Photosynthesis by sugar-cane leaves. A new carboxylation reaction and the pathway of sugar formation

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Abstract
Radioactive products in detached leaf segments were examined after periods of steady-state photosynthesis C14O2. After exposure of Cl4O2 for approx. 1 sec. more than 93% of thelixed radioactivity was located in malate, aspartate and oxaloacetate. After longer periods larger proportions of the radioactivity appeared in 3-phosphoglycerate, hexose monophosphates and sucrose. Similar results were obtained with leaves still attached to the plant. Radioactivity appeared 1st in C-4 of the dicarboxylic acids and C-l of 3-phosphoglycerate. The labelling pattern in hexoses was consistent with their formation from 3-phosphoglycerate. The reaction giving rise to C4 diearboxylic acid appeared to be the only quantitatively significant carboxylation reacton. Evidence is provided that the radioactivity incorporated into the C4 dicarboxylic acid pool is transferred to sugars via 3-phosphoglycerate. A scheme is proposed to account for these observations.