Abstract
The mechanism of photosynhetic inhibition in sink-limited leaves of young apple trees by continuous light (CL) was investigated. By 3 days after CL, the photosynthetic rate (A), stomatal conductance and intercellular CO2 concentration (Ci) had decreased by 38 %, 58 % and 9 %, respectively, while leaf starch concentration accumulated up to 2.8-fold. Therefore, CL trees were sink-limited. However, A, Ci and stomatal conductance approximately recovered after only 1 day of continuous dark (CD) following 3 days of CL, wihile leaf starch concentration was markedly reduced to the control level. Therefore, CL trees were rescued from sink-limit. After 3 days of exposure to CL, the initial slope of the A-Ci curve in CL trees was 17 % lower than that in the controls; that is to say, the carboxylation efficiency of CL trees was reduced. However, the initial slope of the A-Ci curve in CL trees was completely recovered after only 1 day of CD following 3 days of CL. In addition, both the activity and concentration of ribulose-1, 5-bisphosphate carboxylase (RuBPcase) were not significantly affected by 3 days of CL. These results suggested that CL-induced photosynthetic inhibition was due to stomatal limitation, rather than decreased activity of RuBPcase.
