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June 2017 WINES&VINES 57 PRACTICAL WINERY & VINEYARD GRAPEGROWING Ensuring vines have canes of an appropriate diameter at pruning is important for yield consistency. The diameter of canes influences the inflorescence number per shoot and size the following year. 7 In Marlbor- ough, New Zealand, for example, 8-mm diameter Sauvignon Blanc canes have approximately 30% fewer bunches per shoot than those of 11-mm canes, 7 although the ab- solute number of bunches per shoot is affected by other factors during the inflorescence initiation period, 7 including light and temperature exposure to the bud 16 and/or water and nitrogen stress. 10 Grapevine triangle and fruit ripening Grapevine capacity (or leaf area : fruit weight ratio) may influence ripening in two ways. For exam- ple, reducing the capacity by trim- ming Sauvignon Blanc and Pinot Noir shoots from 12 to six leaves shortly after fruit set increased the time from flowering to fruit vérai- son, delaying the onset of ripen- ing. 18,21 Similarly, the reduction in leaf area in this manner either at or before véraison slows the rate of sugar accumulation, increasing the time it takes to attain the soluble solids desirable for har- vest. 19 A reduction in leaf area can be reversed by thinning fruit. The rate of soluble solids accumula- tion on vines trimmed to six leaves with half the fruit removed was similar to that of 12-leaf vines with a full crop. 21 Unlike soluble solids, titratable acidity is little affected by trim- ming. As a result, the leaf area : fruit weight ratio will change the sugar : acid balance of the fruit, and the extended ripening resulting from the lower leaf area resulted in fruit with a lower acidity at any given soluble solids content. 15 How many nodes to retain? Understanding the capacity of a site and the influence of site and vine management on balance helps viti- culturists make informed decisions at pruning. The concept of balanced pruning was introduced by Walter J. Clore in the 1960s as a way to use the productivity measured in the current season to estimate the num- ber of nodes to retain to achieve an adequate yield of mature fruit in the following season. 3 Since no two growing seasons are alike, it can only be an estimate, but developing an understanding of how vines behave in a given vine- yard environment will help viticul- turists achieve maximum yield without compromising future pro- ductivity. It also assumes that yields are unamended by thinning. This method has been suggested for juice grapes, in which pruning weight is used to determine vine INFLUENCE OF SITE CAPACITY AND YIELD ON THE EQUILIBRIUM Low-capacity site with adequate over- wintering reserves Shoot Root The low-capacity site has limited ability to achieve adequate cropping without compromising the growth and carbohydrate reserves in the shoot and root sys- tem (brown). Limiting the crop will enable the shoots and roots to accumulate sufficient reserves for growth in the following spring (yellow). Fruit High-capacity site Low-capacity site excessively cropped with inadequate overwintering reserves