Effect of pO2 during Growth on the Gaseous Diffusional Properties of Nodules of Cowpea (Vigna unguiculata L. Walp.)

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Abstract
Adaptations of nodules of cowpea (Vigna unguiculata L. Walp. cv Vita 3: Bradyrhizobium CB 756) to growth in pO2 ranging from 1 to 80% O2 (volume/volume) involved both readily reversible mechanisms of adjustment and more stable alterations which together resulted in nodules with widely ranging resistance to diffusion of gases. Those grown in subambient pO2 (1-5% O2 were altered such that rapid diffusional adjustment was unable to prevent irreversible loss of nitrogenase on their transfer to higher levels of O2. Those cultured in 80% had adapted to over-supply of O2 such that their transfer to lower levels of O2 limited both nitrogenase and respiratory CO2 release. There was also some evidence for `protective respiration.' Measurement of diffusional properties based on gas exchange kinetics indicated that gaseous permeability values for nodules from 5 to 40% O2 were relatively constant around 20 × 10−3 millimeters per second, while those for nodules from 1% O2 were as high as 67.7 × 10−3 millimeter per second and from 80% as low as 6.8 × 10−3 millimeters per second. Estimates of the thickness of the diffusion barrier ranged from 7.5 micrometers for nodules from 1% O2 to 71.9 micrometers in those from 80% O2.