Plant Water Relations

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 Potential

Water molecules possess a certain amount of kinetic energy. The greater the concentration of water in a system, greater is its kinetic energy or water potential. If two systems containing water are in contact, movement of water molecules occur from the system with higher energy to the system with lower energy. Water potential is expressed in pascals. The value of 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.

Osmosis

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.

Plasmolysis

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 a pressure on its walls known as turgor pressure.

Imbibition

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.

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Practise This Question

Which of the following is true in a fully turgid cell?