ABSTRACT
Allelochemicals play an important role in natural and managed ecosystems. Food crop Oryza sativa L. can produce and release allelochemicals participating in its defense against weeds. However, it remains obscure which allelochemicals in rice are predominantly involved in defense mechanisms against weeds. In this study, many kinds of compounds were systematically isolated and identified from allelopathic rice PI312777 seedlings. Among them, alkaloids, alkylresorcinols, cyclohexenone and urea derivatives, flavonoids and their glucosides, diterpenoids and triterpenes had inhibitory activities on the growth of Echinochloa crus-galli, Cyperus difformis and Leptochloa chinensis weeds associated with rice. However, the presence of these compounds in allelopathic rice seedlings does not necessarily mean that they can be released into the environment to demonstrate their allelopathic effects under natural field conditions. Accordingly, allelochemicals released from selected rice seedlings into the soil were determined and analyzed by LC/MS and HPLC. As a result, phenolic acids, flavonoids, diterpenoids and cyclohexenone and urea derivatives were found in soil-grown allelopathic rice seedlings. Particularly, 3-isopropyl-5-acetoxy-cyclohexene-2-one-1, momilactone B and 5,7,4’-trihydroxy-3,5-dimethoxyflavone occurred in the soil at day 15 after rice seedlings emergence, and reached the concentration initiated for inhibition on associated weeds at day 30 after rice emergence, while other compounds were not detected or they were present as trace only in the soil. These results suggest that 3-isopropyl-5-acetoxycyclohexene-2-one-1, momilactone B and 5,7,4’-trihydroxy-3,5-dimethoxyflavone are 268allelochemicals involved in rice allelopathy and allelopathic rice seedlings can establish their own weed defense mechanism through production and release of these allelochemicals.
