Uptake,Translocation,and Biotransformation Of Organophosphate Esters In Rice(Oryza Sativa L.)

Recently,as an alternative to brominated flame retardants,the production and use of organophosphate esters（OPEs）in China have been increasing.They are widely detected in water,soil,atmosphere,biotas and other environmental media,which leads to potential ecological environmental risks and human health risks.Rice is one of the staple foods in China,and its food safety is related to the national economy and people’s livelihood.Rice can absorb and bioaccumulate organic pollutants in various ways.Previous studies have shown that dietary intake via rice is one of the main exposure ways for Chinese residents to OPEs.At present,there is a lack of research on the uptake,translocation and biotransformation of rice to OPEs.Therefore,the uptake pathway,enrichment,translocation and biotransformation characteristics of 5 typical OPEs by the local japonica rice of Dalian were investigated in this study.The results showed that after 192-h exposure,significant uptake,translocation and biotransformation in rice were observed under 20μg/L OPEs solution exposure.The concentration of 5 OPEs in rice roots after 192-h exposure was 0.96-7.97μg/g dw,while the concentration of shoot was 0.59-1.33μg/g dw.The concentration of TPHP in root was higher than those of other OPEs,while in the shoot the concentration of TPHP was lower.TCEP showed the opposite trend.The uptake and translocation of OPEs in rice roots were in accordance with the first-order reaction kinetics.Logarithm of the root concentration factor（RCF）of OPEs is positively correlated with their corresponding log Kow,while the transport factor（TF）is negatively correlated with log Kow.This indicated that the hydrophobic OPEs are easy to be accumulated in rice root,while the relatively hydrophilic ones are easy to transport to the shoot.The results of hydroponic experiment showed that the biotransformation of OPEs in nutrient solution was negligible.OPEs could degrade in rice tissues and produce corresponding metabolites.The concentrations of metabolites were 1.29-3.10μg/g dw for 192-h exposure in the root and 0.22-0.71μg/g dw in the shoot.The concentration of DNBP in root was the highest,whereas BCIPP was the lowest;The BCEP concentration in shoot was the highest,while DPHP was the lowest.The ratio of OPE diester to OPEs was not significantly different in shoot and root.Significant shoot uptake,root translocation and nutrient solution translocation were observed within 48-h 100μg/L OPEs solution exposure.The concentrations of OPEs in root were 0.11-0.48 at 48 hμg/g dw,higher than those in shoot（0.02-0.12μg/g dw）.The translocation of rice root to nutrient solution was observed,and OPE concentrations in nutrient solution were 0.39-1.10μg/L at 48 h.In addition,OPE biotransformation occurred in both root and shoot,especially in root.The result of rice root exposure experiment suggested a translocation of OPEs from the root in the exposed side to the root on the non-exposed side,as well as the translocation of OPEs from the non-exposed root to nutrient solution.The concentration levels of OPEs at both sides of roots were similar at 48 h（0.94-2.83μg/g dw on the exposed side,0.72-0.86μg/g dw on the non-exposed side）.The translocation direction of OPEs depends on its relative concentration and properties,and reaches the distribution equilibrium in all parts of rice.The biotransformation of OPEs in the root of the exposed side and shoot is greater than that of the non-exposed root,that is,the biotransformation of OPEs was delayed.The results showed that the root exudates significantly hindered the uptake of OPEs in the early stage of culture,and the absorption rate of OPEs was significantly reduced;The absorption of the root exudates to hydrophilic OPEs was hindered.Root exudates promoted the biotransformation of TCEP and TPHP in nutrient solution,but did not change the transport and enrichment of rice.The biotransformation rate of OPEs and diesters translocation in rice shoot were not significantly changed,but the biotransformation rates of TCEP,TPHP and TNBP in roots decreased.In the Cu²⁺exposure experiment,Cu²⁺changed the translocation pathway of the cell membrane in the root of rice,and significantly promoted the uptake of OPEs in root.In addition,Cu²⁺also promoted the translocation of OPEs in rice,including from the shoot to the root in the non-exposed side;It also hindered the biotransformation process of rice root,reducing the biotransformation rate of OPEs at both sides of roots;But Cu²⁺did not change the distribution balance of OPEs in rice.