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Effect of the Hydrogenase System in Rhizobium japonicum on the Nitrogen Fixation and Growth of Soybeans at Different Stages of Development¹

The process of N fixation is energy consuming and inefficient. In most cases 20 to 40% of the energy that is supplied to nitrogenase for the reduction of N₂ is utilized for the reduction of protons to H². Some strains of R. japonicum contain a membrane-bound hydrogenase that is capable of oxidizing all of the H₂ that is produced during N₂ fixation. This study was conducted to evaluate the effects of inoculation of soybeans [Glycine max (L.) Merr.] with Rhizobium japonicum containing H2 recycling capability on N fixation efficiency and soybean yield at various physiological stages of growth.

In these experiments, a H₂-oxidizing strain SR (Hup⁺) and SR3 (Hup^–), a mutant derived from SR which is unable to oxidize H₂, were utilized. Soybeans cv. ‘Wilkin’ were grown under bacteriological conditions utilizing a drip-irrigated nutriculture system. Nodules from plants inoculated with SR evolved no measurable amount of H₂, while nodules from plants innoculated with SR3 evolved between 2.7 and 10 µmoles of H₂/g nodules/ hour depending upon physiological age. Acetylene reduction rates by strain SR were significantly higher than those of nodules formed from SR3 during the vegetative, late pod-fill, and leaf senescence stage of growth, respectively. Dry matter accumulation was significantly higher in plants inoculated with SR than in plants inoculated with SR3 (31% increase during vegetative stage, 25% during flowering, and 27% during pod-fill stage). Nitrogen contents of shoots and seeds were significantly higher in plants inoculated with strain SR, demonstrating increased N fixation by nodules formed by Hup⁺Rhizobium. Our results support the conclusion that strains of R. japonicum selected for inoculation purposes should contain H₂-oxidizing capability as one of their desired characteristics.

Key Words: Soybean yield • Hydrogen oxidation • Energy efficiency • Glycine max

² Former post-doctoral research associate, Laboratory for Nitrogen Fixation Research, Oregon State Univ., currently assistant soil microbiologist and assistant professor of soil science, Agric. Res. Educ. Ctr., Lake Alfred, FL 33850; research assistant; and professor of plant physiology and director of the Laboratory for Nitrogen Fixation Research respectively, Oregon State Univ., Corvallis, OR 97331.

Received for publication July 30, 1979.

This article has been cited by other articles:

				D. MERBERG and R. J. MAIER Mutants of Rhizobium japonicum with Increased Hydrogenase Activity Science, June 3, 1983; 220(4601): 1064 - 1065. [Abstract] [PDF]