New insight into Poa annua control with clethodim from the dose-response study using several grass species and ACCase...

Poa annua L., a widespread allotetraploid weed, possesses both Leu₁₇₈₁-resistant and Ile₁₇₈₁-susceptible ACCase genes. This study aimed to elucidate how P. annua is controlled by the ACCase inhibitor clethodim, despite its resistance to many other ACCase-inhibiting herbicides. We conducted whole-plant and enzyme-level dose-response assays on P. annua, resistant fescue species (Festuca rubra and F. brevipila) with only Leu₁₇₈₁-resistant ACCase, and susceptible species (Poa pratensis and Lolium multiflorum) with only Ile₁₇₈₁-susceptible ACCase. We also investigated the expression ratio of the two ACCase genes in P. annua. The enzyme assay revealed that P. annua's resistance was intermediate between susceptible and resistant species, with a double-sigmoid dose-response curve indicating a functional 50:50 ratio of resistant and susceptible enzymes, i.e., the activity of the susceptible enzyme dilutes that of the resistant enzyme, leading to reduced resistance (the so-called dilution effect). This ratio aligns with the gene expression levels. At the whole-plant level, clethodim was effective against P. annua, but not effective against two fine fescue species at the standard dose. P. annua's dose response fell between that of susceptible and resistant species, though leaning closer to the resistant fescues. Considering the enzyme assay results, the dilution effect may also be involved at the whole-plant level. In any case, the registered dose of clethodim can be redefined as the one controlling P. annua, being set close to its ED₉₀ and located within the narrow margin of dose-response difference between the two grass groups.