Plant Physiology - Musienko, M. M. 2001

Water regime of plants
The role of water in plant life activity

In recent developments in plant Water relations, concepts of water exchange have undergone dramatic changes, with new tenets emerging and innovative research approaches and Methods being devised. L.G. Emelyanov (1992) divides the entire history of water exchange—which traces back to the origins of plant physiology as a science—into five distinct stages: osmotic, colloidal-chemical, thermodynamic, biochemical, and biophysical, each characterized by its own conceptual framework and Research Methods. Researchers' close attention to water exchange is driven by the importance and multifaceted Nature of the Functions it performs, as it acts as the primary regulator of cellular water status and an integral part of cellular METABOLISM. It serves as the leading link in the interaction between plants and the aqueous phase of the soil and atmosphere, one of the main controllable factors of productivity, and a direct indicator of a plant Organism's vitality.

Water is the primordial niche of life; only through evolution did land and atmosphere become a secondary ecological niche. Quantitatively, water holds a dominant position among the chemical compounds of living organisms, with its content reaching up to 85% in mesophyte leaves and up to 99% of fresh mass in roots. Active vital activity is entirely impossible without water. As for dry seeds and spores, where the water content is 3–15%, they remain in a state of anabiosis, with metabolic intensity reduced to a minimum.

High Hydration is characteristic not only of plants but of All living organisms in general, encompassing both terrestrial and aquatic species. An attempt has been made to determine the threshold (i.e., minimum) water content in Cells below which a plant organism perishes. This water has been termed homeostatic water. As it turned out, the content of such water varies among different species, but is always quite substantial. For instance, it constitutes 65–70% in hygrophytes, 45–60% in mesophytes, and 25–27% in xerophytes.

A natural question arises: why do organisms require water in such vast quantities? Could it be needed to drive biochemical reactions? Not at all, since no more than 1% of the water passing through a plant during water exchange is incorporated into biochemical transformations. What, then, is The Significance of the bulk of water in a plant? Its primary significance lies in the fact that, due to its unique physicochemical properties, water constitutes the internal medium where all life processes take place. As Albert Szent-Györgyi aptly put it, water is the "matrix of life." The Emergence of protoplasmic structural formations specific to biological systems is possible only in an aqueous medium (such structures do not exist in inanimate nature), and their functioning is likewise possible only in water, with water itself being an integral part of such structures. Characteristically, all interactions with water are biophysical rather than biochemical, or perhaps even physicochemical, which is why water as such is not consumed or destroyed in the process.

A.L. Kursanov draws a parallel between The Structure of a living Cell and a reinforced concrete structure, in which macromolecules act as the reinforcement framework, while water functions as the concrete mass. Naturally, this analogy is somewhat distant given that Cell Structure is dynamic rather than static. Nevertheless, this analogy carries profound meaning: no functioning biological structure can exist without water.

Emphasizing the Structural Role of water in plants, It is worth recalling that water is precisely what generates hydrostatic pressure (turgor), which determines the characteristic shape of Plant Tissues and Organs. Water is also a thermoregulatory factor, shielding the plant from abrupt ambient Temperature fluctuations thanks to its high heat capacity. Thus, the fact that water constitutes the internal medium of the organism seemingly explains the necessity for a high level of tissue hydration. At the same time, it remains unclear why water, which does not enter into biochemical processes, nevertheless constantly vanishes, necessitating its continuous replenishment from the outside. Furthermore, this is characteristic not just of plants; humans can survive for about a month or more without food, but only a few days without water.

This is explained by the fact that, According to the fundamental principles of general biology, active vital activity is possible only within an aqueous environment that is constantly renewed.

Circulating continuously throughout the organism, water supplies cells with substrates and metabolites while simultaneously removing their Metabolic waste products, including toxic residues. All of this ensures

Homeostasis and the functioning of the organism as a unified whole. It is quite obvious that this unifying transport function of water is of paramount importance for the life activity of both plants and other organisms; therefore, The Mechanism of water transport in the soil–plant–atmosphere continuum cannot be reduced to simple ultrafiltration of water through the plant into the atmosphere.

It should not be forgotten that there exists not only an upward stream but also a downward phloem stream, which is by no means driven by solar-powered water evaporation. Therefore, It is important to remember that a significant role in regulating such flows is played by endogenous control on the part of the plant organism.

Thus, water is a specific medium in which the life activity of plant organisms exclusively takes place;

· a binding transport link between various cells, tissues, and organs that ensures homeostasis and the functioning of the organism as a single whole;

· an indispensable component of protoplasmic structures;

· an obligatory component and participant in a range of biochemical processes;

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

· a factor that stabilizes BODY TEMPERATURE AND prevents overheating.

And, as Antoine de Saint-Exupéry emphasized, it cannot be said that water is necessary for life; rather, water is life itself and the greatest wealth in the world.