Effect of pO2 during growth on the gaseous diffusional properties of nodules of cowpea (Vigna unguiculata L Walp.)

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 oversupply 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.