(CO2)-C-14 PULSE-CHASE LABELING IN CLUSIA-MINOR L

Conditions and maintenance of growth were chosen so that plants of Clu sia minor L. were obtained which showed the C3- and CAM- modes of CO2- exchange, respectively. C. minor is known to accumulate considerable a mounts of citric acid in addition to malic acid during the dark-phase of CAM. (CO2)-C-14-pulse-chase experiments were performed with these p lants. Patterns of labelling during the pulse and redistribution of la bel during the chase in the C3-mode were as expected for C3-photosynth esis. Pulse-labelling in the CAM-mode during the last hour of the ligh t period, during the first part of the dark period and during the last hour of the dark period always led to an almost exclusive incorporati on of label into malate. Redistribution of label from malate after the pulse at the end of the dark period during the chase in the subsequen t light period followed the pattern expected for light-dependent reass imilation of CO2 remobilized from malate in CAM during the light perio d. During the chases in the dark period, label was transferred from C- 14-malate to citrate. This suggests that during accumulation of citric acid in the dark period of CAM in C. minor, citrate is synthesized in the mitochondria from malate or oxaloacetate after formation of malat e via phosphoenolpyruvate carboxylase. The experiment also showed that no labelled compounds are exported from leaves in the CAM-mode during the dark period. In plants of the C3-mode the roots proved to be stro ng sinks.