A simulation model is presented for the distribution and consumption of O-2 in infected cells of soybean root nodule central tissue. It differs from earlier models in closer adherence to observed structure and embodies new morphometric data about the distribution of > 12,000 mitochondria per cell and about the geometry of the gas-filled intercellular spaces near which the mitochondria are located. The model cell is a rhombic dodecahedron and O-2 enters only through interfaces (totalling 26% of the cell surface) with 24 gas-filled intercellular spaces. These spaces are located at the edges of each rhombic face of the cell, forming an interconnected network over the cell suface. Next, O-2 is distributed through the cytoplasm by a leghaemoglobin-facilitated diffusive process, initially between the mitochondria and amyloplasts in the outer layers of the cell and then between > 6,000 symbiosomes (each containing 6 bacteroids) towards the central nucleus. The symbiosomes and mitochondria consume O-2, but impede its diffusion; all O-2 entering symbiosomes is considered to be consumed there. For the calculations, the cell is considered to consist of 24 structural units, each beneath one of the intercellular spaces, and each is divided into 126 layers, 0.2 mu m thick, in and through which O-2 is consumed and diffused. Rates of consumption of O-2 and of N-2 fixation in each diffusion layer were calculated from previously-established kinetics of respiration by mitochondria and bacteroids isolated from soybean nodules and from established relationships between bacteroid respiration and N-2 fixation. The effects of varying the O-2-supply concentration and the concentration and type of energy-yielding substrates were included in the simulations. When the model cell was supplied with 0.5 mM malate, mitochondria accounted for a minimum of 50% of the respiration of the model cell and this percentage increased with increased concentration of the O-2 supply. Gradients of concentrations of free O-2 dissolved in the cytoplasm were steepest near the cell surface and in this location respiration by mitochondria appeared to exert a marked protective effect for nitrogen fixation in layers deeper within the cell. Estimates of N-2 fixation per nodule, calculated from the model cell, were similar to those calculated from field measurements.
