Effect of nitrate-nitrogen on the nodule symbioses of Coriaria and Hippophaë

Nodulated plants of Coriaria arborea and Hippophaë rhamnoides, one year old and grown previously in a rooting medium essentially free of combined nitrogen, were grown for a further 2½ months in the presence in the rooting medium of 0, 10 or 25 mg nitrate-nitrogen per litre of culture solution. The nitrate was labelled with 15N. Plant growth was promoted by the supplied nitrogen, especially in Hippophaë, but nodule growth and nitrogen fixation per plant were depressed, the latter, at the highest level of nitrate, being only 31% (in Coriaria) and 61% (in Hippophaë) of the fixation in plants at zero nitrate level. Although the 15N penetrated (probably indirectly) into the nodules, in both species and at both levels of nitrate the enrichment shown by the nodule tissues as a whole was only about one-fifth of that shown by the rest of the plant. This finding would be explained if four-fifths of the nodule nitrogen was in the endophyte and was wholly unlabelled nitrogen fixed from the atmosphere, while the remaining one-fifth was in the uninfected cells and these were in equilibrium with the tissues of the rest of the plant and carried the same 15N label. The implications of this hypothesis are considered.