Rhizobacteria Pseudomonas fluorescens and Azospirillum sp. association enhances growth of Lactuca sativa L. under tropical conditions

The selection of microorganisms that enhance plant growth and confer biotic and abiotic tolerance to crops constitutes a biotechnology currently gaining importance on a global scale. The aim of this investigation was to evaluate the effects of inoculating rhizobacteria to lettuce (Lactuca sativa L.) on seed germination and vegetative development in order to use isolates as potential biofertilizers under tropical conditions. Five isolates of Pseudomonas fluorescens (Pf) and one of Azospirillum sp. (Az) were inoculated to seeds using a bacterial suspension of 1.5*108 CFU*mL-1. In vitro, none of the isolates promoted germination. In vivo, isolates promoted growth and acted as stress alleviators by conferring tolerance to high temperatures (≥ 30 °C). The highest seedling emergence percentages were induced by the association of P.fluorescens with Azospirillum. This association also promoted the highest leaf-area in 25 d seedlings and exhibited a significantly higher dry-weight in 40 d plants compared to the control (P≤0.05) supporting the advantages of bio-consortiums over individual strains. The strains were able to produce dependent L-tryptophan indole-3-acetic acid (IAA), to solubilize phosphorous in vitro and tolerated at least 5%-salt stress. The results indicate that isolate Pf (26) and Az possess plant growth promoting rhizobacteria (PGPR) traits and should be further assessed. This study suggests that P. fluorescens and Azospirillum act synergically and are able to trigger an induced-tolerance mechanism in lettuce under abiotic stress