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).