A Possible Mechanism of Ammonium Ion Regulation of Photosynthetic Carbon Flow in Higher Plants

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Abstract
Addition of NH4+ to the photosynthesizing leaf cells of Dolichos lablab L. var. lignosis Prain and leaf discs of Vigna sinensis L. savi ex Hassk caused a significant increase in the flow of photosynthetic C toward amino acids with a concomitant decrease toward sugars without affecting the over-all photosynthetic rate. Similar diversion of photosynthetic C away from sugars was also observed in the photosynthesizing isolated chloroplasts of V. sinensis, but the latter differed in that they accumulated organic acids rather than amino acids. To understand the mechanism of NH4+-mediated regulation, the specific and total activities of NAD(P)-glutamate dehydrogenase, glutamine synthetase, pyruvate kinase, alkaline fructose 1,6-bisphosphatase and NAD(P)-glyceraldehyde-3-phosphate dehydrogenase of the cells of D. lablab were checked but none was affected by the added ammonium salts even after prolonged incubation. At certain concentrations, ammonium ions abolished the light activation of NADP-glyceraldehyde-3-phosphate dehydrogenase and alkaline fructose 1,6-bisphosphatase in isolated chloroplasts from dark-adapted Vigna leaves without interfering with the basal dark activity of these enzymes. Based on these observations, a possible mechanism of action of NH4+ in regulating the photosynthetic C flow is postulated.