Effects of different O-2 concentrations on nitrogenase activity, respiration, and O-2 diffusion resistance in Lupinus albus L. cv. Multolupa nodules

Nitrogenase, responsible for N-2 fixation in legume nodules, can be denatur ed by O-2 and functions under microaerobic conditions. However, O-2 is requ ired for ATP production associated with nitrogenase activity; therefore, O- 2 concentration within infected cells is highly regulated by a combination of nodule respiration, leghaemoglobin, and an O-2 diffusion barrier. To inv estigate O-2 diffusion regulation, different concentrations (1%, 21%, and 4 0% (v/v)) were applied to nodulated root systems of Lupinus albus for 3 and 10 days. The application of 1% O-2 for 3 days produced a marked decrease in nitrogen ase activity, up to 30% of that of control (21% O-2) plants. The irreversib le failure of nitrogenase functioning occurred after 10 days and could not be explained by the small variations in the content of nitrogenase componen ts I and II. The internal O-2 concentration seems to be a limiting factor t o N-2 fixation, but nodules can partly maintain their activity if hypoxic c onditions are not too prolonged. The nitrogenase inhibition of the 3-day 40% O-2 treatment was moderate (<20 %), and after 10 days nitrogenase activity returned to control values. Agai n, variations in nitrogenase components were not correlated with the activi ty measured by acetylene reduction in a flow-through system. Root respirati on rates were unexpectedly reduced. These plants seemed to adapt to 40% O-2 by varying the minimum resistance (R-min) of the O-2 diffusion barrier, wh ich was almost doubled from the third day of treatment.