The acetylene reduction assay inactivates root nodule uptake hydrogenase in some actinorhizal plants

Abstract

Actinorhizal nodules do not usually evolve H2 due to the action of an uptake hydrogenase. We have found that nodules of several Frankia symbioses evolved large amounts of H2 gas when returned to air following exposure to 10 kPa C2HT2 during an acetylene reduction assay. Increased H2 evolution in air persisted for several days when intact root systems of Alnus incana (L.) Moench (inoculated with Frankia UGL 011101) were treated with 10 kPa C.H2 for 1 h. Full recovery of uptake hydrogenase activity required 4 to 8 days. Studies with crude homogenates of nodules of the same plants showed that hydrogenase (measured amperometrically with phenazine metho‐sulfate as electron acceptor) was directly affected, since activity in treated nodules was only 10% of that in untreated nodules. A survey of actinorhizal symbioses revealed variation in the effect of an acetylene reduction assay on hydrogen metabolism. Nodules of three species, including Alnus rubra Bong, inoculated with Frankia HFPArD. showed complete inactivation of hydrogenase. H2 evolution in air was 25% of the C2H2 reduction rate and H, evolution in Ar/O2 was equal to the QH2 reduction rate. Two symbioses, Ceanothus americanus L. (soil inoculant) and Batista glomerata Baill. (soil inoculant) showed no change following an acetylene reduction assay. A third group of symbioses showed an intermediate response.