Course Content
Crop Production (Unit 6)
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ASRB NET / SRF / Ph.D. Agronomy
Plant Water Potential and Its Components
  1. 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.

 

  1. Equation for Water Potential

Ψw=Ψs+Ψp+Ψm

Where:

  • Ψs = Solute potential (osmotic potential)
  • Ψp = Pressure potential (turgor or hydrostatic pressure)
  • Ψm = Matric potential

 

  1. 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

 

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