Anthocyanin and proanthocyanidin evolution during ripening of the Refošk grapevine is modulated by deficit irrigation

The effects of two deficit irrigation regimes (50% ETC and 20% ETC) on skin and seed phenolic compounds of the red Vitis vinifera L. cultivar 'Refo & scaron;k' were studied during the veraison - maturation period. The study was undertaken for two successive seasons in the Karst area of North-East Italy, where rainfall is scarce and subterranean drainage limits surface water. To trace the effect of water deficit on both phenol biosynthesis and berry size reduction, phenols were extracted from lyophilised, cryomilled skins and seeds using organic solvents and evaluated in skin and seeds, per units of both dry and fresh weight. The anthocyanin concentration was higher in 20% ETC early after veraison, whereas at harvest the difference was not significant. A decreasing trend with some fluctuations in proanthocyanidin (PA) concentration in skins and seeds was found during maturation with both water regimes. 20% ETC resulted in a decrease in skin PA concentration and increase in seed PA concentration during ripening. As regards PA structural characteristics, there was a trend for higher galloylation and polymerisation of seeds and skins during ripening with the 20% ETC irrigation regime, whereas the effect on prodelphinidins in skins was inconsistent. In 2018, when most of the differences in vine water status between irrigation regimes occurred during veraison - maturation, the concentrations of PAs were affected more significantly than the mean degree of polymerisation and percentage of galloylation. In contrast, in 2019, when differences in vine water status occurred in the pre-veraison-maturation period, the structural characteristics of PAs were affected more significantly than concentrations, especially galloylation of skins and polymerisation of seeds. The results of the study showed that the biosynthesis of PAs, their polymerization and galloylation can be significantly modulated by water deficit regimes applied in the vineyard, in combination with the meteorological course of the season.