Nitrogenase Activity in Alnus incana Root Nodules. Responses to O2 and Short-Term N2Deprivation

Abstract

Abstract O2 and host-microsymbiont interactions are key factors affecting the physiology of N2-fixing symbioses. To determine the relationship among nitrogenase activity of Frankia-Alnus incana root nodules, O2 concentration, and short-term N2 deprivation, intact nodulated roots were exposed to various O2 pressures (pO2) and Ar:O2 in a continuous flow-through system. Nitrogenase activity (H2 production) occurred at a maximal rate at 20% O2. Exposure to short-term N2 deprivation in Ar:O2 carried out at either 17%, 21%, or 25% O2 caused a decline in the nitrogenase activity at 21% and 25% O2 by 12% and 25%, respectively. At 21% O2, nitrogenase activity recovered to initial activity within 60 min. The decline rate was correlated with the degree of inhibition of N2 fixation. Respiration (net CO2evolution) decreased in response to the N2 deprivation at all pO2 values and did not recover during the time in Ar:O2. Increasing the pO2 from 21% to 25% and decreasing the pO2 from 21% to 17% during the decline further decreased rather than stimulated nitrogenase activity, showing that the decline was not due to O2 limitation. The decline was possibly due to a temporary disturbance in the supply of reductant to nitrogenase with a partial O2 inhibition of nitrogenase at 25% O2. These results are consistent with a fixed O2 diffusion barrier in A. incana root nodules, and show that A. incana nodules differ from legume nodules in the response of the nitrogenase activity to O2 and N2 deprivation.