Variability in Stomatal Adaptation to Drought among Grapevine Cultivars: Genotype-Dependent Responses

Leaf stomata are the primary determinants of the plant water relations. Physiological adaptations of stomata in response to water stress have been extensively reported for grapevine. On the contrary, little is known about how the plasticity in stomatal anatomical features may affect their adaptability to drought conditions. In this study, we investigated, at the molecular and anatomical level, the effect of water stress on the stomatal anatomical features of four grapevine varieties extensively cultivated in the north of Italy. Potted plants of Garganega, Glera, Moscato giallo, and Merlot varieties were subjected to a 12–13 day period of water restriction during two consecutive seasons. Stomatal density and size were investigated in newly developed young leaves, 7 days after tip separation, following the occurrence of a water stress event. Furthermore, the gene expression of three key stomagenesis genes (VvEPFL9, VvEPF1, and VvEPF2) was analysed. The response of stomatal anatomical features to drought varied among the studied varieties. Moscato and Glera showed an increase in stomatal density and a decrease in stomatal size. On the contrary, Merlot displayed a reduction in stomatal number, while Garganega remained unchanged in terms of these values. Transcript levels of VvEPFL9 were overall in agreement with stomatal densities measured in the four varieties, showing an up-regulation when drought induced an increase in stomatal density or a down-regulation when the stomatal number decreased. The wide variability in stomatal response observed in the four varieties under study suggests that anatomical changes in stomatal characteristics are genotype dependent. These variations contribute to the intra-specific variability in grapevine’s response to water stress.