Morpho-Physiological Effects of Water Deficit

Morphological Effects

• Roots: Increase in root:shoot ratio. Development of deeper and more prolific roots (adaptation). Root hair proliferation for better absorption.
• Shoots & Leaves: Reduction in shoot growth, plant height. Reduced leaf area, leaf size, and expansion. Leaf rolling (rice), leaf folding, and leaf wilting. Premature leaf senescence and shedding. Reduced tillering/branching.
• Reproductive organs: Poor flowering, reduced pollen viability. Flower and fruit drop. Poor grain filling and reduced seed size.

Physiological Effects

• Water Relations: Decline in leaf water potential (Ψleaf). Reduction in Relative Water Content (RWC). Loss of turgor → wilting.
• Stomatal Behavior: Stomatal closure (ABA-induced). Reduced stomatal conductance → less CO₂ uptake.
• Photosynthesis: Reduced photosynthetic rate due to stomatal closure. Decrease in chlorophyll content → leaf yellowing. Lower Rubisco activity.
• Transpiration: Reduced transpiration due to stomatal closure. But canopy temperature increases (basis of CWSI).
• Nutrient Relations: Lower nutrient uptake and transport. Reduced N assimilation and protein synthesis.
• Respiration: Increased respiration under severe stress → energy loss.

Biochemical & Metabolic Effects

• Accumulation of osmolytes: proline, glycine betaine, sugars → osmotic adjustment.
• Increase in abscisic acid (ABA) → stomatal closure.
• Accumulation of reactive oxygen species (ROS) → oxidative stress.
• Enhanced activity of antioxidant enzymes: SOD, catalase, peroxidase.
• Synthesis of LEA proteins, dehydrins, heat-shock proteins → stress tolerance.

Yield Effects

• Reduction in biomass accumulation.
• Fewer, smaller grains/seeds.
• Reduction in harvest index.
• Maximum yield loss if stress occurs at flowering or grain filling

Key Facts

• Relative Water Content (RWC): 80% → no stress. <50% → permanent wilting.
• Drought stress reduces photosynthesis by 30–70%.
• Root:shoot ratio increases under deficit.
• Reproductive stages are most sensitive to water deficit.
• C₄ plants (maize, sorghum) have higher WUE than C₃ plants (wheat, rice).

Facts – Morpho-Physiological Effects of Water Deficit

• Plant water content = 70–90% of fresh weight.
• Relative Water Content (RWC): 80% = well-hydrated. <50% = permanent wilting.
• Leaf water potential (Ψleaf) in crops under stress: drops from –0.5 MPa (normal) to –2 to –3 MPa (stress).
• Water deficit reduces leaf expansion by 30–50% and plant height by 20–40%.
• Stomatal closure under drought can reduce CO₂ uptake and photosynthesis by 30–70%.
• Transpiration decreases, but canopy temperature rises by 2–5°C (basis of CWSI).
• Root:shoot ratio increases under drought — roots continue to grow while shoot growth slows.
• Water deficit reduces grain yield by 40–70% if it occurs at flowering/grain filling stages.
• Proline (osmolyte) content increases 5–10 times in drought-stressed leaves.
• ABA concentration in leaves can increase 10-fold during drought → induces stomatal closure.
• Reproductive failure: drought reduces pollen viability by 20–80%, causes flower drop in crops like pigeonpea, cotton.
• C₄ plants (maize, sorghum) maintain higher WUE than C₃ plants (rice, wheat) under stress.
• CAM plants (pineapple, agave) survive extreme drought by stomatal opening at night.