Variation in Natural Abundance of ^<15>N among Plant Parts and in ^<15>N/^<14>N Fractionation during N_2 Fixation in the Legume-Rhizobia Symbiotic System

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Sixteen legumes were grown in N-free media so that N was supplied entirely by symbiotic N_2 fixation. The plant tissues were analyzed for natural ^<15>N abundance (expressed as δ^<15>N per mil relative to air N_2) with a ratio mass spectrometer. The nodules of desmodium, centro, siratro, soybean and winged bean showed high enrichment in ^<15>N (~+9‰), while red clover showed slight enrichment (~+2‰). The nodules of 9 other forage legumes (Townsville stylo, white clover, alsike clover, common vetch, Chinese milk vetch, senna, alfalfa, ladino clover, and hairy vetch) showed little enrichment in ^<15>N. In all the legumes investigated, particularly in the ureide-transporting plants such as desmodium, centro, siratro, soybean, winged bean and field bean, the δ^<15>N value of the shoots was negative (~-3.2‰). The δ^<15>N value of the shoots in winged bean and field bean varied by about 1‰~ depending on the Rhizobiwn strains used. The isotopic mass balance of 13 legumes indicated that isotopic fractionation occurs during N_2 fixation by the legume-rhizobia symbiosis with a preference for ^<14>N over ^<15>N, resulting in a δ^<15>N value of -0.2 to -2‰ in the whole plant. The results indicate that ^<15>N/^<14>N isotopic discrimination with a preference for the lighter atom may occur in both N_2 fixation and export of fixed N from nodules.
日本植物生理学会の論文