Environmental Stresses on Crops

Environmental Stresses

• Environmental stresses are unfavorable conditions like water deficit/excess, high radiation, extreme temperatures, salinity, nutrient deficiencies, and pollution.
• Stresses affect crop growth in terms of morphology, physiology, metabolism, and yield.
• Stress is the action, and strain is the reaction.

Drought (Water Stress)

• Drought is a severe water deficiency that inhibits plant growth.
• Drought is categorized into soil drought and atmospheric drought.
• Soil drought leads to atmospheric drought.
• Physiological changes due to drought:
  • Stomata: Lose function and may die, wilting denatures starch in guard and mesophyll cells.
  • Carbohydrates metabolism: Drought depletes starch in wilted leaves and accumulates sugar.
  • Photosynthesis: Reduced stomatal openings inhibit CO2 diffusion, decreasing photosynthesis.
  • Osmotic pressure: Reduced water leads to increased osmotic pressure in plant cells, helping utilize soil moisture better.
  • Permeability: Water and urea permeability increase during drought.
  • Biochemical effects: Starch converts to sugar, nitrate nitrogen increases, and protein synthesis is negatively affected.

Adaptation to Drought

• Drought resistance: Plants' capacity to survive drought with minimal damage.
• Drought-resistant plant types:
  • Ephemerals: Short-lived plants that complete their life cycle during the rainy season and survive as seeds in dry periods.
  • Succulent plants: Store large amounts of water for use during dry periods, adapting with features like thick cuticles, reduced leaf areas, and sunken stomata to minimize transpiration.
  • Non-succulent plants: True drought-tolerant plants that endure drought without water-supplying mechanisms. They exhibit xeromorphy (greyish color, reflective surfaces, smaller leaves, extensive root systems, leaf fall, sunken stomata, thick cuticles, and more).

Methods to Overcome Drought

• Selection of drought-tolerant species.
• Adjusting sowing time to ensure completion of the crop cycle before drought onset.
• Seed hardening with KCl, KH2PO4, CaCl2, or Thiourea.
• Thinning of poorly established plants.
• Mulching to reduce evaporation.
• Foliar spray of antitranspirants like Kaolin, PMA, Waxes, and Silicone oils.
• Foliar spray of KCl.
• Foliar spray of growth retardants like CCC and MC.

High Moisture Stress - Flooding/Waterlogging

• Waterlogging occurs when excessive water saturates the soil, creating anaerobic conditions in the rhizosphere.
• Effect of waterlogging:
  • Oxygen deficiency inhibits plant growth and survival.
  • Flood-sensitive plants die, while tolerant plants can withstand waterlogging for some time.
  • Continuous submergence can be detrimental.

Plant Water Relations in Flooding Stress

• Flooding triggers stomatal closure in C3 plants.
• Reduced root permeability leads to leaf dehydration and wilting.
• Toxic substances (acetaldehyde/alcohol) produced under anaerobic conditions in roots are transported to shoots, affecting plant functions.

Levels of Endogenous PGRs under Flooding Stress

• Flooding stress reduces GA and CK levels in roots.
• ABA and ethylene levels increase in shoots, causing stomatal closure and senescence, respectively.
• Auxin levels decrease, while ACC (ethylene precursor) levels increase.

Mitigation of High Moisture Stress (Waterlogging)

• Providing drainage to remove excess water.
• Spraying cycocel growth retardant to reduce apical dominance and promote lateral growth.
• Foliar spray of 2% DAP + 1% KCl.
• Nipping terminal buds to promote sympodial branches (e.g., in cotton).
• Spraying NAA to control premature fruit/pod fall.
• Spraying brassinolide to increase photosynthetic activity.
• Foliar spray of salicylic acid to increase stem reserve utilization.
• Foliar spray of Boric acid + ZnSO4 + FeSO4 + urea.

Salt Stress

• Salt stress arises from excessive salt accumulation in the soil, decreasing soil solution water potential and leading to exosmosis and wilting.
• Classification of saline soil:
  • Saline soil: Electrical conductivity > 4 dS/m, exchangeable sodium percentage < 15%, pH < 8.5, dominated by Cl- and SO2-4 ions.
  • Alkaline soil (sodic soil): Electrical conductivity < 4 dS/m, exchangeable sodium percentage > 15%, pH > 8.5, dominated by CO-3 and HCO-3 ions.

Classification of Plants Based on Salt Tolerance

• Halophytes: Grow in high salt concentrations:
  • Euhalophytes: Tolerate extreme salt stress.
  • Oligohalophytes: Tolerate moderate salt stress.
• Glycophytes: Sensitive to high salt concentrations.

Effect of Salt Stress on Plant Growth and Yield

• Seed germination: Salt stress delays germination due to reduced α-amylase activity.
• Seedling growth: Early seedlings are more sensitive. Root growth and length are significantly reduced.
• Vegetative growth: Salt injury is more severe at high temperatures and low humidity.
• Reproductive stage: Salt affects panicle initiation, spikelet formation, fertilization, and pollen germination.
• Photosynthesis: High salt concentrations damage thylakoids and reduce chlorophyll b content.

Mechanism of Salt Tolerance

• Maintaining high water potential by reducing transpiration.
• Salt accumulation in older leaves and stems with slower metabolic rates.
• Accumulation of K+ ions to avoid Na+ toxicity.
• Accumulation of toxic ions in the vacuole but not the cytoplasm.
• Accumulation of proline and abscissic acid, linked to salt tolerance.

Relative Salt Tolerance of Crops

• Tolerant: Cotton, sugarcane, barley.
• Semi-tolerant: Rice, maize, wheat, oats, sunflower, soybean.
• Sensitive: Cowpea, beans, groundnut, grams.

Mitigation of Salt Stress

• Seed hardening with NaCl (10 mM concentration).
• Application of gypsum (50% Gypsum Requirement).
• Other options:
  • Improving drainage to leach salts.
  • Using salt-tolerant varieties.
  • Modifying irrigation practices.
  • Applying organic matter.