Influence of Defoliation on Overwintering Carbohydrate Reserves, Return Bloom, and Yield of Mature Chardonnay Grapevines

Abstract

The carbohydrate (CHO) reserve physiology of Chardonnay grapevines growing in the cool climate of New Zealand was examined in relation to subsequent flowering and fruiting using defoliation treatments. Vines were defoliated by removing all but the four basal leaves from shoots at monthly intervals starting at four weeks postbloom. Throughout the following season, CHO reserves in the roots and trunks were measured and detailed assessments of vine flowering and yields were recorded. Previous season’s vine defoliation decreased concentrations of overwintering CHO reserves (mostly starch) in both roots and trunks, with earlier defoliation times resulting in the largest reductions. Roots were most sensitive, with early defoliation reducing starch concentrations at budburst to 1.5% DW compared with 17% DW in non-defoliated vines. Reductions in root and trunk CHO reserves were closely associated with significant decreases in inflorescence number per shoot and flower number per inflorescence (up to 50% less than in control vines). Differences in CHO concentrations in both the root and trunk were maintained to veraison, but at flowering had no effect on the percent fruit set in the first season after defoliation. Therefore, lower yields in the following season were caused by fewer inflorescences per shoot and flowers per inflorescence. Shoot growth and total vine pruning weight were also decreased in vines where CHO reserves were reduced. These findings suggest that restricted CHO reserve accumulation as a consequence of defoliation may have a negative impact on subsequent grapevine flowering and productivity, particularly so in a cool-climate environment where there is no postharvest CHO reserve accumulation period.