Plant Physiology - Musienko M. M. 2001

Water regime of plants
Thermodynamic parameters of the water regime in plants

Ideas concerning the application of a thermodynamic approach to The Study of the Water regime in plants were first actively developed back in the 1930s–1940s by O. M. Alekseyev, but they did not receive due development and remained unknown abroad. In 1960, METABOLISM/2.html">THE CONCEPT OF "water potential" was introduced to characterize the water regime in the soil – plant – atmosphere continuum. As noted in the previous chapter, water potential is a thermodynamic indicator of the state of water in a system. It is a derivative value of two other thermodynamic parameters—water activity and the chemical potential of water. Water activity (a_w) characterizes the effective (real) concentration according to which water participates in various processes. Any intermolecular and other interactions (primarily Hydration) reduce water activity. The activity of pure water is equal to 1. In cellular solutions, water activity is always less than unity. Water activity is calculated using the formula:

where P is the water vapor pressure above the system; P₀ is the saturation vapor pressure over pure water under the same conditions.

Water activity in the protoplasm is determined by its diffusion capacity; therefore, all factors that reduce the velocity of water molecule movement also affect its activity. These factors include hydration, osmotic binding, immobilization, the Temperature factor, and mechanical barriers.

Factors that promote changes in water activity include The transport of water from one part of The Cell to another, or from Cell to Cell; Changes in the content of osmotically active substances; metabolic reactions involving water; changes in turgor pressure; and alterations in the state of water within the protoplasm.

Water in a free state is characterized by greater activity compared to intracellular water; consequently, it constantly tends to move toward regions of lower activity.

Chemical Potential. The concentration of dissolved substances in the vacuole is a measure of the cell's maximum capacity to absorb water. According to the molecular-kinetic theory, molecules of all substances are in a state of rapid chaotic motion, the average velocity of which is determined by temperature.

The chemical potential of water (μ_w) is a value derived from activity. It expresses the maximum amount of Free energy of a water molecule that can be converted into work. Its dimension is J·mol^-1, and it is calculated using the equation:

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where μ_w° is the chemical potential of pure water; R is the universal gas constant; T is the absolute temperature; a_w is the water activity in the system.

In a solution and within a cell, a_w is less than 1, therefore ln a_w is a negative value. Consequently, the chemical potential of water in solutions and in the cell is lower than that of pure water.

The energy level of molecules of a given substance, which is determined by their rate of diffusion (and molecules diffuse in all directions), is called the chemical potential of that substance. When referring to water—and we are interested specifically in this substance—another accepted term is used, namely water potential (ψ). It has the dimension of energy divided by volume, which makes it possible to express the magnitude of water potential in atmospheres, bars, or Pascals (1 atm = 1.013 bar = 0.1 MPa).

Water potential is the algebraic sum of the following components: ψ_s — the osmotic potential, which reflects The Effect of solute particles on water activity; ψ_p — the pressure potential, which determines the effect of mechanical, hydrostatic, or turgor pressure on water activity; ψ_m — the matric potential, which defines the effect of polymer macromolecules on water activity; ψ_g — the gravitational potential, which reflects The Influence of gravity on water activity and plays a significant role only when water is lifted to a certain height, that is:

The values of ψ_s, ψ_m, and ψ_g are always negative, because the presence of solutes and a rigid framework, as well as the action of gravity, reduce water activity. Conversely, the value of ψ_p is positive, because when mechanical pressure is applied to water, the activity of its molecules increases. When the system is in equilibrium with pure water, ψ_H2O = 0. In the soil, plant, and atmosphere, water potential is generally negative.

The water potential gradient determines the direction of water molecule diffusion and bulk water flow.

For the functioning of living organisms, not only their hydration status is important, but above all the state in which this water exists—that is, its concentration, energy level, mobility, reactivity, etc. The state of water is also characterized by its Structure and the ratio between "free" water (with unchanged physicochemical properties) and "bound" water (with altered properties due to interaction with non-aqueous components).

In the Thermodynamics of aqueous systems, concepts such as Gibbs free energy, Entropy, and enthalpy have recently been applied more broadly.

All of this has become possible due to the application of calorimetric Methods in plant physiology, which have made it possible to account for heat release and heat absorption during phase transitions of water within the object (water – ice). Based on the phase transition temperature, as well as the height and width of the heat release peak during the water phase transition, The change in enthalpy (ΔH) is calculated. This parameter, which characterizes the heat capacity of the system, is determined by direct measurement of the heat release of the reaction under study. The value of (ΔH) in experimental studies of the phase transition depends on the number of Hydrogen Bonds formed during the freezing of "free water." Knowing the value of enthalpy, another thermodynamic characteristic of the cell can be calculated—entropy (ΔS), which is a measure of the degradation of internal energy, expressing that fraction of internal energy which cannot be converted into work:

where T is the absolute temperature.

In a plant cell, an increase in entropy may be associated with disordering caused by the action of an ion field on water. Any changes in water mobility during ion hydration immediately affect the entropy CHARACTERISTICS OF THE solution. Therefore, entropy can be used as an indicator of The Nature of hydration (positive or negative). In positive hydration, water mobility decreases, leading to an increase in the orderliness of the entire system and a decrease in entropy.

In negative hydration, conversely, an increase in the mobility of hydration water molecules, system disorder, and increased entropy are observed.

We have attempted to examine only the Basic principles of the thermodynamic approach to assessing the state of water in plants. However, there are numerous difficulties on the path of applying the thermodynamic approach to assessing the state of water in such a complex system as a plant Organism. Overcoming them is possible only by deepening our knowledge of the structure and functioning of living systems, as well as by improving existing Research Methods and developing new, fundamentally different ones.

Thus, the state of water in a plant is characterized by its structural, physicochemical, and thermodynamic parameters, which are extremely closely interrelated.