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

Department of Botany, University of Georgia, Athens, Georgia 30602

3-Mercaptopicolinic acid (3-MPA), an inhibitor of phosphoenolpyruvate carboxykinase, was employed to study the role of organic acid decarboxylation during C₄ photosynthesis. Treatment of detached Panicum maximum leaves with 5 mM 3-MPA inhibited photosynthesis 70 to 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 interpreted as 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.

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