Azospirillum brasilense is one of the diazotroph microorganisms which has been isolated from the rhizosphere of grasses in both tropical and temperate regions. In the last few years there have been several reports, indicating different results as regards significant increases in cereal growth when applying of this bacterium. This research was conducted in the research laboratory of Biology. Dept. in Isfahan University in 2000 to find out the suitable strain and optimum concentration of inoculum for cultivars of wheat as well as the effects on wheat root growth. In the study, wheat seeds (T aestivum) of three cultivars Ghods, Roshan and Omid were inoculated with two Azospirillurn strains (Sp7 and Dolatabad strains) with concentrations of 101 up to
cfu/ml, then the root length, dry weight and number of root branches, being measured and evaluated. Inoculation with 10607 cfu/ml in all cases resulted in the largest root development. However, concentrations of 108 to 1 0q cfu/ml of strains, caused inhibition in root development. Inoculations with Dolatabad strain exhibited more positive effect on root in Ghods cultivar but Sp7 strains caused the most root development in Roshan cultivar. Either strain had a low effect on Omid cultivar.
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