Analysis of reduced photosynthesis in the apple leaf under sink-limited conditions due to girdling

On the 6th day after girdling, the photosynthesis rate of leaves on the 3-year-old stem, 2-year-old stem and new shoot of a branch of a field-grown 4-year-old apple tree decreased. The starch content in the leaves, bark and wood significantly increased, especially in shoot leaves. In the girdled branch, concomitant with the photosynthesis reduction, on the 6th day after girdling, the stomatal conductance of leaves on the 3-year-old stem, 2-year-old stem and new shoot declined. Under this condition, a significant correlation between the photosynthesis rate and stomatal conductance could be recognized. On the other hand, with girdling, the activity of ribulose-1,5bisphosphate carboxylase (RuBPcase) per leaf area decreased on the basis of the decrease in RuBPcase activity per unit enzyme protein. The initial activity of RuBPcase per leaf area in shoot leaves decreased. Moreover, the activity of RuBPcase per leaf area significantly correlated with the photosynthesis rate. Based on these results, it is suggested that the decrease in the photosynthesis rate of the sink-limited branch of a young apple tree 6 days after girdling is mainly caused by closing of the stomatal aperture and reduction of RuBPcase activity per leaf area.