15 N 2 fixation, H 2 evolution and C 2 H 2 reduction relationships in Phaseolus vulgaris

In a study of N 2 fixation in nodules of beans ( Phaseolus vulgaris L.) cv. Venezuela‐350 of different ages and kept under controlled light and temperature conditions, three parameters ‐ 15 N 2 fixation, C 2 H 2 reduction and H 2 evolution—were measured. C 2 H 2 reduction was positively related to the N fixed. A standardization of sampling is necessary to keep experimental errors to a minimum. To decrease the variation between replications, the nodules should be carefully incubated (adequate time and temperature), and the samples for chromatography should also be carefully stored. No significant difference was found in C 2 H 2 reduction in the systems studied ‐ nodulated roots, disturbed whole plants and intact plants—during the first hour of measurement. H 2 evolution was detected in nodules over 20 days of age and was positively related to N 2 fixation and C 2 H 2 reduction. This measurement was essential to account for the equilibrium between the three parameters in the electron balance ratio. Under phytotron conditions, maximum H 2 evolution occurred 20–30 days after planting. Nodulated roots from different plants were used to determine the N 2 fixed and C 2 H 2 reduction, and on average the ratios C 2 H 2 :N 2 varied between 7.3 and 8.3 for all ages. In vivo C 2 H 4 /(3N 2 + H 2 ) electron balance values were over 1, varying from 1.32 to 1.43. These data suggest that overestimation of actual N 2 fixation might be occurring when using the C 2 H 2 reduction technique, even if taking account for H 2 evolution. In this case, this method should be used with great care to avoid an overestimation of on average 32‐43% of the actual value. Variation was smaller between the measures of the three parameters when younger nodules were used.