Twelve populations of eastern black nightshade from different locations in Ontario are resistant to imazethapyr. This study aimed at determining the molecular basis of resistance in these populations and the activity of the resistant acetohydroxyacid synthase (AHAS) enzyme compared to that of the sensitive AHAS in response to different herbicides and branched-chain amino acid concentration. The results of partial AHAS sequencing indicated that all resistant populations had a cytosine₃₃₁ to thymine substitution coding for an alanine₂₀₅ to valine substitution. In vitro AHAS enzyme assays of one resistant population showed that the specific activity of the resistant enzyme was 56% less than that of the susceptible enzyme. AHAS from the resistant population was 72-, 70-, and 64-fold less sensitive than that of the susceptible population to imazethapyr, imazamox, and primisulfuron, respectively. Furthermore, the resistant enzyme was less sensitive to feedback inhibition from branched-chain amino acids compared to the susceptible enzyme. Results confirmed that resistance in resistant populations of eastern black nightshade was conferred by target-site modification and that the Ala₂₀₅Val substitution alters the kinetics and regulation of branched-chain amino acid biosynthesis.

Nomenclature: Imazethapyr; imazamox; primisulfuron; eastern black nightshade, Solanum ptychanthum Dunal SOLPT.