PLANT PHYSIOLOGY AND BIOCHEMISTRY

Lecture Notes

4. WATER RELATIONS

Water Relations in Plant Cells

The Water content in plant Tissues is a highly variable and dynamic parameter. It depends on the plant's age, the season, water availability, Transpiration intensity, and other factors.

Class="center">Forms of Water IN The Cell

Two forms of water are distinguished in Cells: free and bound. Bound water is classified into:

1) osmotically bound (hydrating dissolved substances);

2) colloidally bound (inter- and intramicellar water);

3) capillarily bound (within cell walls and vessels).

The permeability of the Plasmalemma in ROOT cells to water is quite high. In young roots, up to 3/4 of the total intracellular water is contained in vacuoles, 1/4 in cell walls, and only 1/20 in the Cytoplasm.

Water is retained in cells through osmosis and the Swelling of biocolloids. Even when moisture levels fall below the critical threshold, a significant amount of water remains in a bound state.

Cell walls exhibit high hygroscopicity and retain water primarily due to the high hydrophilicity of their pectin and Cellulose components. They contain two fractions:

- a less mobile fraction (in microcapillaries and bound to microfibrils);

- and a mobile fraction (in macrocapillary spaces).

Water content in cell walls accounts for 50% of their mass. Outside The Vascular System, water movement occurs mainly via the apoplast.

In the cytoplasm, water content reaches 95% of its mass. Proteins are the primary type of hydrophilic colloids in the cytoplasm. In hydrophobic regions, water possesses a Structure similar to that of ice. Cell Organelles (Plastids, Mitochondria) typically contain about 50% water. Vacuolar sap generally contains 98% water, along with sugars, salts, ions, organic acids, Enzymes, pigments, mucilage, etc. Vacuolar sap can be viewed as a solution that retains water osmotically due to the selective permeability of the tonoplast.

OSMOTIC WATER UPTAKE

The uptake of water from the external environment is a prerequisite for the existence of any living Organism. It can enter plant cells through the swelling of biocolloids (as seen in seed germination) or via osmotic absorption.

Osmosis is defined as the movement of a solvent into a solution separated from it by a semipermeable membrane (which allows only the solvent to pass).

Let us examine the Factors influencing the Osmotic Pressure of a solution using an osmometer. A bag made of a semipermeable membrane (parchment, cellophane, or colloidal film) equipped with a tube is filled with a sucrose solution. Upon immersing the bag in pure water, the liquid level in the manometer tube rises until the hydrostatic pressure (H) of the liquid Column equals the potential osmotic pressure (P): H = P.

Potential osmotic pressure is calculated using the following formula:

Р = і х С х R х Т

where:

С - molar concentration of the solution;

T - absolute Temperature;

R - gas constant;

i - isotonic coefficient (depends on electrolytic dissociation).

Thus, for dilute solutions, the osmotic pressure at a constant temperature is determined by the concentration of solute particles (molecules, ions).

Potential osmotic pressure is expressed in Pascals and represents the maximum possible pressure exerted by a solution of a given concentration, or the maximum capacity of the solution within a sac (cell) to absorb water.

Osmotic phenomena can also be analyzed through the thermodynamic properties of a system and the chemical potential of substances. The chemical potential of pure water is referred to as water potential (Ψ_H2O). It is highest in chemically pure water (i=0).

The plant cell AS AN OSMOTIC SYSTEM

A plant cell is enclosed by an elastic Cell wall capable of stretching. The vacuole contains numerous osmotically active substances (sugars, organic acids, salts). Let us consider a simplified model of a plant cell. The semipermeable membrane is equivalent to a system comprising the tonoplast and the plasmalemma. When a cell is immersed in pure water, the water will enter the cell According to the laws of osmosis.

The force with which water enters the cell is called suction force (S). It is equivalent to the cell's water potential (Ψ_H2O).

The magnitude of the suction force is equal to the difference between the osmotic pressure of the cell sap (P) and the turgor (hydrostatic) pressure within the cell (H):

S = P - H;

or in thermodynamic terms:

-Ψ_Н2О = -Ψ_Р + Ψ_Н

Turgor pressure is the counter-pressure exerted by The cell wall as it undergoes elastic stretching. Under certain conditions (sufficient H₂O), the volume of the cell sap increases, causing the cytoplasm to expand, press firmly against the cell wall, and stretch it. Consequently, the cell wall enters a state of tension known as turgor.

The relationship between all components of this equation varies depending on the Hydration status. When a cell is fully saturated with water (fully turgid), S = 0 and P = H.

Under conditions of prolonged water deficit, most cells lose turgor, causing the plant to wilt. In such cases, H = 0 and S = P.

The phenomenon of turgor loss can be observed experimentally by immersing tissue samples in a hypertonic solution. In this case, water flows out of the cells, leading to a reduction in protoplast volume and their detachment from the cell walls. This process is called plasmolysis. A cell in this state is referred to as plasmolyzed.

In cases of rapid water loss in young tissues, Cells and Tissues may shrink without plasmolysis. This phenomenon is known as cytorrhysis.

In addition to the osmotic factor, water absorption by the cytoplasm may also be associated with the hydration of proteins and other biocolloids.

The magnitude of osmotic potential varies among different plant species and within different PARTS OF THE same plant. There is a vertical gradient of osmotic concentration and suction force from the roots to the leaves. This is crucial for The transport of water and various dissolved substances. Osmotic pressure governs turgor phenomena, growth, plant movement, etc. It fluctuates throughout the plant's life cycle, which is of significant adaptive importance.