Translocation of amino acids from stem nodules of Sesbania rostrata demonstrated by GC-MS in planta 15N isotope dilution
We report a novel use of the 15N dilution technique to detail the translocation of amino compounds in the legume Sesbania rostrata. The conventional 15N dilution technique follows the dilution of 15N within a labelled plant, as 14N2 is fixed by symbiotic bacteria. In our experiments, stem-nodulated Sesbania rostrata were enriched by feeding with 15N ammonium nitrate for 2 weeks, followed by a 1 week period where the only N available to the plants was via nitrogen fixation of atmospheric N2. We measured the composition, concentration and 15N enrichment of amino compounds in various plant tissues, both above and below the stem nodules, using GC-MS and isotopic abundance mass spectrometry techniques. Approximately 28% of the total N in the stem nodules was derived from internal plant sources. The ureides allantoic acid and allantoin were not abundant in xylem, leaf or nodule tissues. The amides asparagine and glutamine were the major export products from stem nodules although a wide range of other amino compounds are also synthesized. Amino acids within the nodules had a low level of enrichment, demonstrating that a small fraction (≈ 11%) was derived from outside the nodules, and significant cycling of N (28% of xylem N) through the root system was revealed by measurements of 15N distribution and amino acid concentrations.