Isotopic Investigations of Nitrogen Fixation in Non-legume Root Nodules

Abstract

INVESTIGATIONS involving the use of nitrogen-15 and previously reported have been continued as suitable plant material became available. It was shown earlier¹ that when 10 per cent of nitrogen is present in the supplied atmosphere (total pressure 1 atm.), fixation in detached nodules of Alnus, Casuarina, Myrica and Hippophaë is depressed if the proportion of oxygen exceeds 20 per cent (mean for the different genera). In a further experiment, with Casuarina nodules, the extent of this oxygen-inhibition in the presence of a higher level of gaseous nitrogen was examined, with the results shown in Fig. 1. In order to reduce the number of nitrogen-15 assays required, the digests from the triplicate samples of nodules at each gas mixture were pooled prior to assay; thus statistical treatment cannot be applied, but previous data¹ show that this nodule material replicates satisfactorily and indicate for the mean of three samples a standard deviation of the order of 10 per cent of the value of the mean. The results when 10 per cent of nitrogen was provided are similar to those previously reported. Some irregularity is shown in the results at 30 per cent nitrogen; but it is clear that the inhibitory effect of increasing oxygen tension is now considerably mitigated. This could be explained by a competition between oxygen and nitrogen at some stage in the fixation process, evidence for which has been adduced by other authors in respect of Azotobacter² and legume nodules³. The suggestion in Fig. 1 that at 20 per cent oxygen fixation was greater with 10 per cent nitrogen than with 30 per cent could also be explained in this way.