Paraquat Toxicity on the Growth of Rhizobium sp. in a Synthetic Medium

Toxicity of paraquat on the growth of several strains of Rhizobium sp. in Yeast Extract Mannitol medium was studied. Various concentrations of paraquat 100, ranged from O (control) to 100 ppm were applied. Qualitative examination was done using paper disc diffusion technique, and the quantitative examination was conducted based on the change in cell density in medium measured by plate count method.Qualitative data showed that effect or paraquat was species specific. Some strains of Rhizobium sp., namely Rhizobium sp. strain T-37 and QF, were tolerant to paraquat until100 ppm, but other strains were sensitive to paraquat, especially at high concentration.Quantitative examination to the sensitive strains shows that higher concentration of paraquat caused higher toxicity to the growth of Rhizobium. Rhizobium sp. strain G-69 and G-182.paraquat addition at 100 ppm slightly decreased cell density from 10 to 10 CFU/mL. Rhizobium japonicum strains 143 and KS were tolerant to 20 and 40 ppm of paraquat; their cell density increased from 10^6 to a level of 10&7 or 10^8 CFU/mL depend on the strain. This level was not significantly lower than those in medium without paraquat, in which maximal population density reached to 10^8 or 10^9 CFU/mL. Addition of higher paraquat concentration damaged the cell of these strains, and caused population density increased specifically to a level of 10^1 CFU/mL. The data which showed that paraquat was toxic to Rhizobium sp. were important growth inhibitor, because growth inhibition of these bacteria may influence the formation of root nodule on leguminous plants, and in turn will decrease the yield. Due to widely applied paraquat in agricultural and plantation systems, and the role of Rhizobium in nitrigen fixation, these results are important for minimizing the impact of paraquat application.