Calculate the water potential (Ψ) of a plant cell or solution — the driving force behind osmosis and water movement in plants.
Total water potential: Ψ = Ψs + Ψp
Solute potential: Ψs = -iCRT
where i = ionization constant, C = molar concentration, R = pressure constant (0.0831 L·bar/mol·K), T = absolute temperature (K).
Water moves from high Ψ to low Ψ. Pure water at standard conditions has Ψ = 0. Adding solutes makes Ψs negative; turgor pressure inside a cell makes Ψp positive. A typical leaf cell has Ψ near −1.0 MPa, while soil is around −0.05 MPa under field capacity, so water spontaneously flows from soil → root → leaf → atmosphere.
Water potential explains turgor pressure, wilting, osmotic adjustment under drought, and how trees lift water 100+ meters to their canopies via transpiration-driven tension.