Water Potential in Plants and Its Components

Water Potential in Plants

Water potential is the measure of free energy in water per unit volume. The free energy of a molecule determines its ability to perform a task. The free energy of a molecule in a chemical system is called chemical potential. The chemical potential of water is known as water potential.

Chemical potential is expressed as the difference between the potential of a substance or molecule in a given state and the standard state.

What is The Unit of Water Potential?

Water potential is expressed as Mega Pascal or MPa.

What is The Water Potential of Pure Water?

Pure water has the maximum water potential, and it is zero at atmospheric pressure. But in living cells, water potential is the sum of different components such as solute potential, pressure potential, gravitational potential, and matric potential.

Moreover, the higher the water potential in living cells, the better they can absorb water.

This is not an absolute value. It is denoted as psi or Ѱ.

Thus, water potential is expressed as

Ѱ_w = Ѱ_s + Ѱ_p + Ѱ_m + Ѱ_g

Solute Potential

Solute potential (Ѱ_s) or osmotic potential is the effect of a solute on water potential, which reduces its free energy. Macromolecules such as proteins, fats, starch, etc, have a lower solute potential than their monomer forms, such as amino acids, glucose, etc. The monomer molecules have higher solute potential. This is why living cells use macromolecules to store energy to reduce osmotic potential changes.

Pressure Potential

The hydrostatic pressure of a solution or the pressure required to stop further entry of water into a cell is known as pressure potential (Ѱ_p). It helps maintain the turgidity of the cell and is also known as turgor pressure (TP). It is the difference in pressure between the inside and outside of the cell.

A fully turgid plant cell has an equal and opposite pressure called wall pressure. The plant cell wall helps it withstand a wider range of pressure variations.

A plant cell placed in pure water absorbs water and swells. It does not burst due to the negative pressure exerted by the osmotic potential of the solution in the cell sap.

Gravitational Potential

The effect of gravitational pull on the water potential is called gravitational potential (Ѱ_g). It is the sum of the density of water, the height of the water column, and the gravitational acceleration. Even though gravitational potential affects the water potential, it is highly negligible.

Matric Potential

The adsorption affinity of water towards the plant cell surface and colloidal substance is called matric potential (Ѱ_m). In hydrated cells, matric potential is negligible, but a dehydrated cell requires matric potential to absorb water. Matric potential is negligible for a hydrated cell but is important for dehydrated cells and tissues. Under the dehydrated conditions, water forms a very thin layer bound to solid surfaces due to electrostatic interactions. Since such interactions do not affect the solute or pressure potential, they are considered matric potential.

A dry substance or a hydrophilic substance shows a highly negative potential.

Changes of Water Potential in Plants

When there is no solute movement in or out of the cell, water movement causes a change in cell volume.
Water potential and osmotic potential are always negative.
Water potential in plants is zero in a fully turgid cell.
In plants, the pressure exerted by the cell wall and dilution of the cell sap are the reasons why the water potential becomes zero.
The plant cell volume changes, but a small change in it causes large changes in turgor pressure. However, as the cell wall in growing cells expands, leading to a decrease in cell wall pressure, such changes are not visible.