In vitro Interactive Toxicity of Binary Mixtures of Selected Herbicides on Lysinibacillusfusiformis

Aims: To assess the toxicities of some herbicides as individuals and in binary mixtures to Lysinibacillus fusiformis isolated from Oryzasativa plant using dehydrogenase activity as an endpoint. Study Design: The binary mixture consists of combination of any two herbicides selected from the three herbicides (drysate, weedcut and aminoforce) for the study The binary mixture ratios (%) were designed as: 50%:50%; 80%:20% and 20%:80% for the respective mixtures in the concentration range of 0 -10, 000 mg/L. Place and Duration of Study: Silver Press Laboratory, Owerri Nigeria between July, 2016 and August, 2019. Methodology: A laboratory scale study was carried on three toxicants using dehydrogenase inhibition test. The inhibition of dehydrogenase activity of the isolate by toxicant was calculated relative to the control. All the dose-response relationships of the individual toxicants and that of themixtures were described by logistic dose model and Weibullcum model parameter. Results: The results revealed that the median inhibitory concentrations (IC50) observed were 1,067.33 ± 36.68 mg/L for drysate; 2,180.00 ± 147.31 mg/L for weedcut and 4,550.00 ± 62.45 mg/L for aminoforce. Duncan tests indicated that the IC50 of the toxicants were significantly different from each other. Among the individual toxicants, the ascending toxicities ranking were aminoforce > weedcut > drysate. The responses of the test organism to the stresses of the toxicants were dose-dependent and the toxicants also progressively repressed the dehydrogenase activity as the concentration increased. All binary mixtures were strongly synergistic against the organism. Conclusion: Thus, the toxicity of individual compound and synergistic effects of the mixtures of the toxicants indicates potential deleterious effects of both the individual chemicals and their mixtures to the rhizobacteria of Oryza sativa plant.