C4 model plants composed of single-rooted Amarantus cruentus L. leaves were developed to study source-sink relationships. The photosynthetic activity of CO 2 fixation in the rooted leaves cultured under the control condition (10-h light/14-h darkness, 730/miol photons m^s" 1) remained constant and high. When the leaves kept for 8 or 9 d under the control condition were exposed to continuous light (CL) for 4 d, the activity decreased steeply. The starch content increased significantly during the first 2 d in CL whereas the sucrose content increased almost linearly during the 4 d in CL. The close relationship between the decrease in photosynthetic activity and the increase in sucrose indicates that the activity was subjected to feedback inhibition when the end product was forced to accumulate in the leaves. In the treated leaves, the initial activity of ribulose-l,5-bisphoshate carboxylase decreased significantly. A large increase in ribulose-l,5- bisphosphate and decrease in 3-phosphoglyc erate occurred following the deactivation in RuBPcase, which was fully restored by the preincubation with CO2 and Mg2+ and/or P( and Mg 2+ . The activity of phosphoenolpyruvate carboxylase also decreased but not as rapidly as RuBPcase. The alanine and pyruvate levels decreased markedly but the oxaloacetate, glutamine and asparagine levels significantly increased while phosphoenolpyruvate and glutamate were kept at almost constant levels. The malate level first increased slightly and then decreased significantly. The mechanism of coordinated decrease in carboxylation reactions between phtosynthetic carbon reduction cycle and C4 metabolism in response to change in the source-sink balance is discussed.