Selective cluster thinning impacts phenolic composition of Refosco dal peduncolo rosso (Vitis vinifera L.) grapes during fruit maturation

Maturation of berries located in different portion of the cluster is not synchronized, thus potentially impacting fruit quality at harvest. Based on this assumption, an experimental trial was performed to evaluate the effect of selective cluster thinning on the phenolic maturation of Refosco dal peduncolo rosso grapes, an autochthonous variety grown in Friuli Venezia Giulia region (North-Eastern Italy). The trial was conducted in a vineyard of the Scarbolo farm, located Southern of Udine during two consecutive growing seasons. The experiment was carried out with a completely randomized block design with three treatments and 3 replicates of 12 vines each. The treatments were: UN, untreated control ; CT, Cluster Thinning (removal of entire cluster) ; WT, Wing and Tips thinning (removal of the cluster wings and tips in each cluster). All treatments were applied at full veraison and involved a similar reduction of fruits (about 40%). After the treatments, 2 sets of berries were randomly collected and used for basic fruit chemistry parameters and metabolites measurements, respectively. Skins and seeds were separated and grinded under liquid nitrogen, and anthocyanin profile (Sivilotti et al. 2016), skin and seed tannins (Herrera et al. 2015) and their structural characteristics (mDP, %G, %P) (Kennedy & Jones, 2001) were assessed. As expected, both CT ad WT thinning treatments increased soluble solids in grapes at harvest. Moreover, skin tannins were reduced, in particular in the WT treatment, while total anthocyanin concentration was significantly increased in both treatments. In detail, CT and WT treatments resulted in a significant increase of the proportion of hydroxylated and methoxylated forms of anthocyanins, and in a significant reduction of the proportion of the p-coumarated forms when compared with control. The effect of selective cluster thinning and the related changes in grapes composition are discussed, also with regard to a possible impact on wine properties.