Water is the main chemical component in plants; reaches up to 95% by weight in active tissues. The chemical properties of water give it a fundamental role in plant life.
Due to the small size of its molecule, polarity, its ability to form hydrogen bridge bonds with other ionic substances and its high dielectric constant (80 at room temperature) it is the liquid that dissolves the most substances. This means in living beings that water is a system of nutrient supply and waste elimination and is the support in which most metabolism reactions occur.
Due to its large specific heat (physical magnitude that is defined as the amount of heat that must be supplied to the unit of mass of a substance or thermodynamic system to raise its temperature in a unit) it absorbs and stores a significant amount of heat which makes water an excellent thermal insulator, buffering environmental thermal changes.
Capillarity; it is a property of liquids that depends on their surface tension (which, in turn, depends on the cohesion or intermolecular strength of the liquid), which gives it the ability to go up or down a capillary tube. Vegetables take advantage of this quality to facilitate the ascension of raw sap (water and nutrients) through the xylem.
Maintains cellular turgor. Turgor is fundamental for plants because it is the main force for the cellular expansion of plant tissues in the growth process. It is thanks to the turgor, that the petiole, the stem, the leaves and the ripe fruits achieve firmness and stability, helping the plant to preserve its shape and good functioning, Therefore, all plants need the turgor of their cells for their support.
In the hydrodynamic system formed by soil, plant and atmosphere, the plant acts as an intermediary between the water that exists in the soil and that which exists in the atmosphere. Let’s define at this point the concept of water potential (Ψ), which comes to express the free energy that water molecules have to perform work. This water potential is measured in atmospheres or bars (1 bar = 0.987 atm) and ranges from zero to negative values.
The movement of water in soil and plants occurs spontaneously along free energy gradients (water absorption is a passive and non-active work), from regions where water is abundant, and therefore has high free energy per unit volume (higher Ψ), to areas where the free energy of water is low due to its lower presence (lower Ψ). Pure water has a very high free energy because all molecules can move freely. This is the reference state of the water potential; a pure, free body of water, without interactions with other bodies, and at normal pressure, corresponds to an Ψ equal to 0. The Ψ is fundamentally determined by the osmotic effect, associated with the presence of solutes, by the metric forces that adsorb or retain water in solid or colloidal matrices (soils and substrates of culture), by the effect of height and by positive or negative pressures or tensions present in the containers or ducts where it is located. These factors have an additive effect that typically decreases the water potential of the soil or plant with respect to the potential of pure water. Thus, in a particular system, the total water potential is the algebraic sum of four components:
Ψh = Ψo + Ψm + Ψg + Ψp
where Ψ means potential, and the subscripts h, o, m, g, and p, mean water, osmotic, matric, gravitational, and pressure, respectively. The Ψo represents the component determined by the presence of dissolved solutes, decreases the free energy of water and can be zero or assume negative values. As the solute concentration (i.e., the number of solute particles per unit volume of the solution) increases, the Ψo becomes more negative. Without the presence of other factors that alter the water potential, the water molecules of the solutions will move from places with low solute concentration to places with higher solute concentration. Ψo is considered 0 for pure water. The Ψm represents the degree of water retention, due to interactions with solid or colloidal matrices. Such matrices are the colloidal material of the soil or substrate and the cell walls. It can have null or negative values. Finally, the Ψg represents the influence of the gravitational field and is usually positive, although this depends on the position chosen for the reference state. The Ψp represents the hydrostatic pressure and can assume positive or negative values depending on the water is subjected to pressure or stress. Thus, for example, the pressure potential Ψp in cells is positive and represents the pressure exerted by the protoplast against the cell wall, while in the xylem it is negative due to the tension developed by differences in the water potential caused by transpiration. In the SOIL-PLANT-ATMOSPHERE system, water potential can be measured at various points in the water movement pathway from the soil through the plant to the atmosphere. Along this path, the contributions of different components in determining water potential vary.