Whole-canopy source-sink balance at bloom dictates fruit set in cv. Pinot noir subjected to early leaf removal

Severe prebloom leaf removal dramatically affects the source-sink balance in grapevines, leading to a reduction in fruit set. In this study, carried on for two consecutive years, the impact of defoliation at bloom was evaluated with the objective to assess the capacity of developing inflorescences to attract photosynthates from adjacent shoots subjected to varying source/sink manipulations. In Pinot noir trained to a bilateral cordon, untreated vines (UT-UT) were compared to a treatment where shoots on one-half of the vines were subjected to the removal of 10 basal leaves at bloom (UT-LR). Another set of vines underwent sink removal (inflorescences and shoot apex) at bloom on half of the shoots by hand thinning (TFR-UT). A final treatment consisted of removing 10 basal leaves on half of the shoots and removing sinks on the other half of the canopy (TFR-LR). Following treatment application, shoot leaf area retained was similar to 40% of the total in UT-LR and TFR-LR. UT-LR reduced the whole canopy leaf area available per inflorescence by similar to 44% when compared to UT-UT. TFR-LR did not affect this balance. In UT-LR, fruit set was significantly reduced (-36% as compared to UT-UT), whereas it was unaffected by TFR-LR and TFR-UT. Independent of treatments, fruit set in both seasons was correlated with whole-canopy leaf area per inflorescence at bloom and not with the single shoot leaf area retained after early leaf removal.