Enantioselective phytotoxicity of the herbicide imazethapyr in rice

Research increasingly suggests that enantiomer selectivity may be a part of the toxicological effects of chiral contaminants. In this study, we selected Japonica rice variety Xiushui 63 seedlings to evaluate the enantioselectivity of imazethapyr (IM). Significant differences in rice seedling morphology, antioxidant enzyme, oxidant marker and gene transcription were observed between the two IM enantiomers. In the seedling morphological assay, IM enantiomers inhibited elongation of primary roots and shoots, and reduced the number of adventitious roots and the density of root hairs. The inhibitory effects were enhanced with increasing concentrations of IM. The maximal root relative inhibition rate reached 80.4%, 67.0%, and 73.5% for R-(-)-IM, S-(+)-IM and racemate at the concentration of 0.5 mg L(-1), respectively, and the maximal shoot relative inhibition rate reached 77.7%, 26.9%, and 61.7%, respectively. The activities of SOD, POD and CAT and the content of MDA increased at 0.5 mg L(-1) of R-(-)-IM treatment, and were 1.8, 3.3, 1.4, and 2.2 times, respectively, over the activities S-(+)-IM. Real-time PCR showed that R-(-)-IM minimized the transcript abundance of ALS in shoot tissue to 12.2% of the S-(+)-IM, and minimized the transcript abundance of PC in seed to 9.2% of the S-(+)-IM. R-(-)-IM maximized the transcript abundance of beta-amylase in shoots to 8.6-time of the S-(+)-IM. Results from this study imply that R-(-)-IM has stronger toxicity on the growth of rice than S-(+)-IM.