Soil hydraulic constraints on transpiration

Carminati, Andrea [Chair of Soil Physics, University of Bayreuth, Germany] Javaux, Mathieu [UCL]

During drought soil hydraulic conductivity constrains transpiration. To avoid losses in leaf water potential and embolism, stomata promptly close when the soil-plant hydraulic conductance starts to drops. When the soil dries out, its hydraulic conductivity decreases by several orders of magnitude and large gradients in water potential develop around the roots, which causes the xylem water potential to drop to very negative values, with increasing risks of cavitation. Although it is well accepted that stomata regulation allows for decreasing transpiration and preventing cavitation, the relation between soil drying and stomata conductance is difficult to predict. Here we propose that stomata close when the water potential at the root-soil interface drops and the soil-plant hydraulic conductance starts to decrease. In wet soils the relation between leaf water potential ψleaf and transpiration E is linear. In dry soils, the relation becomes nonlinear, with ψleaf rapidly and nonlinearly decreasing for small increases in E. We provide experimental evidence that stomata close when dψleaf/dE starts to decrease. Furthermore, we propose a mechanism, based on ABA production and dilution, that allows plants to respond to changes in soil-plant hydraulic conductance and to remain in the linear part of the E(ψleaf) relation. In summary, we show that soil-plant hydraulics constrains transpiration and that stomata regulation allows for maintaining dψleaf/dE constant. The proposed hydraulic framework places the focus on soil hydraulics for predicting transpiration response to soil drying.