Targeted manipulation of vine balance: Does vine balance directly affect fruit composition?

Vine balance is a concept describing the relationship between carbon assimilation (usually estimated using a measure of canopy size, such as pruning weight) and utilisation of the resulting carbohydrates for fruit production (usually estimated using harvest yield). Manipulating vine balance through leaf area or crop load adjustments affects the proportion of the vine’s carbohydrate production required to mature the fruit. It is commonly considered that composition of the berry, and resulting wine, is strongly affected by vine balance. Field manipulations of vine balance were replicated in three contrasting regions, Hilltops, Murray Darling and Langhorne Creek, for three seasons. These were early defoliation (pre-capfall), late defoliation (pre-veraison), 50% crop removal (pre-veraison) and minimal pruning. Defoliation was also simulated in mature, fruiting, potted vines at Wagga Wagga, by enclosing whole vines in chambers and supplying them with low CO 2 air. This allowed the impact of defoliation on vine carbon assimilation to be separated from any impact on bunch exposure. Changing vine balance affected the fruit maturation rate, but had a less consistent effect on fruit composition. Late defoliation (higher ratio of fruit load to canopy size) reduced total anthocyanin content, despite elongating the maturation period, whereas crop removal had little effect. Interestingly, early defoliation had a limited effect on vine balance, but resulted in increased total tannin content. In both cases, it is possible that the observed compositional effects were caused by changes in bunch environment. Reducing carbon assimilation in the chamber experiment also reduced maturation rate, but did not affect the relationship between sugar and anthocyanin concentrations in the berry. Overall, there was no conclusive evidence that the changes in vine balance achieved had a direct effect on fruit composition.