Photosynthesis with Single-Rooted Amaranthus Leaves. I. Changes in the Activities of RuBP-1,5-bisphosphate Carboxylase and Phosphoenolpyruvate Carboxylase and the Amounts of Intermediates in Photosynthetic Metabolism in Response to Changes in the Source-Sink Balance

C₄ model plants composed of single-rooted Amarantus cruentusL. leaves were developed to study source-sink relationships.The photosynthetic activity of CO₄ fixation in the rooted leavescultured under the control condition (10-h light/14-h darkness,730 µmol photons m^–2s^–1) remained constantand high. When the leaves kept for 8 or 9 d under the controlcondition were exposed to continuous light (CL) for 4 d, theactivity decreased steeply. The starch content increased significantlyduring the first 2 d in CL whereas the sucrose content increasedalmost linearly during the 4 d in CL. The close relationshipbetween the decrease in photosynthetic activity and the increasein sucrose indicates that the activity was subjected to feedbackinhibition when the end product was forced to accumulate inthe leaves. In the treated leaves, the initial activity of ribulose-1,5-bisphoshatecarboxylase decreased significantly. A large increase in ribulose-1,5-bisphosphateand decrease in 3-phosphoglycerate occurred following the deactivationin RuBPcase, which was fully restored by the preincubation withCO² and Mg²⁺ and/or P₁ and Mg²⁺. The activity of phosphoenolpyruvatecarboxylase also decreased but not as rapidly as RuBPcase. Thealanine and pyruvate levels decreased markedly but the oxaloacetate,glutamine and asparagine levels significantly increased whilephosphoenolpyruvate and glutamate were kept at almost constantlevels. The malate level first increased slightly and then decreasedsignificantly. The mechanism of coordinated decrease in carboxylationreactions between phtosynthetic carbon reduction cycle and C₄metabolism in response to change in the source-sink balanceis discussed. (Received May 24, 1999; Accepted September 1, 1999)