Résumé
The present work aims to study the interactive effect of drought stress and high vapour pressure deficit (VPD) on leaf gas exchange, and especially on mesophyll conductance to CO2 (g(m)), in two woody species of great agronomical importance in the Mediterranean basin: Vitis vinifera L. cv. Tempranillo and Olea europaea L. cv. Manzanilla. Plants were grown in specially designed outdoor chambers with ambient and below ambient VPD, under both well-irrigated and drought conditions. g(m) was estimated by the variable J method from simultaneous measurements of gas exchange and fluorescence. In both species, the response to soil water deficit was larger in g(s) than in g(m), and more important than the response to VPD. Olea europaea was apparently more sensitive to VPD, so that plants growing in more humid chambers showed higher g(s) and g(m). In V. vinifera, in contrast, soil water deficit dominated the response of g(s) and g(m). Consequently, changes in g(m)/g(s) were more related to VPD in O. europaea and to soil water deficit in V. vinifera. Most of the limitations of photosynthesis were diffusional and especially due to stomatal closure. No biochemical limitation was detected. The results showed that structural parameters played an important role in determining g(m) during the acclimation process. Although the relationship between leaf mass per unit area (M-A) with g(m) was scattered, it imposed a limitation to the maximum g(m) achievable, with higher values of M-A in O. europaea at lower g(m) values. M-A decreased under water stress in O. europaea but it increased in V. vinifera. This resulted in a negative relationship between M-A and the CO2 draw-down between substomatal cavities and chloroplasts in O. europaea, while being positive in V. vinifera.