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Simple, Rapid Assay for Screening Nitrogen-fixing Ability in Soybean¹

The ability of Rhizobium to fix N₂ in legume root nodules is a property that requires a great deal of further study. A rapid assay for N₂-fixing ability that can be used for screening many plants certainly will be utilized for work on this important symbiosis. We have modified the acetylene-reduction technique to assay for N₂ fixation in nodulated legumes. Inoculated seedlings are grown in sterile conditions in small vials capped with plastic bags. The vials contain vermiculite wetted with an N-free medium. The effectiveness assay is completed within 2 weeks after seedlings have germinated and several thousand seeds can readily be tested in 1 month. Nitrogen-fixing ability is monitored by observing the reduction of acetylene to ethylene by gas chromatography. Cultivars of soybean, Glycine max (L.) Merr., were assayed with this technique and major varietal differences were found. Nitrate and ammonium prevent nodulation and acetylene reduction in the effectiveness assay. Results from screening soybean cultivars and introduction lines may have value for plant-breeding programs and host-Rhizobium interaction studies. This effectiveness assay is useful for screening mutant plants or rhizobia that have defects in their ability to synthesize factors responsible for the establishment of an N₂-fixing root nodule.

Key Words: Nitrogenase • Legumes • Nitrogen metabolism • Rhizobium • Inoculants • Glycine max (L.) Merr. • Acetylene reduction • Nodulation

² Research associate (presently with NifTAL Project, College of Tropical Agric., Univ. of Hawaii, Paia, HI 96779); and professor, Dep. of Bacteriology and the Center for Studies of Nitrogen Fixation, Univ. of Wisconsin, Madison, WI 53706.

Received for publication October 2, 1975.

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