Polygonum chinense Linn. is a medicinal and invasive plant that belongs to the family Polygonaceae. The pharmacological activities and phytochemical constituents of Polygonum chinense are well reported, but the allelopathic effects and potent allelopathic substances of P. chinense remain to be investigated. Hence, this experiment was conducted to separate and characterize potentially allelopathic substances from an extract of the Polygonum chinense plant. The Polygonum chinense plant extracts highly suppressed the growth of cress (Lepidium sativum L.), lettuce (Lactuca sativa L.), barnyard grass (Echinochloa crusgalli (L.) P. Beauv.), and timothy grass (Phleum pratense L.) seedlings in a species- and concentration-dependent way. Two active substances were separated using a series of purification procedures and determined through spectral analysis as (−)-3-hydroxy-β-ionone and (−)-3-hydroxy-7,8-dihydro-β-ionone. These two compounds significantly suppressed the seedling growth of Lepidium sativum (cress) at concentrations of 0.01 and 1 mM, respectively. The extract concentrations necessary for 50% growth inhibition (I50 values) of the cress hypocotyls and roots were 0.05 and 0.07 mM for (−)-3-hydroxy-β-ionone, respectively, and 0.42 and 1.29 mM for (−)-3-hydroxy-7,8-β-ionone, respectively. These findings suggest that these two compounds are in charge of the inhibitory effects of the Polygonum chinense extract and may serve as weed control agents.

The plant Plumbago rosea Linn., belonging to the Plumbaginaceae family, is an important medicinal herb distributed in part of Southeast Asia, and there are many reports of its pharmacological properties. However, the allelopathic activities of P. rosea have not been examined. Thus, the present study was conducted to assess the allelopathic activity of P. rosea and to identify its allelopathic substances. The aqueous methanol stem extract of P. rosea significantly suppressed the seedling growth of barnyard grass (Echinochloa crus-galli L. P. Beauv.), Italian ryegrass (Lolium multiflorum Lam.), timothy (Phleum pretense L.), cress (Lepidium sativum L.), lettuce (Lactuca sativa L.), and alfalfa (Medicago sativa L.). The extract of P. rosea was then purified through chromatographic steps, and two active substances were isolated and determined as 7,4′,5′-tri-O-methyl dihydroquercetin and 7,4′,5′-tri-O-methylampelopsin. The two compounds significantly inhibited the seedling growth of cress, with 7,4′,5′-tri-O-methylampelopsin showing a greater inhibitory effect than 7,4′,5′-tri-O-methyl dihydroquercetin. This result may be due to the 3′-OH group in 7,4′,5′-tri-O-methylampelopsin. The effective concentrations of both compounds required for 50% growth inhibition (EC50 values) of cress seedlings were 0.24 mM and 0.59 mM for root and shoot, and 0.07 mM and 0.21 mM, respectively. These findings suggest that the two compounds may contribute to the allelopathic effect of P. rosea and could be used as a natural source of allelopathic substances.

Double rice‐cropping areas in the dry and wet seasons have been increasing in Myanmar, and hybrid rice has been cultivated throughout the country. However, weeds are often not adequately managed. Therefore, the objective of this study was to investigate the effect of weed infestation on rice production of three hybrid varieties (Palethwe 1, Palethwe 3, and Palethwe Basmati) and three inbred varieties (Manaw Thu Kha, Shwe Thwe Yin, and Sin Thwe Latt) in the dry and wet seasons in a lowland rice field. In the dry and wet seasons, 19 ‐ 20 weed species were naturally infested into the fields. Limnocharis flava was the most abundant weed in both seasons. Weed dry weight and rice grain yield showed a negative linear relationship in both seasons. Manaw Thu Kha showed the highest yield loss in the un‐weeded fields in both seasons, while Sin Thwe Latt and Palethwe Basmati showed the lowest yield loss in the dry season and wet season, respectively. Sin Thwe Latt recorded the lowest yield loss of 30.21% on average in both seasons, and the highest yield loss of 60.37% was recorded for Manaw Thu Kha. The average rice grain yield loss of all the varieties in the un‐weeded fields was 64.37 and 33.66% for the dry and wet seasons, respectively. Thus, the yield loss in the dry season was 1.9‐times greater than that in the wet season. The present research suggests that adequate weed management is necessary for rice cultivation, especially in the dry season in Myanmar.