Effect of Ammonium Chloride and Methionine Sulfoximine on the Acetylene Reduction of Detached Root Nodules of Peas ( Pisum sativum )

Acetylene-reducing activity of detached pea nodules was determined by submerging the nodules in buffer solution [tris(hydroxymethyl)aminomethane-hydrochloride, pH 7.4] containing 100 mM sodium succinate and incubating under a gas phase of 90% O(2) and 10% C(2)H(2). The nitrogenase activity was 4 to 8 mumol of C(2)H(4) formed per g of nodule fresh weight per h and remained constant for at least 4 h. Addition of NH(4)Cl to the buffer solution (at a concentration of 10 mM or more) resulted in a significant decrease of nitrogenase activity, which was more pronounced at higher concentrations of ammonium chloride. The inhibition of nitrogenase activity by NH(4)Cl was reversible; when the NH(4)Cl-containing buffer solution was replaced by buffer without NH(4)Cl, the original activity was partly restored. Treatment of the nodules with NH(4)Cl had almost no effect on the amount of nitrogenase, as measured by the acetylene-reducing activity of ethyl-enediaminetetraacetate-toluene-treated bacteroid suspensions. The effect of NH(4)Cl was largely eliminated by simultaneous addition of 10 mM methionine sulfoximine to the assay solution. This suggests that the assimilation of ammonium ions by glutamine synthetase controls the functioning of nitrogenase activity in the nodules. However, no effect of glutamine, glutamate, or aspartate on the acetylene reduction by detached nodules could be detected.