Mechanisms of Drought Tolerance in Plants

Definition:

• Drought tolerance is a plant’s ability to survive, grow, and yield under low water availability, by maintaining metabolic activity even when tissue water potential is low.
• Plants resist drought mainly via escape, avoidance, and tolerance, with tolerance being the ability to endure water deficit.

Morphological Mechanisms

These are structural adaptations that help plants cope with water deficit.

Root adaptations

• Deep rooting: Taproots and lateral roots grow deep to access water from deeper soil layers (e.g., chickpea, sorghum, pigeonpea).
• High root:shoot ratio: Plants allocate more biomass to roots than shoots to enhance water uptake.
• Root hair proliferation: Increases surface area for absorption.

Leaf and shoot adaptations

• Leaf rolling or folding: Reduces exposed surface area to decrease transpiration (e.g., rice, maize).
• Reduced leaf area: Smaller leaves minimize water loss.
• Waxy cuticle and leaf hairs: Reduce cuticular transpiration (e.g., pearl millet).
• Leaf shedding/abscission: In severe drought, older leaves are dropped to conserve water (cotton, pigeonpea).
• Stem succulence: Some plants store water in stems (cactus, agave).

Reproductive adaptations

• Early flowering or short duration varieties (drought escape strategy).
• Reduction of flower and fruit drop during stress via osmotic adjustment.

Physiological Mechanisms

These involve processes inside the plant to conserve water and maintain turgor.

Osmotic adjustment

• Accumulation of compatible solutes (osmolytes) like proline, glycine betaine, sugars, K⁺.
• Maintains cell turgor and expansion under low Ψw.
• Helps root cells continue water uptake even at low soil moisture.

Stomatal regulation

• ABA hormone signals stomatal closure during water deficit.
• Reduces transpiration while balancing CO₂ uptake for photosynthesis.

Maintenance of Relative Water Content (RWC)

• Plants maintain tissue hydration even under soil water deficit.
• Turgor maintenance ensures cell expansion and growth.

Photosynthesis and water use efficiency (WUE)

• C₄ plants (maize, sorghum, pearl millet) have higher WUE than C₃ plants.
• CAM plants open stomata at night to reduce daytime water loss.

Leaf-level mechanisms

• Leaf rolling/folding, stomatal closure, and reduced leaf area decrease water loss.
• Leaf senescence may occur in older leaves while younger leaves remain active.

Biochemical Mechanisms

• Osmoprotectants: Accumulate to stabilize proteins and membranes (proline, sugars, polyols).
• Stress proteins: LEA proteins, dehydrins, heat-shock proteins (HSPs) protect cells from dehydration.
• Antioxidant defense: ROS (reactive oxygen species) increase under drought; enzymes like SOD, catalase, peroxidase neutralize ROS.
• Hormonal regulation: ABA increases, triggering stomatal closure and activation of stress-responsive genes.
• Compatible solute synthesis: Maintains osmotic balance and protects enzyme function.

Molecular Mechanisms

• Gene expression: Drought-responsive genes activated under stress:
  • DREB (Dehydration-Responsive Element Binding) genes → activate downstream stress genes.
  • CBF/DREB1 genes → protect against dehydration.
  • NAC, MYB transcription factors → regulate osmolyte synthesis, protective proteins.
• Signal transduction: Sensing drought stress triggers secondary messengers like Ca²⁺, ROS, and ABA to initiate protective responses.
• Epigenetic regulation: Some crops “remember” stress and activate quicker response on subsequent drought episodes.

Crop Examples

Key Facts

• RWC >80% = healthy; <50% = permanent wilting.
• Proline content increases 5–10× under drought in tolerant crops.
• ABA can increase 10-fold during drought → stomatal closure.
• C₄ crops use 30–50% less water than C₃ crops for same biomass.
• Deep roots can access water from 1–2 m soil depth.
• Drought stress at reproductive stage causes maximum yield loss (up to 70%).