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ABSTRACT Quinclorac is a systemic herbicide absorbed by germinating seeds, roots and leaves of seedlings. It is a selective compound for crops such as rice, canola, barley, corn, sorghum, and pasture. Quinclorac can be used to control various monocots and dicotyledonous weed species. The biochemical function of this herbicide in the plant has intrigued scientists for nearly four decades. The objectives of this review are to present evidence of three hypotheses on the biochemical functioning of quinclorac and to propose an integrative mode of action. The first theory on the mode of action of quinclorac is supported by evidence of inhibition of incorporation of C14-glucose into cellulose and hemicellulose, thus, affecting the cell wall synthesis. The second hypothesis suggests that quinclorac acts as an auxin in broadleaved weed species. In grass species, however, this herbicide appears to stimulate the activity of the 1-aminocyclopropane-1-carboxylate synthase enzyme and, subsequently, to increase the ethylene production; also, it seems to increase the cyanide acid content to phytotoxic levels. A third hypothesis to explain the harmful effect in some plant species is the formation of reactive oxygen species (ROS). Apparently, these processes are not mutually exclusive; therefore, an integrative theory for the action of quinclorac is suggested. It is theorized that the aforementioned biochemical activities are interconnected and can be the phytotoxic backbone to explain the herbicidal effect depending on the plant species and the plant growth stage, among other factors.
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