Control of Nitrogen Fixation in a Legume Nodule: an Analysis of the Role of Oxygen Diffusion in Relation to Nodule Structure

A mathematical model of oxygen diffusion in a legume root nodule is constructed and validated. The required properties, functions and possible locations of the major resistance to oxygen diffusion are investigated and support is given to the concept of a part water-filled diffusion pathway. The model predicts that in the absence of leghaemoglobin an infected cell would have oxygen damaged nitrogenase in bacteroids close to an air space and anaerobic conditions around those towards its centre. Leghaemoglobin is essential for maintaining a fairly uniform level of nitrogenase activity within the infected cells, but it cannot prevent damage at high oxygen concentrations. Nevertheless, the nitrogenase activity of a nodule can increase with increasing oxygen concentration even though some enzyme damage occurs. In the absence of a variable diffusion resistance, the oxygen tolerance of nitrogenase is related to carbohydrate supply and in a normal atmosphere nitrogen fixation is limited by oxygen diffusion.