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The fruits of Piper retrofractum Vahl. contain a variety of compounds, but no phytotoxic compound with plant growth inhibitory activity has been reported. Therefore, the phytotoxic activity of P. retrofractum fruit extracts was investigated and found the reduction of seedling growth of cress, lettuce, alfalfa, barnyard grass, Italian ryegrass, and jungle rice. Seven bioactive compounds were isolated from the fruit extracts and identified as 3-phenylpropanoic acid (1), (2E,4E)-methyl piperate (2), (2E,4Z)-methyl piperate (3), piperlonguminine (4), dihydropiperine (5), isochavicine (6), and piperine (7). There have been no reports for their phytotoxic activity. The activity of these compounds was determined against the germination and seedling growth of cress and barnyard grass. The total germination percentage (GP) of cress was significantly inhibited by 1, 2, and 6 (40–100% inhibition), while its germination index or progressive total of cumulative germination (GI) was the most sensitive to 1, leading to the longest time required for 50% germination (T50) at 57 h behind control. However, the seedling growth of cress was highly inhibited by 6 with I50 values of 11 µM. The compounds 1 and 5–7 showed significant inhibition on GP of barnyard grass by 55–47%, while 5 and 1 effectively reduced GI and extended T50 to 42–48 h behind control, and 7 showed the most growth inhibition with I50 values of 37 µM. The inhibitory effects of the seven compounds indicate the phytotoxic property of P. retrofractum fruits, which 1, 5–7 may be the main compounds responsible for the phytotoxicity.

Heliotropium indicum L. belongs to the family Boraginaceae. The plant has been used as a folk medicine because it contains substances of various biological activities. It is also identified as a common weed which grows wildly in crop fields in tropical and subtropical regions of the world. However, there is little information on the allelopathic effect in this plant. Therefore, this study was undertaken to investigate the growth inhibitory effect and to identify the growth inhibitory substances in H. indicum. An aqueous methanol extract of H. indicum inhibited shoot and root growth of barnyard grass, foxtail fescue, timothy, cress, lettuce and rapeseed at concentrations higher than 10 mg dry weight equivalent extract/mL. The concentrations required for 50% growth inhibition (I50) of those test plants ranged from 3-282 mg dry weight equivalent extract/mL. The extract was then separated using a sequence of chromatographic fractionations and a growth inhibitory substance was isolated and identified by spectral analysis as methyl caffeate. Methyl caffeate inhibited the growth of lettuce and foxtail fescue at concentrations higher than 1.0 mM. The results suggest that methyl caffeate may contribute to the growth inhibitory effect of H. indicum and may play an important role in the allelopathic effect of H. indicum.

甲農博第1006号

Citronella grass (Cymbopogon nardus) is a perennial herb. Many researchers have studied the exploitation of the active compounds in this plant for medicinal purposes, but there have been no studies on its phytotoxic compounds. Therefore, we investigated the phytotoxic active compounds in the roots of C. nardus. A significant inhibition was observed on the germination of cress (Lepidium sativum L.), lettuce (Lactuca sativa L.), barnyard grass (Echinochloa crus-galli L.), and Italian ryegrass (Lolium multiflorum Lam.) by the aqueous methanol extract of C. nardus roots. The extract decreased the total germination percentage (GP) and germination index (GI, progressive total of daily cumulative germination), and extended the time required for 50% germination (T 50) of all test species. A phytotoxic compound of the extract was isolated and identified as N-octanoyl tyramine by spectral analysis. This compound showed the inhibition on the germination of cress and barnyard grass. The decline in GP, GI, and T 50 was observed at concentrations of 30–2000 μM of N-octanoyl tyramine. The compound also showed the significant inhibition on the growth of cress and barnyard grass seedlings at concentrations greater than 100 and 300 μM, respectively. This study suggests that C. nardus could produce phytotoxic compounds, and N-octanoyl tyramine may be responsible for its phytotoxic activity.

In this study, an allelopathic substance was isolated from an aqueous methanol extract of Hibiscus sabdariffa L. by column chromatography and reverse phase HPLC. The chemical structure of the substance was determined by 1H NMR spectroscopy and mass spectrometry as trimethyl allo-hydroxycitrate. Trimethyl allo-hydroxycitrate inhibited the growth of cress hypocotyls and roots at concentrations greater than 10 mM. The concentrations required for 50% growth inhibition of the hypocotyls and roots of cress were 20.3 and 14.4 mM, respectively. The inhibitory activity of trimethyl allo-hydroxycitrate suggests that the substance may act as an allelopathic substance of H. sabdariffa.

A phytotoxic compound was isolated from the root extracts of Cymbopogon nardus by bioassay-guided purification and characterized to be myrislignan. Myrislignan significantly inhibited seedling growth of cress at a concentration greater than 100 μM (p < 0.05). The concentrations required for 50% growth inhibition of myrislignan on hypocotyl and root growth of cress were 429 μM and 517 μM, respectively. The inhibition on seedling growth indicated that myrislignan had a phytotoxic activity and may contribute to the phytotoxic effect of C. nardus.

The inhibitory effects of aqueous methanol extract of Cymbopogon nardus (L.) Rendle were determined on seedling growth of eight test plant species: alfalfa ( Medicago sativa L.), cress ( Lepidum sativum L.), lettuce ( Lactuca sativa L.), rapeseed ( Brassica napus L.), barnyard grass ( Echinochloa crus-galli L.), Italian ryegrass ( Lolium moltiflorum Lam.), jungle rice ( Echinochloa colonum (L.) P. Beauv.) and timothy ( Phleum pratense L.). The bioassay was conducted with four extract concentrations (0.01, 0.03, 0.1 and 0.3 g dry weight equivalent extract/mL). The extracts inhibited significantly shoot and root growth of four test plants such as cress, lettuce, rapeseed and Italian ryegrass at the concentration ? 0.03 g dry weight equivalent extract/mL. The inhibitions of shoots and roots increased with increasing extract concentrations. The concentrations required for 50% growth inhibition of all test plants ranged 0.007-0.090 g dry weight equivalent extract/mL. Roots of all test plants were more sensitive to the extract than their shoots. Lettuce was most sensitive, follows by cress and timothy. The results suggest that C. nardus may have allelopathic compounds and may be a candidate for isolation and identification of allelopathic compounds to develop an alternative weed management option.

In this study, an allelopathic substance was isolated from an aqueous methanol extract of Hibiscus sabdariffa L. by column chromatography and reverse phase HPLC. The chemical structure of the substance was determined by1H NMR spectroscopy and mass spectrometry as trimethyl allo-hydroxycitrate. Trimethyl allo-hydroxycitrate inhibited the growth of cress hypocotyls and roots at concentrations greater than 10 mM. The concentrations required for 50% growth inhibition of the hypocotyls and roots of cress were 20.3 and 14.4 mM, respectively. The inhibitory activity of trimethyl allo-hydroxycitrate suggests that the substance may act as an allelopathic substance of H. sabdariffa.

In this study, leaves, stalks and roots of Cymbopogon nardus were separately evaluated to determine the most active parts that contained the strong growth inhibitory activity. Each aqueous methanol extracts of Cymbopogon nardus were determined their allelopathic activity by using six test plant species; alfalfa (Medicago sativa L.), cress (Lepidum sativum L.), lettuce (Lactuca sativa L.), barnyard grass (Echinochloa crus-galli L.), Italian ryegrass (Lolium moltiflorum Lam.) and jungle rice (Echinochloa colonum (L.) P. Beauv.). Four extract concentrations (0.01, 0.03, 0.1 and 0.3 g dry weight equivalent extract/mL) were used for the bioassay. The results showed that these three extracts have inhibitory activity and the percent inhibition increased concentration dependently. However, the inhibitory activity of leaf and root extracts was more effective than stalk extract at 95% level of significance. Barnyard grass, Italian ryegrass and jungle rice were the most sensitive to the leaf, stalk and root extracts, respectively. The concentrations required for 50% growth inhibition of C. nardus leaf, stalk and root extracts on all test plants were 0.000-0.025, 0.009-0.077 and 0.003- 0.023 g dry weight equivalent extract/mL, respectively. In addition, separation of these extracts through silica gel column indicated that root extract contained the most active fractions with strong growth inhibition. The present results suggest that C. nardus may have allelopathic compounds and the root extracts have the greatest inhibitory activity. Studies are in progress for the isolation and identification of allelopathic compounds in aqueous methanol extracts of C. nardus roots for the development of natural herbicides.