A Simulation Study of Gaseous Diffusion Resistance, Nodule Pressure Gradients and Biological NitrogenFixation in Soyabean Nodules

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Abstract
A simulation of a normally functioning soyabean nodule, with a variable gaseous diffusion barrier in the inner cortex, has been used to calculate rates of nitrogen fixation and the concentrations of O2, CO2, H2 and N2 at various tissue locations, in response to variations in diffusion-resistance and external O2 concentration. A small diffusion-resistance allowed increased nitrogen fixation in air, but lead to diminished rates at increased external O2 concentrations. Large diffusion-resistances provide increased protection against the effects of high O2 concentrations but diminish nitrogen fixation in air. These effects depend on the respiratory activity (Vmax) of the bacteroids. In general, efficiency (moles of N3 fixed/moles of O2 used) is affected more than N2 fixation rates at increased external O2 concentrations. As a result of differential fluxes and solubilities of the gases involved in nitrogen fixation, significant negative pressure differences (about 24 kPa in air) would be generated between the outer cortex and the nodule central tissues, provided that the structure is sufficiently inflexible, and the central tissue is isolated from undue influences of water and gas. The calculations also show that the concentrations of H2 near the bacteroids remain low (2–3 per cent of concentrations of dissolved N2) and are thus unlikely to inhibit N2 fixation except at high values of the diffusion resistance.