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- water management; ALS activity; tolerance; photolysis; red rice; Oryza sativa; adsorption; availability
Imazethapyr was recently approved for use in rice, but limited information is available regarding its efficacy, environmental fate or potential red rice resistance. Therefore, experiments were conducted to 1) determine the effect of flooding time, and stage of imazethapyr application in red rice control, 2) assess the acetolactate synthase resistance to imazethapyr on red rice ecotypes, 3) determine the relative photolysis of imazethapyr, and 4) determine the effect of soil and moisture on imazethapyr adsorption and availability. When imazethapyr was applied in sequential application of PRE followed by a POST application, to achieve >95% red rice control, flood needed to be established within 14 DAT when imazethapyr was applied EPOST, and 7 DAT when imazethapyr was applied LPOST. Delaying the flood up to 21 DAT reduced rice grain yield for both EPOST and LPOST application timings. Based on enzymatic activity, the mean I50 values were 1.5, 1.1, 1.5, 1.6, 20.8, and 590.6 mM of imazethapyr, respectively, for LA 5, MS 5, TX 4, ??Cypress??, ??CL-121??, and ??CL-161??. CL-161 was 32 times more resistant than CL-121, and at least 420 times more resistant than the average of the red rice ecotypes and ??Cypress??. Results from the ALS assay showed that red rice ecotypes and Cypress had high susceptibility to imazethapyr when compared with the tolerant CL-121 and the resistant CL-161. Measurable enzymatic tolerance to ALS-inhibiting herbicides has not yet developed in these red rice ecotypes. Imazethapyr quantum yield (fI ) was 0.023 ?? 0.002 while the hydroxyl radical rate constant ( I OH k?? ) was 2.8 ?? 0.44 x 1013 M-1 h-1. These results show that imazethapyr is susceptible to both direct and indirect photolysis. The results also show that imazethapyr photolysis in paddy water will be affected by turbidity due to its impact on the availability of sunlight to drive direct and indirect photolysis reactions. Imazethapyr was more available and more concentrated in sandy soil. With higher amounts of water in soil there was greater amount of imazethapyr in soil solution and a lower concentration of herbicide due to dilution. The double centrifuge method provided a better estimate of plant available herbicide.