Photosynthesis

Temperate zone fruit trees have the C3 photosynthetic pathway and utilize light as the energy source to drive the process. The photo-synthetic light response curve is hyperbolic. The rate of photosynthesis of a leaf increases rapidly until it is saturated around 30 percent full sunlight (700 to 1,000 micromoles quanta per square meter per second photosynthetically active radiation). The light compensation point occurs around 20 to 50 micromoles quanta per square meter per second. Light levels above the saturation level have little direct effect on photosynthesis, as other factors become limiting. Several studies show that the areas of the tree canopy that receive 30 percent full sunlight also are the areas that readily initiate flowers and have the largest fruit and highest fruit quality. Because of shading differences in leaf angle, canopy density, and canopy form and shape, the whole leaf canopy likely never becomes photosynthetically saturated.

Fruit tree leaves reach their maximum rate of photosynthesis early in the growing season and become net exporters of photosynthate when they are 10 to 40 percent fully expanded. Early development of apple leaves combined with slow leaf aging and decline ofphotosyn-

thesis translate into a long duration of effective photosynthetic leaf area. This characteristic may form the basis for high yield potential. Results from whole tree studies demonstrate the same general pattern of a high rate of photosynthesis during most of the growing season, declining in the fall around harvest. Apple is unique in having primary spur leaves that form the early canopy and are the sources of photosynthate during cell division. Later in the season, after fruit set, bourse shoot and terminal shoot leaves become the primary sources supplying the fruit with carbohydrates. Spur leaf area is closely related to fruit set, fruit size, fruit soluble solids, long-term yield, and fruit calcium level at harvest (Ferree and Palmer, 1982). Recent work by Wunsche and Lakso (2000) shows that fruit yield is linearly and highly correlated (r2 = 0.78) with spur leaf light interception in a range of different orchard systems.

Early in the season, leaves on spurs with fruit have higher levels of photosynthesis than spurs without fruit, while late in the season this trend is reversed. Leaf efficiency, in a number of studies, is closely related to leaf photosynthetic rate and previous and present light environment. Spur leaves supply the fruit with photosynthate early in the season during cell division, while shoot leaves supply the fruit later in the season during cell expansion. Evaluations in various orchard management systems indicate that spur leaf efficiency follows light level in the canopy. Shoot leaves have higher rates of photosynthesis than spur leaves and also have larger leaf areas.

Apple leaves adjust very quickly to changes in light pattern, and studies on intermittent lighting indicate that apple leaves may be about 85 percent as efficient under alternating light levels as when steady-state light is provided (Lakso, 1994). A single leaf can register multiple levels of photosynthesis. The portion of a leaf in a sunspot will be much higher than the remainder of the leaf that may be in the shade. These characteristics make an apple tree very responsive to the rapid changes in light distribution during the day and also adaptive to longer-term changes that may occur as the crop weight causes limb repositioning.

The skin of young apple fruit photosynthesizes but its contribution to the overall photosynthate of the tree is very small compared to the leaf contribution. The role of fruit photosynthesis may be greater in peach and cherry. Research by Flore and Layne (1996) demonstrates that fruit gross photosynthesis contributes 19.4,29.7, and 1.5 percent of the carbohydrate used by the fruit during Stages I, II, and III, respectively.

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