Plant Water Potential and Its Components
- Concept of Water Potential (Ψw)
- Definition: Water potential is the potential energy of water per unit volume, relative to pure water under standard conditions (atmospheric pressure and room temperature).
- Symbol: Ψw
- Unit: Pressure units → Pascal (Pa), Megapascal (MPa), or Bar
- Reference Point:
- Pure water at standard conditions: Ψw = 0
- Water in plant systems: Ψw is negative (due to solutes, pressure, or matric forces).
Direction of Water Movement:
Water always moves from higher (less negative) Ψw → lower (more negative) Ψw.
- Equation for Water Potential
Ψw=Ψs+Ψp+Ψm
Where:
- Ψs = Solute potential (osmotic potential)
- Ψp = Pressure potential (turgor or hydrostatic pressure)
- Ψm = Matric potential
- Components of Water Potential
(i) Solute Potential (Osmotic Potential, Ψs)
- Due to presence of solutes in water.
- Solute molecules reduce the free energy of water by binding water molecules.
- Always negative.
- The more solutes → the more negative Ψs → stronger tendency to absorb water.
- Governs osmosis.
Example:
- Pure water: Ψs = 0
- 1 M sucrose solution: Ψs ≈ –2.3 MPa (at 25°C).
(ii) Pressure Potential (Ψp)
- Hydrostatic pressure exerted on water inside the cell.
- Can be positive or negative:
- Positive → in turgid living cells (turgor pressure).
- Negative → in xylem vessels due to transpiration pull (tension).
- Maintains cell rigidity, stomatal function, and drives growth by cell expansion.
(iii) Matric Potential (Ψm)
- Results from adsorption of water onto surfaces (cell walls, soil particles, protoplasm).
- Always negative.
- Major role in soil-water relations (capillary water binding).
- In hydrated plant cells, Ψm is usually negligible.
Special Cases
- Turgid Cell: Ψp is positive, balances Ψs → Ψw ≈ 0 (no net water flow).
- Plasmolysed Cell: Ψp = 0 → Ψw = Ψs (negative).
- Soil System: Ψm is very significant (adsorbed water unavailable to plants).
Agronomic Significance
- Explains water absorption by roots (soil Ψw > root Ψw).
- Governs transpiration pull and water ascent in xylem.
- Basis for osmotic adjustment in drought-tolerant crops.
- Used in irrigation scheduling (leaf water potential as stress indicator).
- Helps understand wilting, seed germination (imbibition), stomatal regulation.
Exam Tips
- Pure water Ψw = 0
- Solute potential (Ψs) is always negative
- Pressure potential (Ψp) is usually positive, but negative in xylem
- Matric potential (Ψm) is negative, important in soil
- Water always moves from higher Ψw → lower Ψw