15N Abundance of Nodules as an Indicator of N Metabolism in N2-Fixing Plants

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Abstract
Previous reports of 15N elevation in nodules (compared to other tissues) of N2-fixing plants were expanded. N2-fixing nodules [all inoculated with Rhizobium japonicum] of Glycine max (soybeans), Vigna unguiculata (cowpea), Phaseolus vulgaris (common bean), Phaseolus coccineus (scarlet runner bean), Prosopis glandulosa (mesquite) and Olneya tesota (desert ironwood) were enriched in 15N. Nodules of Vicia faba (fava beans), Arachis hypogaea (peanut), Trifolium pratense (red clover), Pisum sativum (pea), Lathyrus sativus (grass pea), Medicago sativa (alfalfa) and Lupinus mutabilis (South American lupine) were not, nor were the nodules of 9 spp. of N2-fixing nonlegumes. The N of ineffective nodules of soybeans and cowpeas was not enriched in 15N. Thus, 15N elevation in nodules of these plants depends on active N2-fixation. Results obtained so far on the generality of 15N enrichment in N2-fixing nodules suggest that only the nodules of plants which actively fix N2 and which transport allantoin or allantoic acid exhibit 15N enrichment.