Water potential (Ψ) can be defined as a measure of the potential of water to perform work. To perform work, an object must be capable of applying sufficient force to another object for displacement to occur. For water to displace an object, it must be moving. The highest water potential any volume of water could have if the standardized atmospheric pressure is applied to that volume of water is 0. This is the water potential for that of distilled water. Distilled water has the highest potential to move and displace objects.
On the other hand, solute potential (Ψs), also referred to as osmotic potential is negative in plant cells and in distilled water is zero. Solutes reduce the water potential by the intake of potential energy which is available in the water. The solute potential is the reduction in the magnitude of water potential as a result of the dissolution of a solute. Solute molecules have the potential to dissolve in water as water molecules can associate with them through hydrogen bonds – a hydrophobic molecule such as oil that cannot associate with water cannot go into the solution. If there is a decrease in the solute potential, there is an increase in the solute concentration.
As there is a difference in the water potential, water moves from the soil into the root cells of the plants through osmosis. Hence, the solute potential is at times referred to as osmotic potential.
Factors Affecting Water Potential
In plants, the main factors which influence the water potential are pressure, concentration and gravity. The sum of these components is as follows –
Ψ = Ψs + Ψp + Ψg
Ψs denotes the effects of solutes, Ψp denotes the effects of pressure and Ψg denotes the effects of gravity on the free energy of water.
Effect of solutes on water potential –
The solute potential or the osmotic potential indicates the effect of the dissolved solutes on water potential. The solutes reduce the free energy of water by the dilution of water, which mainly is an entropy effect. In other words, combining solutes and water causes an increase in the entropy or disorder of the system and hence decreases the free energy. This indicates that the osmotic potential is independent of the particular nature of the solute.
Effect of pressure on water potential –
The pressure potential is the effect of the hydrostatic pressure on water’s free energy. The positive pressure increases the water potential, while the negative pressure decreases it. Both negative and positive pressure take place in plants. In the cells, the positive hydrostatic pressure is called turgor pressure. The negative hydrostatic pressure that develops in the xylem conduits frequently is tension.
Effect of Gravity on water potential –
The factor of gravity causes water to move downwards till the gravitational force is conflicted by an equal antagonistic force. This gravitational potential relies on the height of water over the reference state water, the water density and the acceleration due to gravity. Symbolically, it is as follows –
Ψg = ρw gh (value of ρwg is 0.01 MPa m-1)
The gravitational factor (Ψg ) generally is left out in criteria of water transport at the level of cells, as variations in this factor among adjacent cells are negligible in comparison with differences in the pressure potential and osmotic potential.
This was a brief on factors affecting water potential. For more on NEET explore NEET BYJU’S.