Combined effects of rootstock and irrigation on gas exchange, fruit quality and yield in 'Merlot' and 'Sangiovese' grapevines

The rootstock/scion combination plays an important role on grapevine tolerance to water stress. An experiment was carried out on 5-year-old potted grapevines (Vitis vinifera L. 'Merlot' and 'Sangiovese') plants to evaluate the combined effect of two rootstocks (SO4 and 1103 Paulsen) and three irrigation regimes (full irrigation, FI; regulated deficit irrigation 1, RDI 1; regulated deficit irrigation 2, RDI 2) on leaf gas exchange, fruit quality and yield. Twelve combinations of variety-rootstock-irrigation regimes were evaluated in this experiment. Control grapevines (FI) were fully-irrigated from budburst through harvest; RDI 1 and RDI 2 vines were subjected to water deficit from fruit set through veraison and from veraison through harvest, respectively, whereas they were fully irrigated for the rest of the irrigation period. Plant water status was monitored by measuring the stem water potential, which reached minimum values of -0.8, -1.7, -2.1 MPa, in FI, RDI 2 and RDI 1, respectively. Significant differences in stomatal conductance (gs) and net photosynthetic rate (Pn) between RDI 1 and the other two irrigation treatments were measured 7 days after the beginning of the differentiation (fruit set). Similarly, RDI 2 grapevines showed lower values of gs and Pn 6 days after the imposition of water deficit at veraison. The highest (2.0 kg) and lowest (1.1 kg) yield per plant were measured in RDI 2-'Sangiovese'-1103 Paulsen and in RDI 1-'Merlot'-SO4 grapevines, respectively. The RDI 1-SO4 grapevines produced the smallest berries in Merlot, whereas there were no significant differences across the different combination of irrigation and rootstock in 'Sangiovese'. The threshold of 22±0.5 °Brix, used to establish the harvest date, was reached first by the RDI 2-'Merlot' grapevines grafted on both 1103 Paulsen and SO4, and after 30 days by the FI and RDI 1 irrigated 'Sangiovese'-SO4 ones.