A number of cultural factors can influence photosynthesis of tree fruit. Nutrient deficiencies of several elements, particularly nitrogen, cause decreases in photosynthesis. Increasing concentrations of ozone are reported to cause a linear decrease in photosynthesis of almond, apple, apricot, pear, plum, and prune, while cherry, peach, and nectarine are unaffected (Retzlaff, Williams, and DeJong, 1991). Certain pesticides, particularly spray oils and the organically approved materials copper and sulfur, cause a reduction in photosynthesis of apple leaves (Ferree, 1979). Research in Ohio shows that mites, apple scab, and powdery mildew also reduce apple leaf photosynthesis. In a simulated leaf injury study, losses of leaf area up to 7.5 percent have no significant effect on apple leaf photosynthesis, but significant reductions occur when losses of leaf area exceed 10 percent. If losses exceed 15 percent, not only is the photosynthetic capacity of the leaf reduced, but the performance of the remaining leaf tissue is also reduced. This reduction is associated with injury to veins and increased circumference of the simulated insect feeding. Thus, injury that results in many small holes is more serious than equivalent area lost in a few large holes. Similar results are reported on cherry.
Although differences among apple cultivars grown under similar conditions are small, spur-type 'Delicious' appears to have higher photosynthetic rates than standard habit 'Delicious'. The effect of rootstock on photosynthesis is nonexistent in some studies, but in others, photosynthetic rates of trees on more vigorous rootstocks are greater than on dwarfing rootstocks. Summer pruning delays the natural decline in photosynthesis of the remaining leaves. Root pruning, which acts mostly through interruption of water relations, causes a temporary reduction in photosynthesis. Orchard management and training systems have little effect on apple leaf photosynthesis as long as similar leaf types and exposures are measured.
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