Water Potential

Water Potential — the Energy Status of Water

Water potential (symbol Ψw, the Greek letter psi) measures the free energy of water in a system, and it is the master concept for predicting which way water will move. Water molecules are always in constant, random motion; the more "free" water molecules present, the greater their combined kinetic energy, and so the higher the water potential. Pure water, having the maximum possible concentration of free water molecules, is given the reference value of zero at standard conditions and no applied pressure — every other value is measured relative to it.

Whenever two water-containing systems are connected, water flows net from the side of higher potential to the side of lower potential — this is simply diffusion driven by potential rather than by concentration alone. Water potential has two components that add together:

• Solute potential (Ψs) — dissolving any solute in water reduces the concentration of free water molecules, which lowers the potential. Ψs is therefore always negative, and becomes more negative as more solute is added.
• Pressure potential (Ψp) — applying pressure raises water potential. Inside a plant cell this arises when water entering by diffusion pushes outward against the (fairly rigid) cell wall, producing turgor — normally a positive value, though the xylem also relies on negative pressure (tension) to pull water upward. …