2.1.1. IAA production
IAA production was investigated by the modified method of
Gordon and Weber (1951), using Azospirillum brasilense sp245 as a
reference strain (Ona et al., 2005). The strains were grown overnight
in Dworkin and Foster (DF) salt minimal medium (Dworkin and
Foster, 1958) and transferred to fresh DF medium amended with
0.1% l-tryptophan. The cultures were incubated for 7 days at 28 ◦C,
and then centrifuged at 15,000
×
g for 10 min. One millilitre of
the supernatant was mixed with 2 mL of Fe-H2SO4 solution (1 mL
of 0.5 M FeCl3·6H2O in 75 mL of 6.13 M H2SO4) and incubated
in the dark for 45 min. The concentration of IAA was calculated
from a standard curve of IAA obtained in the range of
0.5–10.0 g mL−1 by measuring the absorbance of samples and
standards at 450 nm.