominent manifestation of active plant life is growth. D.O. Sabinin (1963) was the first to define growth as a process based on The formation of new structural elements, leading to an increase in the size or mass of a plant; however, defining growth merely as an "irreversible increase in the weight or size of an organ" is insufficient. The most reliable criterion for growth is the formation of new cells and the increase in their volume.

It is customary to distinguish three stages of growth: embryonic, elongation, and internal differentiation. Of these, only the elongation stage is truly associated with an increase in the size and weight of the organism. The other two stages can occur without changes in Cell weight or size.

The growth pattern of any organism, organ, or cell population follows an S-shaped growth curve, which consists of:

- lag phase (initial phase of latent growth). During this initial phase, mechanisms related to the synthesis of Nucleic Acids (DNA and RNA), as well as The Biosynthesis of Enzymes and phytohormones, are active.

- log phase (phase of intensive growth). During this phase, active cell growth occurs through elongation, and new Tissues and Organs emerge, leading to an increase in their size.

- phase of decelerated growth. In this third phase, growth concludes, and inhibitory substances accumulate. The entire plant or its individual parts may enter a state of dormancy.

- stationary phase.

Development refers to the qualitative physiological, biochemical, and morphological changes occurring during the formation of new structural elements, which enable the plant to progress through specific stages of its life cycle, known as ontogeny.

2. Types of plant organ growth.

Unlike other organisms, plants have The ability to form new tissues and organs throughout their ontogeny. These formative processes are localized in specific PARTS OF THE plant organism:

· tips of stems and roots (apical Meristems).

· their cylindrical zone (lateral meristems: procambium, pericycle, cambium, and phellogen).

· the base of young internodes and leaves (intercalary meristems).

The main parts of the shoot (stem, leaves, flowers, buds, etc.) originate in the apical meristem, which is referred to as the shoot apex or growing point.

In perennial plants, stem and ROOT growth is indeterminate. In contrast, leaf growth is always determinate. A leaf passes through four phases during its development:

· primordium formation

· petiole formation

· leaf blade initiation

· leaf blade expansion.

Leaf primordia are formed in the shoot apex. Through intensive division of the apical Cells of the primordium, a finger-like projection is created, consisting of cells that will form the midrib and petiole. Along the edges of the midrib, meristematic tissue initiates the growth of the leaf blade and the formation of its tissues (epidermis, parenchyma). Light conditions significantly influence leaf growth.

The primary Tissues of the stem are formed through The activity of the shoot apex's ground meristem and procambium. Cell elongation in the growth zone leads to shoot lengthening. Growth is stimulated by Gibberellins, which are transported from the roots and leaves. In dicotyledonous plants, the stem thickens due to the activity of the cambium and phellogen.

Unlike the stem, the root apex does not form lateral organs. The root meristem forms the root cap and root tissues. In addition to actively dividing cells, the root apical meristem contains a group of cells capable of replenishing the pool of initials when some are lost or damaged.

3. Phytohormones

Among natural growth regulators, phytohormones are the most well-known. These substances are synthesized within the plant, participate in the REGULATION OF METABOLISM, and largely determine The Nature and rate of morphogenetic processes.

Until recently, five types of phytohormones were universally recognized:

· Auxins.

· Cytokinins.

· Gibberellins.

· Abscisic acid.

· Ethylene.

Phytohormone synthesis occurs in specific tissues and organs.

- Auxins are the first phytohormones to be studied (identified in 1934 by F. Kögl as IAA) and are indole-based compounds. Tryptophan and shikimic acid serve as precursors for IAA in plants. In higher plants, auxin synthesis is most intense in young leaves, buds, active cambium, pollen, and developing seeds. Trace amounts of auxin have also been detected in root tip meristems.

Physiological effects:

· Stimulation of cell elongation during The Development of cambium, vascular bundles, and roots.

· Structure/182.html">Practical Application: ability to stimulate adventitious root formation in cuttings.

· Influences the differentiation of vascular tissue in growing shoots.

· Responsible for apical dominance, where the terminal bud inhibits the growth of lateral buds.

· Delays the opening of lateral buds and the abscission of leaves and fruits.

· Used to regulate leaf and fruit drop, as well as flower abscission.

· Affects cytoplasmic viscosity, enhances Water uptake, and induces cytoplasmic streaming.

· Involved in growth movements, such as Tropisms and nasties.

- Cytokinins are substances that stimulate Cell Division in plant callus tissue cultures. Zeatin was isolated in 1963. Root tips, meristems, xylem sap, germinating seeds, and ripening fruits are rich in cytokinins. Adenine serves as their precursor.

Physiological effects:

· Stimulation of cell division and differentiation.

· Delay of senescence processes.

· Induction of cell division in the presence of auxins.

· Inhibition of aging.

· Stimulation of overall metabolic activity.

The growth-promoting effects of cytokinins are based on the acceleration of cell division, which is linked to enhanced DNA Synthesis.

- Gibberellins were discovered in 1926 by E. Kurosawa, who isolated them from the fungus Gibberella fujikuroi. Gibberellins are most abundant in young, actively growing organs. They are synthesized primarily in young leaves, buds, seeds, and root tips. Mevalonic acid is the precursor for gibberellins.

Physiological effects:

· Stimulate vegetative growth.

· Activate cell elongation and division processes.

· Accelerate seed germination.

· Induce flowering in certain plant groups.

· Promote the formation of parthenocarpic fruits.

· Shift sex expression towards the male phenotype.

· Enhance the activity of numerous enzymes.

By translocating from the tissues and organs where they are synthesized, phytohormones influence physiological processes in other tissues and organs.

- Abscisic acid (ABA) is a natural growth inhibitor that accelerates leaf petiole abscission, inhibits the growth of coleoptile segments, and delays seed germination. ABA exerts a potent inhibitory effect, inducing leaf fall and the transition of woody plants into a dormant state. High concentrations of ABA are found in senescent and mature leaves, buds, and dormant seeds. ABA inhibits cell division and elongation; mevalonic acid serves as its precursor.

- Ethylene was discovered by D.N. Neljubow in 1901. It accelerates fruit ripening and promotes the senescence of all plant parts; it is considered a hormone of aging or maturation. In higher plants, it is synthesized from The amino acid Methionine. It is present in various plant organs (fruits, flowers, leaves, stems, roots) and interacts antagonistically with auxin. It induces growth retardation, accelerates senescence, ripening, and fruit abscission, and promotes the shedding of flowers, Ovaries, and leaves (by forming an abscission layer in leaf petioles and pedicels).

Retardants. An important group of growth regulators are retardants—synthetic substances that slow down vegetative growth, primarily the elongation of stems and shoots. These include ammonium and phosphonium salts, ancymidol, paclobutrazol, chlormequat chloride, and others. Chlormequat chloride is widely used to prevent lodging in wheat grown under high-input agricultural conditions with sufficient moisture. Treating plants with these substances during the formation of lower internodes promotes significant shortening of the straw, which facilitates mechanical harvesting and reduces grain loss. Favorable results are also obtained when treating fruit crops with this preparation, as it accelerates the onset of fruiting in apple and pear trees and increases yields.

4. Seed germination.

Seed germination is a process that primarily requires moisture, as the Hydration of Polysaccharides and other endosperm substances is impossible without it. The pattern of Water uptake and distribution within a seed is determined by its anatomical and genotypic characteristics.

Seed germination represents the resumption of embryo growth resulting from active water absorption and consists of three phases:

1) imbibition, during which water uptake is regulated by the composition of stored nutrients;

2) development (lag phase), during which enzymatic transformation and the initialization of meristematic activity occur;

3) growth, which begins with the rupture of the seed coat and The Emergence of the primary root.

During imbibition, the water content in the seed increases rapidly at first, then more

slowly. The initial reaction to water influx through the seed coat is the activation of metabolism. Almost all organs and tissues of the seed contain, in addition to Proteins, fats, and polysaccharides, low-molecular-weight compounds (sugars, Amino Acids) that are utilized for Respiration During the first minutes of imbibition, generating ATP. After 10–20 hours, the respiration rate decreases slightly due to the depletion of the pool of low-molecular-weight compounds. A subsequent increase in respiration most often coincides with the onset of the Hydrolysis of Proteins, polysaccharides, or fats.

The seed germination phase continues until the plant is established as an independent, photosynthetic organism. Most seeds germinate in the dark. During this process, the seedling attempts to penetrate the soil layer through stem elongation. Once it reaches the surface, Light triggers the growth of leaves and stems. Light not only provides energy for Photosynthesis but also acts as a morphogenetic factor perceived by the phytohormone system.

5. Plant dormancy and its types.

Not all seeds imbibe and germinate under conditions of sufficient moisture. The cause of this may be a state of deep dormancy, which is conditioned by:

· morphological immaturity of the embryo (incomplete development),

· physiological immaturity,

· the presence of inhibitors that suppress enzymatic reactions,

· impermeability of the seed coat to water, gases, etc.

The ability of PLANTS AND THEIR organs to remain in a state of dormancy is a valuable biological trait that helps them withstand conditions unfavorable for survival. A characteristic sign of dormancy is the absence of visible growth in buds, seeds, tubers, and bulbs, as well as a low metabolic rate, which results from the specific Properties of the biocolloids' state. Managing the duration of plant dormancy is of great importance in agricultural practice, and it can be broken using various Methods.

Morphologically and physiologically immature embryos can complete maturation spontaneously through the transformation of internal stimuli. Substances that inhibit germination (derivatives of benzoic and cinnamic acids, coumarin, abscisic acid) are contained in the embryo, endosperm, seed coat, or the pulp and Skin of fruits, and can be broken down or leached out by water. The impermeability of the seed coat to water and gases is overcome by mechanical damage or the application of chemical agents. Germination is stimulated by gibberellins, ethylene chlorohydrin, thiourea, and others.

6. Breaking dormancy.

Exogenous Factors affecting the state of dormancy include light, humidity, Temperature, and atmospheric gas composition. Among these, temperature is of particular importance. For each plant species, germination is possible only under specific temperature conditions. In many cases, the cessation of dormancy is triggered by exposure to cold.

Stratification is the process of breaking dormancy by storing seeds at low temperatures (from 0 to +5 °C) in a mixture of soil with sand or peat for a certain period of time.

Scarification is the artificial mechanical damage to seed coats that prevent the intake of water and air, thereby hindering the seed's interaction with the surrounding environment.

The Effect of cold on breaking dormancy manifests in various ways. It can accelerate embryo maturation, eliminate seed coat impermeability, reduce the content of inhibitors, and more.

Sometimes, the plant's requirement for cold can be replaced by exposure to high temperatures (for example, in soybeans, cotton, and millet).

In some plants, germination is stimulated by fluctuating temperatures. For many plants, the interaction between temperature and light is essential. Often, seed dormancy is broken by low temperatures but maintained at high ones, while at moderate temperatures, it is regulated by light. Frequently, cold can substitute for red light.

7. Plant movements.

Along with metabolism, growth, and development, movement is a characteristic property of plants. Plants lack specific organs for locomotion. However, a range of specific reactions causes changes in THE POSITION OF plant organs in space and microstructures within The Cell. Plant movements are diverse and occur both in response to stimuli and without them.

The movement of organs in plants anchored in a substrate is usually observable with the naked eye and is primarily driven by growth processes and changes in turgor pressure.

Plants perform movements by accumulating or releasing water. If the water uptake is reversible, the movements are considered turgor-driven. In the case of irreversible water uptake, we are dealing with growth movements.

Growth movements are associated with the elongation of organs due to an increase in cell volume resulting from the influx of water into the vacuole and the simultaneous growth of The Cell wall.

Tropisms are Changes in the position of organs in plants anchored in the soil, caused by the unilateral action of an external stimulus. Depending on the Nature of the stimulus (light, gravity, Touch, chemical substances, water, electric current, heat, injury), we distinguish between photo-, geo-, thigmo-, osmo-, hydro-, electro-, thermo-, and traumotropisms. Based on the type of response, tropisms can be positive or negative. As a rule, they are based on growth processes.

- Phototropisms. Among the external factors affecting plant movement, light (blue-green and blue-violet) is of particular importance. In positive phototropism, the bending of an organ toward the light source occurs primarily due to the Inhibition of Growth on the illuminated side of the organ and enhanced growth on its shaded side. The site of light stimulus perception is located closer to the organ's apex than the bending zone, and the stimulus is conducted between these areas.

- Geotropisms. Along with light, gravity is the main factor determining the position of plants in space. Geotropism is the ability of plants to perceive and respond to Earth's gravity. A clear example of a geotropic reaction is the growth direction of trees on mountain slopes: the orientation of the trees is independent of the slope's steepness and is directed away from the Earth's center. Negatively geotropic organs or their parts grow away from the center of the Earth. A geotropic bend, like a phototropic one, is a movement caused by growth. In organs or parts that have completed growth, Secondary Growth induced by geotropism is possible (for example, in lodged cereals and coniferous trees).

- Chemotropism. Chemical substances can also induce directional growth movements. Roots exhibit positive chemotropism primarily toward phosphates, carbon dioxide, and oxygen. This movement helps them approach nutrient-rich and well-aerated soil zones.

- Hydrotropism. This can be viewed as a specific form of chemotropism. The perception of the stimulus that causes hydrotropism occurs in the root tip. The Mechanism of stimulus perception is not sufficiently studied. It is possible that the difference in turgor pressure within the cells is of primary importance, while growth substances participate in subsequent processes.

- Thigmotropism is a directional response triggered by touch; it is observed as positive in tendrils and coleoptiles, and negative in roots.

Other tropisms can be triggered by thermal, electrical, and traumatic stimuli.

Nastic Movements (nasties). Unlike tropisms, in nasties, there is no correlation between the direction of the stimulus and the response. Nasties manifest through changes in turgor pressure. Their naming, like that of tropisms, depends on the stimulus.

Photonasty in flowers. Some flowers (Cactaceae family, Asteraceae, etc.) open upon illumination, while others (campions) close. Each new opening and closing involves growth in length of the upper side of the perianth leaflets, respectively. The movement of leaves that have completed growth is always based on fluctuations in turgor pressure (clover, wood sorrel, mimosa).

Temperature fluctuations can also trigger nastic movements in the perianth leaves of many plants (such as crocuses, tulips, and snowdrops).

Closely related to photonasty and thermonasty are changes in organ positioning based on the time of day, known as nyctinasty, or "Sleep movements."

Seismonastic reactions are the fastest and most conspicuous movements in the plant kingdom, triggered by mechanical stimuli such as shaking or impact. The most well-known example of seismonastic positioning is found in the leaves of the mimosa plant. These movements are driven by underlying cellular physiological processes.

Other Examples of seismonasty include the movement of stamens in barberry.

8. Photoperiodism.

Flowering, fruit and seed formation, the onset of bud dormancy, leaf fall, and seed germination are all closely linked to seasonal changes in day length and temperature. The most profound changes occur during the flowering phase, when the plant transitions from vegetative growth—forming leaves and lateral buds—to reproductive development. The relative duration of light and dark periods is the decisive factor here. These shifts in GROWTH AND DEVELOPMENT patterns, dictated by day and night length, are known as photoperiodic reactions or photoperiodism. Different species respond to changes in day length in various ways. Based on this, plants are classified into three main groups:

· long-day plants.

· short-day plants.

· day-neutral plants.

Flowering in long-day plants is accelerated in northern latitudes, whereas in conditions of short days and long nights (as in southern regions), it may fail to occur entirely. This group includes wheat, rye, barley, flax, and others.

Short-day plants exhibit the opposite response, showing accelerated development in southern regions. Examples include corn, soybeans, millet, hemp, tomatoes, and others.

Plants with a neutral response show no photoperiodic sensitivity to changes in day length (e.g., buckwheat, alfalfa, broad beans, etc.).

The organs that perceive photoperiodic signals are primarily the leaves. Leaves that have completed their growth exhibit maximum photoperiodic sensitivity. Once received, the signal is transmitted throughout the plant as a chemical stimulus to the target organs (for instance, to apical meristems for Flower Formation or to stolons for tuber development).

9. Correlations.

The growth and development of tissues, individual organs, and plant parts are governed by correlative relationships. Correlations represent the mutual influence of different parts on one another, often over significant distances. For example, root growth depends on the supply of assimilates from above-ground organs. Conversely, shoot development is impossible without the minerals and water absorbed by the roots from the soil. Furthermore, the shoot influences the root by supplying auxins, while the root exerts its influence on the shoot via cytokinins and gibberellins. This represents the simplest type of correlation in a plant organism.

When the integrity of the organism is compromised, these correlations are disrupted. This can lead to changes in the growth ratios of individual organs. For example, removing the cotyledons in seedlings leads to impaired growth of primary leaves and lateral roots.

At The Heart of these correlative relationships lies The phenomenon of irritability.

10. Theory of Cyclic Aging and Rejuvenation of Plants.

According to the core tenets of the theory of cyclic aging and rejuvenation in plants (N.P. Krenke, 1940), an individual inevitably ages and dies during its ontogeny. An organism's lifespan is determined by evolutionary factors, which can be modified by environmental conditions. Aging occurs continuously but unevenly—both in the organism as a whole and in its individual parts. During the overall aging process of an individual, the formation of new plant parts necessarily leads to uneven, cyclic rejuvenation. The resulting daughter cells are temporarily rejuvenated. The mechanism and intensity of aging processes differ significantly between cells in a dormant state and those that are actively dividing. Cells in meristematic tissues age most slowly while in a dormant state, whereas actively dividing cells age intensely.

There are two concepts of age: the chronological age of a plant part (the time elapsed from its inception to the present moment) and the total age of that same part (the sum of its chronological age and the age of the plant at the time of that part's inception).

Currently, aging is viewed as the progressive decline of an organism's vital processes with age, ultimately leading to natural senescence. Aging manifests as a progressive disruption of Protein Biosynthesis, a weakening of regulatory systems, the accumulation of inactive structures, and the cessation of physiological Functions. Rejuvenation, conversely, involves the intensification of vital processes linked to increased nucleic acid and Protein Synthesis, activation of cell division and growth, the emergence and accumulation of embryonic tissues, and a general activation of physiological functions.

For a long time, it was believed that the development cycle of a plant organism was determined solely by internal factors, and the transition from intensive growth to fruiting was explained by age. G. Klebs demonstrated the possibility of altering a plant's developmental trajectory through environmental factors (water, mineral Nutrition, etc.). He considered CARBOHYDRATES to be the primary substance determining a plant's transition from vegetative growth to reproduction. In other words, if a plant has a higher ratio of carbohydrates to nitrogenous and mineral compounds, it will transition to fruiting; if the ratio is reversed, vegetative growth will be maintained.

11. Physiology of Flowering.

The process of flowering is the first step toward Sexual reproduction in plants. In 1937, M.Kh. Chailakhyan named the hormone-like substance generated by leaves that signals flowering as "florigen." While various compounds with florigen-like activity are known (gibberellins, auxins, cytokinins, ethylene), they do not possess the universality of florigen, as they are only capable of inducing flowering in specific plant species. Therefore, proponents of the florigen theory argue that these substances cannot be the true flowering Hormones. M.Kh. Chailakhyan himself proposed a modified florigen theory, suggesting that the flowering hormone consists of two complementary substances: one belonging to the gibberellins and the other to the anthesins.

The transition to flowering is marked by several macroscopic signs. For instance, at the onset of reproductive development, the growth pattern of the stem apex changes significantly, and the growth of young internodes accelerates. Many species also accelerate leaf initiation when transitioning to flowering. The final leaves formed before a flower or inflorescence typically have smaller dimensions and a simpler structure than all those preceding them.

The ultrastructure of cellular components also undergoes changes. The earliest of these changes include the following:

1) an increase in the number of Mitochondria and Plastids;

2) the reduction of vacuoles. A significant portion of these changes shows remarkable similarity across various plant species and is characteristic of cells with high metabolic activity.

12. The Role of internal and external factors in flowering

Photosynthesis, the redistribution of assimilates, and the supply of carbohydrates are key factors in the transition to flowering. This transition can be accelerated or delayed by modifying both external and internal factors. Water stress, for instance, promotes earlier flowering in some species.

In woody fruit crops, early flowering can be stimulated by girdling the trunk and branches. This Procedure interrupts the flow of assimilates to The Root System and redirects them toward the fruit-bearing branches. Removing young leaves or competing apical buds can also accelerate flowering in certain plant species.

Chemical substances that alter the activity of acceptors in competing tissues are also utilized. Auxin is capable of both accelerating and inhibiting the initiation of flowering. The effect of auxin largely depends on its concentration, lighting conditions, temperature, Treatment timing, and other factors. It is well-established that auxins play a primary role in the correlations between apical and lateral buds. Therefore, the inhibition of flowering during auxin treatment may be due to the Activation of a dormant lateral meristem.

Cytokinins have an effect on flower formation that is opposite to that of auxin.

Gibberellins are also involved in regulating the transition of a plant from a vegetative to a reproductive state. Gibberellin metabolism is under photoperiodic control.

13. Flowering and fertilization

During flower formation, genes responsible for floral morphogenesis are activated in the apical meristem. The Ovary, formed through complex processes, contains an egg cell (female gamete) with two synergids at one pole, three nuclei (antipodals) at the opposite pole, and a central cell in the middle. In the anthers, four microspores are produced through successive Meiosis and mitosis. If such a microspore lands on a stigma, it germinates. Its nucleus divides mitotically and asymmetrically to form a vegetative and a generative cell. The function of the vegetative cell is to sustain the viability of the pollen grain and the growth of the pollen tube. The generative cell divides to produce two male Gametes (two sperm cells).

The microspore (pollen grain) contains reserve substances, Vitamins, and growth activators and inhibitors.

Pollen germination on the stigma is facilitated by secretions released by its cells. Only compatible pollen grains germinate; others perish. As the pollen tube grows, it gradually moves through the style toward the micropyle. The sperm cells located at the tip of the pollen tube are released upon reaching the embryo sac. One of them fuses with the egg cell to form a zygote, while the second fuses with the central cell to form the endosperm. This process, discovered at the end of the last century by S.G. Navashin, is known as double fertilization. Several pollen tubes from germinated pollen grains may reach the embryo sac, but only the sperm cells from one of them participate in fertilization, while the others degenerate.

14. Physiology of Seed and Fruit development

Pollination, fertilization, and the Formation of the zygote and primary endosperm are similar in most plants. After fertilization of the ovule, a seed is formed, consisting of an embryo, stored nutrients, and a seed coat. The embryo develops from the fertilized egg cell (zygote). The product of the fusion of the central cell with a sperm cell (the triploid nucleus) gives rise to the endosperm.

In cereals, the triploid endosperm nucleus divides rapidly, and within 2-3 days after fertilization, it contains up to 5,000 free nuclei. Subsequently, Cell wall formation begins, and mitochondria, proplastids, The Endoplasmic reticulum, and the Golgi apparatus appear. Meristematic activity is concentrated in the outer cells of the endosperm, which eventually form the aleurone layer—cells filled with protein and lipid droplets.

Embryo development begins after the first four nuclear divisions in the endosperm. Surrounded by the endosperm, the embryo differentiates through successive phases into the scutellum and the radicle with a shoot. For embryo development and seed formation, a supply of organic substances from the leaves and other plant organs is necessary. This is ensured by the fact that during development, the ovule, ripening seed, and developing fruit become dominant centers that produce large quantities of phytohormones. There is a direct correlation between The amount of auxin and the intensity of growth processes in the ovary. In the Initial Stages of embryo development, phytohormones are supplied by other tissues, primarily the endosperm. Later, their synthesis occurs within the embryo itself. During seed formation, auxins are released from the seed into adjacent tissues, activating the growth of the pericarp.

Enhanced growth of the pericarp can be induced artificially by bypassing fertilization. To achieve this, the ovule must be treated with physiologically active substances. The resulting fruits are seedless (parthenocarpic) and are significantly larger than normal ones.

A mature embryo contains high levels of Lipids and protein. Therefore, a mature seed can be viewed as a highly dehydrated storage organ surrounded by a dense and usually impermeable coat. Differences in seed structure are, in most cases, related to the varying ratios of endosperm to embryo size, and in some cases, to the varying thickness of the outer integuments. In cereal grains, the endosperm accounts for 70-80% of the seed's dry mass. This is where the bulk of starch and protein reserves are concentrated. Cereal grains have only one cotyledon, which is why cereals are classified as monocotyledonous plants. In legumes, unlike cereals, the endosperm is short-lived and, during seed maturation, is reduced to a thin layer surrounding the embryo. The site of nutrient storage in legumes is the two cotyledons, which are simultaneously part of the embryo of dicotyledonous plants. The chemical composition of legume seeds, which include some oilseeds (soybean, peanut), peas, and beans, differs significantly from that of cereals. Peanut seeds, for example, can contain up to 50% fat and 30% protein.

Seeds are contained within fruits. Fruit tissues are formed from the ovary. Often, other parts of the flower, such as the receptacle, perianth, or floral bracts, participate in fruit formation. Fruit growth is interconnected with seed development. Phytohormones synthesized during seed development regulate fruit growth, determining the influx of nutrients from the leaves. Consequently, in plants that form fruits, vegetative growth is noticeably reduced. Removing the fruits delays the senescence processes of the plant organism.

15. Transformation of substances during seed maturation

The maturation of fruits and seeds involves the accumulation of nutrient reserves and the attainment of specific dimensions. It is accompanied by metabolic and physiological changes.

The maturation process of cereal grains consists of a sequence of stages: milk, wax, and full maturity. During maturation, the seed loses water, accumulates carbohydrates and proteins, and the total mass of dry matter increases.

In wheat plants, the outflow of organic nitrogen from vegetative organs to the ear begins from the moment of stem elongation. As the ear forms and the grain matures, the absolute content of Cellulose, hemicellulose, and Lignin in the leaves and straw decreases, despite the fact that intensive photosynthesis continues in the plant.

Initially, proteins are synthesized in the grains primarily. Therefore, immature grain has a relatively high protein content. Starting from the end of the milk stage, the qualitative direction of organic matter synthesis in the grain changes: starch synthesis increases sharply, and among proteins, the proportion of alcohol-soluble and alkali-soluble fractions grows. The content and ratio of mineral elements change: the amount of phosphate increases, while that of calcium and magnesium decreases. Simultaneously, morphological changes occur—the grain color shifts from greenish-gray to golden-yellow.

In maturing legume seeds, excluding soybeans, the primary processes involve the synthesis of proteins, starch, and fiber. In soybean seeds, however, fat synthesis outweighs starch production. Generally, legumes accumulate fewer carbohydrates. During the ripening of oilseed crops, Reactions Involving the synthesis of fats and proteins predominate. Fat biosynthesis begins immediately after fertilization and continues until full maturity, varying only in the intensity of the process and the qualitative composition of Fatty acids. Initially, saturated fatty acids prevail, followed by unsaturated ones. For instance, in sunflower seed oil, the proportion of saturated fatty acids drops from 84.6% to 6.7% of the total fatty acid content. A significant increase in unsaturated fatty acid content is also observed during the ripening of flax seeds. Thus, mature seeds yield oil of higher quality.

The ripening of fleshy fruits is also accompanied by changes in the physical and Chemical properties of tissues and the accumulation of nutrients. These nutrients are not utilized for embryo nourishment but rather serve to attract birds and animals, which facilitate seed dispersal. The fleshy tissue of mature fruits (the pericarp) contains significant amounts of sugars and is characterized by softening and the development of aroma.

The ripening process of fleshy fruits can be artificially accelerated using gaseous ethylene. Ethylene is produced during anaerobic processes within fruit tissues during ripening and is released externally. Therefore, placing unripe fruits alongside ripe ones can effectively accelerate their maturation.

To accelerate the ripening of certain crops, other measures are employed, such as two-stage harvesting of cereals and the treatment of cotton crops with defoliants, among others.

The maturation of storage organs like tubers and roots, which accumulate reserve nutrients, has specific characteristics. In most potato varieties, tuberization begins gradually during ontogeny and becomes apparent after the juvenile phase, specifically during the budding stage. Tubers form at the tips of underground shoots (stolons). The process of tuber initiation is dependent on temperature and day length. Tuberization is stimulated by phytohormones synthesized in the leaves. Auxins inhibit tuber formation, gibberellins promote the growth of shoots, including stolons, while cytokinins facilitate tuber development. The interaction between these groups of phytohormones regulates the initiation and growth of tubers.