Water Potential: Water plays a vital role in the living systems. It is a universal solvent, it acts as a reagent and is obtained as a product in many biochemical reactions. It is the major component of all living cells. The protoplasm of a cell consists of about 75-95% water. Here different molecules are dissolved in it.

It helps in maintaining the turgidity of a cell, helps in maintaining internal temperature, etc. Without a constant supply of water, plants could not carry on any of their physiological activities. Water is essential for all the physiological activities of plants. To understand different physiological processes involved in plants, it is important to know some basic processes related to plant-water relations. One such process is Water potential.

Water Potential – Definition

Definition– Water potential refers to the tendency or potential of water to flow from one place to another, which is determined by different internal factors like matric potential, solute potential, pressure potential, etc.

Water Potential – Important points

1. Water potential is a fundamental concept to understand water movement.
2. Water potential is denoted by a Greek letter ψ_w.
3. Its unit is Pascal (Pa).
4. This water potential term was first used by Slatyer and Taylor in 1960.
5. Water molecules possess Kinetic energy. It is a measure of energy available for the movement of water.
6. In liquid and gaseous form, water molecules are in rapid random motion.
7. The greater the concentration of water molecules in a system, the greater is its kinetic energy or water potential.
8. Pure water has greater water potential. By convention, the water potential of pure water at standard temperature and no pressure is taken to zero.
9. The water potential of a solution is always negative. Because in solution, solutes are present, which will decrease the concentration of water molecules and hence its kinetic energy. So, its water potential will also decrease.
10. If two systems are taken in which water is in contact with each other, then the flow of water will be from the system with higher water potential to lower water potential, and it will continue till the water potential of the two systems becomes equal. At this point of equilibrium, the net transfer of water will cease.

Components of Water Potential

For solutions, such as contents of cells, water potential is determined by three internal factors:-

• Solute potential- Ψs
• Matric potential- Ψm
• Pressure potential- Ψp
• Gravity potential- Ψg

Relation between Ψw, Ψs, & Ψp.
The water potential of a cell is the sum of its solute potential and the pressure potential. Ψw =  Ψs + Ψp.

a. Solute potential (Ψs): The amount by which water potential is reduced as a result of the presence of solute in pure water is known as osmotic potential or solute potential. When solutes are added to the pure water, the energy of the water molecules gets reduced.
The solute potential of pure water is zero, so the solute potential is always less than zero or negative. It is represented by Ψs. At atmospheric pressure, Ψw = Ψs.

b. Matric potential (Ψm): Matric refers to the surface, such as cell walls, protoplasm, soil particles, etc. Matric potential is due to the adhesive intermolecular forces and H-bonds that help in the binding of water to the cell wall and cytoplasm. In the case of plant cells and tissues, the matric potential is often disregarded as it is not significant in osmosis. Matrix potential is always negative. It is expressed as Ψm.
This potential is significant in dry seeds. If it is taken into account, then the relationship would be:
Ψw = Ψs+ Ψp+Ψm.

c. Pressure potential (Ψp): If a pressure more than atmospheric pressure is applied to the solution, then its water potential increases. An increase of pressure in a cell increases the turgor pressure and makes the cell turgid. This is pressure potential and denoted by Ψp. It is usually positive. In plasmolyzed cells, it is almost zero. It can be negative in the xylem vessels of a transpiring plant, where water is pulled by an open system creating a tension of about –2 MPa on the leaf surface of a plant.

d. Gravity potential (Ψg): It is due to the effect of gravitational force on the water potential. It depends on the acceleration due to gravity, the height of water above the reference state, and the density of water. But this gravity potential is negligible, so generally, it is not taken into account.

Summary

From the above discussion, we came to the conclusion that for solutions such as the contents of a cell, water potential is determined by three major sets of internal factors, viz. matric potential (Ψm), solute potential (Ψs), and pressure potential (Ψp). Matric potential is often disregarded as it is not significant in osmosis. A fourth potential is called gravity potential, which is due to gravity. It is also generally not taken into account as it is negligible.  So, the relation between Ψw, Ψs, Ψp is written as Ψw = Ψs+ Ψp. The solute potential is always negative. Pressure potential is usually positive, but in xylem vessels under tension, it is negative.

FAQs

Q.1. What is meant by water potential?
Ans: Water potential refers to the tendency or potential of water to flow from one place to another, which is determined by different internal factors like matric potential, solute potential, pressure potential, etc.

Q.2. What is the water potential of pure water?
Ans: The water potential of pure water is always maximum, i.e., 0.

Q.3. Why is the water potential of a cell always negative?
Ans: The water potential of a cell is always negative because of the solute content in the cytoplasm of a cell.

Q.4. Why does pressure increase water potential?
Ans: Pressure increases the kinetic energy of the water molecules present in water and hence increases the water potential.

Q.5. What are the components of water potential?
Ans: Components of water potential are Solute potential (Ψs), Matric potential (Ψm), Pressure potential (Ψp), and Gravitational potential (Ψg).