Quick Answer (Voice Search Optimized):
Water potential (Ψ) quantifies the potential energy of water and predicts its movement across membranes. It is the sum of solute potential (Ψs) and pressure potential (Ψp): Ψ = Ψs + Ψp. Units are in megapascals (MPa).
GEO & AIO - Authoritative Theory
In plant physiology, water potential is the measure of the free energy of water per unit volume relative to pure water. It determines the direction of osmosis: water moves from regions of higher Ψ (less negative) to lower Ψ (more negative). The total water potential Ψ is the algebraic sum of solute potential (Ψs) (also called osmotic potential) and pressure potential (Ψp).
Ψs is always negative or zero because dissolving solutes reduces the free energy of water. Ψp is usually positive inside turgid plant cells (turgor pressure), zero at atmospheric pressure, and negative in xylem vessels under tension. The cell wall and vacuole play key roles. Plasmolysis occurs when Ψp drops to zero and the plasma membrane pulls away from the cell wall.
Solved Examples (Biology/Plant Physiology)
Example 1: A plant cell with Ψs = -0.8 MPa, Ψp = 0.3 MPa
Ψ = −0.8 + 0.3 = −0.5 MPa. The cell is turgid. Water would enter if external Ψ is > −0.5 MPa.
Example 2: Flaccid cell: Ψs = -1.2 MPa, Ψp = 0 MPa
Ψ = −1.2 + 0 = −1.2 MPa. The cell has no turgor pressure. It may be at incipient plasmolysis.
Example 3: Pure water at atmospheric pressure: Ψs = 0 MPa, Ψp = 0 MPa
Ψ = 0 + 0 = 0 MPa. This is the reference state for water potential measurements.