Physiology of crop response to drought

Abstract

Effects of drought can be partitioned into effects on radiant energy interception, conversion of light into dry matter, partitioning of assimilate, and tuber dry matter concentration. The main effect of drought is to reduce radiant energy interception, by reducing both the rate and the duration of growth of leaves, and also the number of leaves produced. Rate of leaf expansion is inversely related to soil moisture deficit (SMD), and genotypes differ both in the threshold at which leaf expansion is constrained by soil moisture, and in their ability to maintain expansion with increasing SMD. Differences between genotypes appear to be related to the ability to maintain leaf water status, rather than in the ability for osmotic adjustment.

The efficiency of conversion of light into dry matter is less sensitive to drought stress than leaf growth. Drought has little effect on the functioning of Photosystem II, and effects on photosynthesis largely reflect stomatal limitation. Prolonged drought, though, may result in a reduction in light-harvesting complexes and down-regulation of carboxylating enzymes. Greater reduction in stomatal conductance to water vapour transfer than photosynthetic rate results in an increase in water use efficiency.

It is suggested that differences between experiments in the pattern of assimilate partitioning to tubers by droughted crops may reflect differences in timing and intensity of water-stress relative to tuber initiation. Tuber dry matter concentration can be related to thermal time accumulated from the time of plant emergence and to SMD. Increasing stability in dry matter concentration over a range of soil moisture conditions could contribute to improved drought tolerance. However, evidence for genotypic differences in stability in dry matter concentration is inconclusive.

The supply of water to shoots depends on the ability of roots to exploit available soil moisture. Compared with cereals, root systems of potatoes tend to be weak and shallowly distributed so that the ability to extract soil moisture effectively is restricted. Drought, while reducing dry matter production increases the root: shoot ratio indicating a shift in the balance of growth in favour of roots. Roots of plants grown in droughted conditions also tend to be thinner. Both responses enable droughted plants to exploit the available soil moisture more effectively. Genotypic differences in rooting depth and in amount of root growth have been found, but the significance of these differences for water extraction and growth of droughted crops is not clear.

Grafting experiments indicate that shoots largely determine changes in both stomatal conductance and partitioning of assimilates in response to drought. Interaction of hydraulic and non-hydraulic regulation of physiological processes provides a mechanism coupling long-term adaptation to soil moisture status with short-term response to evaporative demand. Future work must consider not only effects of water-stress on individual processes, but also the integration of response by the whole plant and the physiological processes underlying long-term responses to drought.