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Acetylene-Ethylene Assay Studies on Excised Root Nodules of Myrica asplenifolia L.¹

ABSTRACT

The acetylene-reducing activity of excised root nodules of Myrica asplenifolia L. [Comptonia peregrina (L.) Coult.] was measured on a large number of samples both in the field and in the laboratory. The apparent K_m for acetylene was determined to be 0.006 atm and was not significantly different in the presence or in the absence of 0.8 atm of N₂. Maximum activity was obtained between 26 and 30C and between 0.15 and 0.25 atm O₂. No significant diurnal fluctuation in activity could be detected. High variation (coefficient of variation ca. 50%) between replicate samples of field-collected nodules was a consistent feature of estimates of acetylene-reducing activity over a range of sites, sample weights, and sample numbers. Therefore, field-collected nodules are unsuitable for physiological experiments unless the effect of the treatments on acetylene-reducing activity is large. In ecological studies where field-collected nodules must be used, extensive sampling is required to permit reliable estimates of the activity. The ratio of the rate of ethylene produced from acetylene to the rate of ¹⁵N₂ incorporated was measured by exposing individual nodule samples sequentially to ¹⁵N₂, then acetylene. The mean ratio for 24 samples was 3.14 ± 0.30.

¹ Canadian IBP contribution no. 217 from the Dep. of Microbiology, Macdonald College, McGill Univ., Macdonald College P.O., Quebec, Canada. Presented before Div. S-3 Soil Science Society of America, Nov. 1, 1972, Miami Beach, Florida.

² Lecturer, Faculty of Forestry, Univ. of Toronto, Toronto, Ontario, Can.; Professor, Dep. of Microbiology, Macdonald College of McGill Univ., Macdonald College P.O., Quebec, Can.; and Research Scientist, Domtar Research Centre, Senneville, Quebec, Can., respectively.

Received for publication December 22, 1972. Accepted for publication July 5, 1973.

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