The effects of row orientation, trellis type, shoot and bunch position on the variability of Sauvignon Blanc (Vitis vinifera L.) juice composition

Abstract

Grape composition is important in determining the flavour and aroma characteristics of the resultant wine. Samples of the juice composition are described by their average value, yet the variability in fruit composition around the mean may also have a large impact on wine quality. The objective of this study was to identify the variance contribution of row orientation and trellis type within the vineyard, and bunch and shoot position within the vine.

For two years primary and secondary bunches were sampled from basal, mid-cane and apical shoots of five year old Sauvignon blanc vines on the Wairau Plains of Marlborough, New Zealand (41º 29' south, 173º 54' East). Vines were trained on either the Scott-Henry (S-H) or vertically shoot positioned (VSP) trellis and located in either east/west (E/W) or north/south (N/S) oriented rows.

The bunch position contributed 30% to 50% to juice composition variance in 1999 and 50% to 90% in 2000. Whilst trellis type and row orientation contributed 42%, 50% and 40% to the brix, TA and pH variance respectively in 1999, they contributed only 26%, 16% and 4% in 2000. This was attributed to a change in canopy management which increased the fruit and leaf exposure, therefore reducing the effects of trellis and row orientation on juice composition variance. Despite bunch position accounting for most of the data variance in 2000, differences between apical and basal shoots were the largest single differences in brix (0.9°), TA (0.9 g/L) and pH (0.07). The least mature bunches on the vine were secondary bunches on basal shoots regardless of trellis type or year. They were from 1.3° - 2.2° brix lower, and 1.0 - 2.7 g/L TA higher than primary bunches on mid cane or apical shoots.

In the first year of the trial differences in fruit exposure caused maturity differences between trellis types. Fruit exposure levels were improved in the second year, and maturity differences were similar to differences in phenology at flowering. The phenology of apical shoots was advanced by 2 to 3 days compared to basal shoots in both years, whilst primary bunches were advanced 1 to 2 days relative to secondary bunches. Variation in the leaf area or leaf area: fruit weight ratio of shoots was not correlated to variation in berry weight or soluble solids. This was probably because of low vine crop loads and remobilisation of carbohydrate reserves.

The results indicated that to reduce variability in the grape crop the viticulturist must promote uniform fruit exposure and try to reduce phenological differences between shoot positions. Further study should consider the relationship between vine crop load and the leaf area or leaf area: fruit weight ratio of individual shoots on the vine. A better understanding of how light exposure on the fruit and leaves contributes to the weighted average juice composition of a vine would also be useful. Whilst the effects of variable fruit exposure on the variance of the data were not clear, biologically significant differences in maturity were not reflected in the data variance, so this line of research is of less concern to the viticulturist/wine maker.