Severe leaf removal improves vine source-sink balance and composition of “Merlot” grapes and wine

Abstract

Basal leaf removal is a widespread cultural practice that alters the microclimate of the fruit zone as well as the source-sink balance of a vine. Severe defoliation may result in vine devigoration via reduced photosynthetic capacity and could be beneficial for vigorous canopies to reduce the concentrations of methoxypyrazines (MPs), a class of volatile compounds that contribute to green aromas, in the fruit.  To test these hypotheses, a study was conducted on vigorous  ‘Merlot’/101-14 grapevines (Vitis vinifera L.) in which severe leaf removal (SLR; removal of over half of the basal leaves on each shoot) was done after fruit set. Two levels of SLR were done on primary shoots on the morning-side of the canopy, while alternate basal leaf removal, where only leaves 1, 3 and 5 from the base of the shoot were removed, was done on the afternoon-side to minimize the risks of berry sunburn and shrivel. Based on several pruning metrics, SLR resulted in the greatest devigoration compared to conventional and no leaf removal treatments. After leaf removal was performed, photosynthesis decreased sharply, particularly in vines that had the highest levels of defoliation. SLR treatments also altered the fruit zone microclimate and had the highest cluster exposure flux availability and berry skin temperatures. Wine composition and sensoric analyses indicate that SLR results in the lowest concentration of MPs. These results suggest that SLR done shortly after fruit set may be a beneficial practice to improve the source-sink balance of high-vigor vines and to enhance grape and wine composition without sacrificing yield.