Effects of soil characteristics and leaf thinning on potassium levels in vines and on wine pH and acidity in Cabernet-Sauvignon and Syrah

Abstract

This study investigated the effects of leaf thinning on potassium levels in petiole and different grape tissues during ripening in two red varieties: Shiraz (SH) and Cabernet Sauvignon (CS), cultivated in the Costers de Segre Designation of Origin (NE, Spain). Petiole and grape berries were collected from early veraison to harvest in two plots with differences in vigor, in which leaf thinning was partially applied. Differences in vigor were examined using the Normalized Difference Vegetation Index (NDVI). Petiole samples were dried and finely ground. Grape skins, pulp and seeds were separated, dried and finely ground. Samples were digested with a mix of nitric acid and hydrogen peroxide in a closed vessel microwave digestion system. K was measured by spectroscopy atomic emission. Grape parameters such as pH, acidity and berry weight were analysed the day of collection according to the OIV methods.  K  concentrations  were  related  to  these  parameters  obtained  during  ripening.  For  both  varieties, K levels decreased since early veraison until harvest. High differences in K levels occurred between both varieties, which were in agreement with the differences in soil K. Leaf thinning increased K levels in petiole. K levels in seeds followed similar patterns in both varieties: up to 5 g/kg in CS and up to 3.5 g/kg in SH, with greater values in the most vigorous areas, but without differences between treatments. K levels in pulp decreased during ripening in both varieties. However, in skins K levels decreased in SH (up to 7 g/kg) while increased in CS (up to 10 g/kg) and they were slightly higher in the no leaf thinning treatment. Some correlations were found between K in petiole and in pulp in both varieties, but there were only positively correlated with K in skins in SH and negatively correlated with K in seeds in CS. Small differences in pH were observed between both treatments in both varieties, with slightly higher values in the leaf thinning treatment. Greater differences were found in sugar content between treatments, which was more pronounced in CS than in SH. For total acidity there were some differences between treatments and plots during the ripening period but not near harvesting. Must pH positively correlated with K in petiole and in seeds in both varieties, with better fits in the most vigorous plots.