Water is often the most limiting resource determining the growth and survival of plants. This can be seen in both the yield of crop species and the productivity of natural ecosystems with respect to water availability.
The natural distribution of plants over the earth’s land surface is determined chiefly by water: Day rainfall (RR) and by evaporation demand (potential evapotranspiration, PEPE) which depends on temperature and humidity. This leads to such diverse vegetation groups as the lush vegetation of tropical rain forests, the shrubby vegetation of Mediterranean climates, or stands of tall trees in temperate forests. Climates can be classified according to the Thorniness Index: (RPEYOTE).
Agriculture also depends on rainfall. Crop yield is water-limited in most regions in the world, and agriculture must be supplemented with irrigation if the rainfall is too low. Horticultural crops are usually irrigated.
Plants require large amounts of water just to satisfy the requirements of transpiration: a large tree may transpire hundreds of liters of water in a day. Water evaporates from leaves through stomachs, which are pores whose aperture is controlled by two guard cells. Plants must keep their stomachs open in order to take up CO, as the substrate for photosynthesis. In the process, water is. lost from the moist internal surfaces of the leaf through the stomachs pores. Water loss also has a benefit in maintaining the leaf temperature through evaporation cooling.
The ratio of water lost to CO, taken up is around 300:1 in most land plants, meaning that plants must transpire large quantities of water on a daily basis in order to take up sufficient CO, for normal development.
In this section, we will examine plant water relations and the variables that plant physiologists use to describe the status and movement of water in plants, soil and the atmosphere.
One of the challenging aspects of understanding plant water relations is the range of pressures from positive to negative that occur within different tissues and cells. Positive pressures (turgor) occur in all living cells and depend on the semipermeable nature of the plasma membrane and the elastic nature of the cell walls. Negative pressures (tensions) occur in dead cells and depend on the cohesive strength of water coupled with the strength of heavily signified cell walls to resist deformation. These play an important role in water trans through the xylem.
Knowledge of plant water relations is important because water is essential for both plants and animals. It serves as a medium for the dissolution of substances. A huge amount of water is taken up daily by plants and a considerable amount is lost in transpiration. The water requirement of different categories of plants is different.
Water molecules possess a certain amount of kinetic energy. The greater the concentration of water in a system, the greater is its kinetic energy or water potential. If two systems containing water are in contact, the movement of water molecules occurs from the system with higher energy to the system with lower energy. Water potential is expressed in pascals. The value of the water potential of pure water at standard temperature is 0.
If a certain amount of solute is added to pure water, the concentration of water decreases, and thus the water potential decreases. The amount by which the water potential decreases is called solute potential. This is always negative and the value of solute potential decreases with an increase in the amount of the dissolved solutes. The value of water potential increases when a pressure more than atmospheric pressure is applied to pure water. When water enters a plant cell by diffusion and exerts a pressure on the walls of the cell, the cell is termed as turgid. This increases the pressure potential. This value is usually positive. Water potential is the sum of solute potential and pressure potential.
It is the movement of water across a semi-permeable membrane. Water moves from a region of its higher concentration to the region of its lower concentration till equilibrium is reached.
In this again there are two processes- Endosmosis and Exosmosis.
Endosmosis is a process in which inward diffusion of water through a semipermeable membrane occurs when the surrounding solution is less concentrated while exosmosis is a process in which the outward diffusion of water through a semipermeable membrane occurs when the surrounding medium is more concentrated.
It is a process that occurs when water moves out of the cell and the cell membrane shrinks away from the cell wall. This occurs when the cell is placed in a hypertonic solution (which has more solutes). Water is lost from the cytoplasm and then from the vacuole. When the cell is placed in an isotonic solution, no net movement of water occurs and when it is placed in a hypotonic solution, water moves into the cell and exerts pressure on its walls known as torpor pressure.
It is a process in which water is absorbed by solids and their volume increases. An example of this can be- absorption of water by seeds and dry wood. In this, the movement of water is along the concentration gradient.