Crop Adaptation to Water Deficit

Concept

• Water deficit (drought stress) occurs when water availability falls below crop demand.
• Crops adapt through morphological, physiological, and biochemical mechanisms to avoid, tolerate, or escape drought.

Types of Adaptation Strategies

i) Drought Escape

• Completing life cycle before onset of severe drought.
• Mechanism: short duration, early maturity.
• Examples: Early-maturing varieties of wheat, barley, pulses, pearl millet.

ii) Drought Avoidance

Plant maintains high tissue water potential despite soil water deficit.

Mechanisms:

• Efficient water uptake
• Deep, prolific roots (sorghum, chickpea).
• More root:shoot ratio.
• Mycorrhizal association for better absorption.

Reduced water loss

• Stomatal closure (ABA mediated).
• Thick cuticle, waxy leaf surface.
• Leaf rolling (rice).
• Reduced leaf area (pearl millet, sorghum).
• Leaf shedding in severe drought.

iii) Drought Tolerance

• Plant endures low tissue water potential and maintains metabolic activity.
• Mechanisms:
• Osmotic adjustment → accumulation of solutes (proline, glycine betaine, sugars, K⁺).
• Cell membrane stability → protects against desiccation.
• Protective proteins (LEA proteins, dehydrins).
• Antioxidant defense against drought-induced ROS.

iv) Morphological Adaptations

• Deep and extensive root system (sorghum, chickpea).
• Narrow/rolled leaves → reduce transpiration (rice, maize).
• Sunken stomata, hairiness, cuticular wax.
• Leaf abscission during severe drought (cotton, pigeonpea).
• C₄ plants (maize, sorghum) & CAM plants (pineapple, agave) → higher water use efficiency (WUE).

Physiological & Biochemical Adaptations

• Stomatal regulation → balances photosynthesis vs water loss.
• Osmotic adjustment → maintains turgor under low Ψw.
• Accumulation of osmoprotectants → proline, betaine, trehalose.
• ABA (abscisic acid) → induces stomatal closure under drought.
• Antioxidant enzymes (SOD, catalase, peroxidase) → protect against oxidative stress.

Agronomic & Genetic Adaptations

• Breeding drought-tolerant varieties (e.g., ICRISAT pearl millet, ICAR chickpea).
• Short duration crops escape terminal drought (e.g., mungbean, barley).
• Use of mulching, conservation tillage, rainwater harvesting to support adaptation.
• Genetic engineering: DREB, HSP, and LEA genes for stress tolerance.

Key Facts

• Drought resistance = Escape + Avoidance + Tolerance.
• Relative Water Content (RWC) is a key indicator of adaptation; >80% = healthy, <50% = stress.
• Pearl millet & sorghum: deep roots + waxy cuticle → classic drought avoiders.
• Chickpea: deep tap root → drought avoidance.
• Rice (SRI method) improves root growth & water productivity under deficit.
• C₄ crops use 30–50% less water than C₃ crops to produce same biomass.
• CAM plants open stomata at night → extreme drought adaptation.