Impact of defoliation on Plavac mali grape composition in the conditions of Hvar vineyards

Background: Plavac mali is a well-known native Croatian cultivar used for the production of premium wines. One of the locations known for this variety is the island of Hvar, in the southern Dalmatia subregion. Grape quality is influenced by the canopy microclimate, and one of the main practices for manipulating the canopy is defoliation. Aim: The aim was to investigate the impact of defoliation treatments on grape composition in two vineyards with different microclimates. Materials and Methods: The study was carried out in 2020 at two sites on the island of Hvar: the Selca vineyard, situated inland, and Sveta Nedjelja, situated on the southern side of the island. The experiment was set up as a randomized block design with three replicates of 20 vines per treatment. The defoliation treatments were control (C), basal defoliation before véraison (T1), basal defoliation at véraison (T2), and apical defoliation above the fruiting zone at véraison (T3). Total soluble solids (TSS), titratable acidity, and pH were determined according to OIV (2019) methods, and organic acids (tartaric, malic, citric) by HPLC. Individual polyphenolic compounds were extracted from freeze-dried grape skins and quantified by HPLC with diode-array and fluorescence detection, with results expressed as mg/kg of skin dry weight. Data were analyzed by one-way ANOVA with Duncan's multiple range test (p < 0.05) in XLSTAT. Results and Discussion: At both sites, the defoliation treatments affected TSS and polyphenolic profiles, increasing TSS and reducing the content of polyphenols. Treatments at Sveta Nedjelja accumulated excessive TSS, reaching over 110 °Oe. The highest content of polyphenolic compounds was found in T2 at the Selca vineyard and in T1 at the Sveta Nedjelja vineyard. Conclusion: In the context of a warm Mediterranean climate, this viticultural practice and its performance should be reconsidered.