The effects of environmental and methodological factors on acetylene reduction by nodules of Acacia pulchella R.Br., Kennedia coccinea Vent, and K. prostrata R.Br. grown in pots were studied.
The time course of acetylene reduction, measured for each species during assays lasting up to 3 h, showed that there was an initial lag phase before maximum rates were reached and that rates then declined rapidly. This decline in activity was less marked for K. prostrata than for the other two species. Rates of acetylene reduction did not differ significantly between detached nodules, roots with attached nodules and whole plants.
Delaying the introduction of acetylene into assay vessels indicated that the decline in nitrogenase activity during assays at higher temperatures was only partly caused by the acetylene substrate. As weights of nodules were small in relation to the volume of assay vessels, the decline in activity is unlikely to result from changes in pO2. The initial lag phase was shorter and the decline in activity with time more marked with increasing temperature. This finding has important implications for conducting and interpreting field assays, and in determining optimum temperatures for nitrogenase activity.
Calibration factors relating acetylene reduction to nitrogen fixation were estimated from measurements of nitrogen accumulation and acetylene reduction during growth of young plants in a nitrogen-free sand. Ratios for A. pulchella (2·5:1) and K. coccinea (2·4:1) were lower than theoretical ratios (3 to 4:1), probably because of the decline in rates of acetylene reduction during assays. Lowering of irradiance to about 4 % of full sunlight over 2 d depressed acetylene reduction rates by 58% for A. pulchella and by 36% for K. coccinea.