The behaviour of Azospirillum spp. in the presence of plant growth hormones.

Tests were carried out to determine the effects of commercial preparations of indole-3-acetic acid (IAA), gibberellin (GA) and kinetin, separately and in combination, on Azospirillum lipoferum and A. brasilense. These hormones showed no effect upon morphology, growth rate, or oxygen uptake. Some random effects of GA, kinetin and the combination of hormones--but not of IAA alone--were observed on the number of viable cells. IAA, in the concentrations 0.001-1.0 microgram/ml, inhibited nitrogenase in A. lipoferum but not in A. brasilense. GA and kinetin separately and in combination with IAA had no influence on nitrogenase activity.