Physiological basis for isoxadifen-ethyl induction of nicosulfuron detoxification in maize hybrids

Isoxadifen-ethyl can effectively alleviate nicosulfuron injury in the maize. However, the effects of safener isoxadifen-ethyl on detoxifying enzymes in maize is unknown. The individual and combined effects of the sulfonylurea herbicide nicosulfuron and the safener isoxadifen-ethyl on the growth and selected physiological processes of maize were evaluated. Bioassays showed that the EC50 values of nicosulfuron and nicosulfuron plus isoxadifen-ethyl for maize cultivar Zhengdan958 were 18.87 and 249.28 mg kg-1, respectively, and were 24.8 and 275.51 mg kg-1, respectively, for Zhenghuangnuo No. 2 cultivar. Evaluations of the target enzyme of acetolactate synthase showed that the I50 values of nicosulfuron and nicosulfuron plus isoxadifen-ethyl for the ALS of Zhengdan958 were 15.46 and 28.56 μmol L-1, respectively, and were 0.57 and 2.17 μmol L-1, respectively, for the acetolactate synthase of Zhenghuangnuo No. 2. The safener isoxadifen-ethyl significantly enhanced tolerance of maize to nicosulfuron. The enhanced tolerance of maize to nicosulfuron in the presence of the safener, coupled with the enhanced injury observed in the presence of piperonyl butoxide, 1-aminobenzotriazole, and malathion, suggested cytochrome P450 monooxygenases may be involved in metabolism of nicosulfuron. We proposed that isoxadifen-ethyl increases plant metabolism of nicosulfuron through non-P450-catalyzed routes or through P450 monooxygenases not inhibited by piperonyl butoxide, 1-aminobenzotriazole, and malathion. Isoxadifen-ethyl, at a rate of 33 mg kg-1, completely reversed the effects of all doses (37.5-300 mg kg-1) of nicosulfuron on both of the maize cultivars. When the two compounds were given simultaneously, isoxadifen-ethyl enhanced activity of glutathione S-transferases (GSTs) and acetolactate synthase activity in maize. The free acid 4,5-dihydro-5,5-diphenyl-1,2-oxazole-3-carboxylic was equally effective at inducing GSTs as the parent ester and appeared to be the active safener. GST induction in the maize Zhenghuangnuo No. 2 was faster than in Zhengdan 958.