Plant Physiology - Musienko, M. M. 2001

Water regime of plants
The water potential gradient is the driving force behind the uptake and movement of water within cells, tissues, individual organs, and the organism as a whole

The bulk of Water enters the plant from the soil through The ROOT System. The distribution of the root system across specific soil horizons is often determined by soil moisture patterns. Soil consists of three phases: solid (mineral particles, humus), gaseous (soil air), and liquid (soil solution).

The soil water regime depends on all processes that govern the influx of moisture into the soil, its movement through profile horizons, retention, and loss (evaporation). From a physiological perspective, several Forms of soil moisture are distinguished based on their availability to plants:

·gravitational water, which fills large pores between soil particles and is readily available to plants;

·capillary water, which fills the capillary pores of the soil, is retained by surface tension forces, and is likewise readily available to plants;

·film water, which surrounds colloidal soil particles. Such water can be absorbed by root Cells from the outer layers of Hydration shells. At the same time, the closer water molecules are to the colloidal particles, the stronger they are retained and, as a rule, the less available they are to plants;

·hygroscopic water, which is adsorbed by dry soil when placed in an atmosphere with 95% relative humidity. This form of water is completely unavailable to plants.

Unavailable water constitutes the so-called dead storage, the magnitude of which depends on the physicochemical properties and Structure OF THE soil. L. Briggs and H. Shantz proposed calling this dead storage the wilting coefficient of plants. This is the soil moisture threshold at which irreversible signs of plant wilting appear, their turgor is not restored, and growth and yield formation cease.

The liquid phase of the soil—the soil solution—directly influences the Water regime of plants. The concentration of this solution determines its water potential. Soil solution water is held with varying degrees of strength: readily available — 0.5 MPa, moderately available — 1.0–1.2 MPa, and poorly available — 2.5–3.0 MPa. It moves through the soil driven by water potential gradients between different PARTS OF THE soil solution system.

At full field capacity (the moisture content at which all soil pores are filled with water), the soil water potential is close to zero, making water easily accessible to the roots. However, when the soil water potential drops below that of the root Cell, the plant wilts.

All water within a plant forms a single, interconnected system that is in direct contact with soil water and water vapor in the surrounding atmosphere.

Water always moves in the direction of a more negative water potential—that is, from a system with higher energy to one with lower energy. It should be borne in mind that the water potential gradient is the driving force behind water transport.

The difference in osmotic pressure between the soil solution and the vacuolar sap of root cells determines the direction of the water flow from the soil into the plant. The Mechanism of vector-like water movement along the water potential gradient from Cell to Cell is still insufficiently understood. Evidence suggests that plant cells are polar with respect to permeability. Furthermore, differences in membrane permeability, as well as the intensity and direction of metabolic processes in different Regions of the same cell, drive the vectorial Movement of water. The Plasmalemma, Cytoplasm, and tonoplast function as a unified whole, acting as a single semipermeable membrane.

There is Evidence indicating the presence of Myosin-like Proteins within plant membrane structures, which are associated with the functioning of membrane pores. The contractile activity of these proteins may underlie not only The regulation of pore radius but also the very formation of pores on the membrane surface. Membrane pores or channels should perhaps not be viewed as "hollow" structures through which molecules can move unhindered. On the contrary, diffusing water molecules encounter variable resistance, which they can overcome if they possess a sufficient kinetic energy reserve required to break bonds. After penetrating a certain distance through bond Cleavage, the resulting space is occupied by another molecule. Thus, membrane pores seem to form transiently and then close again. When the kinetic energy of water falls below the bond energy within the pores, the molecule cannot diffuse, and the pore is considered closed.