Identifying the Potential to Use Vineyard Water Status to Alter Anthocyanins and Other Phenolic Compounds in Red Winegrapes
The data from field experiments with Cabernet Sauvignon grown near Lodi show that clear differences in vine water status were obtained at different stages of development by withholding water. Although water status of vines irrigated every 2-3 days (Continual treatment) decreased until veraison, when water was withheld from the beginning of the season (Early Deficit) or from times closer to veraison (Veraison Stress and Moderate Veraison Stress) vine water status decreased significantly more rapidly resulting in differences in vine water status of about 3 bars at veraison. When water was withheld after veraison (Late Deficit), vine water status rapidly decreased to values that were about 4 bars more stressed than the Continual vines. This was sufficient stress to cause some leaf senescence and abscission. The general response to water deficits was for phenol ics and anthocyanins to increase in concentration although this is not uniformly the case. For anthocyanins, the accumulation that occurs at veraison was induced earlier in vines that were water stressed. The minor anthocyanins were more responsive to early stress, whereas the major anthocyanin compound, malvidin-3-glucoside, accumulated to the highest concentration in Late Deficit fruit. The responses of juice phenolics in general and cinnamates specifically to water stress were complex and depended on the specific compound. Some accumulated during ripening and some decreased in concentration after veraison. In general, most juice cinnamates were present at harvest in highest concentrations in Late Deficit fruit. Nonanthocyanin skin phenolics were predominately flavanoids which accumulated during ripening. The accumulation was more rapid and to higher concentrations in vines that were exposed to early stress (Early Deficit and Veraison Stress) and lowest in vines that were exposed to high water status early in fruit development. Analysis is continuing of the specific nonanthocyanin skin phenolics. A new field trial has been established in a premium Cabernet Sauvignon vineyard (Knight’s Valley). In this trial similar data will be developed but more samples will be collected late in fruit ripening for determination of the relationship of the concentration of specific compounds with Brix, TA, and “hang time” and their responses to early and late season water deficits.