The influence of vine vigour and canopy ideotype on fruit composition and aroma of Sauvignon Blanc

Abstract

The influence of soil texture on canopy growth, vine yield, and fruit composition of Sauvignon blanc were assessed on a mature vineyard from the Rapaura area in Marlborough. The subject vines were mature Sauvignon blanc in a commercial vineyard trained to four cane VSP and planted with a north south orientation. Row and vine spacings were typical of the area at 3m x 1.8m. Four areas of different soil texture were identified using trunk circumference measurements, visual assessment of the surface soil and aerial photographs to identify changes in vine growth. Soil pits were excavated at a later date to determine root numbers and record the soil texture in the different areas. Two crop treatments were imposed on half of the plots at approximately 50% veraison: unthinned crop and 50% crop. A shading treatment was also imposed at 50% veraison where three sets of tagged shoots had bunches that were exposed to sunlight, bunches that had some natural shading from leaves or bunches that had paper bags fastened over each to provide a completely shaded environment. Vine vegetative vigour for each plot was assessed during the growing season using the Point Quadrat method and at pruning using bud counts and pruning weights. Thirty berry samples were collected from the 32 plots and analysed weekly for soluble solids and berry weight. Harvest of bunches from the tagged shoots occurred on two different dates with the first harvest picked when fruit from one soil and crop treatment had reached 21 °Brix. The second picking took place just before the commercial harvest, which coincided with last soil and crop treatment reaching 21 °Brix. Bunches from each tagged shoot were weighed, frozen and later analysed for soluble solids, pH, organic acids, and methoxypyrazines.

Vines on very stony textured soils had small trunk circumferences with a high root density and yield to pruning weight ratio compared to those growing on deep silt soils. Vines on deep silt soils had more vigorous canopies with large shoots and a higher leaf layer number. The target soluble solids was reached first by the vines on the very stony plots despite the higher crop load with soil texture appearing to be a dominant factor by influencing the time of flowering.

Cluster shading decreased soluble solids, consistent with other studies, whilst crop thinning resulted in an earlier harvest. This was contrary to popular opinion that crop thinning at veraison would have no impact on sugar accumulation. pH and organic acids were unaffected by shading or bunch thinning. Vines growing on deep silt soils had a significantly higher level of total acidity and malic acid than those on the stony soils.

At harvest, methoxypyrazine levels were very low compared to previously reported figures for Marlborough, which may have been a result of sample preparation. IBMP was significantly higher on deep silt soils, however, with no impact from the shading or crop treatment. The findings are consistent with the hypothesis that IBMP synthesis may be increased by the number of leaves surrounding bunches (Roujou de Boubee, 2003).

The results suggest that soil texture plays a dominant role in influencing Sauvignon blanc flavour and aroma and due consideration should be given to vineyard layout and site selection prior to new plantings taking place.