Oxaloacetate as the Source of Carbon Dioxide for Photosynthesis in Bundle Sheath Cells of the C4 Species Panicum maximum

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Abstract
3-Mercaptopicolinic acid (3-MPA), an inhibitor of phosphoenolpyruvate carboxykinase [EC 4.1.1.32], was employed to study the role of organic acid decarboxylation during C4 photosynthesis. Treatment of detached P. maximum leaves with 5 mM 3-MPA inhibited photosynthesis 70-75%. Oxygen was found to have no effect on the degree of inhibition. The postillumination 14CO2 burst associated with P. maximum photosynthesis was almost abolished by 5 mM 3-MPA. The turnover rates of malate and aspartate during C4 photosynthesis were severely reduced as well as the rates of formation of C3 cycle intermediates in P. maximum leaves treated with 3-MPA. These results are direct evidence for the fixation of CO2, arising from the decarboxylation of oxaloacetate, by the C3 cycle in bundle sheath cells of P. maximum leaves.