ABSTRACT:
Salicylic acid (SA) is an important signal molecule, regulating oxidative stress response in plants. In this study, we evaluated the influences of SA (1mg L-1, 10mg L-1and 50mg L-1) on the accumulation of clothianidin (CLO), dinotefuran (DFN) and difenoconazole (DFZ) (5mg L-1) and pesticide-induced (CLO-10mg L-1, DFN-20 mg L-1, and DFZ-10mg L-1) oxidative stress in cucumber plants. Exogenous SA at 10mg L-1 significantly reduced the half-lives of three pesticides in nutrient solution and prevented the accumulation of pesticides in roots and leaves. And the role of SA in reducing residues was related to the major accumulation sites of pesticides. By calculating the root concentration factor (RCF) and translocation factor (TF), we found that SA at 10mg L-1 reduced the ability of roots to absorb pesticides and enhanced the translocation ability from roots to leaves. Roots exposed to high concentrations of three pesticides could reduce biomass, low chlorophyll content, promote lipid peroxidation, and alter the activities of a range of antioxidant enzymes, respectively. Exogenous SA at low concentrations significantly mitigated these negative effects. Hence, we speculated that application of exogenous SA at 10 mg L-1 could effectively alleviate the accumulation of pesticides and induce stress tolerance in cucumber planting systems.
Keywords: Pesticides, Cucumber, Uptake, Translocation, Salicylic acid, Oxidative stress
Introduction
As a kind of plant protection product, pesticides play an important role in controlling diseases, insects, weeds, and increasing yield of crops (J. Liang & Tang, 2010). However, its misuse has caused a series of environmental safety and food safety problems. For example, some neonicotinoids have raised concerns about their toxicity to bees, which put them at risk of being banned (Jiang et al., 2018; Jiang, Zhang, Lin, Liu, & Mu, 2019). Moreover, overuse of pesticides may cause pesticide pollution in agricultural products as well as soil and water (K. Wang, Wu, & Zhang, 2012). And once the excessive accumulation of pesticides will cause phytotoxicity by injuring the physiological structure of plants (Mahapatra, De, Banerjee, & Roy, 2019). Therefore, it is necessary to investigate the uptake, translocation and accumulation behaviors of systemic pesticides in plants and make effective strategies to reduce pesticide stress.
The active expression of pesticides is closely related to its uptake and translocation behaviors in plants. On the one hand, the uptake and translocation behaviors of pesticides affect the efficacy and duration, which is beneficial to choose the application technology of pesticides (Huang et al., 2019); On the other hand, it is helpful to further understand the dynamic distribution and accumulation of pesticides in various parts of plants and clarify the dynamics residues of pesticides in the environment (Hingmire, Oulkar, Utture, Ahammed Shabeer, & Banerjee, 2015; Utture et al., 2011). Previous studies have reported that the uptake and translocation behaviors of systemic pesticides in plants are related to their physical and chemical properties, such as octanol/water partition coefficient (logKow ), water solubility and molecular weight (Y. Li, Long, et al., 2018; Namiki, Otani, Motoki, Seike, & Iwafune, 2018; Qiu et al., 2016), but also are related to the plant species, growth conditions, growth stage, application method and other factors (Ge et al., 2016; Hwang, Lee, & Kim, 2015; Y. Li, Yang, et al., 2018).
When plants subjected to various biological and abiotic stresses, a large number of reactive oxygen species (ROS) can accumulate in the cells, which can destroy the cell molecular structure and interfere with the physiological process of plants (Noctor, Mhamdi, & Foyer, 2016). For defense, plants have a variety of complex enzymatic and non-enzymatic antioxidant systems that they can resist oxidative stress including superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), ascorbate peroxidase (APX) and other enzymes and non-enzymatic substances they can remove different types ROS (Safari, Akramian, Salehi-Arjmand, & Khadivi, 2019). As a result of human production activities, many wastes remained in the environment for a long time, which can produce stress responses to plants such as heavy metals, pesticides, and engineered nanomaterials (Lian et al., 2020; Rodriguez-Serrano et al., 2006; Safari et al., 2019). Salicylic acid (SA) is a well-known endogenous signaling molecule related to plants stress resistance (Q. J. Wang et al., 2016; X. Y. Zhao et al., 2020; X. Y. Zhao et al., 2019). When plants are in adverse environments, SA can regulate various physiological and biochemical processes of plants to alleviate stress (W. Y. Song, Peng, Shao, Shao, & Yang, 2014; Q. Wang et al., 2013; Xu, Fan, Dong, Kong, & Bai, 2014). It is worth noting that SA can inhibit the accumulation of pesticides in plants, which is vital for reducing pesticide residues and ensuring the safety of agricultural products (Kaya & Yigit, 2014; C. Wang & Zhang, 2017). For phytotoxicity caused by pesticides, most previous studies focused on exogenous SA to reduce the toxicity of herbicides on nontarget plants (Akbulut, Yigit, Kaya, & Aktas, 2018; Kaya & Doganlar, 2016). However, the regulatory functions of SA under commonly used systemic pesticide stress and the effects of exogenous SA on the accumulation of pesticides in various parts of the plants were rarely investigated.
Cucumber(Cucumis sativus L. ) is a widely cultivated cash crop in China. In the present study, the three pesticides, clothianidin (CLO), dinotefuran (DFN) and difenoconazole (DFZ) were selected as the representatives. CLO is a neonicotinoid insecticide with excellent systemic and osmotic activity that can be used as a soil, spray and seed treatment, for control of sucking and chewing insects (Jeschke, Nauen, Schindler, & Elbert, 2011). DFN is a systemic neonicotinoid insecticide with translaminar activity, and it can be applied on foliage, soil and nursery boxes by spray, drench, broadcast and pricking-in-hole treatments (Corbel, Duchon, Zaim, & Hougard, 2004). DFZ is a systemic fungicide of sterol demethylation inhibitors with a novel broad-range activity protecting the yield and crop quality by foliar application or seed treatment (Dong et al., 2013). The three pesticides all could be absorbed by plant roots, which can be used for soil treatment, but there are significant differences in water solubility and logKow of the three pesticides. The physical and chemical properties of these three pesticides are shown in Table S1 . We investigated the effects of exogenous SA on the uptake, translocation and accumulation behaviors of these three pesticides in cucumber plants by measuring the concentration of pesticides in various parts of the cucumber plants and calculating the root concentration factor (RCF) and translocation factor (TF) values. Moreover, the effects of SA on oxidative stress induced by these three pesticides to the cucumber plants were also studied by measuring biomass, photosynthetic pigment content, antioxidants content and antioxidant enzyme activities. This study provides a new perspective to understand the uptake and translocation behaviors of pesticides and pesticide pollution in plants.