Mechanical Mid-Shoot Leaf Removal on Ortrugo (Vitis vinifera L.) at Pre- or Mid-Veraison Alters Fruit Growth and Maturation

Abstract

Production of high-quality sparkling wines relies on an optimal balance between sugar and acidity in the fruit at harvest. Warming trends favor a too-fast sugar increase at the expense of adequate acidity. The removal of photosynthetically active leaves from the distal part of the canopy around veraison has been proposed as a simple technique to delay sugar accumulation. Over three seasons (2015 to 2017), we compared the effects of mechanical mid-shoot leaf removal in the white variety Ortrugo, either performed before veraison (PRE-LR) or at a total soluble solids (TSS) concentration of ~12 Brix (POST-LR), with non-defoliated vines (C). PRE-LR shifted the sugar increase in all the three seasons, but the effects held until harvest only in 2015 and 2016 (−1.1 Brix and −1.4 Brix, respectively, if compared to C). Acidity was unaffected. POST-LR was less effective in changing the composition of grapes during ripening. Both leaf removal treatments reduced berry size (−5%) and yield (−19% PRE-LR and −14% POST-LR, in comparison with C). Finally, the treatments changed the berry morphology and the proportion between its components, with some peculiarities due to the seasons, but the relative skin mass generally increased. Overall, the efficacy of the technique was below expectations, primarily because the delay in general ripening expected from removing a significant portion of the mature and functioning leaves was offset by a decrease in malic acid due to excess heat summation for this variety.