The quantitative relationship between nitrogen fixation and the acetylene-reduction assay

Acetylene.reduction activity of detached soybean nodules was positively correlated with the nitrogen content of the host plants grown in nitrogen-free media. Acetylene reduction by nodulated root systems was related to light intensity preceding sampling; activity of detached nodules was lower and less influenced by light. Comparison of acetylene reduction and 15N 2 uptake by detached soybean nodules gave ratios of acetylene reduced to nitrogen fixed (C2~: NHs ratio) ranging from 2·7 to 4·2 in different experiments. The mean value for four experiments was 3·3±0·6 (S.E.). Different effects of oxygen were obtained with detached soybean nodules when 0·1 atm nitrogen and 0·1 atm acetylene were used as substrates. When the acetylene concentration was reduced to 0·005 atm the effects upon both assays were similar but the ratio of the two assays was still affected by p02.