Pre-veraison water deficit and pre-harvest rewatering effects on the quality of Merlot berries

The effect of deficit irrigation strategies on red grape varieties has been widely studied. In general, an improvement of some important berry and wine compositional traits (e.g. colour and aroma compounds) is achieved by maintaining grapevines under water deficit conditions from pre-veraison to harvest. However, such strategy reduces the berry weight and yield, and can increase berry sugar concentration resulting in high alcohol wines. In this experiment, we tested a regulated deficit irrigation (RDI) strategy, imposed in pre-veraison and released 30 days after veraison, with the aim of triggering the very well recognized water deficit effects on berry composition while preventing yield reduction and excessive sugar concentration at harvest. The experiment was set-up in a Merlot vineyard in the experimental farm of the University of Udine. Irrigation was fully controlled by a sub-surface irrigation system and rainfall was excluded by a transparent roofing encompassing the experimental rows. Grapevines were irrigated to 100% Etc until berry set, when irrigation was withheld in water deficit plots (WD) and stem water potential (Ψs) was maintained between -1.2 and -1.5 MPa until 90 DAA (a month after veraison), when irrigation was partially restored. Control plots (C) were irrigated 100% Etc for all the growing season. At harvest, WD resulted in enhanced anthocyanin and tannin concentration compared to C berries, but no differences in yield and sugar concentration were observed. Restoring irrigation before harvest, commonly linked to a “dilution effect” by viticulturists, avoided berry shrinkage and yield losses and did not deleted the water stress effect.