A modelling framework to predict transpiration reductions during drought based on soil hydraulics

There is increasing need for mechanistic and predictive models of transpiration and stomatal response to drought. Global measurements of transpiration showed that the decrease in soil moisture is a primary constrain on transpiration. Additionally, a recent meta-analysis indicated that stomatal closure is explained by the loss in soil hydraulic conductivity, more than that of the xylem. Despite these evidences on the role of soil drying as a key driver of transpiration reduction, the mechanisms by which soil drying impacts transpiration, including the effect of different soil hydraulic properties, are not fully understood. Here, we propose that stomata regulate transpiration in such a way that the relation between transpiration and the difference in water potential between soil and leaves remains linear during soil drying and increasing vapor pressure deficit (VPD). The onset of hydraulic nonlinearity sets the maximum stomatal conductance at a given soil water potential and VPD. The resulting trajectory of the stomatal conductance for varying soil water potentials and VPD depends on soil and plant hydraulics, with the soil hydraulic conductivity and root length being the most sensitive parameters. From this hydraulic framework it follows that stomatal closure is not simply a function of soil moisture, soil water potential or leaf water potential. Instead, it depends on transpiration demand and soil-plant hydraulics in a predictable way. The proposed concept allows to predict transpiration reductions during drought with a limited number of parameters: transpiration demand, plant hydraulic conductivity, soil hydraulic conductivity and active root length. In conclusion, this framework highlights the role of the soil hydraulic conductivity as primary constrain on transpiration, and thus on stomatal conductance and photosynthesis