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Inhibition of Photorespiration and Increase of Net Photosynthesis in Isolated Maize Bundle Sheath Cells Treated with Glutamate or Aspartate

Department of Biochemistry, The Connecticut Agricultural Experiment Station, Box 1106, 123 Huntington Street, New Haven, Connecticut 06504

Net photosynthetic ¹⁴CO₂ fixation by isolated maize (Zea mays) bundle sheath strands was stimulated 20 to 35% by the inclusion of L-glutamate or L-aspartate in the reaction mixture. Maximal stimulation occurred at a 7.5 mM concentration of either amino acid. Since the photosynthetic rate and the glutamate-dependent stimulation in the rate were equally sensitive to a photosynthetic electron transport inhibitor, 3-(p-chlorophenyl)-1,1-dimethylurea, it was concluded that glutamate did not stimulate CO₂ fixation by supplying needed NADPH (NADH) through glutamate dehydrogenase. Treatment of the bundle sheath strands with glutamate inhibited glycolate synthesis by 59%. Photorespiration in this tissue, measured as the O₂ inhibition of CO₂ fixation (the Warburg effect), was inhibited by treatment with glutamate. The stimulation in net photosynthetic CO₂ fixation probably results from the decrease in photorespiratory CO₂ loss. This metabolic regulation of the rate of glycolate synthesis and photorespiration observed with isolated bundle sheath strands could account for the inability to detect rapid photorespiration in the mature intact maize leaf.