Water Potential (symbol Ψ, The Greek Letter Psi)

Water potential is a property that characterizes water's propensity to shift across different regions as a result of various physical and chemical factors, including concentration, pressure, gravity, and other variables. It is typically expressed in pressure units such as pascals (Pa) or bars (bar) and is always assessed relative to a reference point, which is typically pure water under standard temperature and pressure. The movement of water is always from regions of high water potential to regions of low water potential until equilibrium is achieved. Various factors such as solute concentration, pressure, and temperature can influence the water potential of a solution.

Water potential is a fundamental term derived from thermodynamics. Water molecules possess kinetic energy, which means that in liquid or gaseous form they move about rapidly and randomly from one location to another. The greater the concentration of water molecules in a system, the greater the total kinetic energy of water molecules in that system and the higher is its so-called water potential. Pure water therefore has the highest water potential. If two systems containing water are in contact (such as soil and atmosphere, or cell and solution) the random movements of water molecules will result in the net movement of water molecules from the system with the higher water potential (higher energy) to the system with the lower water potential (lower energy) until the concentration of water molecules in both systems is equal. This is diffusion involving water molecules.

Note the following main points:

1) Pure water has the maximum water potential, which by definition is zero.

2) Water always moves from a region of higher water potential to a region of low water potential.

3) All solutions have lower water potentials than pure water and therefore have negative values of Ψ (at atmospheric pressure and a defined temperature).

4) Osmosis can be defined as the movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane.