In viticulture, leaf removal (LR) in the cluster area is an effective practice against fungal diseases. Pre-flowering LR often has additional consequences compared to post-flowering LR, such as yield limitation, higher fruit maturity index and higher anthocyanin concentration in the must at harvest, with positive effects on wine composition. The phenotypic mechanisms behind the accumulation of thiol precursors in grapes remain incompletely understood, as do the ways in which they can be improved by agronomic practices. A six-year field trial on the Swiss white grape variety Vitis vinifera Petite Arvine evaluated the effects of 1) timing of LR (i.e., ‘separated flower buds’ or ‘flowering’ stages) and 2) compensating for leaf area removed in the bunch area by increasing canopy height (i.e., 100 or 150 cm canopy height) on grape and wine composition, in particular varietal thiols and thiol precursors. Pre-flowering LR reduced harvest yield by decreasing the number of berries per cluster. Unpredictable climatic conditions prior to flowering, such as cold temperatures and low light levels, significantly influenced fruit set and amplified the negative effects of pre-flowering LR, increasing the risk of excessive yield loss. Adjusting the timing of LR from the ‘separated flower buds’ stage to the ‘flowering’ stage provided a means to manage yield loss effectively. Compared to pre-flowering LR, LR applied at the flowering stage decreased acidity and yeast assimilable nitrogen concentrations, increased the concentration of the aroma precursor Cys-3MH in the must at harvest, and slightly enhanced the overall quality of the white wine. Meanwhile increasing trimming height to compensate for leaf removal in the cluster area increased the leaf-to-fruit ratio, and slightly improved both grape ripeness at harvest (higher TSS, lower tartaric acid) and wine sensory profile (higher colour intensity, floral, volume and overall impression). The combination of LR at flowering and increased canopy height (Flow-150) appeared to be a good compromise, mitigating yield loss and slightly improving wine composition in terms of Cys3MH accumulation in the must. Further research is needed to better understand and optimise the physiological mechanisms underlying the formation of aroma precursors in the fruit.
