Acetylene-induced Changes in the Oxygen Diffusion Resistance and Nitrogenase Activity of Legume Root Nodules

Legume symbioses such as pea, lucerne and clover exhibit a substantialdecline in nodular respiration and nitrogenase activity whensubjected to gas streams containing C₂H₂. Assuming a constantrate of O₂ diffusion into the nodule this decrease in respiratoryO₂ consumption would lead to an increase in internal O₂ concentrationwhich would inactivate nitrogenase. No such inactivation hasbeen observed indicating that a change in diffusion resistanceis involved in the C₂H₂ response. Root nodules of C₂H₂ responsivesymbioses are distinguished by their tolerance to high (80 percent) O₂ levels. The nitrogenase of soya-bean and sainfoin,which do not respond to added C₂H₂, is denatured at O₂ levelsabove 40 per cent. Even in O₂-tolerant systems (e.g. pea) theenzyme is damaged if the increase in O₂ levels is rapid (ca.45 s) indicating that an induced change rather than a ‘permanent’feature is involved. The O₂-tolerant, C₂H₂-responsive behaviourpattern of white clover nodules was reversed by pulse feedingwith nitrate-N indicating that the overall C: N ratio in thenodules is involved in determining the response.