Model of gas exchange and diffusion in legume nodules : I. Calculation of gas exchange rates and the energy cost of N2 fixation.

A mathematical model is described which allows the estimation of rates of O2, CO2, N2, and H2 exchange from legume nodules under steady state conditions of N2 fixation. Calculated rates of gas exchange under defined conditions of nodule size, relative growth rate (RGR), specific total nitrogenase activity (TNA), nitrogenase electron allocation coefficient (EAC), uptake-hydrogenase activity (HUP) and nature of the N export product compared favorably with experimentally-obtained rates reported in the literature. Therefore the model was used to predict the effects of varying each of these nodule characteristics on the rates of gas exchange, and on the apparent respiratory cost (CO2/NH3) and sucrose cost (sucrose consumed/NH3) of N2 fixation.The model predicted that, all other characters being equal, ureide-producing nodules would consume 8% less sucrose per N fixed than asparagine-producing nodules, but would display an apparent respiratory cost which would be 5% higher than that in asparagine-producing nodules. In both ureide-producing and asparagine-producing nodules, the major factor affecting the apparent respiratory cost of N2 fixation was predicted to be EAC, followed by TNA, nodule RGR and nodule size. The relative importance of HUP in improving the apparent respiratory cost of N2 fixation was predicted to be largely dependent upon its potential role in the regulation of EAC.