Yield reduction through cluster or selective berry thinning similarly modulates anthocyanins and proanthocyanidins composition in Refosco dal peduncolo rosso (Vitis vinifera L.) grapes

‘Refosco dal peduncolo rosso’ is a late-ripening and low-yielding red grape variety, mainly cultivated in Northeastern Italy (Veneto and Friuli Venezia Giulia), and characterized by relatively high anthocyanins and average-to-low tannins concentrations. Under Friuli's climatic conditions, it is often challenging to maintain the Refosco dal peduncolo rosso grapes hanging on the vines long enough to match complete berry maturation before the rain season starts. Therefore, winegrowers normally perform cluster thinning in order to enhance or accelerate ripening. This study compared the effects of selective berry thinning (cluster shoulders and tips removal) and classical cluster thinning on fruit technological maturity, anthocyanin profile, and skin and seed proanthocyanidins concentration. Our results revealed that both thinning treatments induced a significant increase in total soluble solids, as well as total anthocyanins through the specific enhancement of OH- and di-substituted monoglucosylated anthocyanins. Additionally, skin high molecular weight proanthocyanidins was reduced by selective berry thinning, while mean degree of polymerization and percentage of galloylation were significantly decreased by both thinning treatments. These results showed that the yield reduction obtained by both methods were profitable to improve the maturation of Refosco dal peduncolo rosso grapes. However, the application of selective berry thinning provided a significant reduction of both skin high molecular weight proanthocyanidins and percentage of prodelphinidins. Therefore, the first evidences on grapes composition favored the cluster thinning technique as less time consuming, but more research on wine and sensory effects is needed to confirm the potential of selective berry thinning.