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

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Abstract
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.