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.