Figure 3. VitiSim simulations of the canopy net CO2 fixation of Concord vines with 100 shoots or 350 shoots/vine (minimally-pruned) and long-term Geneva, NY weather.

The effects of pruning level on the individual demand and supply curves do not appear very striking. However, when the combined demand of the crop and the shoot canopy are subtracted from the net CO2 supply available from canopy photosynthesis (Figure 4), there are several differences observed in the balance. Compared to the 100-shoot vine, the minimallypruned vine requires more reserves initially due to the greater number of growing shoots.

However, minimally-pruned vines also have a more rapid canopy completion and earlier cessation of shoot growth, resulting in an earlier decline in carbon demand by shoots. This results in a more positive carbon balance than lower-shoot number vines in the immediate post-bloom period critical for fruit

set, flower bud development, and early root growth.

Conversely, post-veraison, the minimally- pruned vine has a strong carbon deficit as the supply is similar but the crop demand is much higher than the lower shoot number vine. These results may explain why Cornell researchers have found stable high yields over many years with minimally-pruned vines, if drought stresses are limited, yet these vines struggle to reach acceptable sugar levels in the crop.

A practical response has been developed by our colleagues, the late Bob Pool and currently Terry Bates, by using high node numbers, then mechanically thinning the crop

at 75 to 80 days after budbreak (about 30 days after bloom) to reestablish the vine crop load (dashed line in Figure 4). It may also explain why we have not found any reduction in fine root growth, and found slightly earlier root growth with minimally-pruned vines, even with 20% to 25% higher crop levels.6

Although Concord vines may not represent the shoot numbers and crop in wine grape systems, we believe the general patterns and the effects of pruning level shown will be similar in most species and varieties. Hopefully a better understanding of the seasonal dynamics of carbon balance and vine growth will help improve our understanding of vine balance.