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The allelopathic impact of aqueous extract (2.5 and 5%) and residues (2 and 4%) of five aquatic weeds namely Alternanthera philoxeroides, Alternanthera sessilis, Conyza stricta, Polygonum barbatum and Echinochloa crus-galli was investigated on the germination and early growth of wheat (Triticum aestivum). The aqueous extracts and residues of all tested weeds at different concentrations suppressed the germination/emergence percentage, mean germination/emergence time, and germination/emergence index and early seedling growth of wheat. Of the species tested, A. philoxeroides and A. sessilis showed the higher inhibition than C. stricta, P. barbatum and E. crus-galli. The results revealed that the inhibitory effect was proportional to the concentrations of the extracts and residues. Higher concentrations of extracts and residues had the stronger inhibitory effect. Aqueous extract caused more detrimental effect as compared to soil incorporated residues. Phytotoxins were identified as caffeic acid, chlorogenic acid, ferulic acid, 4-hydroxy-3-methoxybenzoic acid, gallic acid, p-coumaric acid, m-coumaric acid, syringic acid and vanilic acid. It can be concluded that rice aquatic weeds contain allelopathic compounds in their tissues which may cause allelopathic effects on subsequent wheat crop in rice-wheat cropping.

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