Ninety-seven inbred lines of sunflower were screened in the field by treatment with a combination of imazamox and malathion, an inhibitor of cytochrome P450 monooxygenases (P450s), to identify sunflower lines with natural tolerance to the herbicide reversed by malathion. One tolerant line, named TolP450-1, was selected and characterized in the field and in the greenhouse to evaluate its response to the herbicides imazamox, prosulfuron, and atrazine at different plant development stages (germination, emergence, and third difoliate) with and without malathion. For all herbicides and all development stages analyzed, TolP450-1 showed significantly higher tolerance compared with the susceptible line RHA266. In all cases, the tolerance was reversed by malathion. This sunflower line, tolerant to multiple herbicides, may be useful in helping to manage herbicide-resistant weeds by allowing additional herbicides to be used in this oilseed crop.

Nomenclature: Atrazine; imazamox; malathion, 2-(dimethoxyphosphinothioylthio) butanedioic acid diethyl ester; prosulfuron; sunflower, Helianthus annuus L