How Do Crops Balance Water Supply and Demand when Water Is Limiting? ^†

Abstract

Plants are sessile organisms requiring mechanisms that enable them to balance water supply and demand in dry environments. Demand (D) is largely driven by canopy size (transpirational leaf area), although differences in transpiration per unit leaf area also occur. Supply (S) is primarily driven by water capture via the root system. Drought stress can be defined as the situation where supply of water cannot meet demand of the crop, such that water availability is the limiting factor for biomass accumulation. Under such conditions, plants will need to reduce D in order to meet the limited S, access more water to increase S, or increase the efficiency with which water is utilised. We used sorghum, a model C4 crop species, to demonstrate how the stay-green trait can modulate canopy development and root architecture to enhance adaptation. We show how stay-green positively impacts the balance between S and D under post-flowering drought, including insights at the molecular level. We provide examples of how canopy and root traits impact the S/D balance in other cereals under water limitation. For example, on the supply side, the extent of genetic variation for root angle (RA) has been evaluated in sorghum, wheat and barley, and genomic regions associated with RA have been mapped. Furthermore, the relationship between RA and grain yield has been explored in barley and sorghum field trials. The capacity to manipulate components of S and D to optimise the S/D balance should assist crop improvement programs to develop enhanced ideotypes for dry environments.