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 ofPapaver somniferum resulted in a 3-fold or greater increasein their rate of dark ¹⁴C fixation, while even 0.1 mM NH₄⁺ nearlydoubled that rate. The most rapid increase in the labeling ofa metabolite occurred in aspartate, and was accompanied by adecrease in the steady-state level of labeled phosphoenolpyruvate.No change in labeled pyruvate level occurred, and alanine labelingdeclined. Ammonium ion addition had no effect on the respiratoryrate of these cells in the dark. We conclude that NH₄⁺ stimulatesphosphoenolpyruvate carboxylation in this system, but has nodetectable effect on the pyruvate kinase reaction in the dark.Our results are compared with earlier findings and the possibleregulatory function of ammonia is discussed. (Received July 23, 1979; )