4.4. Mechanisms of alleviates phytotoxicity by exogenous SA
In our study, three concentrations of exogenous SA were supplemented to assess the effects of SA on pesticides-induced oxidative stress in cucumber plants. Based on the above results, we found that low concentration of SA (1mg L-1 and 10mg L-1) had significant effects on relieving oxidative stress of cucumber plants induced by the three pesticides. Whereas, high concentration of exogenous SA (50 mg L-1) had little effect on alleviating oxidative stress of cucumber plants. Therefore, the effects of exogenous SA on reducing oxidative stress depends on its concentrations. In accordance with the present results, previous studies have demonstrated that low concentration of SA can improve the tolerance of plants to adversity. In contrast, high concentration of SA induced a higher level of oxidative stress, leading to a decrease in tolerance to environmental stress (Bai, Dong, Kong, Xu, & Liu, 2014; Z. Liu, Ding, Wang, Ye, & Zhu, 2016; Pasternak et al., 2019). In the previous part of this study, we found that exogenous SA could impair the absorption capacity of roots to pesticides and thus reduce the accumulation of pesticides in cucumber plants. These results matched those observed in earlier studies. SA could reduce the accumulation of chlorpyrifos in wheat plants by decreasing uptake and translocation abilities of chlorpyrifos (C. Wang & Zhang, 2017). SA could reduce the accumulation of isoproturon in wheat by promoting its degradation (L. Liang et al., 2012; Yi Chen Lu et al., 2014). SA could reduce the residual of flurochloridone in sunflower leaves by the foliar treatment (Kaya & Yigit, 2014). SA was able to reduce the concentration of thiamethoxam, hymexazol and chlorantraniliprole in roots and leaves of cucumber plants and the uptake ability of roots, the bioconcentration ability of plants, and the translocation ability from roots to leaves were weakened (T. Liu et al., 2020). Taken together, we speculated that SA mitigated phytotoxicity by weakening the uptake ability of pesticides in plants and enhancing the capacity of antioxidative stress. However, the exact mechanism of SA on plant stress resistance needs to be further illuminated.