Hormetic behavior of chickpea bacterium Mezorhizobium ciceri to different concentrations of Imazethapyr and Trifluralin

Rhizobium is terrestrial bacteria that can coexist and form nodules in the leguminous family. The legume-rhizobia symbiosis results in great quantities of nitrogen fixation throughout the world. Rhizobial populations become non-target microbiomes for the herbicide applied in the field. Various herbicides such as Trifluralin and Imazethapyr can affect legume-rhizobia symbiosis negatively or in hermetic way. Hormesis is a dose-response relationship phenomenon characterized by a low-dose stimulation and a high-dose inhibition. This study aimed to investigate the effect of Imazethapyr and Trifluralin herbicides at two pH value (5.5 and 7) on the growth of Mezorhizobium ciceri at in vitro conditions.

To investigate the effect of Imazethapyr and Trifluralin herbicides on M. ciceri bacteria, an experiment was conducted as a completely randomized design in Gonbad Kavous University. M. ciceri strain were grown and maintained on yeast extract mannitol agar medium. Yeast extract manitol broth medium was used at two pH equal to 5.5 and 7. Moreover, Imazethapyr (0.021, 0.042, 0.084, 0.168, 0.336, and 0.672 g.L-1) and Trifluralin (0.337, 0.675, 1.35, 2.7, 5.4, and 10.8 g.L-1) herbicides accompanied with no-herbicide control added to the medium containing of 105 cell.ml-1 of M. ciceri at in vitro condition and the measurement of bacterial population was calculated at 600 nm light absorption, using spectrophotometer. The experiment was carried out with 4 replications and repeated two times. Population trends at different herbicide were fitted using three and four parameters logistic models and the Brain-Cousens equation was used when the hormesis response was observed.

The bacterial population of M. ciceri affected by Imazethapyr and Trifluralin concentrations in pH=5.5 followed the three parameter logarithm logistic model and in pH=7 followed the five parameter Brain-Cousense model. The maximum bacterial populations in pH=5.5 and pH=7 were 22.8×106 from zero concentration and 378.5×106 from 0.021 dose of Imazethapyr, respectively. At 0.021 and 0.042 g.L-1 of Imazethapyr doses, the bacterial population increased by 25.6% and 10.25%, respectively, compared to the control, which indicates the bacteria's hormetic behavior. The amount of hormesis in Trifluralin from doses (0.337 and 0.675 g.L-1) was recorded equal to 1.01×108 cell.mL-1. The increasing percentage of bacterial population from these doses compared to the control was 20.92 and 16.14, respectively. The value of “e” parameter, which indicates the amount of herbicide required for 50% reduction of the bacterial population, in pH=5.5 was obtained from Imazethapyr equivalent to 1.03 10-1 g.L-1 and from Trifluralin equal to 2.93 g.L-1.

This study showed statistical evidence of M. ciceri growth stimulation against minimum doses of Imazethapyr and Trifluralin in neutral acidity as Hormesis. This bacterium's population growth reaction in both herbicides under acidic conditions followed the logarithmic logistic model, and no hormesis was observed. In general, with increasing the dose of herbicides used in this experiment, M. ciceri bacterial population growth decreased. However, depending on the culture medium's pH, the bacterial reaction process was different so that in neutral acidity and sub-lethal doses was accompanied by an increase in population growth and then as the doses increased the growth rate decreased.