The Relationship Between Hydrogen Evolution, Hydrogen Exchange, Nitrogen Fixation, and Applied Oxygen Tension in Soybean Root Nodules

Abstract

Hydrogen evolution and exchange and nitrogen fixation by detached soybean root nodules were studied at various external oxygen tensions (pO2). The main findings were (1) Increasing pO2 stimulated hydrogen evolution up to about 50% oxygen, above which it was inhibited in the absence but not in the presence of nitrogen. (2) 10% deuterium inhibited hydrogen evolution by decreasing the effects of rising pO2. (3) Nitrogen inhibited hydrogen evolution competitively when two different pO2 values were used to produce different H-donor concentrations. When nitrogen fixation was not limited by lack of nitrogen (10% nitrogen, 20% oxygen), about 5 [mu]moles hydrogen/g/hr were evolved, suggesting that a metabolic pool of molecular hydrogen exists in these nodules under natural physiological conditions. At high pC2 up to 25 moles of hydrogen were evolved for every mole of nitrogen fixed. (4) The exchange reaction (HD formation from deuterium) was affected by pO2 in a similar way to nitrogen fixation, an average of 2-3 moles of HD being formed for every mole of nitrogen fixed. (5) Carbon monoxide inhibited hydrogen evolution only at high external pO2 while HD formation was inhibited at all pO2 values. This inhibitor reduced nitrogen fixation more than either hydrogen evolution or HD formation. These results are discussed in terms of current theories of the pathways of nitrogen fixation in legume root nodules.