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

C-4 model plants composed of single-rooted Amarantus cruentus L. leaves wer e developed to study source-sink relationships. The photosynthetic activity of CO2 fixation in the rooted leaves cultured under the control condition (10-h light/14-h darkness, 730 mu mol photons m(-2) s(-1)) remained constan t and high. When the leaves kept for 8 or 9 d under the control condition w ere exposed to continuous light (CL) for 4 d, the activity decreased steepl y. The starch content increased significantly during the first 2 d in CL wh ereas the sucrose content increased almost linearly during the 4 d in CL, T he close relationship between the decrease in photosynthetic activity and t he increase in sucrose indicates that the activity was subjected to feedbac k inhibition when the end product was forced to accumulate in the leaves. I n the treated leaves, the initial activity of ribulose-1,5-bisphoshate carb oxylase decreased significantly. A large increase in ribulose-1,5-bisphosph ate and decrease in 3-phosphoglycerate occurred following the deactivation in RuBPcase, which was fully restored by the preincubation with CO2 and Mg2 + and/or P-i and Mg2+ The activity of phosphoenolpyruvate carboxylase also decreased but not as rapidly as RuBPcase. The alanine and pyruvate levels d ecreased markedly but the oxaloacetate, glutamine and asparagine levels sig nificantly increased while phosphoenolpyruvate and glutamate were kept at a lmost constant levels. The malate level first increased slightly and then d ecreased significantly. The mechanism of coordinated decrease in carboxylat ion reactions between phtosynthetic carbon reduction cycle and C-4 metaboli sm in response to change in the source-sink balance is discussed.