Diffusion Limitation of Oxygen Uptake and Nitrogenase Activity in the Root Nodules of Parasponia rigida Merr. and Perry.

Parasponia is the first non-legume genus proven to form nitrogen-fixing root nodules induced by rhizobia. Infiltration with India ink demonstrated that intercellular air spaces are lacking in the inner layers of the nodule cortex. Oxygen must diffuse through these layers to reach the cells containing the rhizobia, and it was calculated that most of the gradient in O(2) partial pressure between the atmosphere and rhizobia occurs at the inner cortex. This was confirmed by O(2) microelectrode measurements which showed that the O(2) partial pressure was much lower in the zone of infected cells than in the cortex. Measurements of nitrogenase activity and O(2) uptake as a function of temperature and partial pressure of O(2) were consistent with diffusion limitation of O(2) uptake by the inner cortex. In spite of the presumed absence of leghemoglobin in nodules of Parasponia rigida Merr. and Perry, energy usage for nitrogen fixation was similar to that in legume nodules. The results demonstrate that O(2) regulation in legume and Parasponia nodules is very similar and differs from O(2) regulation in actionorhizal nodules.