Managing moderate water deficit increased anthocyanin concentration and proanthocyanidin galloylation in “Refoˇsk” grapes in Northeast Italy

Grapevines in Karst between Italy and Slovenia are grown on a shallow "Terrarossa” red karst loam laying on carbonate rocks. In Karst, water stress conditions occur during the summer season both because of the rain scarcity and the poor available water content of the soils. Moreover, irrigation can be used to only partially supply grapevine requirements, where possible. An irrigation trial was carried out in a vineyard of “Refoˇsk” during the seasons 2018–2019 to understand the effects of two deficit irrigation regimes (20% and 50% ETC) on yield parameters and basic maturation, but mainly on polyphenol content, composition and extractability. During the summer season Ψstem was regularly checked in order to follow the plant water status, and at harvest time berry samples were collected to determine the total concentration of anthocyanins and proanthocyanidins and their extractability in a wine-like solution. Moreover, the structural characteristics of seed and skin proanthocyanidins were analyzed. In both seasons, differences in plant water status were ascertained, but values of Ψstem never highlighted situations of severe water stress for both deficit irrigation regimes. 20% ETC resulted in a significant increase in the concentration of both total and extractable anthocyanins; on the other hand, proanthocyanidin concentration was not significantly affected by the irrigation regime. As regards the structure of proanthocyanidins, 20% ETC accounted for a higher degree of galloylation in both total and extractable seed and skin proanthocyanidins. A more limited irrigation regime resulted in both higher anthocyanin concentration and degree of galloylation of seed and skin proanthocyanidins - attributes that might account for a better wine quality potential.