Improving industry capacity to manage the yield and wine quality relationship through understanding the influence of vine carbon balance on berry composition

Abstract

Vine balance is a concept describing the relationship between whole vine carbon uptake and its utilisation, as sugar, for fruit production. 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, for three seasons. In addition, the effect of defoliation was simulated, without changing bunch environment, by enclosing whole vines in chambers and supplying them with low CO2 air to reduce photosynthesis.

Changing vine balance consistently altered the rate of ripening, but did not correlate with treatment effects on fruit composition, where they occurred. Late defoliation extended the maturation period, but reduced total anthocyanin content. Crop removal shortened the maturation period, but had little effect on the fruit. Interestingly, early defoliation had no clear effect on vine balance, but resulted in both increased anthocyanin and increased tannin content. The chamber experiment also extended the maturation period, but had no effect on the relationship between sugar and anthocyanins. Overall, there was no conclusive evidence that the changes in vine balance achieved had any significant effect on fruit or wine composition when fruit was harvested at the same sugar ripeness.

Summary

The concept of vine balance is an empirical one and is resistant to a specific definition. However, the importance of balanced vines for quality fruit production is widely acknowledged in the global wine industry and significant time and effort are spent on managing the relationship between crop load and vine vigour. This is often to the detriment of vine yield. Despite this, measurements of vine balance are not often made, and more commonly used metrics, such as the Ravaz index, are retrospective in nature and limited in their applicability across seasons. What the various definitions and metrics have in common, is an acknowledgement that, in essence, vine balance is a source-sink relationship. For the purposes of this project vine balance was defined as the ratio between yield and canopy size.

This multi-disciplinary project presented an opportunity to address the extent to which vine balance directly drives berry composition. By studying the physiology and molecular biology of fruit responses to variations in assimilate supply, whether driven by environmental variability, direct manipulation of crop load or changing carbon dioxide (CO2) concentrations under controlled conditions, this project aimed to determine the extent to which vine balance directly influences fruit composition.

This project utilised three different approaches to comprehend vine balance: to apply crop and canopy treatments in a consistent manner across trials sites that differed significantly in production aim and climatic conditions, thereby generating fruit from vines that differ in vine balance through multiple causes; to directly manipulate vine carbohydrate supply to the fruit independently of environment; and to not only assess the fruit, and resulting wine, from these experiments, but to utilise molecular biology tools to examine the impact of changes in vine balance on the key genes that regulate the fruit composition. In all cases, physiological maturity of the fruit was defined as a total soluble solids concentration of 24°Brix.

The field component of the project utilised three sites in three states, all with Shiraz vines of a similar age, at Langhorne Creek, Murray Valley and Hilltops. Early defoliation, crop thinning, late defoliation and minimal pruning management strategies were imposed in the same way at each site and reapplied on the same vines in three consecutive seasons shortly before flowering or veraison.

Despite wide differences in vine yield between the sites and the seasons (ranging from 1.1 to 30.1 kg/vine), the impact of the treatments was largely consistent. The early defoliation treatment improved the anthocyanin to sugar ratio and the tannin content of the fruit, but had no effect on vine balance; the crop thinning treatment had no effect on fruit composition, despite drastically reducing yield at all sites; the late defoliation reduced the anthocyanin to sugar ratio, despite elongating the maturation period; and the minimal pruning treatment had no effect on anthocyanin to sugar ratio despite the vines not being fully adjusted to the treatment by the project end.

The direct manipulation of fruit carbohydrate supply was achieved using chambers to reduce CO2 concentration in the air supplied to the vines by approximately 50%, thereby reducing photosynthesis and carbohydrate availability (simulating a high yield to canopy size vine, without affecting the bunch environment). When applied during the maturation period the system slowed the rate of sugar accumulation in the fruit by about 50%. However, doing this had no effect on the anthocyanin to sugar ratio of the fruit during maturation or at harvest.

Wine was made in small lots (50 kg ferments) from all the field treatments and using micro-vinification (1 kg ferments) from the chamber treatments. In all cases, the wine composition reflected the anthocyanin to sugar ratio of the fruit used and therefore the effects observed in the fruit were also observed in the wine. In fact, the field effects were often even more exaggerated in the wine. The extraction of anthocyanins from the fruit to the wine generally modified the proportion of different forms of anthocyanins, with treatments T2 (early defoliation) and T4 (late defoliation) impacting on extractability.

The gene expression data supported the results seen in the fruit and wine composition, with no clear effect of vine balance per se on the expression of the key regulatory genes for anthocyanin and phenolic compounds. However, in the early defoliation treatment, which did not affect vine balance, but did improve fruit composition, a small increase in flavonol synthase (FLS) gene expression across sites and seasons was observed in flowers and very young berries. Although this change in gene expression is likely to be a result of increased light exposure of the fruit it could be used to develop a molecular marker for an increase in anthocyanins and tannins at a very early stage of berry development. Interestingly, UFGT, which encodes the final step in anthocyanin synthesis, was reduced in the late defoliation treatments over several sampling timepoints, suggesting that the negative impact of this treatment on fruit anthocyanin content was not simply driven by reduced enzyme activity from increased temperature of exposed fruit.

Overall, neither the field management strategies, nor the direct manipulation of fruit carbohydrate supply, supported a significant direct role for vine balance, defined as yield to canopy size ratio, in determining fruit composition at harvest. This was supported by both sensory and chemical analysis of the wines made from that fruit and questions the benefit of using non-selective crop thinning to improve fruit quality.

However, the negative effect of late defoliation and the positive effect of early defoliation do suggest a significant role for bunch environment on fruit, and resulting wine, composition. While this is far from a new concept, the results from this project indicate that focusing vineyard management on bunch environment rather than yield control would be a more successful means to produce fruit, and wine, with a composition typically seen as being of higher quality. Furthermore, the project results suggest that successful implementation of bunch environment management can benefit fruit composition across a wide range of climates and viticultural targets, enhancing the quality of wines produced in any given wine region.