Soil Rather Than Xylem Vulnerability Controls Stomatal Response to Drought

The current trend towards linking stomata regulation to plant hydraulics emphasizes the role of xylem vulnerability. Using a soil-plant hydraulic model, we show that xylem vulnerability does not trigger stomatal closure in medium-wet to dry soils and we propose that soil hydraulic conductivity loss is the primary driver of stomatal closure. This finding has two key implications: transpiration response to drought cannot be derived from plant traits only and is related to soil-root hydraulics in a predictable way; roots and their interface with the soil, the rhizosphere, are key hydraulic regions that plants can alter to efficiently adapt to water limitations. We conclude that connecting below- and aboveground hydraulics is necessary to fully comprehend plant responses to drought.