Optimal plant growth-promoting concentration of Azospirillum brasilense inoculated to cucumber, lettuce and tomato seeds varies between bacterial strains

Azospirillum brasilense is a well-known plant growth-promoting rhizobacteria (PGPR) that can have positive impact on plant growth when present in sufficient density. The production of phytohormones by Azospirillum has been proposed as one of the major mechanisms responsible for the observed plant growth effects. In particular, this plant growth-promoting effect could be mediated by IAA (indole-3-acetic acid), a phytohormone produced by Azospirillum in substantial amount, because the effect of inoculation with higher inoculum density on root morphology mimicked the increasing concentration of exogenous IAA applied to the seed. This study evaluated the effect of inoculation with different bacterial concentrations of three A. brasilense strains (Sp7, Sp7-S and Sp245) on the early seedling growth and endogenous plant IAA of cucumber, lettuce and tomato. Seeds were inoculated with 8, 7 and 6 log cfu ml(-1) of each strain before germinating in Petri dishes. The early seedling growth of vegetables was affected by the inoculation with the A. brasilense strains. In cucumber, seeds inoculated with Sp7 at log 8 and 6 cfu ml(-1), Sp7-S and Sp245 at log 8 and 7 increased seedling growth, vigor index and endogenous plant IAA by up to 55%. In lettuce, the inoculation with log 6 of Sp7, log 7 and 6 of Sp7-S, and log 8 and 7 of Sp245 yielded superior seedling growth with improved seedling vigor. In addition, log 7 and 8 of Sp7 and Sp7-S, respectively, increased IAA concentration by more than 20%. In tomato, Sp7 at log 6, Sp7-S and Sp245 at log 7 exerted more positive effects on root biomass. IAA concentrations increased up to 171% by inoculation with all concentrations of Sp7 and Sp7-S and more than 300% by Sp245 at log 8. The bacteria-elicited promotion of plant IAA production might improve plant function via effects on metabolism and morphology. These results demonstrate that seed inoculation with the strains at the studied range of cell concentrations stimulated early seedling development. However, the optimal promoting concentration varied between strains. Strain Sp7 appeared to be more beneficial at log 6 cfu ml(-1) while Sp7-S and Sp245 were more beneficial at log 7 or higher.