Effects of different O2 concentrations on nitrogenase activity, respiration, and O2 diffusion resistance in Lupinus albus L. cv. Multolupa nodules

Nitrogenase, responsible for N2 fixation in legume nodules, can be denatured by O2 and functions under microaerobic conditions. However, O2 is required for ATP production associated with nitrogenase activity; therefore, O2 concentration within infected cells is highly regulated by a combination of nodule respiration, leghaemoglobin, and an O2 diffusion barrier. To investigate O2 diffusion regulation, different concentrations (1 %, 21 %, and 40 % (v/v)) were applied to nodulated root systems of Lupinus albusfor 3 and 10 days. The application of 1 % O2 for 3 days produced a marked decrease in nitrogenase activity, up to 30 % of that of control (21 % O2) plants. The irreversible failure of nitrogenase functioning occurred after 10 days and could not be explained by the small variations in the content of nitrogenase components I and II. The internal O2 concentration seems to be a limiting factor to N2 fixation, but nodules can partly maintain their activity if hypoxic conditions are not too prolonged. The nitrogenase inhibition of the 3-day 40 % O2 treatment was moderate (<20 %), and after 10 days nitrogenase activity returned to control values. Again, variations in nitrogenase components were not correlated with the activity measured by acetylene reduction in a flow-through system. Root respiration rates were unexpectedly reduced. These plants seemed to adapt to 40 % O2 by varying the minimum resistance (Rmin) of the O2 diffusion barrier, which was almost doubled from the third day of treatment.