Role of hydraulic traits in stomatal regulation of transpiration under different vapour pressure deficits across five Mediterranean tree crops

The differential stomatal regulation of transpiration among plant species in response to water deficit is not fully understood, although several hydraulic traits have been reported to influence it. This knowledge gap is partly due to a lack of direct and concomitant experimental data on transpiration, stomatal conductance, and hydraulic traits. We measured sap flux density (J(s)), stomatal conductance (g(s)), and different hydraulic traits in five crop species. Our aim was to contribute to establishing the causal relationship between water consumption and its regulation using a hydraulic trait-based approach. The results showed that the species-specific regulation of J(s) by g(s) was overall coordinated with the functional hydraulic traits analysed. Particularly relevant was the negative and significant relationship found between the Huber value (H-v) and its functional analogue ratio between maximum J(s) and g(s) (J(smax)/g(smax)) which can be understood as a compensation to maintain the hydraulic supply to the leaves. The H-v was also significantly related to the slope of the relationship between g(s) and J(s) response to vapour pressure deficit and explained most of its variability, adding up to evidence recognizing H-v as a major trait in plant water relations. Thus, a hydraulic basis for regulation of tree water use should be considered. We found coordination between stomatal regulation of tree transpiration and hydraulic traits, with the Huber value being the trait with the most predominant role in the stomatal control-water use relationship.