Microbiological studies on plant growth promoting rhizobacteria for improving the productivity of some plants

Ninety one rhizobacterial isolates were isolated from the rhizosphere soil of wheat (Triticum aestivum L.) and faba bean (Vicia faba L.) grown in different locations at Sharkia governorate, Egypt. These rhizobacterial isolates were screened for indole acetic acid (IAA) production, siderophores and cellulase enzymes production, phosphate solubilization and nitrogenase enzyme activity. All isolates were able to produce IAA in presence or absence of tryptophan (L-TRP) although much variation was observed between them. Sixty eight isolates were able to solubilize Ca3 (PO4)2 on modified Bunt and Rovira medium. Sixty four isolates showed positive reactions with the chrome azurol S (CAS) agar medium, while 65 isolates showed positive cellulase reactions with carboxymethyl cellulose medium (CMC) and 16 isolates gave positive reactions with nitrogenase enzyme activity.Out of the ninety one isolates, 42 isolates were selected for identification based on their efficiencies as plant growth promoting rhizobacterial (PGPR); five of them which proved to be the most efficient PGPRs were identified as Azospirillum brasilense, Serratia marcescens, Pseudomonas fluorescens, and Bacillus brevis as well as Azotobacter sp.. All of these isolates were used as bio-fertilizers either as single inoculants, as a mixture or combined with either of two levels of chemical fertilization in pot trail experiments conducted in insect net proof house to evaluate the response of maize and soybean plants grown in loam sandy soil, to inoculate with these selected strains. Significant increases were found in the fresh and dry weights of plants as well as, total nitrogen, phosphorus and potassium content as compared with their respective control treatments. Co-inoculation between PGPRs species resulted in the most increases in these parameters than those inoculated than single inoculation; but, variations were noticed among the isolates.