Drought Tolerance

What is Drought Tolerance?

• Definition: Drought tolerance is a plant’s ability to survive, grow, and produce yield under water-limited conditions.
• Plants can resist drought in three main ways (Levitt, 1972):
  1. Drought Escape: Complete life cycle before severe drought occurs (short-duration crops).
  2. Drought Avoidance: Maintain tissue water by efficient water uptake or reduced water loss.
  3. Drought Tolerance: Survive even when tissue water potential is very low — the plant endures stress.

So, drought tolerance is different from avoidance — the plant actually withstands low water levels rather than just avoiding stress.

Mechanisms of Drought Tolerance

Plants tolerate drought through morphological, physiological, and biochemical adaptations.

Morphological Adaptations

• Deep roots: Tap roots reach water in deeper soil layers (e.g., chickpea, sorghum).
• High root:shoot ratio: More biomass allocated to roots than shoots to enhance water uptake.
• Leaf adaptations: Small, rolled, or hairy leaves reduce water loss.
• Leaf shedding: Some plants drop older leaves during severe drought (pigeonpea, cotton).

Physiological Adaptations

• Osmotic adjustment: Plants accumulate solutes like proline, glycine betaine, and sugars to retain water in cells.
• Stomatal regulation: Hormone ABA triggers stomatal closure to reduce transpiration.
• Maintaining Relative Water Content (RWC): Ensures turgor pressure remains high for cell expansion and growth.
• Water Use Efficiency (WUE): Ability to produce more biomass per unit of water (higher in C₄ crops like maize, sorghum).

Biochemical & Molecular Adaptations

• Osmoprotectants: Proline, sugars, and ions stabilize proteins and membranes.
• Protective proteins: LEA proteins, dehydrins, heat-shock proteins prevent cell damage.
• Antioxidant defense: SOD, catalase, and peroxidase protect cells from oxidative damage caused by drought.
• Stress signaling: ABA and other transcription factors (like DREB genes) help activate drought response genes.

Crop Examples

Why is it Important?

• Ensures crop survival under drought.
• Stabilizes yield in rainfed and semi-arid areas.
• Helps breeding programs identify traits like deep roots, osmotic adjustment, and high WUE.
• Guides agronomic practices: mulching, deficit irrigation, and conservation tillage.

Key Facts

• RWC >80% = no stress; <50% = permanent wilting.
• Proline content increases 5–10× in drought-tolerant crops.
• Drought stress reduces yield by 40–70% if it occurs at reproductive stage.
• C₄ plants use ~30–50% less water than C₃ plants to produce same biomass.
• CAM plants survive in deserts by opening stomata at night.
• Pearl millet and sorghum are considered the most drought-tolerant cereals.