Physical and morphological constraints on transport in nodules.

For active nodule nitrogen fixation, O(2), N(2), and carbohydrate must be transported throughout the nodule. No quantitative analysis of these transport processes in the nodules has been presented. By invoking several simplifying assumptions, a second-order differential equation for the various gradients and concentrations in the nodule was solved. Even though the nodule can only be approximated in this analysis, it indicates clearly that intercellular gas spaces must exist in nodules for adequate O(2) distribution. To preserve low O(2) concentrations and protect the nitrogenase, these gas spaces cannot be in direct contact with the ambient atmosphere. It is hypothesized that a gas barrier exists in the cortical region of the nodule to limit O(2) diffusion. This barrier would not substantially inhibit N(2) transport. Carbohydrate transport from the vascular tissue via diffusion in the liquid phase can adequately accommodate the requirements within the nodule.