A MODEL OF THE REGULATION OF NITROGENASE ELECTRON ALLOCATION IN LEGUME NODULES .2. COMPARISON OF EMPIRICAL AND THEORETICAL-STUDIES IN SOYBEAN

In N-2-fixing legumes, the proportion of total electron flow through n itrogenase (total nitrogenase activity, TNA) that is used for N-2 fixa tion is called the electron allocation coefficient (EAC). Previous stu dies have proposed that EAC is regulated by the competitive inhibition of H-2 on N-2 fixation and that the degree of H-2 inhibition can be a ffected by a nodule's permeability to gas diffusion. To test this hypo thesis, EAC was measured in soybean (Glycine max L. Merr.) nodules exp osed to various partial pressures of Ha and N-2, with or without chang es in TNA or nodule permeability to gas diffusion, and the results wer e compared with the predictions of a mathematical model that combined equations for gas diffusion and competitive inhibition of N-2 fixation (A. Moloney and D.B. Layzell [1993] Plant Physiol 103: 421-428). The empirical data clearly showed that decreases in EAC were associated wi th increases in external pH(2), decreases in external pN(2), and decre ases in nodule permeability to O-2 diffusion. The model predicted simi lar trends in EAC, and the small deviations that occurred between meas ured and predicted values could be readily accounted for by altering o ne or more of the following model assumptions: K-i(H-2) of nitrogenase (range from 2-4% H-2), K-M(N-2) of nitrogenase (range from 4-5% N-2) the allocation of less than 100% of whole-nodule respiration to tissue s within the diffusion barrier, and the presence of a diffusion pathwa y that is open pore versus closed pore. The differences in the open-po re and closed-pore versions of the model suggest that it may be possib le to use EAC measurements as a tool for the study of legume nodule di ffusion barrier structure and function. The ability of the model to pr edict EAC provided strong support for the hypothesis that H-2 inhibiti on of N-2 fixation plays a major role in the in vivo control of EAC an d that the presence of a variable barrier to gas diffusion affects the H-2 and N-2 concentration in the infected cell and, therefore, the de gree of H-2 inhibition.