3.4.2 ROS generation and proline content
The values of H2O2 content and proline content are shown in Fig. 4 C, D . From the data in Fig. 4 C , it could be seen that after single root exposure to CLO, DFN and DFZ, the values of H2O2 content increased by 313 %, 300 % and 424 %, respectively, compared with the controls. Also, the low concentration (1mg L-1 and 10 mg L-1) of SA have significant effects on reducing the H2O2 content of roots. With SA supplementation at 1mg L-1, the H2O2 content in roots significantly decreased by 74%, 62% and 52%, respectively, compared with the single root exposure of CLO, DFN and DFZ treatments. With SA supplementation at 10mg L-1, the H2O2 content in roots significantly decreased by 78%, 82% and 62%, respectively, compared with the single root exposure of CLO, DFN and DFZ treatments (Fig. 4 C ).
In the present study, it was found that the three pesticides all enhanced accumulation of proline and more increased when the SA at 1mg L-1 and 10 mg L-1 were supplemented. With SA supplementation at 1mg L-1, the proline content in roots significantly in- creased by 35%, 29% and 35%, respectively, compared with the single root exposure of CLO, DFN and DFZ treatments. With SA supplementation at 10mg L-1, the proline content in roots significantly increased by 44%, 52% and 83%, respectively, compared with the single root exposure of CLO, DFN and DFZ treatments (Fig. 4 D ).
3.4.3.Antioxidant enzymes assays and lipid peroxidation
The activities of SOD, APX and GST in roots were markedly affected under CLO, DFN and DFZ treatments compared with the controls, respectively. The SOD activity increased by 150 %, 191 % and 210 % at the single root exposure of CLO, DFN, and DFZ treatments, respectively, compared with the controls, while APX activity was significantly inhibited (Fig. 4E, F ). Similarly, single root exposure to CLO, DFN, and DFZ also led to increasing GST activity, respectively (Fig.4 G ). From the dates in Fig. 4 E, F, G , exogenous SA also could alter the antioxidant enzyme activities. The SOD activity significantly decreased in the roots of cucumber plants treated with 1mg L-1and 10 mg L-1 SA compared with the treatments with pesticides alone, and it was similar to the control groups (Fig. 4 E ). Similarly, SA at 1 mg L-1 and 10 mg L-1 also increased the APX activity, suggesting that the phytotoxicity caused by three pesticides were alleviated, respectively (Fig. 4 F ). With supplementing exogenous SA (1mg L-1 and 10mg L-1), GST activity in roots was elevated, which was beneficial to promoting pesticide metabolism and alleviating phytotoxicity (Han, Mhamdi, Chaouch, & Noctor, 2013; Kaya & Doganlar, 2016).
As for single root of CLO, DFN, and DFZ exposure, MDA contents in cucumber roots significantly increased by 40%, 59% and 67%, respectively, compared with the unstressed plants (Fig. 4H ). With SA supplementation at 1mg L-1, the MDA content in roots significantly decreased by 56%, 57% and 37%, respectively, compared with the single root exposure of CLO, DFN and DFZ treatments. With SA supplementation at 10mg L-1, the MDA content in roots significantly decreased by 61%, 61% and 56%, respectively, compared with the single root exposure of CLO, DFN and DFZ treatments.