Distribution of photosynthates towards the grapes: effects of leaf removal and cluster thinning applied before veraison in cv. Verdejo

Abstract

Context and purpose of the study – The relationship between grape production and leaf surface is a highly debated aspect in terms of the impact it may have on the composition and quality of grapes, especially in areas that focus their cultivation on high-quality wine. In many occasions, the limitation of the unitary production level in these areas is claimed to be the main factor for achieving high quality levels in the wine, forgetting the importance of the source-sink relationship and other environmental factors and management of the canopy. Taking this consideration into account, this work seeks to know the response of the vine as a whole, and the individual shoot as well, to the application of various alternatives of leaves and clusters removal, carried out in the phase immediately before veraison, in cv. Verdejo, in Spain.

Material and methods – Throughout the 2017-2019 period, the productive and qualitative response of cv. Verdejo, grafted on 110-R, was studied through the application, in the pre-veraison phase, of the following experimental treatments: T, control treatment (normal shoots); DT, total defoliation (total leaf removal of a shoot of each spur, fully respecting the other shoot); A, cluster abscense (total defoliation of a shoot of each spur and complete cluster elimination of the other shoot); DP, partial defoliation (leaves removed from the cluster node on one shoot of each spur, fully respecting the other shoot). The design of the experiment is based on 4 random blocks, with an elementary plot of 6 vines to be measured, in a vineyard conducted on a vertical trellis, planted in 2012 with vine distances of 2.8 x 1.4 m, and pruned as a bilateral Royat cordon with 8 spurs (2-bud) per vine.

Results – The application of treatment A caused a noticeable reduction of productivity and Ravaz index, without hardly affecting the vegetative growth, despite removing the leaves from one of the two shoots of each spur, probably favored by the reduction in half of the sinks (clusters). On the contrary, the treatment DT caused light  reduction of vegetative growth, as well as sugar concentration, as a consequence of reduction of half of photosynthates source (leaves). The treatment DP did not have noticeable effects on productive or vegetative behaviour. Treatments T (control) and A favored an increase in pH, a reduction in acidity and a slight increase in potassium, compared to DT and DP, probably due to the higher overall leaf-cluster ratio in the former. Regarding the type of shoot within each treatment, the HT type hardly reduced grape production compared to N type, within the DT treatment, but showed a decrease in both vigor and sugar concentration, related to the reduction in the ratio source-sink. The HP shoot type did not show notable differences with respect to N, within the DP treatment. The comparison between HT-type shoots showed a significant increase in sugars and a slight reduction in total acidity in that of treatment A versus that of DT, probably derived from the non-existence of sinks in the other shoot (RT) of treatment A. In summary, the complete removal of leaves on one of the two shoots of each spur, without reducing the crop load, produced a limitation of photosynthates that affected the quantitative and qualitative response of the plant, while the removal of leaves from the node of the cluster or the alternate combination of cluster thinning and leaves removal did not cause significant alteration or redistribution of photosynthates in the vine.