Development of Crop Water Deficit

Concept

• Crop water deficit occurs when water loss (transpiration + evaporation) > water uptake by roots.
• It is the imbalance between crop water requirement (ETc) and soil water supply.
• Expressed as:

Water Deficit = ETc − Water SupplyWater \ Deficit

Causes of Water Deficit

• Soil factors:
  • Low soil moisture (below Field Capacity).
  • Poor water holding capacity (sandy soils).
  • High salinity → lowers soil water potential.
• Plant factors:
  • Shallow root system.
  • High leaf area index (LAI) → more transpiration.
  • Poor stomatal regulation.
• Atmospheric factors:
  • High temperature, radiation, wind speed.
  • Low relative humidity → increases vapor pressure deficit (VPD).

Process of Development

• Initial Phase (Soil Drying): Soil water decreases from Field Capacity toward PWP. Root uptake becomes slower.
• Plant Response Phase: Root water absorption cannot match transpiration. Leaf water potential (Ψleaf) drops. Relative water content (RWC) falls (<80%).
• Physiological Stress Phase; Stomata close → photosynthesis reduced. Turgor loss → leaf rolling, wilting. Growth slows down (cell division, expansion inhibited).
• Severe Deficit Phase
  • RWC <50%.
  • Irreversible wilting.
  • Premature leaf senescence & flower drop.
  • Yield losses.

Effects of Crop Water Deficit

• Morphological: reduced leaf area, leaf rolling, wilting, reduced height.
• Physiological: stomatal closure, reduced photosynthesis, increased respiration, reduced nutrient uptake.
• Biochemical: accumulation of proline, glycine betaine, ABA.
• Yield: fewer flowers, poor grain filling, smaller fruits.

Quantification

• Relative Water Content (RWC) = indicator of plant hydration.
• Leaf Water Potential (Ψleaf) measured with pressure chamber.
• Crop Water Stress Index (CWSI): based on canopy temperature vs air temperature.

Agricultural Significance

• Helps identify critical stages of irrigation (flowering, grain filling).
• Guides deficit irrigation practices (regulated deficit irrigation, partial root drying).
• Basis for breeding drought-tolerant crops with:
  • Deep roots.
  • High osmotic adjustment capacity.
  • High WUE.

Key Facts

• RWC >80% = no stress; <50% = permanent wilting.
• Most crops experience maximum yield loss if deficit occurs at reproductive stage (e.g., flowering in rice, tasseling in maize, boll formation in cotton).
• A plant may lose up to 10–20% of its fresh weight water before wilting appears.
• Drought stress reduces photosynthesis by 30–70% depending on severity.