Basal defoliation at berry set modulates berry weight, must composition, and secondary metabolite profile in ‘Muscat Hamburg’ grapes

Abstract

Grape yield and quality largely depend on achieving an optimal balance between crop load and sunlight exposure, which is essential for the development of a desirable phenolic and aromatic profile in grapes. Improving canopy microclimate conditions and adjusting the ratio between older, less photosynthetically active leaves and younger, more active leaves may enhance grape composition. Basal defoliation is expected to improve grape and wine quality by increasing the concentration of polyphenolic and volatile compounds. The aim of this study was to evaluate the effect of basal defoliation performed at berry set on yield parameters, basic must composition, and the profile of polyphenolic and volatile compounds in grapes of ‘Muscat Hamburg’ grown on the Croatian hillside (2023.). At berry set, six leaves per shoot were removed in the cluster zone in the defoliation treatment, while no leaf removal was applied in the control. The trial was arranged in a randomized block design with three replicates of eight vines each. At harvest, yield per vine and average cluster weight were determined. From each replicate, 10 clusters were sampled for basic must analysis and 100 berries were collected for secondary metabolite analysis. Volatile compounds in grapes were analyzed by GC–MS and polyphenolic compounds were determined by HPLC. Data were subjected to one-way ANOVA. Basal defoliation at berry set did not affect total yield but significantly reduced cluster and berry weight. It increased sugar concentration and decreased total acidity in the must. Defoliation at berry set enhanced the content of total polyphenols, flavonols, flavan-3-ols, and resveratrol-3-O-glucoside, while no effect was observed on total anthocyanins, hydroxycinnamic acids, or hydroxybenzoic acids. It also increased the concentration of total volatile compounds, particularly terpenes and aldehydes, without affecting higher alcohols, fatty acids, esters, or C13-norisoprenoids, and reduced volatile phenols in berries.