Interaction of Irrigation and Pruning Level on Grape and Wine Quality of Shiraz Vines

Abstract

We examined the effect of irrigation (trickle irrigation three times a week replacing 0.6 of Class A pan evaporation) and no irrigation on xylem water potential, leaf and bunch temperatures, sugar and acid levels and pH of the berries, and wine quality of Vitis vinifera cv Shiraz in the hot, arid climate of Griffith, N.S.W., for four years from 1975. Four pruning levels: 20, 40, 80 and 160 nodes, were imposed on the two irrigation treatments.

Irrigation compared to no irrigation and pruning to a high node number compared to a low node number both delayed sugar accumulation due to an increase in yield and increased total sugar production. In one year irrigation combined with severe pruning hastened sugar accumulation, but in the other years severe pruning of irrigated vines delayed ripening compared to nonirrigated vines although the yield levels were similar. The delay in ripening was not completely explained by the increase in yield.

The irrigated and nonirrigated vines were under sufficient moisture stress to stop photosynthesis for most of the day. The nonirrigated vines reached critical stress levels earlier than the irrigated vines, but accumulated berry sugar more rapidly than the irrigated vines. The differences between the irrigated and nonirrigated vines in grape sugar accumulation occurred early during berry development, possibly at Stage II when sugar accumulation begins. The differences in grape juice acidity and pH also occurred early during berry maturation.

The acidity and pH of juice from berries from irrigated vines were higher than that from nonirrigated vines. The pH of the juice increased during fermentation for all treatments, but the juice acidity from the nonirrigated vines decreased and that from irrigated vines increased during fermentation. This cancelled the irrigation effect on juice acidity so that irrigation had no effect on wine acidity.

Differences were established between wines from the irrigated and nonirrigated vines, but could not be quantified because of the subjective nature of evaluation.