PHOTOSYNTHETIC PHYSIOLOGY OF SPUR PRUNED AND MINIMAL PRUNED GRAPEVINES

Canopy development, photosynthetic performance and yield characteristics of Riesling grapevines managed by either conventional spur pruning or minimal pruning were compared over a growing season. Leaf area development 4-5 weeks after budburst was 4-5-fold greater on the minimal pruned vines due to the 6-7-fold greater number of buds that burst to produce shoots. By time of flowering (8 weeks after budburst) there was less than a 2-fold difference between the pruning treatments in leaf area per vine. At time of harvest the leaf area of spur pruned vines on a Y-shaped trellis exceeded that of minimal pruned vines. Average photosynthetic rates of leaves on shoots on minimal pruned vines were 40% higher than on spur pruned vines at 4 weeks after budburst, but average rates were similar the following week and thereafter. Calculated instantaneous photosynthetic rates for entire vines were 3-6-fold higher for the minimal pruned vines at 4-5 weeks after budburst. However, by time of flowering, vines in both treatments had similar photosynthetic rates. At harvest, spur pruned vines showed somewhat greater instantaneous carbon gain than minimal pruned vines. Carbon gain per vine per day estimated from hourly measurements of irradiance over the canopy showed a similar trend to the instantaneous rates. Leaf conductances did not differ with pruning treatment. Calculated instantaneous water loss per vine was 2-5-fold higher for minimal pruned vines 4-5 weeks after budburst, but from flowering onwards spur pruned vines were likely to use more water than minimal pruned vines. Minimal pruned vines yielded twice the quantity of fruit of spur pruned vines, but only one-quarter the dry weight of new canes. Total carbon invested in fruit, new canes and leaves, however, was similar in both pruning treatments, accounting for 60-70% of the estimated carbon gain by the vines.