Q.Briefly describe water potential. What are the factors affecting it?

Water potential (ψw), usually measured in pressure units (pascals, Pa), expresses the chemical/free energy status of water at a point relative to pure water at standard atmospheric pressure and temperature, whose water potential is defined as zero. Water always moves — by diffusion or osmosis — from a region of higher water potential to a region of lower water potential. Water potential is the algebraic sum of two main components: ψw = ψs + ψp, where ψs is the solute (osmotic) potential — always negative or zero, since dissolved solutes lower the free energy of water — and ψp is the pressure potential, which is usually positive in a turgid living cell (due to wall pressure resisting the entry of more water) but can be negative, as in the tension found in xylem vessels. In soils …

Water potential (ψw) is the potential energy of water, expressed relative to the potential energy of pure water at standard atmospheric pressure and temperature (which is taken as the reference value of zero). It is essentially a measure of the tendency of water to move, diffuse, or evaporate, and — critically — it determines the direction in which water will spontaneously move: water always moves from a region of higher water potential to lower water potential. Water potential is conventionally expressed in pressure units, most commonly pascals (Pa) or megapascals (MPa).

Components / factors affecting water potential:

ψw = ψs + ψp

• Solute potential (ψs), also called osmotic potential: This is the contribution of dissolved solutes to water potential. Any solute dissolved in water reduces the free energy (and hence the water potential) of that water, so ψs is always negative, becoming more negative as solute concentration increases; it equals zero only in pure water. This is the dominant factor determining the water potential of the cell sap/vacuolar contents.
• Pressure potential (ψp): This is the effect of physical (hydrostatic) pressure on water potential. In a living, walled plant cell, as water enters by osmosis the protoplast swells and presses against the rigid cell wall, which pushes back (wall pressure); this generates a positive pressure potential that raises the cell's overall water potential and opposes further net entry of water — this is what produces turgor in cells. Pressure potential can, however, become negative, as happens under tension in the water columns of xylem vessels during active transpiration. …

Define water potential as free energy relative to pure water (zero reference) that governs the direction of water movement, then give the equation ψw = ψs + ψp, explain each term's sign and physical meaning (solutes lower it, pressure/turgo …

• Omitting the equation/relationship between solute potential and pressure potential and simply listing vague 'factors' without explaining their sign or physical basis.
• Saying pressure potential is always positive — it can be negative under tension, such as in the xylem. …