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Studies on the Mechanism of Glycerate 3-Phosphate Synthesis in Tomato and Maize Leaves

¹ Département de Biologie, Centre d'Etudes Nucléaires de Saclay, B.P. No. 2, 91,190

The net carbon incorporation in maize (Zea mays) and tomato (Lycopersicum esculentum) leaves was mainly the result of the carboxylation of ribulose 1,5-diphosphate. In both of these organisms synthesis of glycerate 3-phosphate was studied during short chase experiments (2 or 3 seconds in ¹⁴CO₂ then 8 to 27 seconds in unlabeled CO₂). Changes in the radioactivity in the individual carbon atoms of glycerate 3-phosphate, malate, and aspartate are consistent with the formation, in both leaves, of 2 molecules of glycerate 3-phosphate for each CO₂ molecule incorporated. The CO₂, before reacting with ribulose 1,5-diphosphate, is first incorporated in an intracellular CO₂ pool which has a different composition according to the species. This pool is constituted in tomato by volatile compounds (50 nanomoles per gram of fresh weight) more or less in equilibrium with atmospheric CO₂. In maize the pool consists of carbon atoms 4 of malate and aspartate (for at least 80% of the pool) and volatile compounds which correspond, in all, to 540 nanomoles per gram of fresh weight where atmospheric CO₂ enters through an irreversible reaction.