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A Model of the Regulation of Nitrogenase Electron Allocation in Legume Nodules (II. Comparison of Empirical and Theoretical Studies in Soybean)

In N2-fixing legumes, the proportion of total electron flow through nitrogenase (total nitrogenase activity, TNA) that is used for N2 fixation is called the electron allocation coefficient (EAC). Previous studies have proposed that EAC is regulated by the competitive inhibition of H2 on N2 fixation and that the degree of H2 inhibition can be affected by a nodule's permeability to gas diffusion. To test this hypothesis, EAC was measured in soybean (Glycine max L. Merr.) nodules exposed to various partial pressures of H2 and N2, with or without changes in TNA or nodule permeability to gas diffusion, and the results were compared with the predictions of a mathematical model that combined equations for gas diffusion and competitive inhibition of N2 fixation (A. Moloney and D.B. Layzell [1993] Plant Physiol 103: 421–428). The empirical data clearly showed that decreases in EAC were associated with increases in external pH2, decreases in external pN2, and decreases in nodule permeability to O2 diffusion. The model predicted similar trends in EAC, and the small deviations that occurred between measured and predicted values could be readily accounted for by altering one or more of the following model assumptions: K1(H2) of nitrogenase (range from 2–4% H2), Km(N2) of nitrogenase (range from 4–5% N2), the allocation of less than 100% of whole-nodule respiration to tissues within the diffusion barrier, and the presence of a diffusion pathway that is open pore versus closed pore. The differences in the open-pore and closed-pore versions of the model suggest that it may be possible to use EAC measurements as a tool for the study of legume nodule diffusion barrier structure and function. The ability of the model to predict EAC provided strong support for the hypothesis that H2 inhibition of N2 fixation plays a major role in the in vivo control of EAC and that the presence of a variable barrier to gas diffusion affects the H2 and N2 concentration in the infected cell and, therefore, the degree of H2 inhibition.