Variation in Natural Abundance of15N among Plant Parts and in15N/14N Fractionation during N2Fixation in the Legume-Rhizobia Symbiotic System

摘要

Sixteen legumes were grown in N-free media so that N was supplied entirely by symbiotic N2fixation. The plant tissues were analyzed for natural15N abundance (expressed asδ15N per mil relative to air N2) with a ratio mass spectrometer. The nodules of desmodium, centro, siratro, soybean and winged bean showed high enrichment in15N (∼+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 in15N.In all the legumes investigated, particularly in the ureide-transporting plants such as desmodium, centro, siratro, soybean, winged bean and field bean, theδ15N value of the shoots was negative (∼−3.2‰). Theδ15N value of the shoots in winged bean and field bean varied by about 1‰depending on theRhizobiumstrains used. The isotopic mass balance of 13 legumes indicated that isotopic fractionation occurs during N2fixation by the legume-rhizobia symbiosis with a preference for14N over15N, resulting in aδ15N value of−0.2 to−2‰in the whole plant.The results indicate that15N/14N isotopic discrimination with a preference for the lighter atom may occur in both N2fixation and expor