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

Abstract

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 15 N. 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 15 N 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 15 N label. The implications of this hypothesis are considered.