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.