Hydrogen evolved by nitrogenase may be recycled by a hydrogenase present in some legume nodules. Anoka and Portage cultivars of soybeans were inoculated with each of 8 and 24 strains, respectively, of Rhizobium japonicum and surveyed for H2 evolution and C2H2 reduction rates nodule weight, and plant dry weight. Six of the strains (3Ilb 110, USDA 122, USDA 136, 3Ilb 6, 3Ilb 142, and 3Ilb 143) which exhibited no H2 evolution in air were shown to take up H2. The relative efficiencies of nitrogenase energy utilization based on C2H2 reduction rates of nodules relative efficiences of nitrogenase energy utilization based on C2H2 reduction rates of nodules ranged from 0.96 to 1.0 for the six strains. Nodules formed by strain WA 5099-1-1 evolved small amounts of H2 in air and had a relative efficiency of 0.92. Nodules formed by the remaining 25 strains had relative efficiencies ranging from 0.41 to 0.80. A H2-evolving (3Ilb 123) and non-H2-evolving (3Ilb 143) strain were tested on seven soybean cultivars to determine the effect on the expression of hydrogenase. Nodules formed by strain 3Ilb 143 exhibited an efficiency of 1.0 on the following cultivars: Amsoy 71, Anoka, Bonus, Clark 63, Kent, Peking, and Portage. Relative efficiencies from 0.63 to 0.77 were determined for the five cultivars nodulated by strain 3Ilb 123. From the experiments with these cultivars, the capacity to recycle H2 produced from the nitrogenase system appears to be determined by the R. japonicum strain.

The interaction between the ATP-dependent evolution of H2 catalyzed by nitrogenase and the oxidation of H2 via a hydrogenase has been postulated to influence the efficiency of the N2-fixing process in nodulated legumes. A comparative study using soybean (Glycine max L. Merr.) cv produced increases in plant dry weight and total N2 fixed of 11 and 15%, respectively. This apparent increase in the efficiency of N2 fixation for nodulated legumes capable of reutilizing the H2 evolved from nitrogenase is considered and it is concluded that provision of conclusive evidence of the role of the H2-recycling process in N2-flxing efficiency of legumes will require comparison of Rhizobium strains that are genetically identical with the exception of the presence of hydrogenase.utilization during N2 fixation and therefore increase the quantity of N2 fixed and plant yield.The function of hydrogenase within legume nodules has been discussed by Dixon (6, 7) who has proposed the following possible roles: (a) serves as a means for reducing H2 concentrations in nodules below inhibitory levels; (b) acts as a mechanism for protecting nitrogenase from 02 inactivation by using H2 as a substrate to support respiration; and (c) provides a system whereby the H2 evolved from nitrogenase is metabolized and a portion of the otherwise wasted energy is conserved (4, 6). The proposal that H2 metabolism leads to ATP synthesis or to available reducing power which increases the efficiency of energy utilization within the nodule has been favoted in most discussions (1,(6)(7)(8)15). Dixon (6) has shown that the oxidation of H2 via hydrogenase in pea nodule bacteroids apparently is linked to ATP production. At this time there are no published reports supporting the conclusion that hydrogenase in nodules is capable of providing the reducing potential via the appropriate electron carriers, to support N2 fixation directly. If energy is the major limitation of N2 fixation in leguminous species as has been proposed (4, 9), then the coupling of the oxidation of H2 to energy-yielding processes conceivably could increase the rate of N2 fixation. Alternatively the conservation of energy through H2 recycling processes might decrease the demand for photosynthate and consequently contribute to increased dry matter. production. The possibility also must be considered that oxidation of H2 via hydrogenase is not coupled to useful energy-producing processes and therefore is not beneficial to the plant. This paper describes an investigation for the purpose of comparing the yield and efficiency of N2 fixation by legumes with nodules that recycle H2, with those that lack an active hydrogenase and evolve the H2 produced during N2 fixation.In regard to their capability to catalyze nitrogenase-dependent H2 evolution and H2 oxidation via a hydrogenase, leguminous symbionts that have been tested may be classified into two categories (13,15). The first includes symbionts that evolve H2 produced by the nitrogenase system. The second group is like the first, with the exc...

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