| Abstract
| - 3-Isopropyl-5-acetoxycyclohexene-2-one-1 (1), momilactone B (2), and 5,7,4‘-trihydroxy-3‘,5‘-dimethoxyflavone (3) were isolated and identified from an allelopathic rice accession PI312777. Thesethree compounds at low concentrations could inhibit the growth of weeds Echinochloa crusgalli andCyperus difformis associated with rice, especially mixtures of the compounds had stronger inhibitoryactivity than did individual compounds. Studies with hydroponic culture, continuous root exudatestrapping system (CRETS), and direct resin adsorption methods showed that a total of 7.6 n moles 1,2, and 3 were exuded from living roots of each seedling into the environment at 10 days after seedlingswere transplanted. Furthermore, 1, 2, and 3 were found in the soil growing PI312777 seedlings atday 15 after seedlings emergence and reached a total of 39.5 μg/g soil at day 30. The results indicatedthat PI 312777 seedlings could release sufficient quantities of 1, 2, and 3 into the environment to actas allelochemicals inhibiting the growth of associated weeds. Investigations on the distribution of 1,2, and 3 in PI 312777 plant, and its root exudates showed that the levels of 1, 2, and 3 were significantlyhigher in the shoots and root exudates than in the roots, and only trace 1 was observed in the roots.The results suggest that the roots of rice seedlings are not major site of synthesis or accumulation1, 2, and 3, but a pathway for their release into the environment. The levels of 1, 2, and 3 in the rootexudates were over 2-folds higher under direct resin adsorption than under hydroponic culture andCRETS, and hence, it is the preferred method to collect and identify active allelochemicals in riceexudates in future studies on rice allelopathy. Keywords: Oryza sativa L.; allelochemical; release; root exudates; weed-suppressive activity
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