2.
Indirect evidence suggests that legumes can adjust rapidly theresistance of their root nodules to O
2 diffusion. Here we describeexperiments using O
2 specific micro-electrodes and dark fieldmicroscopy to study directly the operation of this diffusionbarrier. The O
2 concentration sensed by the electrode decreasedsharply in the region of the inner cortex and was less than1.0 mmol m
3 throughout the infected tissue in nodulesof both pea (
Pisum sativum) and french bean (
Phaseolus vulgaris).In a number of experiments the ambient O
2 concentration wasincreased to 40% while the electrode tip was just inside theinner cortex. In 13 out of 21 cases the O
2 concentration atthis position either remained low and unchanged or increasedirreversibly to near ambient values. In the remaining casesthe O
2 concentration increased after 1 to 2.5 min and then decreasedto its former value. These results are ascribed to an increasein resistance of the barrier in response to increased O
2 fluxinto the nodule. It was shown microscopically that air spacesboth at the boundary between the infected zone and the innercortex, and within the infected zone started to disappear 3min after nodules were exposed to high ambient O
2 concentrationsand had disappeared completely after 8 min. These spaces werenot changed by exposure of the nodule for 10 min to either N
2or air. Key words: Oxygen, root nodules, air spaces
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