Decrease in the amount of free energy of water molecules due to the addition of the solute in water is known as the solute potential. The solute potential of pure water is zero since it does not contain any amount of solute. The more the amount of solute the lower is the water potential and the solute potential is negative. i.e., the solute potential of a solution is always negative.
In accordance with the free energy, water potential is defined as the free energy difference of molecules in water to that in a solution. The water potential is represented by the letter ‘psi’ and is measured in bars. The addition of solutes lowers the free energy of water and thus lowers the water potential. The water potential of pure water at atmospheric pressure is zero. The flow of water occurs from a region of high water potential to a region of low water potential. For a solution at atmospheric pressure water potential is equal to the solute potential. Solute potential is one of the components to determine the water potential.