STIMULATION OF DARK CO2 FIXATION BY AMMONIA IN ISOLATED MESOPHYLL-CELLS OF PAPAVER-SOMNIFERUM L

Addition of 2 mM ammonium ion to isolated mesophyll cells of Papaver somniferum resulted in a 3-fold or greater increase in their rate of dark 14C fixation, while even 0.1 mM NH4+ nearly doubled that rate. The most rapid increase in the labeling of a metabolite occurred in aspartate, and was accompanied by a decrease in the steady-state level of labeled phosphoenolpyruvate. No change in labeled pyruvate level occurred, and alanine labeling declined. Ammonium ion addition had no effect on the respiratory rate of these cells in the dark. We conclude that NH4+ stimulates phosphoenolpyruvate carboxylation in this system, but has no detectable effect on the pyruvate kinase reaction in the dark. Our results are compared with earlier findings and the possible regulatory function of ammonia is discussed. © 1979 Oxford University Press.