Model of gas exchange and diffusion in legume nodules : II. Characterisation of the diffusion barrier and estimation of the concentrations of CO2, H 2 and N 2 in the infected cells.

The rates of nodule O2, CO2, N2 and H2 exchange calculated in the previous modeling study (D.B. Layzell et al., 1987, Planta 173, 117-127) were combined with information on the diffusion characteristics of each gas, and the structural characteristics of soybean nodules, to produce a comprehensive mathematical model of nodule structure and function. The model assumed that an aqueous barrier to gas diffusion exists in the nodule cortex which may be regulated to maintain an O2 concentration of 10 nM in the centre of the infected cells of the central zone. The model was used to predict the concentration of N2, CO2 and H2 in the infected cells as the physical and physiological characteristics of the nodule were varied. The model predicted that (a) the diffusion barrier may be represented by plugs of water in the intercellular spaces of a layer of cells between the inner and outer cortex, the depth of which may be varied to vary the resistance of the barrier; (b) facilitated diffusion of O2 by oxyleghemoglobin is essential to the regulation of free O2 concentration in the infected cells; (c) the diffusion barrier is less effective in regulating CO2 flux than the fluxes of other gases with the result that the total gas pressure in the central zone is less than atmospheric pressure; (d) concentrations of N2 and HCO 3 (-) in the infected cells are saturating with respect to nitrogenase activity and phosphoenolpyruvate carboxylase activity respectively and (e) under atmospheric conditions the concentration of H2 in the infected cells is similar to, or greater than the K i . (H2) for N2 fixation, which may account for values of nitrogenase electron allocation coefficient below 0.75.