Direct evidence for changes in the resistance of legume root nodules to O ₂ diffusion

Indirect evidence suggests that legumes can adjust rapidly the resistance of their root nodules to O2 diffusion. Here we describe experiments using O2 specific micro-electrodes and dark field microscopy to study directly the operation of this diffusion barrier. The O2 concentration sensed by the electrode decreased sharply in the region of the inner cortex and was less than 1 0 mmol m ~
3 throughout the infected tissue in nodules of both pea (Pisum sativum) and french bean (Phaseolus vulgaris). In a number of experiments the ambient O2 concentration was increased to 40% while the electrode tip was just inside the inner cortex. In 13 out of 21 cases the O2 concentration at this position either
remained low and unchanged or increased irreversibly to near ambient values. In the remaining cases the O2 concentration increased after 1 to 2-5 min and then decreased to its former value. These results are ascribed to an increase in resistance of the barrier in response to increased O2 flux into the nodule.
It was shown microscopically that air spaces both at the boundary between the infected zone and the inner cortex, and within the infected zone started to disappear 3 min after nodules were exposed to high ambient O2 concentrations and had disappeared completely after 8 min. These spaces were not
changed by exposure of the nodule for 10 min to either N2 or air