Errors in the Acetylene Reduction Assay: Determined Using a Flow-Through System

Abstract

Measurements on both attached nodulated roots and detached nodules have revealed that nitrogenase activity in many legume symbioses declines rapidly in the presence of acetylene, with a concurrent reduction in respiration. These observations were made using an open, flow-through system which measures the rate of C₂H₄ production rather than its cumulated concentration (Minchin et al., 1983). The reduction in nitrogenase activity begins within a few minutes of exposure to acetylene and continues for 30–60 min before a new steady-state is attained. This lower value commonly represents 40–60% of the maximum (pre-decline) rate of C₂H₄ production. This maximum rate represents the pre-assay rate of nitrogenase activity as determined by ¹⁵N₂ fixation. When C₂H₂ is removed respiratory activity returns to the pre-exposure level, demonstrating that permanent damage to the nodules is not involved in the decline. A decline and recovery also occurs when N₂ is replaced with argon or helium. This suggests that the decrease in nodule activity is linked to the cessation of ammonia production.