Course Content
Crop Production (Unit 6)
0/29
ASRB NET / SRF / Ph.D. Agronomy
Plant Water Relations

Plant Water Relations

  • Water constitutes 80–90% of growing plant tissues
  • Concerned with absorption, transport, and utilization of water by plants.
  • Water is essential for:
    • Cell turgidity (growth, expansion)
    • Photosynthesis & metabolic reactions
    • Nutrient transport
    • Cooling effect (transpiration)

The central concept that explains water movement is Water Potential (Ψw).

 

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 & room temperature).
  • Introduced by Slatyer & Taylor (1960).
  • Symbol: Ψw
  • Units: Pressure units → Pascal (Pa), Megapascal (MPa), or Bar
  • Reference Point:
    • Pure water at standard conditions → Ψw = 0
    • Water in plants/soil → Ψw is negative (due to solutes, pressure, or matric forces).

Principle: Water always moves from higher (less negative) Ψw → lower (more negative) Ψw.

 

Agronomic Significance

  • Explains root water absorption and xylem transport.
  • Helps in irrigation scheduling (critical Ψw values for crops).
  • Wilting occurs when soil Ψw ≤ –1.5 MPa (Permanent Wilting Point).
  • Drought tolerance: Plants with osmotic adjustment (more negative Ψs) survive better.
  • Used in crop breeding for stress resistance.

 

General Facts

  • Introduced by: Slatyer & Taylor (1960).
  • Pure water: Ψw = 0 (at standard state, 25°C & 1 atm).
  • In plants/soil: Ψw is always negative.
  • Units: Pascal (Pa), Megapascal (MPa), or Bar. 1 MPa ≈ 10 bar.
  • Direction of water movement: Always from higher (less negative) Ψw → lower (more negative) Ψw.

 

Typical Values

  • Leaf water potential: –0.2 to –1.5 MPa (normal conditions).
  • Drought stress leaf Ψw: may reach –3 to –5 MPa.
  • Soil water potential: Field capacity: –0.03 MPa
    • Permanent wilting point (PWP): –1.5 MPa (Briggs & Shantz, 1912)

 

Exam-Ready One-Liners

  • Pure water = Ψw 0 (reference standard).
  • Solute potential = always negative.
  • Pressure potential = positive in turgid cells, negative in xylem.
  • Matric potential = important in soils, negligible in cells.
  • Water movement = higher Ψw → lower Ψw.
  • PWP occurs at Ψw = –1.5 MPa.
  • Imbibition pressure in seeds can reach 200–500 atm.

 

error: Content is protected !!