The role of oxygen diffusion from the shoots and nodule roots in nitrogen fixation by root nodules of Myrica gale

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Abstract
Nitrogenase activity (acetylene reduction) and O2 uptake by microbially induced root nodules of M. gale L. were measured before and after removal of nodule roots. There was no significant effect of nodule root removal when respiration was measured in the gas phase (0.05-0.2 atm PO2 [partial pressure of oxygen] (1 atm = 101.325 kPa)) or acetylene reduction in a stirred aqueous phase at 0.2 atm PO2. However, when acetylene reduction was measured in 0.05 atm PO2 in an unstirred aqueous phase, there was a 66 to 76% reduction in activity. Nodule roots are important for O2 uptake when the nodules are present in an aqueous phase at low PO2, which is probably the normal environmental conditions for many of the nodules. Other measurements showed that diffusion of O2 from the shoot to the root nodules is not important for N fixation. These measurements were done on whole plants with the shoots in air (0.20 atm PO2) and the roots in water at the desired PO2 value. With 0.0 atm PO2 in the root environment, the rate of acetylene reduction was only 4% of the rate at 0.2 atm PO2. Thus, only small amounts of O2 are transported from the shoot to the nodules.