PLANT PHYSIOLOGY AND BIOCHEMISTRY

Lecture Notes

11. PLANT GROWTH AND DEVELOPMENT

Class="right">“The most essential feature of a plant is that it grows: its very name points to this.”

Timiryazev K.

Ontogeny (being + origin) is the individual development of an Organism from a zygote (or vegetative primordium) to its natural death.

During ontogeny, the hereditary information of an organism (the genotype) is realized under specific environmental conditions, resulting in The formation of the phenotype—the sum of all traits and properties of an individual organism.

Development refers to qualitative Changes in the Structure and Functional activity of PLANTS AND THEIR parts throughout The process of ontogeny.

The Emergence of qualitative differences between Cells, Tissues, and Organs is termed differentiation. The process of differentiation is also interpreted as Cell specialization—the adaptation of Cells and Tissues during ontogeny to perform specific Functions.

METABOLISM/2.html">THE CONCEPT OF “development” also encompasses age-related changes.

Growth is the irreversible increase in the size and mass of cells, an organ, or the entire organism, associated with the formation of new structural elements. The concept of “growth” reflects the quantitative changes that accompany The Development of an organism or its parts. The hierarchy of these concepts is illustrated in the following diagram:

Stages of Ontogeny in Higher Plants

The development of higher plants is divided into four stages: 1) embryonic; 2) juvenile; 3) reproductive; 4) senescence.

Embryonic Stage

The embryonic stage of ontogeny in seed plants covers the Development of the embryo from the zygote to the mature seed.

The zygote is formed As a result of the fusion of a sperm cell from the pollen tube (male gametophyte) with the egg cell of the embryo sac (female gametophyte). Double Fertilization, first discovered and described by S. Navashin, occurs within the embryo sac. The embryo sac is located within the nucellus, which is surrounded by the integument. The nucellus provides Nutrition for the embryo and, less frequently, transforms into a storage tissue known as the perisperm. The integuments serve a protective function and eventually develop into the seed coat.

Embryos pass through several successive developmental phases. For most dicots, these include the proembryo, globular, Heart, torpedo, and maturation stages.

The egg cell within the embryo sac is already polarized: The Nucleus is shifted toward the chalazal pole, while a large vacuole occupies the micropylar half.

After fertilization, the zygote retains the polarization of the egg cell and remains in a latent state for some time (from 2–4 hours to 1–3 days). During this period, RNA Synthesis increases, the volume of the zygote expands, and the triploid nucleus begins to divide, forming the endosperm; phytohormones (Auxins and Cytokinins) are supplied from the nucellus and Placenta, which are essential for endosperm development.

During the first division of the zygote, which occurs perpendicular to its axis of polarization, the daughter cell facing the micropyle is significantly larger and, upon subsequent division, forms a single-row filament of cells called the suspensor. It performs the following functions:

- by elongating, it pushes the embryo deep into the endosperm tissues;

- it absorbs substances from the nucellus and integument and transfers them to the embryo;

- it synthesizes phytohormones.

In some species, the part of the suspensor cells adjacent to the embryo later becomes part of the apical meristem and the ROOT cap of the embryonic root. The two synergids adjacent to the egg cell function as haustoria, absorbing substances from the nucellus. The antipodal cells located near the endosperm perform a similar haustorial function.

The second cell of the two-celled embryo, oriented toward the chalazal pole, undergoes two longitudinal divisions along the axis of polarization to form a quadrant. Subsequently, in most dicots, each of these cells divides transversely, resulting in the formation of an octant. The four distal (peripheral) Cells of the octant will eventually give rise to the cotyledons and the SHOOT apex, while the four proximal cells will form the hypocotyl and the basal portion of the root. An embryo consisting of one, two, four, or eight cells is referred to as a proembryo.

As the embryo develops, the concentration of IAA and cytokinins increases. The terminal cell of the suspensor, which borders the octant, becomes the hypophysis—the initial cell of the root pole—and becomes morphologically distinct.

All 8 cells of the proembryo divide periclinally (parallel to The surface of the octant). This developmental phase is known as the globular stage. The outer cells go on to form the protoderm, while the inner cells give rise to the primary cortex and the central cylinder. During this phase, cytokinin is particularly essential. It is supplied by the endosperm, which plays a major role in embryo development.

The next phase is the heart stage. In the morphologically upper part of the globular embryo, intense bilateral Cell Division occurs, leading to the initiation of the primordia of two symmetrically placed cotyledons. Conversely, in the region between them (the future shoot apex), cell division slows down sharply. During this period, the influx of IAA, cytokinin, and adenine is necessary for normal development.

The clear chronological sequence of embryo differentiation into distinct specialized regions is noteworthy. This is facilitated by correlative interactions between cells of different regions. The Increasing complexity of these relationships indicates the ESTABLISHMENT OF THE future organism's own hormonal system.

The torpedo stage of embryo development is associated with cell division occurring primarily transverse to the longitudinal axis, along with more intense cell growth in the cotyledon primordia and the hypocotyl zone. Procambial cells in the hypocotyl become clearly distinguishable by their elongated shape along the axis. The root promeristem is formed. In addition to IAA and cytokinin, gibberellin is required (for hypocotyl growth).

In some dicots, the cotyledons and hypocotyl bend and fold during growth. The shoot apex is initiated between the cotyledons, and the suspensor disintegrates. If the cotyledons function as storage organs, they fill almost the entire volume of the maturing seed, and Proteins, starch, and fats are deposited in them during the final stages of embryo formation.

The delivery of nutrients to ovules, and subsequently to maturing seeds and fruits, is determined by these areas becoming dominant centers: large quantities of phytohormones are synthesized in their tissues, resulting in an increased attracting effect.

In The final stage of maturation, the seed loses a significant amount of Water and, in most species of the temperate zone, enters a state of dormancy. This transition is associated with a decrease in free auxins, cytokinins, and Gibberellins in the tissues and an increase in ABA content.

Juvenile Stage

The juvenile stage in seed plants begins with the germination of seeds or vegetative propagation organs and is characterized by rapid accumulation of vegetative biomass.

Plants during this period are not yet capable of sexual reproduction.

The process of seed germination is divided into phases of imbibition, sprouting (emergence of the radicle), heterotrophic growth of the seedling, and the transition to autotrophic nutrition.

Imbibition. The absorption of water by the seed serves as the trigger for germination. It occurs as a result of increased permeability of the seed coats to water and the Hydration of Biopolymers within the cells. Consequently, oncotic pressure (imbibition pressure) develops, causing the seed coats to rupture. Imbibition is practically independent of Temperature, O₂ content, or light.

Sprouting begins when the seed moisture content reaches 40-65%. It occurs through the elongation growth of the embryonic root or hypocotyl, which pushes the root tip out of the seed. In grasses, it pierces the coleorhiza (root sheath). Cell division usually occurs later. Elongation growth is possible due to a decrease in ABA during imbibition. The emergence of the root ensures the anchorage of the germinating seed in the soil and improves water absorption.

Heterotrophic growth. Following the root, shoot growth begins. Germinating in the dark, both the root and shoot are oriented primarily by the gravitational vector. The growth of the axial PARTS OF THE embryo and seedling is supported by phytohormones. Notably, in grasses, IAA and cytokinins are initially supplied to the embryo from the endosperm, GAs are released from a bound state in the embryonic axis, and after a few hours, gibberellin synthesis is induced in the scutellum. The epithelial cells of the scutellum begin to digest storage substances in the endosperm. Conditions are created for acid Digestion and the absorption of nutrients by the epithelium (symport of sugars and Amino Acids with H⁺ ions across the Plasmalemma). Epithelial cells grow into the endosperm and carry out heterotrophic nutrition of the developing embryo.

Growth of the embryonic root is accompanied by the appearance of zones of cell division, elongation, and differentiation. The root itself begins to synthesize cytokinins and GAs, which are transported to the shoot. The shoot elongates due to the stretching of the hypocotyl (dicots) or mesocotyl (grasses). The hypocotyl bends strongly, forming a hook that facilitates its movement through the soil. IAA is synthesized in the bud of dicots and at the apex of the coleoptile (a colorless sheath leaf) of grasses.

When the etiolated shoot reaches the soil surface, light-growth and photomorphogenetic reactions occur: the growth of the hypocotyl or mesocotyl is sharply inhibited, while the growth of the epicotyl (the first true internode) and leaves is enhanced. The Ethylene content in the hook zone decreases, and the hook straightens. The plant turns green and transitions to a phototrophic type of nutrition.

As a result of continued growth of the primary, lateral, and adventitious roots, along with shoot formation and stem thickening, the plant accumulates significant vegetative biomass by the end of the juvenile period.

The duration of the juvenile period varies among different plants: from a few weeks (annual herbs) to decades (in woody plants).

Seedlings differ from adult plants in many parameters:

✵ there are differences in leaf shape (cucumbers, cotton);

✵ in internal structure (ferns);

✵ in growth habit (ivy).

✵ less robust apical meristem.

The juvenile stage is characterized by a complete absence of flowering. A key phenomenon here is competence, which refers to the readiness to respond specifically to a particular inductive stimulus. In this case, a juvenile plant lacks the competence to respond to factors that trigger the development of sexual or vegetative reproductive organs. This may be due to the absence of receptor proteins in the target organs—which are involved in the induction of generative reproduction—that would otherwise respond to hormonal signals.

However, the absence of flowering is not a definitive indicator of juvenility, as many plants remain in a vegetative state for long periods even when mature. Juvenile growth is typically characterized by a higher capacity for root formation, a trait widely utilized in horticulture.

The juvenile state is maintained by a specific hormonal balance. The transition from the juvenile to the mature growth type is most evident in woody species. In beech trees, for example, all developmental phases—from juvenile to mature—can coexist simultaneously.

The stage of Senescence and Death

The stage of senescence and death (the senile stage) spans the period from the total cessation of fruit-bearing to the natural death of the organism. This is a period of progressive decline in vital activity. Its duration is determined to some extent by the overall lifespan of the plant, which varies significantly across different species and ecological groups:

✵ ephemerals live for 2-4 weeks;

✵ annual and biennial plants live for 1-2 years;

✵ some perennial herbs live for 10-30 years;

✵ grapevines and tau-sagiz live for up to 100 years;

✵ trees (such as linden, spruce, oak, and sequoia) can live for over 1000 years.

This indicates that the lifespan of each species is genetically determined. While Aging and death are the final phases of ontogenesis for any plant, the term "senescence" can also be applied to individual plant organs.

Plants exhibit various types of senescence:

✵ annual plants die off completely;

✵ in perennial herbs, the aerial part dies off completely every year;

✵ in many plants, lower leaves age and die off during the growth process;

✵ in deciduous trees, all leaves age and fall simultaneously in the autumn. Prior to the abscission of a leaf or fruit, an abscission layer consisting of dense, transversely oriented cells forms at the Base of the petiole or pedicel. Within this layer, cell walls and middle lamellae partially dissolve. The entire process is induced by ethylene, which is synthesized by the aging leaves. The senescence of isolated leaves can be delayed by cytokinins, and in some plants, by auxins and gibberellins. ABA and ethylene accelerate the senescence process.

Hypothetical mechanisms of senescence

1) The hypothesis proposed by the German plant physiologist H. Molisch is based on the observation that in monocarpic plants—annuals, biennials, and certain perennial herbs (such as agave or bamboo, which flower only once)—senescence occurs immediately after flowering and fruit maturation. Senescence is triggered by the massive translocation of nutrients to the developing reproductive organs, leading to the death of the plant due to the exhaustion of its vegetative parts. This view is supported by the fact that removing flowers can delay senescence.

However, this hypothesis is challenged by the fact that removing male flowers in dioecious plants also delays senescence, even though there is no significant nutrient translocation. Furthermore, in some plants, senescence is induced by specific photoperiods. It is possible that senescence arises from a specific balance of phytohormones, which is established under the Influence of External and/or internal factors.

2) According to V. Kazaryan (1959), the primary role in senescence processes is played by the functional correlation between roots and leaves: the cause of senescence may be a lag in root system development. During fruit formation, root growth is suppressed due to a decrease in the supply of assimilates. Reduced root activity leads to disruptions in mineral nutrition, water balance, Photosynthesis, and other vital processes, thereby lowering the plant's overall vitality. The suppression of root growth, regardless of its cause, leads to a reduced supply of physiologically active substances—primarily cytokinins—to the aerial organs, as well as a decrease in the proliferative activity of apical shoot Meristems, which triggers the senescence of the entire organism. (Proliferation refers to the growth of tissue through the formation and multiplication of new cells.)

3) M. Krenke's theory of cyclic aging and rejuvenation of plants. Every organism ages and dies. Krenke defines aging as a long-term process that begins with the fertilization of the egg cell. The organism's transition from the embryonic stage to the juvenile stage, and subsequently to the adult stage, are all gradual processes associated with aging. This aging process is cyclic. It is linked to rejuvenation—the formation and DEVELOPMENT OF NEW, young organs—which results in every part of a plant being characterized by both its own specific age and the overall age of the plant.

Senescence is of great biological significance. It serves as a method for plants to adapt to unfavorable environmental conditions. Moreover, senescence promotes faster evolution by accelerating the turnover of generations, thereby facilitating the "recycling" of genetic material.