Factors Affecting the Phytotoxicity of Norflurazon

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Abstract
Norflurazon [4-chloro-5-(methylamino)-2-(α,α,α-trifluoro-m-tolyl)-3(2H)-pyridazinone] decreased the total chlorophyll content and the chlorophyll a/b ratio of nine plant species. Grain sorghum [Sorghum bicolor(L.) Moench.], wheat (Triticum aestivumL.), and sicklepod (Cassia obtusifoliaL. ♯3CASOB) were the most susceptible plants, and cotton (Gossypium hirsutumL.) was the most tolerant. The soil properties most closely correlated with norflurazon activity were organic matter content and clay component, not clay content. A high treatment rate was necessary for effective control of plant growth in soil high in organic matter content and high in montmorillonite or vermiculite. Approximately twice as much norflurazon was required to reduce chlorophyll content 50% when applied to the soil preplant incorporated as was required when applied preemergence.