IMAZETHAPYR RESISTANCE IN COMMON SUNFLOWER (HELIANTHUS-ANNUUS)

Resistance to imazethapyr was identified in a population of common sun flower that had been treated with imazethapyr for seven consecutive ye ars. The imazethapyr resistant biotype of common sunflower was approxi mately 170 times more resistant to imazethapyr than the susceptible bi otype based on the rate required for 25% control. Resistance was due t o altered acetolactate synthase (ALS) that is less sensitive to imazet hapyr. The imazethapyr concentration required to inhibit in vitro ALS activity by 25% was 210-fold higher in the resistant biotype than in c he susceptible biotype. Differences in absorption, translocation, and metabolism of imazethapyr in common sunflower biotypes were not suffic ient to explain the resistance to imazethapyr.