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).