Solute potential is always ≤ 0 — dissolved solutes lower water potential.
Pressure potential is usually ≥ 0 in turgid plant cells, 0 at flaccid.
How water potential works
Water moves by osmosis toward the region of lower (more negative) water potential. If Ψ is 0, water is in equilibrium and there is no net movement.
- • Ψs — solute (osmotic) potential: negative in solutions
- • Ψp — pressure (turgor) potential: positive when water presses against a wall
- • Units: 1 MPa = 1000 kPa = 145.04 psi
Classic example
A cell with Ψs = −0.9 MPa and Ψp = +0.4 MPa has Ψ = −0.5 MPa. If placed in a solution of Ψ = −0.7 MPa (lower potential), water moves out of the cell into the solution and the cell shrinks.
Turgor pressure
Ψp is what makes plants stand upright. Water entering the vacuole pushes the cell wall outward.
Wilting
When soil water potential drops below root potential, roots lose water to the soil and the plant wilts.
Xylem transport
A water potential gradient from soil → root → leaf drives the entire transpiration stream.
More plant science tools
Pair water potential with our molarity and dilution calculators for solution-based experiments.
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