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

Adaptations of nodules of cowpea (Vigna unguiculata L. Walp. cv Vita 3: Bradyrhizobium CB 756) to growth in pO(2) ranging from 1 to 80% O(2) (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 pO(2) (1-5% O(2) were altered such that rapid diffusional adjustment was unable to prevent irreversible loss of nitrogenase on their transfer to higher levels of O(2). Those cultured in 80% had adapted to over-supply of O(2) such that their transfer to lower levels of O(2) limited both nitrogenase and respiratory CO(2) 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% O(2) were relatively constant around 20 x 10(-3) millimeters per second, while those for nodules from 1% O(2) were as high as 67.7 x 10(-3) millimeter per second and from 80% as low as 6.8 x 10(-3) millimeters per second. Estimates of the thickness of the diffusion barrier ranged from 7.5 micrometers for nodules from 1% O(2) to 71.9 micrometers in those from 80% O(2).