Factors Affecting the Acetylene-Induced Decline during Nitrogenase Assays in Root Nodules of Myrica gale L

Our goal was to determine why the rate of acetylene reduction by nodules of actinorhizal plants declines after an initial peak value. The decline was eliminated by pretreatment with argon, indicating that the decline is initiated by cessation of ammonia synthesis. When O2 concentration was decreased during the decline, the rate of acetylene reduction increased. This shows that during the decline there is either O2 toxicity or competition between respiration and nitrogenase for reductant. The decline was not eliminated when uptake hydrogenase was inactivated by pretreatment with acetylene, showing that cessation of H2 oxidation is not the primary cause of the decline. The effects of a variety of other treatments on the decline were also studied. Overall, we conclude that the cessation of ammonia formation is the primary cause of the acetylene-induced decline. We hypothesize that the supply of reductant for nitrogenase depends on amino acids that are depleted following cessation of ammonia formation. We also conclude that the initial peak rate of acetylene reduction provides the best measure of nitrogenase activity.