4.1. The dissipation and accumulation behaviors of pesticides
In this study, the half-lives of individual pesticides in planted nutrient solution were significantly shorter than that in unplanted nutrient solution. Plant intake was an important factor, and previous studies have shown that the half-lives of pesticides were relevant to the factors as the cultivation modes of plants, the existence of plants, and soil types (Lockridge, Verdier, & Schopfer, 2019; Šudoma et al., 2019). Lv et al. (2016) found that during the experimental period, the pH level of the unplanted systems keep constant, but the pH of the planted systems significantly decreased. Plants can secrete secondary metabolites during the growth period, including low-molecular-weight substances (e.g., organic acids, amino acids, and fatty acids) and high-molecular-weight substances (e.g., polysaccharides and proteins) (L. Zhao et al., 2016). Substances that produced by plant roots metabolism might be responsible for changes in pH of nutrient solutions. Therefore, the pH level of nutrient solution might be one reason for the change of half-lives.
The CLO and DFN were mainly accumulated in leaves, and the DFZ was mainly accumulated in roots, which could be attributed to the water solubility and partition coefficient octanol/water (logKow) of the pesticides and differences in physiological structure between organs (Ge et al., 2016; Qiu et al., 2016). This finding was in agreement with the previous result, which showed that the concentrations of imidacloprid and thiamethoxam detected in rice leaves were higher than roots, but for difenoconazole, the concentration detected in rice roots was higher than leaves (Ge et al., 2017).
The RCF values of DFZ was highest, followed by CLO and DFZ. The larger the logKow of pesticide, the easier it is to accumulate in the organs with more lipids. The order of lipid content of root, stem and leaf was root > stem >leaf (Qiu et al., 2016). Thus, DFZ was more easily accumulated in the roots of cucumber plants. Previous studies have shown that the RCF values increased with increasing logKow values of pesticides, which was in accord with our results (Namiki et al., 2018; Qiu et al., 2016). Moreover, among the three pesticides, the highest TF value was the CLO followed in declining order by DFN and DFZ. The higher the solubility of pesticide, the easier it is to accumulate in the leaves (Qiu et al., 2016). The logKow of DFZ was higher, while water solubility was poorer than CLO and DFN. Thus, the TFstemand TFleaf of DFZ were all lower than CLO and DFN, which again indicating that DFZ mainly accumulated in roots, and its upward migration ability was weak. The logKow of CLO and DFN was comparative, and the TFstem and TFleaf was similar, respectively. For CLO and DFN, TFleaf were much higher than TFstem, indicating that both had stronger ability to migrate to the leaves, which could possibly be related to the higher water content in the leaves.