Seasonal Changes in the Photosynthetic Rate in Apple Trees

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Abstract
Seasonal changes in photosynthesis of apple trees (M. domestica Borkh.) were monitored to examine the effect of source-sink interactions on photosynthesis and photorespiration. Elevated photosynthetic rates were observed during 2 periods of the growing season and correlated with the fruiting process. The 1st period of increased photosynthetic rates was during the blood period, when spur leaves on flowering shoots exhibited up to 25% higher photosynthetic rates than vegetative spur leaves on a leaf area basis. CO2 assimilation rates were also higher in fruiting trees than nonfruiting trees during the period of rapid fruit growth from July to Sept. Photorespiration, dark respiration, leaf resistance, and transpiration exhibited no seasonal changes which correlated to the presence or absence of fruit. These data represent the first comprehensive examination of the effects of flowering/fruit formation on photosynthesis and photorespiration in perennial plants.