California Sea Grant College Program

Rice (Oryza sativa), early watergrass (Echinochloa oryzoides); EWG; biotypes resistant (R) and susceptible (S) to thiocarbamates and aryloxyphenoxy propionate herbicides), and late watergrass (Echinochloa phyllopogon; LWG; R and S biotypes) were exposed to clomazone for 7 d. Whole-plant change in fresh weight and B-carotene concentrations were measured. For growth, the No Observed Effect Concentrations (NOECs) were 7.9, 0.21, 0.21, 0.46, and 0.46 µM clomazone for rice, EWG (R), EWG (S), LWG (R), and LWG (S), respectively, while the concentrations causing 25% inhibition in response (IC25s) were 5.6, 0.46, 0.42, 0.92 and 0.79 µM clomazone, respectively. For rice, EWG (R), EWG (S), LWG (R), and LWG (S), the B-carotene NOECs were 0.21, < 0.08, < 0.08, 0.08 and 0.46 µM clomazone, respectively and IC25s were 0.42, 0.08, 0.08, 0.33 and 0.54 µM, respectively. At a field-relevant clomazone application rate resistance to clomazone was observed in EWG (R) and LWG (R) as measured by growth and B-carotene, respectively. Due to similar sensitivity between rice and late watergrass, use of clomazone in rice culture may require the use of a safening technique.

Rice and EWG (S) were exposed to 14C-labeled clomazone to determine accumulation, biotransformation and mass balance. In terms of extractable 14C, rice contained more total residues than EWG (p < 0.05), but the concentration in EWG was significantly higher (p < 0.01). More metabolized residue was measured in EWG compared to rice (p < 0.01). Both species produced hydroxylated forms of clomazone, B-D-glucoside conjugates, and unidentified polar metabolites. The suspected active metabolite, 5-ketoclomazone, was found at a significantly higher concentration in EWG vs. rice (21 ± 2 vs. 5.7 ± 0.5 pmol g-1, respectively; p < 0.01). Qualitatively and quantitatively, more clomazone metabolism was observed in the relatively sensitive EWG compared to rice.

R and S biotypes of EWG and LWG were exposed to clomazone alone and in combination with P450 inhibitors. Growth reduction (GR25, GR50, and GR75) values were lower for S biotypes than for R biotypes (p < 0.05) with exposure to clomazone alone indicating resistance to clomazone in the R biotypes. In combination with clomazone disulfoton increased 20-d growth in LWG (R) by as much as 5-fold, compared to application of clomazone alone. Oxydemeton methyl increased 20-d growth by as much as 6-fold. Disulfoton and oxydemeton methyl reduced !)-carotene levels compared to treatment with clomazone alone. For EWG (S), disulfoton increased 4-d growth by 46% and increased B-carotene levels by 52%. Demeton-S increased B-carotene levels by 48%. At 6800 g ai ha-1 PBO increased growth and B-carotene levels by 28 and 48%, respectively. These results indicate that P450 enzymes serve both to activate and detoxify clomazone.