A search for growth inhibitors in rice root exudates was undertaken in order to clarify the allelopathic system in rice (Oryza sativa L.). Rice seedlings inhibited the growth of cress (Lepidium sativum L.) and lettuce (Lactuca sativa L.) seedlings when the cress and lettuce were grown with rice seedlings. The putative compound causing the inhibitory effect of rice seedlings was isolated from their culture solution, and the chemical structure of the inhibitor was determined by spectral data as momilactone B. Momilactone B inhibited the growth of cress and lettuce seedlings at concentrations greater than 3 and 30 microM, respectively. The concentration of momilactone B was 3.4 and 1.1 nmol per seedling in the culture solutions of husked and non-husked rice seedlings, respectively. These results suggest that rice seedlings may release momilactone B into the environment and the stress caused by the husk-treatment may increase the amount of momilactone B released. Thus, momilactone B may play an important role in rice allelopathy.

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A new allelopathic substance that promoted the shoot growth of different plant species but inhibited the root growth was isolated as an amorphous powder from mucilage of germinated cress (Lepidium sativum L.) seeds. This substance was identified as sodium 2-O-rhamnopyranosyl-4-deoxy-threo-hex-4-enopyranosiduronate (designated lepidimoide) from the mass and the nuclear magnetic resonance and infrared spectra coupled with some chemical evi- Fig. 1). The allelopathy was not caused by contact with cress seeds, but was due to the mucilage of the germinated cress seeds, since seeds or seedlings of different species placed apart from the cress seeds on an agar plate also showed the effect (2). Longman and Callow (3) and Ray et al. (5) reported that the mucilage of germinated cress seeds contained the polysaccharides that reduced binding of Pythium aphanidermatum to cress roots. However, our recent studies demonstrated that the allelopathic substance(s), the Mr of which was below 5 x 103, was different from the mucilage polysaccharides (1).In this article, we report isolation and identification of an allelopathic substance from mucilage of germinated cress seeds and also report on some preliminary aspects of its biological activity. MATERIALS AND METHODS Plant MaterialThree thousand cress seeds (Lepidium sativum L.) were allowed to imbibe in distilled-deionized H20 for 1 h and put on a stainless steel net (3 mm mesh) in a stainless steel tray (40 x 40 x 3 cm3) containing 1.6 L of H20. The seeds on the net, in contact with the H20, were cultured at 250C in the dark for 2 d. The culture solution was aerated with an air pump during the culture period. PurificationThe culture solution was filtered through Toyo No. 1 filter paper and evaporated to dryness in vacuo at 350C. The concentrate was divided into acetone-soluble and -insoluble fractions. The biological activity of growth promotion of Amaranthus caudatus L. hypocotyls and growth inhibition of the roots was detected in the acetone-insoluble fraction. This fraction was dissolved in 10 mL of H20 and was then fractionated into three parts, of Mr above 105, from 105 to 5 X 103, and below 5 x 103 by molecular exclusion chromatography (Mol cut, Millipore Corp.). The biological activity was detected in the fraction of Mr below 5 x 103, and this was evaporated to dryness in vacuo at 350C. HPLCThe concentrate (approximately 150 mg) dissolved in H20 was purified by reverse phase HPLC (Waters, ,uBondasphere 5 ,um C18-100A; 19 mm x 15 cm; 100% H20, flow rate 5 mL/min; 214 nm detector). The biological activity was found in fractions with retention times of 5 to 8 min. The eluate was concentrated in vacuo at 350C and further purified by a second HPLC step (YMC Packed Column AQ-324 S-5 120A ODS; YMC Co. Ltd., Kyoto, Japan; 100% H20, 1 mL/min, 214 nm detector). The eluate at retention time 17.0 to 17.8 min showed biological activity and was evaporated to dryness in vacuo at 350C. It gave 6.5 mg of an amorphous powder.We checked whether its occurrence could be ascribed to con...

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