Effects of rootstocks and irrigation levels on grape quality of Vitis vinifera L. cv. Shiraz

The influence of two rootstocks (SO4 and 1103P) on grape quality and berry chemical composition was studied in a factorial experiment, in field grown grapevines of cv. Shiraz (Vitis vinifera L.), subjected to five irrigation levels [0% (T1), 25% (T2), 50% (T3), 75% (T4) and 100% (T5) of irrigation depth (IW, mm): Class A pan evaporimeter (CPE)]. Spectrophotometric analyses of total anthocyanins (TA), total phenolics (TP) and total antioxidant activity (AA) in grape extracts were performed. Also, total soluble solids (TSS), total acidity, pH, total sugar content, ash, juice yield and color index of red grapes (CIRG) of berry samples were determined. TA, TP, AA, TSS, total sugar content, ash, and CIRG values decreased together with increasing irrigation levels. On the contrary, T4 and T5 irrigation treatments increased total acidity, pH and juice yield of samples compared to the effects of T1, T2 and T3 irrigation treatments for both rootstocks. Moreover, T1 or T2 treatments caused an increase in TA, TP, AA, TSS, total sugar content, ash, and CIRG index values of grape samples in comparison to that of vines irrigated with T3, T4 and T5 levels. Grape quality response to irrigation levels was altered by rootstocks and quality of grapes harvested from vines grafted on SO4 was higher compared to those from 1103P under all irrigation treatments. Based on the findings, it was suggested that T2 irrigation level might be sufficient to guarantee Shiraz yield potential without significant loss in grape quality under the study conditions. Also, the results make it possible to recommend use of SO4 rootstock under non-limiting water conditions because of its positive on grape quality parameters, while 1103P might be better choice under water-limiting conditions.