Drought Stress Responses: Avoidance and Tolerance

Under conditions of drought, plants lose water to the atmosphere. When water uptake cannot keep pace with water loss, transpiration is fed mainly from the vacuoles. Usually the water permeability of the tonoplast is considerably higher than that of the plasma membrane. This allows fast equilibration of the intracellular water potentials upon changes in the cell’s water status. Water flows via the plasma membrane into the apoplast, from where it evaporates into the intercellular spaces. Because of the decrease in volume caused by water loss, the osmotic pressure in the protoplast is increased; that is, the osmotic potential becomes more negative. At the same time, the hydrostatic pressure decreases.

When the so-called turgor loss point is reached, the cell wall is completely relaxed and the pressure (ψ_p) is zero. Consequently, the water potential of the cell is then equal to its osmotic potential (ψ_s). In this state a plant shows substantial wilting. With further loss of water, wilting increases, as the cell walls not only are relaxed but also, in responding to the intracellular suction, bend inwards.

This will finally result in cytorrhysis—that is, the complete collapse of the cell—provided that the cell wall rigidity is low enough to allow folding. As described for the case of freezing dehydration, this happens because the water-imbibed cell wall does not allow any air to penetrate. Suction develops and, correspondingly, a negative water potential (-1 to -2 MPa) of the cell arises.

Besides the escape of water-limited conditions through the timing of, for instance, germination or flowering (Fig. 6.7), principally two different strategies to cope with water scarcity can be distinguished: avoidance and tolerance. It is important to note, however, that these categories merely represent distinct areas within a continuum of responses.

Fig. 6.7. Drought adaptation strategies ranging from escape to tolerance of severe dehydration. (Modified from Verslues and Juenger (2011))

They can even both be displayed by the same plant, depending on the severity of the water limitation (Fig. 6.7). Avoidance refers to a balancing of water uptake and water loss that maintains the water status. This can be achieved by restricting water loss, by increasing the water supply or by water storage. It has been argued that avoidance is an idealised concept. In reality, avoidance mechanisms achieve later onset of drought stress (Lawlor 2013). Tolerance mechanisms help a plant endure a moderate lowering of the water potential. They protect cells against damage potentially arising from water loss—for instance, by compatible solute accumulation. Key cellular functions are maintained, enabling resumption of growth after resupply of water.

An extreme form of drought tolerance is shown by poikilohydric plants, which withstand near complete tissue dehydration (desiccation). Lichens and certain mosses, on the one hand, and resurrection plants such as Craterostigma plantagineum and Xerophyta humilis, on the other hand, have evolved specific adaptations that allow them to enter a dormant or quiescent state under dry conditions. These adaptations are substantially different from those that allow homoiohydric plants to continue their physiological activity during less severe water limitation. Owing to the accumulation of protective proteins and osmolytes, cells of poikilohydric plants survive a very low water status and are able to resume metabolic activity following rehydration. This enables poikilohydric plants to inhabit extremely dry environments such as the deserts in Southern Africa, where a diversity of resurrection plants is found.

Finally, another useful categorisation of strategies similar to the avoidance-tolerance distinction is the differentiation between isohydric and anisohydric plant species. It refers to two different types of stomatal regulation in response to soil drying. Isohydric plants such as maize or poplar maintain the water potential of their cells at relatively constant values (at around -2 MPa) by reducing stomatal conductance early upon the onset of water shortage. In contrast, anisohydric species (e.g. sunflower) allow a decline in cellular water potential upon a drought-induced drop in soil water potential.

The two strategies are associated with different physiological risks. Rapid stomatal closure can lead to early onset of a negative carbon balance because of the reduced CO₂ assimilation. On the other hand, the decrease in water potential can result in hydraulic failure. When water loss through transpiration is substantially greater than the uptake of water by roots, high xylem water tension can develop, which eventually leads to cavitation of xylem vessels and conductivity loss (McDowell et al. 2008). Overall, anisohydric species tend to be more common in drought-prone habitats than isohydric species.