Comparative actions of clomazone on beta-carotene levels and growth in rice (Oryza sativa) and watergrasses (Echinochloa spp)

Seedlings of rice, early watergrass (thiobencarb-resistant and thiobencarb-susceptible biotypes, R and S, respectively), and late watergrass (thiobencarb-resistant and thiobencarb-susceptible biotypes, R and S, respectively) were hydroponically exposed to clomazone at concentrations ranging from 0.08 to 7.9 microM. Whole-plant growth (mg fresh wt) and beta-carotene concentrations (microg g(-1) fresh wt) were measured after a 7-day exposure period. For growth, the no observed effect concentrations (NOECs) were 7.9, 0.21, 0.21, 0.46 and 0.46 microM clomazone for rice, early watergrass (R), early watergrass (S), late watergrass (R) and late watergrass (S), respectively, while the concentrations causing 25% inhibition in response (IC25) were 5.6 (+/-1.6), 0.46 (+/-0.06), 0.42 (+/-0.08), 0.92 (+/-0.45) and 0.79 (+/-0.08) microM clomazone, respectively. Clomazone inhibits beta-carotene synthesis via inhibition of the non-mevalonate isoprenoid synthetic pathway. For assessment of clomazone effects, beta-carotene levels proved to be a more sensitive toxicological endpoint than growth. For rice, early watergrass (R), early watergrass (S), late watergrass (R) and late watergrass (S), the beta-carotene NOECs were 0.21, <0.08, <0.08, 0.08 and 0.46 microM clomazone respectively, while IC25 values were 0.42 (+/-0.26), 0.08 (+/-0.02), 0.08 (+/-0.02), 0.33 (+/-0.09) and 0.54 (+/-0.15) microM, respectively. No evidence was found that the thiobencarb-resistance mechanisms present in early and late watergrasses impart resistance to clomazone. Due to similar sensitivity between rice and late watergrass, use of clomazone in rice culture will require the use of a safening technique.