ACETYLENE-INDUCED AND ARGON-INDUCED DECLINES IN NITROGENASE ACTIVITY IN CORIARIA-ARBOREA

When assayed in a continuous flow system, 4-month-old Coriaria arborea plants underwent a decline in nitrogenase activity and nodule respiration upon exposure to acetylene (C2H2) (10 kPa). Neither nitrogenase nor respiratory activity was recovered following initial exposure and instead a new equilibrium level was reached. It is suggested C2H2-induced declines in Coriaria can be explained by a variable nodule diffusion resistance mechanism. The size of the decline in nitrogenase activity was reduced with increasing plant age such that 12-month old plants had a barely detectable decline. Prior adaptation to non-atmospheric pO2 concentrations also affected the size of the decline. At 35 kPa O2 the decline was Significantly smaller than at 21 kPa O2 but at 12 kPa O2 the extent of the decline was greater. Both the effect of plant age and pO2 on the C2H2-induced decline offer support to the operation of a variable diffusion resistance mechanism. Coriaria also underwent a decline in respiratory activity when Ar replaced N2 in the assay gas stream. Lag-phase measurements showed the Ar-induced decline was accompanied by an increase in nodule diffusion resistance.