Mechanism of symbiotic nitrogen fixation

Abstract

Two new lines of evidence are presented which confirm the conclusion previously reached by Wilson and Umbreit (Arch. Mikrobiol. 8: 440, 1937) that H2 is a specific inhibitor for symbiotic N fixation. These are: (1) The rate of uptake of both free and combined N by red clover plants is essentially logarithmic during the period of the expts. Because of this fact the specific unimolecular rate constant, g, may be estimated from the slope of the line obtained when the logarithm of the quantity of N taken up is plotted against time. Comparison of g values shows that these are decreased significantly by replacing part of the N in the atmosphere by H2 only with inoculated plants dependent on free N for their supply of this element. The effect is not caused by the reduction of the pN2 since in the'' range of pN2 employed, fixation is independent of the partial pressure of this gas. The rate of uptake of combined N aa measured by the g values is not affected by the presence of H2 in the atmosphere. (2) If red clover plants are kept in an atmosphere containing H2, fixation of free N is inhibited, but if air is substituted for the H2-containing atmosphere, fixation immediately increases. If the plants are kept in the H2-containing atmosphere and combined N furnished them, uptake of this combined N takes place at a rate comparable to that of plants kept in air throughout the expt. Likewise, if the plants are kept in air for several weeks until fixation of N is well under way, then transferred to an atmosphere which contains H2, inhibition of N fixation takes place almost immediately. If, however, at the time that the plants are transferred to the H2-containing atmosphere, combined N is also supplied, there is no inhibition in the development of the plant. The inhibition is evidently reversible and not associated with any particular stage or rate of growth of the plant.