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Agronomy QUESTION #2078
Question 1
ALS (acetolactate synthase) inhibitor herbicides, which include sulfonylureas, imidazolinones, and triazolopyrimidines, control weeds by:
  • Disrupting cell membrane integrity causing rapid desiccation
  • Inhibiting the first enzyme in branched-chain amino acid (valine, leucine, isoleucine) biosynthesis, arresting cell division in meristematic tissue — used at very low doses (grams per hectare) due to high enzyme affinity✔️
  • Blocking photosystem II electron transport causing photo-oxidative damage
  • Mimicking auxin hormones at supra-optimal concentrations causing uncontrolled growth and plant death
Correct Answer Explanation
ALS inhibitors block acetolactate synthase, preventing synthesis of branched-chain amino acids. Cell division in apical meristems ceases within days. They are active at extremely low doses (5–75 g a.i./ha for sulfonylureas) because of high target affinity. ALS mutation is the most common mechanism of herbicide resistance worldwide due to the single amino acid substitutions that confer resistance.