COMPARATIVE ALLELOPATHIC POTENTIAL OF NATIVE AND INVASIVE WEEDS IN RICE ECOSYSTEM

Invasive weeds pose serious threat to aquatic ecosystems such as wet land paddy rice. An investigation was made to compare the allelopathic effects of aqueous extracts and residues of two invasive weeds viz., Alternanthera philoxeroides and A. sessilis with those of three native weeds viz., Conyza stricta, Polygonum barbatum and Echinochloa crus-galli of rice (Oryza sativa L.) in Pakistan. All weeds under study showed phytotoxic effects on germination and seedling growth of rice through their aqueous extracts as well as residues to variable degree compared with control treatment. However, significantly lower germination percentages (10 and 35%), germination/emergence index (0.5 and 0.4), shoot lengths (2 and 7 cm), root lengths (1.5 and 1.9 cm), seedling biomass (5 and 6 mg), seedling vigor indices (25 and 100) and higher mean germination/emergence times (6.8 and 8 days) were observed in case of 5% aqueous extract of A. sessilis and 4% residue of A. philoxeroides, respectively. Overall, phyto-inhibitory effects of water extracts were more severe than residues. The highest suppressive action of A. philoxeroides and A. sessilis seem not only due to their higher total phenolic contents (116 and 106 mg L^-1) but complex interaction of potent phenolic compounds namely 4-hydroxy-3-methoxybenzoic acid, chlorogenic acid, ferulic acid, gallic acid, m-coumaric acid, p-coumaric acid and vanillic acid as shown by their HPLC analysis. It can be concluded that these two invasive weed species are bigger threat to local wet land rice ecosystems and may result in greater yield losses of rice crop in the country.