N2 fixation (C2H2 reduction) in 2,4-dichloro-phenoxyacetic acid (2,4-D) treated wheat inoculated with free-living diazotrophs

N2 fixation associated with 2,4-D-treated wheat seedlings was investigated using a number of free-living, non-associative, O2- and acid-tolerant diazotrophs. Not all diazotrophs tested formed an N2-fixing association. Even strains within one species behaved differently. Azospirillum was found to have a narrow pH and pO2 range for nitrogenase activity (C2H2 reduction) in 2,4-D-treated wheat seedlings, whereas Derxia gummosa was capable of forming an effective N2-fixing association under a wide range of pH and pO2.