Transpiration response to soil drying vs. increasing vapor pressure deficit in crops - physical and physiological mechanisms and key plant traits.

The water deficit experienced by crops is a function of atmospheric water demand (vapor pressure deficit, VPD) and soil water supply over the whole crop cycle. We summarize typical transpiration response patterns to soil and atmospheric drying and the sensitivity to plant hydraulic traits. We explain the transpiration response patterns using a soil-plant hydraulic framework. In both cases of drying, stomatal closure is triggered by limitations in soil-plant hydraulic conductance. However, traits impacting the transpiration response differ between the two drying processes and act at different time scales. A low plant hydraulic conductance triggers an earlier restriction in transpiration during increasing VPD. During soil drying, the impact of the plant hydraulic conductance is less obvious. It is rather a decrease in the belowground hydraulic conductance (related to soil hydraulic properties and root length density) that is involved in transpiration downregulation. The transpiration response to increasing VPD has a daily time scale. In the case of soil drying, it acts on a seasonal scale. Varieties that are conservative in water use on a daily scale may not be conservative over larger time scales (e.g. during soil drying). This potential independency of strategies needs to be considered in environmental-specific breeding for yield-based drought tolerance.