Dual nitrogen species involved in foliar uptake of nitrogen dioxide in Arabidopsis thaliana

Foliar uptake of nitrogen dioxide (NO₂) is governed by its reactive absorption mechanism, by which NO₂ molecules diffuse through cell wall layers and simultaneously react with apoplastic ascorbate to form nitrous acid, which freely diffuses across plasmalemma. However, whether free diffusion of nitrous acid is the sole mechanism of foliar uptake of NO₂ remains unknown. The involvement of ammonia-inhibitable nitrite transporters in the foliar uptake of NO₂, as reported in nitrite transport in Arabidopsis roots, is also unknown. In this study, we treated Arabidopsis thaliana leaves with methionine sulfoximine (MSX) to inhibit incorporation of ammonia into glutamate and exposed them to 4 ppm ¹⁵N-labeled NO₂ for 4 h in light followed by quantification of total nitrogen, reduced nitrogen, and ammonia nitrogen derived from NO₂ using mass spectrometry and capillary electrophoresis. The total nitrogen derived from NO₂ in leaves without MSX treatment was 587.0 nmol NO₂/g fresh weight, of which more than 65% was recovered as reduced nitrogen. In comparison, MSX treatment decreased the total nitrogen and reduced nitrogen derived from NO₂ by half. Thus, half of the foliar uptake of NO₂ is not attributable to passive diffusion of nitrous acid but to ammonia-inhibitable nitrite transport. Foliar uptake of NO₂ is mediated by a dual mechanism in A. thaliana: nitrous acid-free diffusion and nitrite transporter-mediated transport.