Population Density and In vitro Characterization of Selected PGPRs from Tobacco Rhizosphere Soils

Present study revealed that PGPRs such as Bacillus, Pseudomonas, Trichoderma and Streptomyces spp. present predominantly in tobacco rhizosphere soils. Population density of PGPR was found to be higher in northern light soils (NLS) followed by Karnataka light soils (KLS) than in southern light soils (SLS) and traditional black soils (TBS) whereas lowest was recorded in chewing tobacco soils. Moreover, among the PGPRs, bacterial PGPRs such as B. subtilis and P. fluorescens were found to be higher than actinomycetes (Streptomyces spp.) and fungi like T. viride. Similarly, of the bacterial PGPRs, P. fluorescens population was the highest (11.98 x10⁵) in NLS while B. subtilis was the least (5.99 x10⁵ cfu/gm soil dry wt.) in chewing tobacco. The population density of T. viride recorded least in chewing tobacco soils of West Bengal and Tamil Nadu (3.18 x 10³ and 2.77 x 10³ cfu/gm soil dry wt., respectively). The population density of PGPR strains positively coincided with soil nutrients like total organic matter, phosphorous and potassium contents. The results on biochemical characterization indicated that most of the strains were efficient in hydrolyzing starch, gelatin and casein. Among the different carbon and nitrogen sources tested, maltose and potassium nitrate, respectively were the good supplementary sources. Of the different amino acid and vitamin sources evaluated, alanine and riboflavin, respectively were found to be limiting substrates for maximum growth of PGPR strains. Antagonistic activity of Streptomyces sp. against Pythium aphanidermatum showed 37.52 per cent of inhibition over control.