4.3. The role of exogenous SA in pesticide-induced oxidative stress in cucumber plants
Previous studies indicated that excessive long-term application of pesticides could produce phytotoxicity once it exceeds the limits of non-target plant, which would affect plant growth by decreasing biomass and chlorophyll content and so on (Kaya & Yigit, 2014; C. Wang & Zhang, 2017). The roots system could absorb and transport water and nutrients to ensure the growth of the plants (Yu et al., 2020). When the plant roots are exposed to excessive pesticides, the physiological structure of the roots can be damaged, resulting in slow growth (C. Wang & Zhang, 2017). Notably, compared with CLO and DFN, the fresh biomass of plants decreased by more than half when single root exposure of DFZ, which possibly due to the more residue of DFZ in the roots. After single root exposure of DFZ, the root morphology of cucumber plants changed compared with the control groups, which mainly showed that the lateral root decreased, the root volume increased, the diameter increased, and the root length became shorter (Fig. S2 ). In this study, the fresh biomass and total chlorophyll content significantly increased with SA supplementation at 1mg L-1 and 10mg L-1 compared with the treatments of pesticides alone, respectively. Previous studies have demonstrated that low concentration of SA promoted plants growth, while high concentration of SA inhibited plants growth and low concentration of SA also could increase chlorophyll content (Kong, Dong, Zhang, et al., 2014; Pasternak et al., 2019; Y. Song et al., 2016).
Single root exposure of CLO, DFN and DFZ, the H2O2 content in cucumber roots significantly increased, indicating that the ROS balance in the plants was out of balance and might have a toxic effect on the plants(J. Xu et al., 2014). Similarly, it was also reported that some herbicides markedly increased the H2O2 content in non-target plants (Q. Li et al., 2019; Spormann, Soares, & Fidalgo, 2019). However, the ROS imbalance of cucumber plants was alleviated by exogenous SA (1mg L-1 and 10mg L-1). Similarly, the research showed that application of 5 mg L-1 exogenous SA significantly eased the phytotoxicity of wheat inducing by isoproturon and also reduced the abundance of O 2.− and H2O2 (L. Liang, Lu, & Yang, 2012). Consistent with our results, exogenous SA had also been reported to increase the content of free proline in a variety of stressed plants (Q. Li et al., 2019; S. Liu, Dong, Xu, & Kong, 2013; Safari et al., 2019). Proline is a small molecular organic solvent, which is widely found in plants in a free state. It can regulate cell osmotic pressure, stabilize sub-cellular structures such as membranes and proteins, remove reactive oxygen species and other (Ashraf & Foolad, 2007). It also has been proved that free proline can remove reactive oxygen species caused by heavy metal Hg contamination (F. Wang, Zeng, Sun, & Zhu, 2008). This fact was in accord with our study that as low concentration of SA-induced the increasing level of free proline coincided with the decreasing trend of H2O2 content in pesticides-stressed plants. Hence, increasing proline accumulation appeared to be an adaptive response which enhanced plants resistance to the pesticide stress.
Adverse environmental conditions stimulated the plants to induce oxidative stress and increased the activity of antioxidant enzymes to withstand stress (Akbulut et al., 2018; Lian et al., 2020). Exogenous SA at low concentrations also regulated antioxidant enzyme activities to resist stress (Kong, Dong, Xu, et al., 2014). In this study, after addition of SA at 1mg L-1 and 10mg L-1, the activities of APX and GST increased compared with the treatments of pesticides alone, respectively, and the SOD activity returned to the control level. Malondialdehyde (MDA) is the primary substance produced during lipid peroxidation, serving as an indicator of oxidative damage of membrane lipids. Under pesticide stress, cucumber plants treated with exogenous SA had lower MDA content than untreated ones. These results indicated the critical role of SA in plants resistance to stress and reduction of oxidative damage. This finding was further verified by the positive role of exogenous SA on the antioxidant enzyme activities and proline content. It has also been reported that exogenous SA can lower MDA content in other plants under stress of pesticides, heavy metals and nanomaterials (Safari et al., 2019; C. Wang & Zhang, 2017).