The impact of cluster thinning on fertility and berry and wine composition of 'Blauer Portugieser' (<i>Vitis vinifera L.</i>) grapevine variety

Aim: Two different yield reductions based on cluster thinning (CT) were performed to determine their impact on vine growth, yield, and grape and wine composition of 'Blauer Portugieser' grapevine variety. Methods and results: Two levels of cluster thinning (limited CT1 – 20-30 % and severe CT2 – 40-50 % cluster reduction) were applied at the pea-size berry (BBCH 75) phenological stage in 2007, 2008 and 2011. The potential impact of CT was determined by measurements of vine growth and fertility potential, berry weight, berry colour, soluble solids content, titratable acidity, pH and total phenolics. Additionally, for the first time, individual phenolic compounds were identified and quantified in berry skin and wine by HPLC-MS. In general, CT of 'Blauer Portugieser' significantly decreased titratable acidity in grape and wine, and increased pH and chromatic parameters in grape and alcohol content and volatile acidity in wine. A significant decrease in yield per vine (of 0.92 kg of grape/vine), together with an increase in soluble solids (of 2.8 °Brix) in grape and pH and total extract content in wine was only observed in severe CT (CT2). Furthermore, CT2 significantly increased the content of total anthocyanins, flavonols and hydroxycinnamic acids, but not total flavanols, in grape and wine. CT2 significantly increased the content and proportion of p-coumaroyl pentose in grape and wine, catechin in grape, epicatechin in wine, quercetin-3-glucuronide (the main flavonol in 'Blauer Portugieser') in grape and wine, the content of myricetin-3-glucoside in grape, and the content of 3-glucosides of laricitrin, myricetin and quercetin in wine. Finally, CT2 increased the content and the proportion of 3-glucosides of delphinidin, petunidin and peonidin but decreased the proportion of malvidin-3-glucoside in grape and wine. Conclusion: A significant impact on yield and grape and wine composition was observed, particularly in the CT2 treatment, in which the yield loss was compensated by higher contents of soluble solids in grape, alcohol in wine, and phenolic compounds in grape and wine. Significance and impact of the study: The present study is the first report on the impact of different levels of cluster thinning on yield and grape and wine composition of 'Blauer Portugieser' variety. Grape and wine composition has been evaluated with an emphasis on a detailed profile of individual and total phenolic contents. The results are undoubtedly useful for winegrowers, who, until now, lacked technological guidelines to optimize 'Blauer Portugieser' yield and wine quality.