2.3. Determination of CLO, DFN and DFZ in cucumber plants
For “pesticide exposure experiments- uptake and translocation” part, the contents of pesticides in plant tissue samples were analyzed. The QuECHERS method was used to prepare nutrient solution and plant tissue samples (Ge et al., 2017). The specific sample preparation steps and instrumental analysis conditions are shown in the supplementary materials .
The recoveries of three pesticides in nutrient solution and plant tissues were used to verify the feasibility of the analytical method. External standard method was used for quantitative analysis. The results of quality assurance and quality control are shown in the supplementary materials .
2.4.Determination of antioxidant enzyme activity
The extraction of enzyme liquid was carried out according to the method described by Andrews et al. (2005). The method of SOD determination referred to Farouk and Al-Amri (2019). APX activity was determined by the reduction value of absorbance at 290 nm per unit time using the method of Kaya and Doganlar (2016). GST activity was tested according to Habig, Pabst, and Jakoby (1974). The specific measurement steps are shown in the supplementary materials .
2.5. Determination of the total chlorophyll andH2O2content in cucumber plants
The pigment was extracted by methanol extraction following the method of L. Zhao et al. (2019). The specific measurement steps are shown in thesupplementary materials . The content of H2O2 in cucumber roots was determined by H2O2 kits.
2.6. Determination of the malondialdehyde (MDA) content
The level of lipid peroxidation was evaluated by the determination of malondialdehyde (MDA) content based on the method of Heath and Packer (1968). The specific measurement steps are shown in thesupplementary materials .
2.7. Determination ofproline content
The content of proline in plant roots was determined according to the method of Bates, Waldren, and Teare (1973). The specific measurement steps are shown in the supplementary materials .
2.8. Data processing and statistical analysis
In order to compare the difference in the behaviors of three pesticides between supplementing different concentrations of SA and without SA, the root concentration factor (RCF) and translocation factor (TF) were calculated. The root concentration factor (RCF) was used to indicate the ability to absorb target compounds by plant roots in nutrient solution, RCF > 1 indicates that the compound is easily absorbed by the roots, and calculated as follows (Qiu et al., 2016): (Eqn 1)
\(\mathrm{RCF=}\frac{\mathrm{concentration\ in\ root\ (mg/kg)}}{\mathrm{concentration\ in\ nutrient\ solution\ (mg/L)}}\)
Translocation factors (TF) was used to indicate the ability of leaves and stems to transport target compounds from roots, TF>1 indicates that the excellent capacity for stems and leaves to translocate the pesticides from roots, and calculated as follows (Ge et al., 2016): (Eqn 2 and Eqn 3)
\(\ \mathrm{\text{TF}}\mathrm{\text{stem}}=\frac{\mathrm{concentration\ in\ stem\ (mg/kg)}}{\mathrm{concentration\ in\ root\ (mg/kg)}}\)
\(\ \mathrm{\text{TF}}\mathrm{\text{leaf}}\mathrm{\ }=\frac{\mathrm{concentration\ in\ leaf\ (mg/kg)\ }}{\mathrm{concentration\ in\ root\ (mg/kg)}}\)
The concentration of pesticides in nutrient solution was calculated using the first-order equation Ct=C0e-kt. The half-life was calculated by the equation t1/2 =ln2/k, where Ct is the concentration of target pesticides at time t (days), C0 is the concentration of target pesticides at the initial time, and k is the first-order rate constant (day -1).
An independent sample t-test was used to compare the differences between different treatments and pesticides. All statistical analyses were statistical significance at the 0.05 level.