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

Lecture Notes

TOPIC No. 3. CHARACTERISTICS OF PLANT WATER RELATIONS

Outline

1. Water content in plants: state, properties, and physiological role.

2. Osmotic properties of Cells and their role in plant life.

3. The ROOT System as an organ of water absorption.

4. Forms of soil water and their availability to plants.

5. Root pressure: manifestations and significance.

6. Water transport in plants.

7. Transpiration: metrics and biological significance.

8. Using physiological indicators to determine irrigation needs.

1. Water content in plants: state, properties, and physiological role.

Water content in plant Tissues is a dynamic and variable parameter, depending on the age of the Tissue and organ, soil moisture availability, and the balance between Water uptake and loss. For instance, leaves of trees and shrubs, as well as zones of active stem and root growth, contain up to 85% water. In other Organs with lower metabolic activity (such as bark or mature stems), the total water content is relatively low, ranging from 35% to 45%. Water content is also high in certain fruits, bulbs, and tubers. For example, potato tubers contain 70-85% water, depending on their maturity.

Depending on its state, water in Cells and Tissues is classified into two forms: free and bound.

Free water is highly mobile. It accounts for up to 75% of the total water content in young tissues and is primarily located within vacuoles.

Bound water is categorized into osmotically bound (Hydration of ions and molecules), colloidally bound, and capillary water (found within Cell walls and xylem vessels).

The high hygroscopicity of The Cell wall is crucial for facilitating water movement throughout the plant. Compared to The Cell wall, the Cytoplasm is more highly saturated with water (95%). The hydration status of a cell is maintained by the processes of water uptake and loss, collectively known as water exchange.

Properties of water:

· High heat capacity, which ensures thermoregulation of the plant Organism;

· High thermal conductivity (transfer of energy from warmer to cooler PARTS OF THE organism due to thermal motion), allowing it to evaporate even at 0⁰С;

· Water as a dipole (a system of two point electric charges equal in magnitude and opposite in sign);

· Cohesion - the force of attraction between water molecules;

· Adhesion - the force with which water adheres to the walls of vessels;

· High capacity for dissolving gases.

The properties of water determine its physiological significance; therefore, water is:

· a specific medium in which all physiological processes related to the vital activity of plant organisms occur;

· a connective transport link between various cells, tissues, and organs, ensuring Homeostasis and the functioning of the organism as a single entity;

· an essential component of protoplasmic structures, providing them with Organization;

· a mandatory participant in a series of biochemical processes;

· a factor that maintains turgor, and consequently, the shape of tissues, organs, and entire plants;

· a factor that stabilizes BODY Temperature AND prevents overheating;

· a universal solvent;

· a factor that ensures the elastic state (turgor) of the plant organism.

2. Osmotic Properties of the cell and their role in plant life.

Osmosis is the slow diffusion of solvent molecules and substances through semipermeable membranes. The phenomenon of osmosis is of great importance in biological processes. In an osmotic system, when a solution is separated from water by a semipermeable membrane that allows only water molecules to pass, a unidirectional movement occurs along the activity gradient toward the solution. The additional pressure that must be applied to prevent this unidirectional Movement of water molecules is called osmotic pressure.

Osmotic potential is a component of water potential that determines the ability of water to perform work at a specific point in a system. The activity of water molecules, i.e., their kinetic energy, depends on the concentration of the solution: the lower the concentration, the higher the water activity, and vice versa. Therefore, osmosis occurs in an osmotic system consisting of a semipermeable membrane with two solutions of different concentrations, or a solution and a solvent, on either side. In this case, the osmotic potential of a solution separated from a pure solvent by a membrane is realized as an equivalent value of osmotic pressure.

Solutions with the same osmotic pressure are called isotonic. When two solutions of different concentrations are separated by a semipermeable membrane, water moves from the solution with the lower concentration to the one with the higher concentration. The solution with the higher concentration has a greater osmotic pressure and is called hypertonic, while the one with the lower concentration has a lower pressure and is called hypotonic.

The cell and all cellular Organelles surrounded by cell membranes are osmotic systems. The cell wall, composed primarily of Cellulose and pectin, is highly permeable to dissolved substances. Often, when studying osmotic phenomena, the cell is viewed as an osmotic system in which Introduction/36.html">Biological Membranes surrounding the protoplasm act as the semipermeable barrier, and the vacuolar sap—containing osmotically active substances such as sugars, Amino Acids, minerals, etc.—acts as the working solution.

Osmotic pressure is a parameter of diffusion pressure. It is caused by a decrease in the chemical potential of the solvent in the presence of a dissolved substance. The chemical potential of water is referred to as water potential.

The water potential of biological systems includes several components:

Gravitational potential characterizes The change in water activity as it moves within the Earth's gravitational field. It plays a significant role in xylem and phloem transport in tall trees.

Osmotic potential is determined by the concentration of dissolved substances: P=RCTi.

Matric potential characterizes the decrease in the activity of water molecules due to the hydration of colloidal substances and adsorption at phase boundaries. When water is absorbed, a colloidal micelle swells. Swelling is The process of liquid or vapor absorption by a high-molecular-weight substance, accompanied by an increase in its volume. The phenomenon of swelling is caused by capillary and colloidal effects.

When a cell absorbs water, the cytoplasm is pressed against the cell wall, generating a counter-pressure from the cell wall onto the cell contents, which creates hydrostatic pressure. Water activity increases under METABOLISM/18.html">The Influence of pressure; therefore, hydrostatic potential has a positive value.

At full saturation with water (maximum turgor pressure), the positive pressure potential balances the negative osmotic potential, meaning the WATER POTENTIAL OF the cell equals zero. In this state, no further water absorption occurs. If, however, the osmotic potential exceeds the pressure potential, the water potential will have a negative value, and the cell is capable of absorbing water.

Water uptake is determined by the difference between osmotic (P) and turgor (T) pressure. This value is called the suction force (S):

Class="center">S = Р - Т.

The process of water absorption by the cell continues until the cell is fully saturated and the cell wall has reached its maximum stretch. At this point, the cell reaches its maximum possible volume, the concentration of the cell sap becomes minimal, and the turgor pressure becomes maximal. If such a cell is placed in a concentrated solution, it will lose water. The volume of the vacuole will decrease, as will the pressure of the cytoplasm against the cell wall, resulting in a decrease in turgor pressure. Upon reaching the minimum cell volume and continuing water loss, the protoplasm begins to pull away from the cell wall. The process of water loss by the cell continues until the concentrations of the cell sap and the external solution equilibrate. The phenomenon of the protoplasm pulling away from the cell wall is called plasmolysis, and the cell is referred to as plasmolyzed. In the event of low humidity in the air and water loss by the cell, a free space does not form between the protoplast and the cell wall; instead, the protoplast, decreasing in volume, pulls the cell wall inward. The cell surface becomes wavy. This state of the cell is called cytorrhysis. In wilted plants, the turgor pressure of the cells becomes less than zero, i.e., a negative value. This is explained by the fact that during cytorrhysis, the elastic forces of the cell wall do not compress the protoplast but rather stretch it. The sum of the forces of cytorrhysis and osmotic pressure determines the suction force of a wilted cell:

S = Р + Т.

By absorbing water, such a cell will increase in volume, the concentration of cell sap and osmotic pressure will decrease, and turgor will appear and begin to rise. When the cell is fully saturated with water, the turgor (T) and osmotic (P) pressures equalize, and the suction force (S) becomes zero. Thus, a plant cell is a self-regulating osmotic mechanism.

3. The Root System as an organ of water absorption.

Almost all water absorbed by a plant enters through the root. Normal physiological processes in a plant can only occur if the water supply to its tissues remains constant. Water losses due to transpiration must be compensated by its intake. Great importance is attached to the size of the root system, its growth rate, and its absorptive activity. This is primarily due to the fact that water diffusion in the soil is very slow: no more than 1 cm per day. Therefore, in the soil, it is not the water that moves to the root, but the root that grows toward the water.

This circumstance determines the specific ORGANIZATION OF THE root system, which is characterized by its vast size and ability to branch. The total surface area of the roots usually exceeds the surface area of the above-ground organs by 140-150 times.

Even in a one-year-old apple seedling, 5-7 orders of root branching are formed with a total length of 250 m, and about 3 km including root hairs. In mature trees, the root system is measured in tens of kilometers. Moreover, about half of it is formed by roots up to 5 m long. The density of roots in the arable soil layer for water absorption by woody and herbaceous plants should be 0.3-0.5 cm per 1 cm of soil.

Root Morphology. The unique ability of roots to occupy significant volumes of soil is associated with A large number of growth points (meristematic tissues account for 10% of root mass compared to only 1% in stems), a high rate of growth processes (110 cm per day), and the property of positive hydrotropism, i.e., The ability to grow toward more moist areas of the soil.

The absorptive function of the root is determined by the peculiarities of its Anatomical Structure. The root is conventionally divided into four zones: Cell Division, elongation, absorption (or root hairs), and the conduction zone. The cell division zone is protected by the root cap and requires an insignificant amount of water. Cells in this zone are characterized by large nuclei, the absence of vacuoles, and the Primary Structure of cell walls. Their water potential is determined mainly by matric force, i.e., the ability of protoplasm and cell wall colloids to swell.

Intensive water absorption begins in the elongation zone. Here, the intensive synthesis of cytoplasmic Proteins takes place. A significant increase in cell volume is achieved through The formation of a large vacuole, which acts as a reservoir for osmotically active substances. This gives rise to the second component of water potential - the osmotic one. Simultaneously with the increase in vacuole volume, the cell walls soften and stretch. The elasticity of the cell wall reduces its resistance to water absorption.

The root Hair zone is the primary absorptive zone of the root. Here, there are 230-500 root hairs per 1 mm of root surface, which increases the active surface area by 10-15 times.

4. Forms of Water in soil and their availability to plants.

Clay particles (aluminosilicates) and humus substances, forming colloids, can retain a significant amount of hydration water. Such water is conventionally called bound water. Water contained in soil capillaries is considered free. A certain portion of water is part of the mineral Components of the soil. It is chemically bound and practically inaccessible to the plant.

There are various terms for defining the availability of soil moisture. THE CONCEPT OF "field capacity" is widely used to characterize the maximum reserves of soil moisture that can be utilized by a plant. Minimum reserves are denoted by the concept of "permanent wilting point." It is the lower limit of soil moisture at which plant growth is possible. Available soil moisture is understood as The amount of water that lies between the level of full field capacity and the permanent wilting point.

The absorption of water by plants from the soil is a more complex process than simple water uptake by roots from a vessel filled with it. The water-holding forces of the soil counteract the plant's water uptake. The concept of "water-holding forces" includes A number of forces caused by various factors. First of all, the soil contains not pure water, but a solution, the concentration of which determines the magnitude of the osmotic pressure that counteracts water absorption by the root system. Osmotic resistance to absorption is significant only in saline or excessively fertilized soils with highly soluble salts.

Solid soil particles and organic colloidal substances are wetted by water, a portion of which also fills large soil capillaries. Such water is called gravitational water. It is mobile and, obeying the force of gravity, can move down the soil profile after atmospheric precipitation. At the same time, water is retained in small soil capillaries by the surface tension forces of the menisci. This is capillary moisture. The force of its retention in the soil is small, and therefore, like gravitational water, this water can be absorbed by root hairs.

Air-dry soil also contains a small amount of water, which depends on its granulometric composition. This water is called hygroscopic. The force of its adhesion to soil particles is about 1000 atm, and therefore such water cannot be absorbed by plants at all. The property of soil colloids to swell in water increases its water-holding capacity and increases the amount of firmly bound, so-called imbibitional water. Peat soils have many organic substances and therefore contain a large amount of such water.

5. Root pressure, its manifestations and significance.

The root system actively moves water. This can be easily verified by damaging the stem, when liquid appears On the surface. This phenomenon is called bleeding. The force that directs the movement of the aqueous solution in living cells and vessels is called root pressure. The liquid that is released is called xylem sap. It is an aqueous solution of Mineral Substances that enter through the root system and are transported with the upward flow through xylem elements. In addition to mineral elements, xylem sap may contain organic substances - products of metabolism (organic acids, simple CARBOHYDRATES, amino acids, Vitamins). Xylem sap varies in chemical composition among different plants and depends on the species, as well as the plant's GROWTH AND DEVELOPMENT phase.

The phenomenon of bleeding manifests itself differently in various plants. In some plants (fuchsia, nettle, sunflower, tomatoes), it is easily detected, while in others, it is almost imperceptible. Bleeding is particularly pronounced during sap flow in woody plants in the spring, such as birch or maple. The appearance of liquid droplets can also be observed on young, undamaged plant organs under conditions of high air humidity. The phenomenon of droplet release at the tips of plant leaves is called guttation. This is primarily water released through specialized cells called hydathodes. The content of dissolved substances in the liquid during guttation is 8-10 times lower than during bleeding. Both bleeding and guttation are driven by root pressure. In herbaceous plants, root pressure is low, while in woody plants, it reaches 2-3 atmospheres.

Root pressure can be defined as the pressure that develops in the xylem due to the METABOLIC ACTIVITY OF the root. It is the force that causes a one-way flow of water with dissolved substances in the plant, independent of transpiration. Root pressure in plants is the lower-end engine that ensures the lifting and movement of water throughout the plant organism.

6. Water Movement in the plant.

The path of the upward flow can be divided into two parts, which differ in structure, length, and physiological properties.

The first part consists of two sections of living cells. It is very short (a few mm or even fractions of a mm): in the root - from The surface of the root hair to the Vessels of the central cylinder - the radial path; in the leaf - from the vessels of the conducting bundles to the leaf mesophyll - the short-distance path.

The second part consists of xylem vessels and tracheids, which were formed from the procambial Cells of the root and stem. Mature vessels and tracheids lack cytoplasm, resemble empty tubes, and perform a water-conducting function. This part of the path, which begins from the vessels of the root's central cylinder and ends in the vessels of the leaf's conducting bundles, is called the long-distance path.

The radial path begins in the cell walls of the root hairs. To absorb water, cells require a suction force that exceeds the suction force of the soil solution. The absorbed water moves to the vessels of the conducting system in two ways: apoplastic (through the free space) and symplastic (through the Cell Cytoplasm).

In the apoplastic pathway, water moves from the cell walls of the root hairs through the free space of the primary cortex cells to the endodermis, where it encounters the water-impermeable Casparian strips, and is therefore forced to move toward the passage cells of the endodermis. The further path of water proceeds either through the cytoplasm of the parenchyma cells of the central cylinder in the direction of the increasing suction force gradient or along the walls of these cells, eventually reaching the conducting vessels of the xylem.

In the symplastic pathway, water moves from the cell walls of root hairs into the cytoplasm, then gradually travels through the cytoplasm of the primary cortex and perivascular parenchyma cells until it reaches the vascular vessels.

The force that drives water into the xylem elements via the root system is known as root pressure. It acts as the lower engine of the upward water flow.

Due to its limited force, this lower terminal engine cannot consistently supply the plant with water, particularly in the case of tall woody plants.

This function is primarily performed by the upper terminal engine: the suction force of transpiration. During transpiration, a suction force is generated first in the mesophyll cells of the leaf or other organs, and subsequently at the ends of the xylem vessels; this force is dozens of times greater than root pressure. It creates a gradient within the conductive system, thereby establishing a directed movement of water from the soil to the aerial organs.

Thus, the driving force of the upward water current in the xylem elements consists of the lower engine, which pushes water, and the upper engine, which pulls it. The upper terminal engine can develop a force of 10-15 atmospheres or more, which determines its leading role in plant water exchange. However, there are periods when the relative importance of these engines shifts. For instance, in winter and early spring, when trees lack leaves, the primary role in water transport belongs to the lower terminal engine. For the upper terminal engine, the energy source is the Sun—the energy of its rays absorbed by the leaf and utilized for water evaporation. For the lower terminal engine, the energy source is Respiration.

The continuity of the water stream, in addition to the upper (transpiration) and lower (root pressure) terminal engines, is maintained by Intermolecular Forces of cohesion (between water molecules) and adhesion (between water molecules and the hydrophilic walls of conductive vessels), collectively known as tensile forces. The magnitude of adhesion reaches 300-350 atm. This creates continuous water columns in the vessels that firmly link the cells of the root and leaves.

Transpiration of water from the leaf surface creates a significant negative hydrostatic pressure gradient in the xylem, inducing water tension that is transmitted to the vessels of the stem and root.

The velocity of Water movement through the xylem is relatively low. For deciduous trees, it averages 20 cm/h per 1 cm² of wood cross-section, while for conifers, it is 5 cm/h.

7. Transpiration, its indices, and biological significance.

Transpiration is the physiological process of water evaporation by a plant. The leaf serves as the primary organ of transpiration in plants. Transpiration plays a leading role in the Uptake and Transport of water and dissolved nutrients. Evaporation is a physical process in which water transitions from a liquid to a gaseous state, losing a significant amount of energy in the process; consequently, the temperature of an actively transpiring leaf is 4-6 degrees lower than the ambient temperature.

Since plant epidermis is often covered by a cuticle, water vapor escapes through Stomata—Pores in the epidermis that facilitate gas exchange. A stoma is a narrow intercellular opening (slit) bordered by two guard cells.

Stomata are present in almost all aerial plant organs, but they are most abundant in leaves. Depending on the plant species, the number of stomatal openings ranges from 10 to 600 per 1 mm² of leaf surface. In many plants (75% of species), including most woody species, stomata are located on the underside of the leaf. The diameter of the stomatal slit is 3–12 μm. Stomata facilitate communication between the external environment and the plant's intercellular space (gas exchange and transpiration). From the surface of the stomata, which accounts for 1% of the leaf surface area, 50–80% of the water that could evaporate from an open water surface of the same area is lost.

The process of stomatal transpiration can be divided into several stages:

- Evaporation of water from the surface of mesophyll cells into the intercellular spaces. At this stage, the plant can regulate transpiration by increasing the content of osmotically and colloidally bound water and by reducing the hydration of cell walls.

- Exit of water vapor from the intercellular spaces through the stomatal slits. Transpiration of water from the leaf surface through stomata occurs at nearly the same rate as from an open water surface.

- Diffusion of Water vapor from the leaf surface into the more distant layers of the atmosphere. This stage is regulated by air temperature, wind speed, and air humidity.

The specific term (transpiration) for the process of water evaporation by a plant is justified because it is not merely a physical phenomenon of evaporation, but a complex physiological process. Transpiration is of vital importance in the life of a plant. Primarily, it creates a continuous flow of water from the root system to the leaves, integrating all plant organs into a single whole, and protects the plant organism from overheating. Water-soluble mineral and some organic nutrients are also transported with the transpiration stream. The more intense the transpiration, the faster The transport of these substances occurs.

Transpiration can be stomatal (through stomata), cuticular (through the cuticle), and lenticular (through lenticels).

Stomatal transpiration is usually the primary form, but for plants of different ecological groups, The Significance of both types of transpiration varies and depends on environmental conditions.

The total area of stomata ranges from 1 to 2% of the entire leaf surface. Guard cells of the stomatal apparatus differ from other epidermal cells in that they contain chlorophyll. Furthermore, their cell walls are unevenly thickened: the outer walls are thin, while those surrounding the stomatal slit are thickened. The unequal thickness of the cell walls causes Changes in the volume and shape of the guard cells, which leads to the opening and closing of the stomata.

Cuticular transpiration occurs through the surface of the cuticle covering the leaf epidermis. As a rule, it is significantly lower than stomatal transpiration. However, young leaves exhibit a high intensity of cuticular transpiration because their cuticle layer is not yet as robust as that of older leaves. While cuticular transpiration in an old leaf accounts for 5–10% of total transpiration, in a young leaf, it often accounts for 40–70%. The pores of the cuticle, through which water not only evaporates but also penetrates into the cytoplasm of epidermal cells, have a diameter of 0.5–1.0 nm. Cuticular wax can exist in an amorphous or crystalline state. Under drought conditions, the waxy coating of the cuticle becomes crystalline, the pores close, and cuticular evaporation ceases. Naturally, young plants are particularly sensitive to water supply and dehydrate easily.

Lenticular transpiration occurs via lenticels—groups of loosely arranged periderm cells that protrude to the surface as small bumps or ridges, through which gas exchange takes place in perennial stems and roots.

A plant is capable of regulating the intensity of transpiration. By closing its stomata, the plant reduces transpiration and simultaneously increases its body temperature. However, when stomata are closed, the plant cannot absorb carbon dioxide from the air for Nutrition. Therefore, the stomatal apparatus of plants responds to changing environmental conditions by opening or closing the stomatal slit accordingly.

Transpiration drives the movement of vast quantities of water through the plant and has an adaptive significance closely linked not only to water exchange but also to other metabolic processes, including Photosynthesis, respiration, and mineral nutrition. Therefore, when studying the water regime of various plants, it is extremely important to analyze such transpiration metrics as its intensity, transpiration coefficient, and transpiration efficiency.

- Transpiration intensity is a value indicating the amount of water in grams that a plant evaporates per hour from a specific leaf surface area. This value ranges from 0.15 to 1.47 g per 1 dm² per hour.

- Transpiration ratio is the amount of water in grams transpired by a plant to produce one unit of dry matter. For various plant species, this value ranges from 125 to 1000, most commonly hovering around 300. This metric depends on environmental conditions and serves as an indicator of a plant's water requirements. For instance, in wheat, it typically ranges between 220 and 750 units.

- Transpiration efficiency is the reciprocal of the transpiration ratio; it represents the amount of dry matter (in grams) accumulated by a plant per 1 liter of water transpired.

- Another useful metric is relative transpiration, defined as The ratio of the transpiration rate per unit of leaf area to the evaporation rate from an equivalent surface area of free water over the same period.

8. Utilizing physiological indicators to determine irrigation needs.

The foundation of irrigated agriculture lies in a scientifically sound, rational irrigation regime that accounts for mineral nutrition systems tailored to specific crops within a given soil and climatic zone. However, the most effective results are achieved by determining irrigation schedules based on the physiological state of the plants themselves. Methods of physiological monitoring include:

· Water-absorbing capacity of the leaf.

· Concentration, osmotic pressure, and suction force of cell sap.

· Permeability and viscosity of the cytoplasm.

· Water-retention capacity and water deficit of plant tissues.

· Status of the stomatal apparatus.

· Electrolyte leakage and electrical resistance of tissues.

· Water potential values and growth response.