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.