Diversity of Pseudomonas spp. isolated from rice rhizosphere populations grown along a salinity gradient

Along the coastline of Tamil Nadu, five sites were chosen to assess the diversity of Pseudomonas populations isolated from rice (Oryza sativa) cultivated along a salinity gradient. One of these sites was under organic farming while the other four were under inorganic farming. A total of 256 Pseudomonas strains isolated from these five sites were analyzed using both phenotypic (substrate utilization patterns and antibiotic resistance assay) and genotypic (PCR-RFLP of 16S rDNA) characteristics. The results derived from this study indicate that soil salinity affects rhizosphere Pseudomonas populations. It was observed that increasing salinity led to decreasing diversity. Fluorescent pseudomonads were the dominant species found in the non-saline site, while in the saline sites they were replaced by salt-tolerant species, in particular Pseudomonas alcaligenes and P. pseudoalcaligenes. An interesting observation was the increase in diversity found in the saline site under organic farming. Organic farming was found to be capable of mitigating the harmful effects of saline stress to a large extent, and restoring the Pseudomonas diversity, thereby making it comparable with the diversity encountered in the non-saline site.