Uptake, Assimilation, and Novel Metabolism of Nitrogen Dioxide in Plants

To understand the uptake and assimilation of nitrogen dioxide (NO₂ ) in various plants, quantification of both inorganic nitrogen such as nitrate, nitrite and ammonium ions, and organic nitrogen (or Kjeldahl nitrogen) is vital. Thus, we first describe the quantification of these ions by the capillary electrophoresis method. It is noteworthy that the nitrite ion concentrations in plant tissues are somewhat controversial, and that we have previously reported possible causes of nitrite ion contamination from experimental tools. Details of fumigation of plants with NO₂ , and of nitrogen analysis of fumigated plant tissues are described. According to plant physiology textbooks, the total nitrogen taken up into the plant body should equal the sum of the inorganic nitrogen plus organic or Kjeldahl nitrogen. However, we have unexpectedly discovered that about one-third of the total nitrogen derived from NO₂ taken up in the leaves of Arabidopsis thaliana is converted to neither inorganic nor Kjeldahl nitrogen, but instead to an as yet unknown nitrogen. We hereafter designate this nitrogen unidentified nitrogen (UN). Some details for the determination of the UN also are described.