Responses to clomazone and 5-ketoclomazone by Echinochloa phyllopogon resistant to multiple herbicides in Californian rice fields.
BACKGROUND
Late watergrass [Echinochloa phyllopogon (Stapf.) Koss.] is a major weed of Californian rice that has evolved P450-mediated metabolic resistance to multiple herbicides. Resistant (R) populations are also poorly controlled by the recently introduced herbicide clomazone. The authors assessed whether this cross-resistance was also P450 mediated, and whether R plants also had reduced sensitivity to photooxidation. Understanding mechanism(s) of resistance facilitates the design of herbicide management strategies to delay resistance evolution.


RESULTS
Ratios (R/S) of R to susceptible (S) GR(50) were near 2.0. [(14)C]Clomazone uptake was similar in R and S plants. Clomazone and its metabolite 5-ketoclomazone reduced chlorophyll and carotenoids in S more than in R plants. The P450 inhibitors disulfoton and 1-aminobenzo-triazole (ABT) safened clomazone in R and S plants. Disulfoton safened 5-ketoclomazone only in S plants, while ABT synergized 5-ketoclomazone mostly against S plants. Paraquat was more toxic in S than in R plants.


CONCLUSIONS
Cross-resistance to clomazone explains failures to control R plants in rice fields, and safening by P450 inhibitors suggests that oxidative activation of clomazone is needed for toxicity to E. phyllopogon. Clomazone resistance requires mitigation of 5-ketoclomazone toxicity, but P450 detoxification may not significantly confer resistance, as P450 inhibitors poorly synergized 5-ketoclopmazone in R plants. Responses to paraquat suggest research on mechanisms to mitigate photooxidation in R and S plants is needed.
Yasuor
Hagai
H
Weed Science Program, Department of Plant Sciences, University of California, Davis, CA 95616, USA. hyasuor@ucdavis.edu
TenBrook
Patti L
PL
Tjeerdema
Ronald S
RS
Fischer
Albert J
AJ
D018533:Biomass; D002140:California; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D004222:Disulfoton / Q000494:pharmacology; D031747:Echinochloa / Q000187:drug effects* / Q000502:physiology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D053209:Herbicide Resistance; D006540:Herbicides / Q000494:pharmacology*; D007555:Isoxazoles / Q000494:pharmacology*; D012275:Oryza; D023303:Oxazolidinones / Q000494:pharmacology*; D010269:Paraquat / Q000494:pharmacology; D010860:Pigments, Biological / Q000378:metabolism
C532841:5-ketoclomazone; D065607:Cytochrome P-450 Enzyme Inhibitors; D004791:Enzyme Inhibitors; D006540:Herbicides; D007555:Isoxazoles; D023303:Oxazolidinones; D010860:Pigments, Biological; D004222:Disulfoton; C095255:clomazone; D003577:Cytochrome P-450 Enzyme System; D010269:Paraquat