Stomatal restrictions to photosynthesis in grapevine cultivars grown in a semiarid environment

Diurnal changes in the leaves of field-grown grapevine (Vitis vinifera L.) cultivars Syrah and Tempranillo were followed over summer 2009 with respect to gas exchanges. Net photosynthetic rate (AN) of both cultivars rapidly increased in the morning, decreasing slowly until the late afternoon, when reached the lowest values. Stomatal conductance (gs) changed in parallel with AN, indicating that AN was greatly affected by gs. This pattern was repeated every day throughout the summer, with slight modifications according to plant water status. Under severe water stress situations, when as a result of drought gs decreased below 0.05 mol H2O m-2 s-1, intrinsic water use efficiency (WUEi) declined sharply in Tempranillo, which did not happen in Syrah, where despite stomatal closure kept increasing WUEi. Water stress intensified leaf to air vapour pressure deficit (VpdL) response however instantaneous WUE (WUE inst) levels plunged to very low with high VpdL in both cultivars.