Co-inoculation of Urea and DAP Tolerant Sinorhizobium meliloti and Pseudomonas aeruginosa as Integrated Approach for Growth Enhancement of Brassica juncea.

Two plant growth promoting rhizobacteria--Sinorhizobium meliloti RMP1 and Pseudomonas aeruginosa GRC(2) were studied for integrated nutrient management to obtain improved yield of Brassica juncea. Low concentrations of urea and diammonium phosphate (DAP) stimulated the growth of both S. meliloti RMP1 and P. aeruginosa GRC(2). 1 M of urea and 0.35 M of DAP was found lethal for RMP1, while 1.3 M and 0.37 M concentrations of urea and DAP proved to be toxic for GRC(2). Lc(50) was observed as 0.49 M of urea and 0.15 M of DAP for RMP1, and 0.66 M urea and 0.18 M of DAP for GRC(2). Urea and DAP adaptive variants of RMP1 and GRC(2) was isolated. Adaptive bacterial variants had better growth rates at sub-lethal (Lc(50)) concentrations of urea and DAP as compared to non-adaptive variants. They also retained plant growth promoting attributes similar to non adaptive variants. GRC(2) and RMP1 did not affect the growth of each other and were chemotactically active for DAP, urea as well as root exudates of B. juncea. Both the isolates colonized well in the rhizosphere of B. juncea, as their populations were recorded ≈5 log(10) cfu g(-1) after 120 days. Interestingly, the colonization ability was found even better when both strains were co-inoculated, as their population was recorded in the range of ≈6 log(10) cfu g(-1) after 120 days. In field trials, application of RMP1 and GRC(2) resulted in significant increase in biomass and yield of B. juncea as compared to control. However, yield was better with application of half dose and full dose of recommended fertilizers. Interestingly, the biomass as well as yield improved further when both isolates were applied together along with half dose of recommended fertilizers.