Uptake,Translocation,and Bioavailability Of Ten Pesticides In The Soil-cucumber Plant System

The applied pesticides in the field will enter the soil directly or indirectly and interactwith soil components complexly,which eventually exist in the forms of free dissolved,bound and fixed states.However,it is the free pesticides that can be uptake by plants and produce bioavailability.Therefore,it is of great significance to explore the uptake,translocation,and the bioavailability of pesticides in soil plants continuum for the risk assessment of residual pesticides in soil.In this study,a widely cultivated vegetable,cucumber,was selected as the research object.The uptake and translocation and the main influencing factors of the 10 commonly used pesticides（thiamethoxam,imidacloprid,clothianidin,simazine,isoproturon,alachlor,metolachlor,pretilachlor,acetochlor and butachlor）in the soil-cucumber plant systems were studied through indoor pot experiments.The uptake mechanism and its main influencing factors for the aforementioned pesticides were also explored.A method was established to accurately evaluate the bioavailability of pesticides to cucumber plants in soils.The effects of the physicochemical properties of pesticides and soils on the bioavailability of pesticides to cucumber plants in soils were also investigated.The main research results are summarized as follows.（1）All 10 pesticides could be detected in cucumber plants after transplanting for 5and 7 d at the soil concentration of 10 mg/kg.The ranges of the detected concentrations were 2.05-21.51 mg/kg（roots）and 0.03-40.13 mg/kg（shoots）respectively.The uptake of clothianidin,simazine,imidacloprid,acetochlor,pretilachlor and metolachlor by cucumber roots principally belongs to active uptake,while the uptake of thiamethoxam,butachlor,alachlor and isoproturon is passive uptake.According to the uptake pathway,the uptake of metolachlor,imidacloprid,pretilachlor and simazine by cucumber roots was mainly carried out in apoplastic pathway,but the uptake of butachlor,alachlor,clothianidin,isoproturon,thiamethoxam and acetochlor is the symplastic pathway.（2）The uptake and translocation of the 10 pesticides in soils by cucumber plants are related to their physicochemical properties,soil type and plant variety.The detected concentrations and translocation factors of pesticides in cucumber plants were significantly negatively correlated with the octanol-water partition coefficient(log K_OW)（R²≥0.7909,p<0.01）,but had no significant correlation with molecular weight,water solubility（S_W）and p Ka,indicating that K_OW is the main factor affecting the uptake and translocation of different pesticides by cucumber plants.With the increasing of log K_OWof the pesticides,the uptake and translocation capacity for the pesticides in the cucumber roots and shoots decreased.There were differences in the uptake of acetochlor by cucumber plants in different types of soil.The uptake concentration was highest in the red soil（0.011-1.591 mg/kg）,followed by fluvo-aquic soil（0.001-1.007mg/kg）,yellow-brown soil（0.007-0.534 mg/kg）,yellow-cinnamon soil（0.002-0.321mg/kg）and black soil（0.001-0.064 mg/kg）.Different varieties of cucumber plants also had differences in the uptake of clothianidin in the soil.The detected concentration of clothianidin in the roots of cucumber plants was significantly positively correlated with the protein content of the roots（R²≥0.7494,p<0.05）,indicating that the root protein content also affected the uptake of pesticides by cucumber plants.（3）The concentrations of the 10 pesticides in the soil were analyzed using four different chemical extraction methods.The correlation between the uptake concentration of pesticides by cucumber plants and the concentrations measured by different chemical methods was as follows:in situ pore water method（R²≥0.7795,p<0.01）,PU film passive sampling method（R²≥0.3732,p<0.05）water extraction and acetonitrile extraction methods（R²≤0.3622,p>0.05）,indicating that the in situ pore water method can be preferably used to characterize the bioavailability of different pesticides to cucumber plants in soil,followed by the PU film passive sampling methods.However,the water extraction and acetonitrile extraction method cannot be used to characterize the bioavailability of different pesticides to cucumber plants in soil.（4）Taking acetochlor as an example,a combination of biological and chemical methods was used to evaluate the bioavailability of pesticide to cucumber plants in different soils.The results of biological method showed that the IC₅₀ of acetochlor to cucumber plants were significantly correlated with the organic matter content and cation exchange capacity of soils（R²≥0.7859,p<0.05）,but not significantly correlated with the p H and clay content（R²≤0.1430,p>0.05）.It showed that the organic matter content and cation exchange capacity were the main factors affecting the toxicity of acetochlor to cucumber plants,while p H and clay content had less effect.The results of the chemical method showed that the in situ pore water method and the PU film passive sampling method could be effectively used to characterize the bioavailability of acetochlor to cucumbers plants in different soils（R²≥0.6571,p<0.01）.