All of the processes of carbon gain and loss can be influenced by stress. In fact, stress can be defined as any factor, in excess or in deficit, that detrimentally affects the normal functioning of a plant, altering any aspect of a plant's carbon balance .
The ecological influence of water is the result of its physiological importance. The major role of water can be summarized by listing its most important functions under four general headings: constituent, solvent, reactant, and maintenance of turgidity .
A water deficit therefore can affect the majority of aspects related to growth: anatomy, morphology, and physiology .
The system that describes the behavior of water and water movement in soils and plants is based on a potential-energy relationship. Water has the capacity to do work; it will move from an area of high potential energy to one of low potential energy. The potential energy in aqueous systems is expressed by comparing it with the potential energy of pure water. The water in plants and soils usually is not chemically pure because of solute and it is physically constrained by forces such as polar attractions, gravity, and pressure. As a result, the potential energy is less than that of pure water. In plants and soils, potential energy of water is called water potential. It is usually negative; the more negative the value, the lower is the water potential.
Water potential (kPa) is the sum of several components of potential:
1. tym, matrix potential, the force with which water is held to plant and soil constituents by forces of adsorption and capillarity;
2. tys, solute potential (osmotic potential), the potential energy of water as influenced by solute concentration;
3. ty p, pressure potential (turgor pressure), the force caused by hydrostatic pressure, and it usually has a positive value; and
4. ty^, gravitational potential, which is always present but usually is insignificant in short plants, compared with the other three.
Because gravity can be omitted in most cases, water potential remains ty = — tys + typ. In a transpiring plant, the absolute value of tys will exceed that of typ, yielding a negative value.
The concept of water potential fulfills two main functions: It governs the direction of water flow across cell membranes and is the driving force for water movement from the soil into the roots, and it is a measure of the water status of a plant. Water potential is the most commonly measured parameter which is closely connected to plant functions. Thus, a decrease in ty under given conditions relative to ty of well-watered plants can be correlated with yield and productivity. The water movement through the Soil-Plant-Atmosphere Continuum (SPAC) is explained as occurring along a decreasing waterpotential gradient, ty. The potential gradients among the components of the system constitute the driving force for the flow within the system. Processes might be noted as analogous to the Ohm's Law:
Difference in water potential (Aty)
In other words, the water flow through each component of the system is determined by the existing potential gradient and by the resistance in the own segment.
It is assumed that forces causing movement are tensions originated by the large mass flux generated during transpiration and, occasionally, by the root pressure when a plant transpirates very slowly. Thus, the direction and, partly, the water movement rate in the SPAC depend on the gradient of water potential. The higher water potential in the soil, the higher the gradient. Irrigation increases the soil's water potential and decreases the resistance to flow. Hence, root water absorption is favored. Water balance and the water movement within the plant are determined by water relationships at cell level. The hydric potential in a plant cell is the same throughout when the cell has hydric equilibrium. However the potential components might vary. When the cell undergoes changes in its hydric potential then water comes in or goes out of the cell. These changes affect its turgor, volume and solute concentration. During the daytime the behavior of stomata depends on the guard cell's ^p. This potential depends on capability of the plant to absorb water and to replace water losses provoked by transpiration. When ^p comes close to zero, the stomata begins to close. Then the resistance to the water vapor transport goes up and transpiration is consequently limited. This function of the stomata lessens the development of severe water stress that would damage tissues. A plant becomes water stressed when hydric and pressure potentials are sufficiently lowered so as to alter normal performance . These low values of hydric potential at the leaf level can be reached due to various causes: low values of hydric potential in the soil, high transpiration flows, or elevated resistance to flow:
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