14CO2Pulse-Chase Labelling inClusia minorL.

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Abstract
Conditions and maintenance of growth were chosen so that plants of Clusia minor L. were obtained which showed the C3- and CAM- modes of CO2-exchange, respectively. C. minor is known to accumulate considerable amounts of citric acid in addition to malic acid during the dark-phase of CAM. 14CO2-pulse-chase experiments were performed with these plants. Patterns of labelling during the pulse and redistribution of label during the chase in the C3-mode were as expected for C3-photosynthesis. Pulse-labelling in the CAM-mode during the last hour of the light period, during the first part of the dark period and during the last hour of the dark period always led to an almost exclusive incorporation of label into malate. Redistribution of label from malate after the pulse at the end of the dark period during the chase in the subsequent light period followed the pattern expected for light-dependent reassimilation of CO2 remobilized from malate in CAM during the light period. During the chases in the dark period, label was transferred from l4C-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 malate 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 strong sinks.

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