Effect of Argemone mexicana L. on seed germination and early seedling growth of six crop species was investigated. Aqueous extract of A. mexicana inhibited germination, root and shoot growth of all the test species. The species showed differential response. Germination was reduced by the aqueous shoot extract in the order: pearlmillet > mustard > wheat> carrot> corn > turnip. Root length was reduced in the order:corn> pearl-millet> wheat > mustard > turnip> carrot, whereas shoot length was reduced in the order: pearl-millet> wheat> corn > turnip> mustard > carrot. Decaying A. mexicana in sandy loam soil at a rate of 5,10,20 g / 400 g soil substantialy inhibited germination and seedling growth of pearl-millet. Bioassay of the extract of A. mexicana revealed three phenolic inhibitors and paper chromatography revealed the presence of p-hydroxybenzoic, vanillic and salicylic acids.

The phytotoxicity of atrazine and three phenolic compounds (i.e., benzoic, p-coumaric and caffeic acids) was investigated. The germination of six test species was reduced by atrazine in the order: pearl-millet > wheat> turnip> carrot> corn> mustard. Whereas, the phenolic compounds affected the germination of pearl-millet in the order: caffeic acid> benzoic acid> p-coumaric acid. Atrazine in conjunction with the three phenolic compounds exhibited synergistic effect on the process of germination and seedling growth.

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