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Cyanobacteria represent less a investigated group of prokaryote, in terms of their plant growth promoting potential, especially in relation to the production of phytohormones. The present investigation was aimed towards analyzing growth kinetics, indole acetic acid (IAA) production and acetylene reduction activity (ARA) as an index of nitrogen fixation in two selected cyanobacterial strains belonging to the genus Anabaena, as influenced by tryptophan supplementation and light: dark conditions. Interesting observations were recorded in terms of enhancement of IAA production accompanied by protein and chlorophyll accumulation in the two cyanobacterial strains grown in media without tryptophan and incubated under light: dark or continuous light conditions. Colorimetric and chromatographic analyses supported the observations that tryptophan is not essential as a precursor for IAA biosynthesis in these …