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

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Abstract
Legume symbioses such as pea, lucerne and clover exhibit a substantial decline in nodular respiration and nitrogenase activity when subjected to gas streams containing C2H2. Assuming a constant rate of O2 diffusion into the nodule this decrease in respiratory O2 consumption would lead to an increase in internal O2 concentration which would inactivate nitrogenase. No such inactivation has been observed indicating that a change in diffusion resistance is involved in the C2H2 response. Root nodules of C2H2 responsive symbioses are distinguished by their tolerance to high (80 per cent) O2 levels. The nitrogenase of soya-bean and sainfoin, which do not respond to added C2H2, is denatured at O2 levels above 40 per cent. Even in O2-tolerant systems (e.g. pea) the enzyme 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 behaviour pattern of white clover nodules was reversed by pulse feeding with nitrate-N indicating that the overall C: N ratio in the nodules is involved in determining the response.