Imazethapyr resistance in common sunflower (Helianthus annuus)

Resistance to imazethapyr was identified in a population of common sunflower that had been treated with imazethapyr for seven consecutive years. The imazethapyr resistant biotype of common sunflower was approximately 170 times more resistant to imazethapyr than the susceptible biotype based on the rate required for 25% control. Resistance was due to altered acetolactate synthase (ALS) that is less sensitive to imazethapyr. The imazethapyr concentration required to inhibit in vitro ALS activity by 25% was 210-fold higher in the resistant biotype than in che susceptible biotype. Differences in absorption, translocation, and metabolism of imazethapyr in common sunflower biotypes were not sufficient to explain the resistance to imazethapyr.