Abstract
We have unexpectedly discovered that about one-third of the nitrogen derived from 15N-labeled nitrogen dioxide (NO2) taken up by Arabidopsis thaliana plants was converted to neither inorganic nor Kjeldahl nitrogen, but instead to an as yet unknown nitrogen compound(s). We here refer to this nitrogen as unidentified nitrogen (UN). In this study, the generality of the formation of the UN across species, nitrogen sources and cultivation environments for plants has been studied. All of 12 plant species studied were found to form the UN. Tobacco plants fed with 15N-nitrate appeared to form the UN. And the leaves of naturally fed vegetables, grass and roadside trees were found to possess the UN. Our present findings imply that there is a novel nitrogen metabolism for the formation of the UN in plants. We propose that UN-bearing compounds may provide a chemical basis for the biological action of reactive nitrogen species (RNS).
