Oxygen Diffusion Pathways and Nitrogen Fixation in Legume Root Nodules

The theoretical analysis presented in this paper suggests that the nature of the diffusion pathway from the surface of a nodule to the infected zone depends on the morphology of the nodule; in particular the cross-sectional area of the intercellular spaces in the inner cortex. If the diffusion barrier contains continuous pathways there will be no pressure difference between the atmosphere and the infected zone. The conditions under which the intercellular spaces of the inner cortex could be air-filled, water-filled, or a combination of both are explored. An experiment to resolve this issue is suggested. Information obtained using cryo-scanning electron microscopy and oxygen electrode data have been used to illustrate various points. Expressions are derived for the diffusion resistance of the nodule and its component parts. To provoke further discussion a simple mechanism for the control of diffusion is described in general terms.