The argon-induced decline in nitrogenase activity commences before the beginning of a decline in nodule oxygen uptake

Replacement of N-2 by argon in the air around nodules directs nitrogenase electron flow in its total onto H+ resulting in increased nodule H-2 evolution (total nitrogenase activity (TNA)). However, argon application induces a so-called argon-induced decline in nitrogenase activity (Ar-ID) connected with decreased nodule oxygen permeability. Consequently, TNA measurements tend to underestimate total nitrogenase activity. It is unclear whether the decline in oxygen diffusion into nodules induces the Ar-ID, or whether a decline in nitrogenase activity is followed by lower nodule O-2 uptake. The objective of the present work was to examine the time sequence of the decline in nodule H-2 evolution and O-2 uptake after argon application. In addition, the reliability of TNA values, taken as quickly as possible after the switch to Ar/O-2, was tested through comparative measurement of N-15(2) uptake of the same plants. Short-term TNA measurements in an optimized gas exchange measurement system yielded reliable results, verified by parallel determination of N-15(2) uptake. A five min application of Ar/O-2 was without effect on the subsequent H-2 evolution in ambient air. A parallel experiment on control plants revealed that a decrease in nodule oxygen uptake began several minutes after the onset of the decline in H-2 evolution. We conclude that the primary effect of the replacement of N-2 by argon differs from oxygen diffusion control. A gas exchange system allowing an immediate taking of TNA yields reliable results and does not disturb nodule activity. Gas exchange measurements provide a powerful tool for studying nodule physiology and should be combined with material from molecular studies. (C) 2010 Elsevier GmbH. All rights reserved.