THE ACETYLENE-INDUCED DECLINE IN NITROGENASE ACTIVITY IN ROOT-NODULESOF ELAEAGNUS-ANGUSTIFOLIA

The rate of C2H2 reduction by nodulated seedlings of Elaeagnus angusti folia (Russian olive) was followed as a function of time. Our goals we re to: 1) determine whether there is an C2H2-induced decline in nitrog enase activity; and 2) investigate the mechanism of any decline. We fo und a peak rate of C2H2 reduction at 1.5 min after the introduction of C2H2 that was followed by a rapid decline in activity to 56% of the p eak value. After the decline there was a partial recovery to 67% of th e peak value at 60 min. When the pO(2) was decreased during the declin e there was no significant effect (p<0.05) on nitrogenase activity. Wh en the C2H2 reduction assay was preceded by an incubation in a gas mix ture (20 kPa O-2) with Ar substituted for N-2, there was little declin e in nitrogenase activity as a function of time, but the rate of C2H2 reduction per gram nodule was reduced by approximately 50%. From these results we conclude that Elaeagnus angustifolia exhibits a pronounced C2H2-induced decline and consequently the initial peak rate C2H2 redu ction must be determined to obtain a valid measure of nitrogenase acti vity. We further suggest that cessation of NH3 formation initiates the decline and that the decline is not caused by a change in nodule perm eability to gases.