Introduction
Water is a vital factor for the survival of all living organisms. It is essential for many functions performed inside the body of all plants. It aids in cell enlargement due to turgor pressure and cell division, resulting in increased plant growth. It is required for seed germination, plant root growth, and soil organism nutrition and multiplication. Water is required for the hydraulic process in the plant. Gradients in hydrostatic pressure drive the flow of water through plants and soil over macroscopic distances. It is driven by gradients in water potential over microscopic distance (e.g., across semipermeable membranes). The evaporation of water from the leaves is primarily controlled by stomata. Thus, it is very important to study plant-water relations.
Well-watered plants are turgid. Their cells are surrounded by a strong but slightly pliable wall. During the course of a day, a well-hydrated leaf may transpire multiple times its own volume of water. Although a leaf can lose a lot of water through evaporation, the net loss of water is usually very small.
Properties of Water
Water’s ability to form strong hydrogen bonds is one of its most important characteristics. These have a significant impact on several biologically important bulk properties. Hydrogen bonds, for example, ensure that macromolecules like proteins and DNA (deoxyribonucleic acid), are surrounded by a shell of water molecules that acts as a spatial buffer between the macromolecules. This prevents them from adhering to each other and precipitating. It has high cohesive strength, allowing it to withstand the extremely high tensions that develop in the xylem and thus maintain liquid water continuity throughout the plant. Because of its polarity and ability to form hydrogen bonds, it is an excellent solvent for ions and small organic molecules such as sugars. The polar nature of fatty acids is especially important in allowing them to organize themselves into membranes that bind cells and organelles.
Water Potential
The chemical potential of water is the difference in Gibbs free energy per mole between the water sample and pure water at standard temperature and pressure. It is measured in the units of Joules per mole.
The chemical potential divided by the volume of one mole of water is called ‘Water potential’ and is denoted by the symbol ‘ψ’.
Water potential (ψ) is made up of two main components, hydrostatic pressure (P) and osmotic pressure (π), with the result that ψ = P – π.
Water potential can be applied to any sample of water, whether it is inside a cell, the cell wall, xylem vessels, or the soil. Water potential is always negative in plants because suction (negative hydrostatic pressure) is used to draw water into a plant, which reduces the water potential. It is zero in pure water at atmospheric pressure.
Osmotic Potential
The attraction of solutes for water is measured by osmotic pressure. It is the hydrostatic pressure that must be applied to a solution in order to prevent water from flowing into it when separated from pure water by a semipermeable membrane (the membrane that allows water but not solute to pass through).
The value of osmotic pressure is simply determined by the number of solute molecules in the solution. It describes the osmotic relations of the plant cells. The relationship between osmotic pressure and solute concentration is π = cRT, where π is the osmotic pressure, c is the concentration of the solution, R is the universal gas constant, and T is the absolute temperature, for dilute solutions.
Gravity’s Effect
The concept of water potential generally excludes the effects of gravity, but in some cases, such as tall trees or dealing with water flow in soil, gravity must be considered. Where the effect is significant, total water potential, Φ (Pa), is defined as the sum of the water potential (ψ) and a gravitational term (ρgh), where ρ (kg m−3) is the density of water, g (9.8 m s−2) is the acceleration due to gravity, and h is the height (relative to a given reference) in the gravitational field.
Types of Water Flow
There are three different ways by which water flows inside the plant. These are:
1) Water flow by osmosis
2) Water flow by diffusion in liquid or gas
3) Water flow in bulk
Osmotic flow occurs across semipermeable membranes, in which molecules of a solvent pass from a less concentrated solution to a more concentrated one. The flow of water is driven by differences in water potential.
Diffusion is the movement of solute molecules from a high concentration to a low concentration of molecules. Diffusive flow is caused by concentration gradients and random thermal motion of molecules. It is fast over short distances but slow over long distances.
Hydrostatic pressure gradients drive bulk flow. Over long distances, it is much faster than diffusive flow because the molecules are all moving in the same direction, and thus, their movement is cooperative. Water flows in bulk in xylem vessels, cell wall interstices, and soil water-filled pores.
All three types of flow are involved at various stages as water in the transpiration stream moves from the soil to the roots, through the plant, and out through the stomata.
Water in the Soil
Soil is porous and holds water in its pores through capillary action. The water holding capacity of soil is totally based on the principles of soil and plant water relations. The distribution of pore sizes in the soil, as well as its depth, determines how much water the soil can hold for plant use. Large pores drain quickly and contribute little to the reservoir. They drain because gravity creates a suction that the capillary effects are unable to withstand. They also drain quickly because the flow rate in large pores is much faster than in small pores.
Water Flow from the Soil through Roots
Roots are branched and follow winding paths through the soil. Roots are the medial agents in the water relations of plants and soil. Despite their complex geometry, any given segment of the root can be thought of as a cylinder through which water flows down a pressure gradient in the soil.
With fewer roots and drier soil, the soil’s ability to supply water to the root surface may be limited. The stomata must then close to ensure that transpiration does not exceed supply, or the plant will desiccate.
Radial flow from a root to the xylem is partially mediated by cell walls. Water enters the symplast in the root cortex and leaves it in the xylem parenchyma cells next to the vessels, because impervious walls block the way at the endodermis and possibly hypodermis. The resistance to water flow across the membranes involved is significant, and it often accounts for the majority of the drop in water potential between the soil and the evaporating surfaces of the leaves, at least in well-watered plants.
Water flows in the xylem vessels of roots and then stems and branches under the influence of surface tension and cohesive force. From the xylem, the water flows in the substomatal cavities of the leaf, and then through stomata, it transpires out of the plant’s body.
Conclusion
Water acts as an excellent solvent and helps in the uptake and distribution of minerals, nutrients and other solutes for growth. The protoplasm of the cells is nothing but water in which thousands of different molecules are dissolved and several more particles are suspended. Most of the herbaceous plants have 10 to 15 percent of their fresh weight, as dry matter and rest water. A watermelon contains more than 92 percent water. The distribution of water within the plant varies from organ to organ.