Calculate plant water potential (Ψ) from solute and pressure components. Understand water movement in plants using the fundamental equation: Ψ = Ψs + Ψp.
Last updated: March 2026
1 for non-ionic (sucrose), 2 for NaCl, 3 for CaCl₂
Turgor pressure (positive) or tension (negative)
Water potential (Ψ, the Greek letter psi) is a measure of the free energy of water in a system. It determines the direction water will move: from areas of higher water potential to areas of lower water potential. This fundamental concept explains water movement in plants, soils, and biological systems.
Water potential is typically expressed in units of pressure (bars, megapascals, or kilopascals). Pure water at atmospheric pressure and standard temperature has a water potential of zero. Any factor that reduces the free energy of water (dissolved solutes, negative pressure) makes water potential more negative.
In plant physiology, water potential has two main components: solute potential (Ψs, also called osmotic potential) and pressure potential (Ψp). Solute potential is always negative because dissolved solutes reduce water's free energy. Pressure potential can be positive (turgor in cells) or negative (tension in xylem). The equation Ψ = Ψs + Ψp combines these to determine net water movement.
Plant Cell with 0.5 M Sucrose Solution
This flaccid cell (no turgor) has negative water potential due to dissolved sucrose. Water will move INTO this cell from pure water (Ψ = 0). If the cell had turgor pressure of +3 bars, total Ψ would be -1.239 + 3 = +1.761 bars (positive).
Pure water at atmospheric pressure has Ψ = 0. Adding solutes or applying negative pressure (tension) lowers free energy, making Ψ negative. Most biological systems contain solutes, so Ψ is typically negative.
Ψs (solute/osmotic potential) is the effect of dissolved solutes (always negative). Ψp (pressure potential) is physical pressure: positive for turgor in cells, negative for tension in xylem, zero in flaccid cells.
Water moves from higher (less negative) Ψ to lower (more negative) Ψ. Example: soil Ψ = -0.3 bars, root Ψ = -0.6 bars → water flows from soil into roots.
The ionization constant (i) accounts for how many particles a molecule produces in solution. Sucrose doesn't ionize (i=1), NaCl splits into Na⁺ and Cl⁻ (i=2), CaCl₂ into Ca²⁺ and 2Cl⁻ (i=3).
Yes! Turgid plant cells often have positive Ψ due to turgor pressure (Ψp) exceeding the negative Ψs. This pushes water out. Root pressure and guttation are examples of positive Ψ in plants.
Bars are common in plant physiology (1 bar ≈ 0.1 MPa ≈ 100 kPa). This calculator uses bars. The pressure constant R = 0.00831 L·bar/(mol·K) matches bar units. Convert if using MPa or kPa.
Use a pressure chamber (Scholander bomb) for Ψ, psychrometer for Ψ, or osmometer for Ψs. This calculator is for theoretical calculations or when you know component values.
Matric potential (Ψm) from surface adhesion is important in soils but usually negligible in plant cells. The full equation is Ψ = Ψs + Ψp + Ψm. This calculator focuses on the plant cell equation.
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