This study evaluated the effects of some environmental conditions on IAA
biosynthesizing capacity of four nitrogen fixing bacteria, namely Paenibacillus
cineris TP-1.4, Bacillus megaterium MQ-2.5, Klebsiella pneumoniae OM-17.2,
and Pseudomonas boreopolis CP-18.2. Carbon source, pH, NaCl, and tryptophan
supplement treatments were set to investigate the effects of those environmental
factors on IAA synthesis. The IAA synthesizing capacity of bacterial strains in
liquid medium was measured spectroscopically following incubation by
Salkowski's reagent method. The results showed that, under the sucrose
amendment, the IAA concentrations produced by all four bacterial strains were
significantly higher than those of the other four carbon source added treatments.
Two of the four bacterial strains produced the highest yield of IAA in liquid
medium at pH 7 (TP-1.4 and OM-17.2), whereas pH 8 was optimum for the other
two strains (MQ-2.5 and CP-18.2). The MQ-2.5 strain could synthesize IAA
fairly well in up to 5% NaCl a nd produced the highest amount of IAA with 1%
NaCl. Furthermore, IAA synthesizing capability of tested bacterial strains
increased sharply along with increasing tryptophan content in culture medium
except for the TP-1.4 strain. From the current study, these isolates emerged as
possible alternatives for future IAA production for plant growth and yield
enhancement. Hence, they have a great potential to be used as bio -inoculants for
plant growth promotion in eco-friendly and sustainable agriculture.