Rice allelopathic potential and its modes of action on Barnyardgrass (Echinochloa crus-galli)

Six lines of rice (AC1423, Kouketsumuchi, Taichung Native 1, Woo Ca Chin Yu , YH-1 and CN-15) showing higher biological suppression of barnyardgrass in field trials and control (Nancheum), were screened against barnyardgrass i n pot culture in green house. In petri-plate bioassays, the aqeous extracts of different rice accessions varied in their inhibitory allelopathic effec ts on barnyardgrass and the inhibition followed the order:Woo Co Chin Yu > Kouketsumuchi > AC1423 > CN-15 > Taichung Native I > YH-1. The inhibition i n weed growth increased with increasing concentration of extract. The radic al length was found more sensitive to aqueous extract than hypocotyl length . The phytotoxicity was density-dependent i.e. the growth inhibition was ne gatively and significantly correlated with barnyardgrass densities in pots. Such a result was contrary to the predicted effect of resource competition and provided an evidence for a hypothesis of chemical interference. Bioche mical analysis demonstrated that aqueous extracts significantly blocked act ivity of superoxide dismutase and catalase, thereby increasing free radical s, consequently resulting in higher lipid peroxidation and altered membrane potential in barnyardgrass seedling. Activities of ATPase and amylase at d ifferent germination time courses of barnyardgrass were also significantly inhibited by aqueous extracts, but the reverse was true in peroxidase and I AA oxidase. This was attributed to combined action of allelopathic compound s, suggesting that phytotoxicity of allelopatliic extracts might result fro m generalized cellular disruption, rather than from Specific mechanisms.