Acetylene reduction, H2 evolution and (15)N 2 fixation in the Alnus incana-Frankia symbiosis.

Acetylene reduction, (15)N2 reduction and H2 evolution were measured in root systems of intact plants of grey alder (Alnus incana (L.) Moench) in symbiosis with Frankia. The ratios of C2H2: (15)N2 were compared with C2H2:N2 ratios calculated from C2H2 reduction and H2 evolution, and with C2H2:N2 ratios calculated from accumulated C2H4 production and nitrogen content. It was possible to calculate C2H2:N2 ratios from C2H2 reduction and H2 evolution because this source of Frankia did not show any hydrogenase activity. The ratios obtained using the different methods ranged from 2.72 to 4.42, but these values were not significantly different. It was also shown that enriched (15)N could be detected in the shoot after a 1-h incubation of the root-system. It is concluded that the measurement of H2 evolution in combination with C2H2 reduction represents a nondestructive assay for nitrogen fixation in a Frankia symbiosis which shows no detectable hydrogenase activity.