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ALS-Inhibiting Herbicides

An introduction to ALS-inhibiting herbicides

Carl E. Whitcomb

Lacebark Inc., Publications and Research, Stillwater, Oklahoma

Herbicides that inhibit acetolactate synthase (ALS), the enzymecommon to the biosynthesis of the branch-chain amino acids (valine,leucine, and isoleucine), affect many species of higher plantsas well as bacteria, fungi, yeasts, and algae. The novel mechanismof action attributed to ALS inhibitors, their effect on thereproduction of some plant species, their potency at extremelylow concentrations, and the rapid evolution of resistance tothese herbicides in some plants and microorganisms are characteristicsthat set ALS inhibitors apart from their predecessors. Thisclass of chemicals affects seedling growth. Older plants exhibitvaried signs of malformation, stunting, and reduced seed production.These herbicides are so potent that they can affect plants atlevels that are undetectable by any standard chemical protocol.Weeds quickly become resistant to ALS inhibitors, presumablybecause these herbicides have a single mode of action and becausemany have long residual activity. Concern now is directed towardsdeveloping the technology to detect very low concentrationsof ALS inhibitors in the environment and their indirect effectson plant and animal health.

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