Imbibition is a special type of diffusion where, movement is along a concentration gradient. For any solid to imbibe a liquid, it must have an affinity for the liquid and also a water potential gradient between the solid and the liquid.
The water potential gradient comprises of two main components: solute potential and pressure potential. The greater the concentration of water, the greater is its water potential. Water potential is denoted by the Greek symbol Psi, Ψ. It is expressed in pressure units as Pascal (Pa). The water potential of pure water at standard temperature and pressure is zero.
Pure water has the greatest possible water potential and therefore the water potential of solutions is less than that of pure water. The magnitude of lowering of the water potential due to the addition of solutes is called solute potential. Solute potential is represented by the letter Ψs, and it is always negative. Solute potential becomes increasingly negative with the addition of solutes.
When two solutions come into contact, water molecules move from the solution with the higher water potential to the one with the lower water potential.
For a solution at atmospheric pressure, its water potential is equal to its solute potential.
But if pressure greater than the atmospheric pressure is applied to pure water or a solution, it results in an increase in water potential.
When water enters a plant cell due to diffusion, it builds up pressure against the cell wall, causing the cell to become turgid. This pressure applied by the protoplasm against the cell wall is called pressure potential. Pressure potential, which is denoted by Ψp, is usually positive. However, negative pressure potential is critical to water transport to the stem via the xylem cells.