Changes in grape phenylpropanoid composition induced by leaf removal in two ‘Pinot noir’ vineyards

Beyond the biological significance, secondary metabolites are crucial in the determination of grape and wine quality attributes, and their composition, can be significantly modulated by exogenous (site, climate, soil properties) and endogenous factors (variety, clone, rootstock) and finally by agricultural practices. The grape composition is undoubtedly crucial for Pinot noir production and any improvements in polyphenol complexity are welcomed. Leaf removal is an important canopy management practice, leading either to an improvement of fruit-zone microclimate and grape quality (Haselgrove et al., 2000). Different results can be achieved depending on the timing of performing, and pre-flowering leaf removal seems to be more promising even if further results are needed (Sternad Lemut et al., 2011). In this study, leaf removal was performed in two ‘Pinot noir’ vineyards (located in Vipava Valley, Slovenia and in S. Michele all’Adige, Italy) at different phenological stages: 10 days before flowering (pre-flowering leaf removal) and at veraison, while untreated vines were used as a control. For each shoot 4-to-6 leaves were removed manually. At harvest, the grapes from all the treatments were collected separately, processed, and a comprehensive LC-MS/MS metabolic profiling approach (Vrhovsek et al., 2012) was adopted in order to highlight the induced quantitative changes of several dozens of phenolic compounds including a number of up to date very poorly studied ones. The results revealed a number of changes related both to vineyard location and to the timing of leaf removal. As related to vineyard location, the occurrence of secondary metabolites was 40%-higher in Italian samples, probably due to the different ripening status of the grapes (25 Brix in Italy vs 22 Brix in Slovenia). On the other hand, comparing leaf removal treatments, the most significant changes were observed in the group of 18 flavonols. The amount of total flavonols was increased by 216% and 109% in Slovenia and by 56% and 20% in Italy, in the pre-flowering and in the veraison treatments, respectively, as compared with the controls. Pre-flowering leaf removal has also triggered significant changes in total hydroxycinnamic acids (both vineyards) and anthocyanins (in Slovenia), while other phenolic groups showed some changes in occurrences within individual group members, but were not significant in total amount. Despite location differences, grape polyphenols included in the study were positively affected by both leaf removal treatments, particularly when performed at early phenological stages. Metabolic profiling technique represents a powerful tool, but a lot of efforts and data are still required towards a comprehensive understanding of the mechanisms behind it.