Main content area

A simulation study of gaseous diffusion resistance, nodule pressure gradients and biological nitrogen fixation in soyabean nodules

Sheehy, J.E., Bergersen, F.J., Minchin, F.R., Witty, J.
Annals of botany 1987 v.60 no.3 pp. 345-351
Glycine max, root nodules, nitrogen fixation, pressure, hydrogen, oxygen, nitrogen, carbon dioxide
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 O,, CO,, H, and N, at various tissue locations, in response to variations in diffusion-resistance and external O, concentration. A small diffusion-resistance allowed increased nitrogen fixation in air, but lead to diminished rates at increased external O, concentrations. Large diffusion-resistances provide increased protection against the effects of high O, concentrations but diminish nitrogen fixation in air. These effects depend on the respiratory activity (VmU[) of the bacteroids. In general, efficiency (moles of N, fixed/moles of O, used) is affected more than N, fixation rates at increased external O, 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 H, near the bacteroids remain low (2-3 per cent of concentrations of dissolved N,) and are thus unlikely to inhibit N, fixation except at high values of the diffusion resistance.