Drought, heat and salt stress in crops

Escape, avoidance and tolerance

J. Levitt's classic framework describes three strategies, still used in exams and in breeding:

• Escape: completing the life cycle before stress arrives, for example early-maturing varieties that flower before the dry season or the hottest weeks.
• Avoidance: keeping tissues unstressed despite a stressful environment, for example deep roots that reach stored water, closing stomata, leaf rolling, waxy or hairy leaves, or excluding salt at the roots.
• Tolerance: functioning while tissues are stressed, for example osmotic adjustment, protective proteins and antioxidants, or storing salt safely in vacuoles.

Many traits combine these, and the best strategy depends on the timing and severity of stress in the target environment.

Drought

Water stress develops as soil dries and leaf water potential falls. The order of damage matters:

1. Cell expansion is the most sensitive process and slows first, so leaves and stems grow less even under mild stress.
2. Stomata close, driven partly by the hormone abscisic acid (ABA) from roots and leaves, so CO₂ uptake and photosynthesis fall.
3. Under severe stress, leaves senesce and xylem may cavitate, as air bubbles break the water columns.

Timing matters most. Stress at flowering causes pollen sterility, poor fertilisation and grain abortion; in maize, the gap between pollen shedding and silk emergence widens (the anthesis–silking interval), and grain set collapses. Stress during grain filling reduces grain weight.

Plant responses include deeper rooting, osmotic adjustment (accumulating proline, sugars and potassium to keep turgor), leaf rolling, smaller leaf area, and remobilisation of stem reserves to the grain.

Heat

High temperature affects crops in several ways:

• Reproductive failure: pollen development and fertilisation are very heat-sensitive. In rice, a few hours above about 35 °C at anthesis can cause high spikelet sterility; wheat and maize suffer when temperatures exceed about 30 to 35 °C around flowering.
• Shorter grain filling: crops develop faster in heat, so grain filling ends earlier with smaller grains.
• Higher respiration and photorespiration, especially with warm nights.
• Damage to proteins and membranes, at more extreme temperatures.

Plants respond by producing heat-shock proteins, which act as molecular chaperones, by cooling leaves through transpiration (if water is available), and in some rice varieties by flowering early in the morning, when it is cooler.

Salinity

R. Munns described salt stress as acting in two phases:

1. Osmotic phase (within minutes to days): salt in the soil lowers its water potential, making it harder for roots to take up water, much as in drought. Growth slows quickly.
2. Ionic phase (days to weeks): sodium and chloride accumulate in older leaves to toxic levels, those leaves die, and the plant loses photosynthetic area. Sodium also competes with potassium.

Tolerance mechanisms include excluding sodium at the roots, keeping a high potassium-to-sodium ratio, storing sodium in vacuoles away from enzymes, and making compatible solutes such as proline and glycine betaine. Barley, cotton, sugar beet and date palm are relatively tolerant; beans, many fruit trees and rice at the seedling stage are sensitive.

Cold, frost and flooding

• Chilling injury (above freezing, roughly 0 to 12 °C) damages membranes of tropical and subtropical crops such as maize, rice, cotton, banana and tomato.
• Freezing injury occurs when ice forms in tissues; hardy crops such as winter wheat cold-acclimate in autumn, increasing sugars and protective proteins.
• Flooding and waterlogging starve roots of oxygen, forcing them into fermentation. Rice and other tolerant species form aerenchyma (air channels). The SUB1A gene lets "scuba rice" survive up to about two weeks fully submerged by staying dormant, while deepwater rices instead elongate rapidly to keep leaves above water.

Shared responses

Different stresses trigger similar defences:

• Reactive oxygen species (ROS) build up when photosynthesis is disrupted; plants respond with antioxidant enzymes and compounds.
• ABA, the main stress hormone, coordinates stomatal closure and stress genes.
• Compatible solutes, heat-shock and other protective proteins.
• Stresses often come together, as heat with drought, and their combined effect is usually worse than either alone.

Measuring stress tolerance

Breeders compare yields of varieties under stress and without it. A widely used measure is the stress susceptibility index of R. A. Fischer and R. Maurer (1978).

Managing abiotic stress

• Match variety duration and sowing date to avoid stress at flowering.
• Conserve and supply water: mulches, conservation agriculture, supplementary irrigation at critical stages.
• Manage salinity: leaching, drainage, tolerant crops, good irrigation water.
• Use tolerant varieties, from conventional breeding, marker-assisted selection (as with SUB1 rice) and newer tools.
• Protect against heat and cold with shading, windbreaks, irrigation timing and frost protection in high-value crops.

Questions people ask

Sources

1. Levitt, J. Responses of Plants to Environmental Stresses, 2nd edition, volumes 1 and 2. Academic Press, 1980.
2. Munns, R. and Tester, M. "Mechanisms of salinity tolerance." Annual Review of Plant Biology 59, 651–681 (2008).
3. Fischer, R. A. and Maurer, R. "Drought resistance in spring wheat cultivars. I. Grain yield responses." Australian Journal of Agricultural Research 29, 897–912 (1978).
4. Jagadish, S. V. K., Craufurd, P. Q. and Wheeler, T. R. "High temperature stress and spikelet fertility in rice." Journal of Experimental Botany 58, 1627–1635 (2007).
5. Xu, K. et al. "Sub1A is an ethylene-response-factor-like gene that confers submergence tolerance to rice." Nature 442, 705–708 (2006).

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