Diffusion of Oxygen in Relation to Structure and Function in Legume Root Nodules

Abstract

Although substantial rates of oxidative phosphorylation by bacteroids of Rhizobium are required to support sustained N₂ fixation in legume nodules, the enzyme nitrogenase is extremely sensitive to oxygen. This apparent paradox indicates that nodules must exercise effective control over internal oxygen concentration. Structural features including lenticel development, the thickness and arrangement of cells and air spaces in the inner and outer cortex, the presence or completeness of the common endodermis as well as the distribution of infected cells, uninfected cells and air spaces in the nodule medulla are important to gaseous ventilation of the organ. Among these, the organisation of cells and water / gas- containing extracellular voids in the inner cortex bounding the infected medulla zone are critical components to overall diffusive resistance. These elements also appear to provide the means for the oper- ation of a reversible variable diffusion barrier to control gas movement. Inhibition of nodule functioning by a widely diverse range of factors, e. g. restricted H₂O or photosynthate supply, exposure to acetylene or combined N, appears to be mediated through increased diffusion resistance.