A2 Drought and water stress

Topic

Drought represents a disruption of the water balance within the soil-plant-atmosphere continuum; this disruption occurs when the evaporative demand of the air surrounding the plant rises while the soil's capacity to supply water to the roots declines. The greater the atmospheric evaporative demand and the lower the soil's supply capacity, the more profound the disruption to the water balance and the more severe the drought stress experienced by the plant.

This disruption to the water balance causes leaf water potential to drop; the greater the disparity between atmospheric evaporative demand and soil supply capacity, the further that potential falls. Depending on how low it drops, this decline in leaf water potential triggers a cascade of physiological responses within the plant, with each response activated sequentially at specific thresholds.

The first threshold crossed as leaf water potential falls triggers stomatal closure, whereby the plant limits the loss of water vapor to the air. If leaf water potential continues to decline, the plant loses tissue turgor; this loss of turgor inhibits plant growth before the drop in leaf water potential reaches the threshold where photosynthesis itself is affected—consequently, growth slows down prior to photosynthesis.

Within the plant, if leaf water potential drops further—beyond the threshold affecting photosynthesis—xylem embolism occurs. This involves the formation of air bubbles (cavitation) that block the conduits through which water rises from the roots to the leaves. Xylem embolism further reduces the plant's ability to replenish water in the leaves, leaving leaf water potential increasingly vulnerable to further decline. When leaf water potential drops below a critical threshold, xylem embolism ceases to be an isolated event and irreversible cellular damage occurs in the plant tissues, leading to the plant's death if the disruption of water balance is not reversed before that point is reached.

The sequence and intensity with which stomatal closure, growth reduction, xylem embolism, and irreversible cellular damage are triggered depend on whether the species follows an isohydric or anisohydric strategy: an isohydric species triggers stomatal closure while leaf water potential is still high—maintaining relatively stable leaf water potential at the cost of halting growth earlier—whereas an anisohydric species delays stomatal closure and allows leaf water potential to drop much further before reacting, thereby remaining more exposed to xylem embolism and irreversible cellular damage.

Ultimately, drought ceases to be merely a disruption of the water balance within the soil-plant-atmosphere continuum and becomes a specific physiological outcome for the plant: depending on the species' isohydric or anisohydric strategy and the severity of the water balance disruption, the plant may simply slow its growth and regain water balance when atmospheric evaporative demand decreases or soil water supply is restored, or it may suffer xylem embolism and irreversible cellular damage leading to death.

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