4.2. The effect of exogenous SA on the behaviors of pesticides
The effect of exogenous SA on pesticides half-lives depended on its concentrations. In the present study, when the exogenous SA was 10 mg L-1, it had the greatest impacts on the half-lives of the three pesticides. In addition, among the three pesticides, addition of SA at three concentrations all reduced the half-lives of DFZ in nutrient solution. And with supplementing SA at 10 mg L-1, the half-lives were reduced by more than half compared with the without SA treatments, which indicating that exogenous SA has the strongest effects on promoting the degradation of DFZ in nutrient solution. This might be related to the physical and chemical properties of pesticides. Importantly, the lower half-lives of individual pesticides mean that they keep shorter time in the environment, which can alleviate environmental pressure (Mohapatra et al., 2019).
Addition of SA at three concentrations all could block the accumulation of three pesticides in various parts of cucumber plants except for DFZ with SA supplementation at 50mg L-1. When the SA was 1mg L-1 and 10mg L-1, the inhibition of accumulation of the three pesticides was the most significant. Therefore, exogenous SA could prevent the accumulation of pesticides in plants, but had a dose effect. Previous study had shown that SA could reduce residues by promoting the intracellular herbicides catabolism (Y. C. Lu, Zhang, & Yang, 2015). Noteworthy, addition of SA at 1mg L-1 and 10mg L-1 significantly reduced the concentrations of CLO and DFN in roots and leaves, respectively; but for DFZ, the concentration was significantly reduced in roots. For these phenomena, we speculated that it might be related to the accumulation behaviors of three pesticides in various parts of cucumber plants. And the effects of SA suppressing the pesticide entry into plant tissues might be related to the concentrations of pesticides (Kaya & Yigit, 2014). Exogenous SA played the strongest role in promoting the degradation of pesticides in the sites with high concentration of pesticides. This idea could be proved again by showing that when SA was 10mg L-1, the concentration of DFZ in the root was 18.70mg kg-1, equivalent to 60% of the without SA treatments, which also indicated that exogenous SA had the greatest effect on reducing the DFZ residue in cucumber plants. In summary, the role of SA in reducing pesticide residues in a particular plant might be related to the major sites of accumulation, which provided a basis for selecting the application site of SA on the plants.
Addition of SA at 10 mg L-1, the RCF values of three pesticides all decreased, indicating that SA can inhibit pesticide uptake by roots and impaired the ability to accumulate pesticides. The TFleaf values of three pesticides all improved after application of SA, indicating that the pesticides moved faster in cucumber plants than the without SA treatments. In our study, we found that SA can improve the upward migration ability of three pesticides. It was difficult to explain this result, but it might be related to the change of pesticide behavior and pesticide metabolism level in plants after the addition of SA (Kong, Dong, Xu, Liu, & Bai, 2014). This finding also accorded with the earlier research, which showed that with SA supplementation, the TF value of isoproturon in wheat plants was higher than the without SA treatments (Yi Chen Lu et al., 2014).