BayCEER Workshop 2019

Soil rather than xylem vulnerability controls stomatal response to drought

Stomata control up to 80% of terrestrial evapotranspiration and their response to soil drying determines plant drought tolerance and terrestrial fluxes. The current trend towards linking stomata regulation to plant hydraulics emphasise the controlling role of xylem vulnerability.  However, stomata have been shown to close much before the xylem cavitates, which compromises the use of xylem vulnerability models to predict stomatal and transpiration response to drought. Here, we show that stomatal closure during drought is controlled by the loss of hydraulic conductivity of the soil, rather than that of xylem. We support this thesis with a soil-plant hydraulic model and a meta-analysis of stomatal functioning across species. We provide a mechanistic explanation, based on hydraulic and hormonal signals diluted by transpiration, linking soil-plant hydraulics to stomata closure. The model predicts that stomata close at the onset of hydraulic limitation maintaining a linear relationship between leaf water potential and transpiration rate. A meta-analysis across species proves that this relationship is linear and that the onset of soil hydraulic limitation predicts stomatal closure much better than the xylem vulnerability curve. We conclude that transpiration (and thus photosynthesis) responses to drought cannot be derived from plant traits only and that they are related to soil and root hydraulics in a predictable way.