A Powell amaranth population suspected to be resistant (R) to linuron was discovered in a carrot field in Keswick, Ontario, Canada, in 1999. Dose–response analysis with different herbicides and DNA sequencing of the psbA gene encoding the D1 protein of photosystem II were done to confirm the resistance and identify its basis. A calculated resistance factor indicated a 12-fold increased resistance when linuron was applied to an R population compared with a susceptible (S) population. Moreover, the R population showed 6.4- and 3.1-fold greater resistance to two other phenylurea herbicides (diuron and monolinuron), 1.8- and 1.4-fold greater resistance to two triazine herbicides (metribuzin and prometryn), and 2.6-fold greater resistance to the triazinone metribuzin. R population was also cross-resistant to bentazon and bromoxynil when compared with S population, with a calculated resistance factor of 1.4 and 2.2, respectively. The partial nucleotide sequence of the psbA gene of R populations differed at two locations when compared with S populations. The first mutation coded for a Val₂₁₉Ile substitution in the deduced amino acid sequence of the D1 protein, and the second mutation was silent and encoded for a proline at position 279 in both R and S populations. The Val₂₁₉Ile substitution in the psbA gene is most likely the cause of this Powell amaranth population resistance to linuron and other PSII inhibitors. This is the first recorded instance of a Val₂₁₉Ile substitution in an Amaranthus species.

Nomenclature: Linuron; green pigweed (Canada); Powell amaranth (USA); Amaranthus powellii S.; Wats; carrot; Daucus carota L.