Delivery of N2 to bacteroids in simulated soybean nodule cells: consideration of gradients of concentration of dissolved N2 in cell walls, cytoplasm, and symbiosomes

Abstract

The previously published simulation of physiological functions occurring in infected cells of soybean nodules has been extended to include consideration of the diffusion of N2 from the outside of a nodule to the nitrogen-fixing bacteroids, in relation to published values for the apparentKm(N2) for nitrogen fixation in the soybean nodule system. Nitrogen fixation is driven by bacteroid respiration, so increases in the average relative oxygenation (Y) of cytoplasmic leghaemoglobin lead to increased bacteroid respiration, increased nitrogen fixation, and greater differences in concentration of dissolved N2 between the cell surface and the innermost bacteroids (d[N2]). Over the range ofY considered, values for d[N2] were from 5.2- to 6.2-fold greater than the corresponding values for d[O2], because of facilitation of O2 flux by cytoplasmic leghaemoglobin. Gradients of [N2] within symbiosomes are small relative to cytoplasmic values and at the symbiosome surface [N2] was greater than 0.4 mol/m3 at the greatest rates of nitrogen fixation calculated. Therefore, it is unlikely that values for [N2] anywhere in the infected cell are low enough to affect rates of nitrogen fixation significantly, unless low external atmospheric N2 pressures are used experimentally.