To comprehend plant-water relations, an understanding of certain standard terms is necessary. Water potential (Ψw) is a concept fundamental to understanding water movement.
Solute potential (Ψs) and pressure potential (Ψp) are the two main components that determine water potential. Water molecules possess kinetic energy. In liquid and gaseous form they are in random motion that is both rapid and constant.
The greater the concentration of water in a system, the greater is its kinetic energy or ‘water potential’. Hence, it is obvious that pure water will have the greatest water potential. If two systems containing water are in contact, random movement of water molecules will result in net movement of water molecules from the system with higher energy to the one with lower energy.
Thus water will move from the system containing water at higher water potentialto the one having low water potential. This process of movement of substances down a gradient of free energy is called diffusion. Water potential is denoted by the Greek symbol Psi or Ψ and is expressed in pressure units such as pascals (Pa). By convention, the water potential of pure water at standard temperatures, which is not under any pressure, is taken to be zero.
If some solute is dissolved in pure water, the solution has fewer free water and the concentration of water decreases, reducing its water potential. Hence, all solutions have a lower water potential than pure water; the magnitude of this lowering due to dissolution of a solute is called solute potential or Ψs. Ψs is always negative. The more the solute molecules, the lower (more negative) is the Ψs .
For a solution at atmospheric pressure (water potential) Ψw = (solute potential) Ψs. If a pressure greater than atmospheric pressure is applied to pure water or a solution, its water potential increases. It is equivalent to pumping water from one place to another.
Can you think of any system in our body where pressure is built up? Pressure can build up in a plant systemwhen water enters a plant cell due to diffusion causing a pressure built up against the cell wall, it makes the cell turgid (see section 11.2.2); this increases the pressure potential.
Pressure potential is usually positive, though in plants negative potential or tension in the water column in the xylem plays a major role in water transport up a stem. Pressure potential is denoted as Ψp. Water potential of a cell is affected by both solute and pressure potential.
The relationship between them is as follows: Ψw = Ψs + Ψp