ABSTRACT
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.
ABSTRACT
A mathematical model is presented to explain the regulation of nitrogenase electron allocation to N2 fixation (EAC) in legume nodules. The model is based on two assumptions: (a) that H2 inhibits N2 fixation in a competitive manner; and (b) that O2, H2, and N2 move into and out of nodules by diffusion and their movement is impeded by a diffusion barrier, the permeability of which is controlled to maintain a very low infected cell O2 concentration. When the model was used to simulate nodules displaying a range of values for total nitrogenase activity (TNA), maximum EAC values were predicted to be between 0.69 and 0.71, and a negative correlation was predicted to exist between EAC and TNA. These predictions were in good agreement with empirically derived values reported in the literature and support the suggestion that H2 inhibition of N2 fixation is a major determinant in the regulation of nitrogenase EAC in legume nodules. Two versions of the model were constructed. A closed-pore model assumed that the diffusion barrier consisted of a solid shell of water of variable thickness in the nodule cortex. An open-pore model assumed that a small number of gas-filled intercellular spaces connected the nodule central zone with the root atmosphere and these pores were opened or closed by water to provide variations in the nodule's permeability to gas diffusion. Because of differences in the diffusivity of gases in the gaseous and aqueous phases, the model predicted that, at a given infected cell O2 concentration, an open-pore diffusion barrier would result in less H2 accumulation in the infected cells than a closed-pore diffusion barrier. Therefore, the model may be used to test specific hypotheses about the physical structure of the barrier to gas diffusion in legume nodules.