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Polycavernoside E (1), a new polycavernoside analog, was isolated from a marine Okeania sp. cyanobacterium. The relative configuration was elucidated primarily by analyzing the two dimensional nuclear magnetism resonance (2D NMR) data. The absolute configuration was clarified by comparing the electronic circular dichroism (ECD) data of 1 with those of known analogs. Polycavernoside E (1) exhibited moderate antitrypanosomal activity against Trypanosoma brucei rhodesiense. Furthermore, the isolation of polycavernoside E (1) from marine cyanobacteria provides additional evidence that marine cyanobacteria, and not red algae, are responsible for the biosynthesis of polycavernosides.

Logging operations of Liriodendron tulipifera L., as timber trees, and fallen leaves in autumn from ornamental trees produce a large amount of leaf waste. In this study, the allelopathy of L. tulipifera fresh and fallen leaves was investigated for the development of potential applications of leaf waste. The extracts of fresh and fallen leaves of L. tulipifera showed growth inhibitory activity against weed species, Vulpia myuros (L.) C.C.Gmel., Echinochloa crus-galli (L.) P.Beauv., and Lolium multiflorum Lam., under laboratory conditions. The powder of L. tulipifera fresh and fallen leaves also inhibited the germination of E. crus-galli under greenhouse conditions. A potent allelochemical was isolated from fresh and fallen leaf extracts through a bioassay-guided separation process, and was identified as lipiferolide. Lipiferolide inhibited the growth of L. multiflorum and Lepidum sativum in a concentration-dependent manner. This investigation suggests that the leaf waste of L. tulipifera from logging operations and fallen leaves is potentially useful for the purpose of weed control, such as through the use of soil additive materials from leaves or the creation of foliar spray from leaf extracts. The development of weed control materials using L. tulipifera leaf waste may be a means with which to minimize waste, reducing environmental impacts and economic concerns.

Chamaecyparis obtusa (Siebold et Zucc.) Endl. is cultivated in the temperate region of East Asia for its high-quality and profitable timber. The timber-processing industry produces a large amount of waste, such as tree bark, branches, and leaves, and finding ways to minimize such waste is economically and environmentally efficient. In the present study, the allelopathic activity and allelochemicals of the C. obtusa leaves were investigated to develop potential application methods. C. obtusa leaves were phytotoxic and associated leaf extracts significantly suppressed the growth of the weeds; Echinochloa crus-galli, Phleum pratense and Lolium multiflorum under laboratory conditions. The leaf powder applied onto the soil surface also suppressed the germination of E. crus-galli and P. pratense under greenhouse conditions. Hinokiic acid and (+)-dihydrosesamin were isolated from the leaves and structurally identified. Both compounds suppressed the growth of cress and P. pratense in a concentration dependent manner. When the leaves of C. obtua were applied as a soil additive, hinokiic acid and (+)-dihydrosesamin in the leaves potentially cause the growth inhibitory activity by leaching into the soil. These experiments showed that the leaves and the leaf extracts of C. obtusa are phytotoxic. These experiments also demonstrated that the leaves and the leaf extracts of C. obtusa have growth inhibitory potential against several plant species, and the leaves produce allelochemicals. Therefore, the leaves and leaf extracts of C. obtusa may be useful for weed suppression purposes. The leaf biomass of C. obtusa is frequently large and the timber processing industry produces significant leaf waste. The development of weed control products using waste leaves may be a solution to minimize the timber processing waste to reduce environmental impact and provide economic value. However, leaf material should be evaluated for weed suppression and herbicidal activity as a soil additive and also possible as a foliar spray under field conditions.

Plant parts and extracts that are rich in bioactive substances with allelopathic potential can be explored as a possible alternative to herbicides for natural weed control in sustainable agriculture. In the present study, we investigated the allelopathic potential of Marsdenia tenacissima leaves and its active substances. Aqueous methanol extracts of M. tenacissima showed significant inhibitory activities against the growth of lettuce (Lactuca sativa L.), alfalfa (Medicago sativa L.), timothy (Phleum pratense L.), and barnyard grass (Echinochloa crusgalli (L.) Beauv.). The extracts were purified through various chromatography steps, and one active substance was isolated and determined by spectral data to be a novel compound, assigned as steroidal glycoside 3 (8-dehydroxy-11β-O-acetyl-12β-O-tigloyl-17β-marsdenin). Steroidal glycoside 3 significantly inhibited the seedling growth of cress at a concentration of 0.03 mM. The concentrations needed for 50% growth inhibition of the cress shoots and roots were 0.25 and 0.03 mM, respectively. These results suggest that steroidal glycoside 3 may be responsible for the allelopathy of M. tenacissima leaves.

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.

Osmanthus fragrans Lour. has been cultivated for more than 2500 years because of the fragrance and color of the flowers. The flowers and roots have been used in tea, liquors, foods, and traditional Chinese medicine. The species contains more than 180 compounds including terpenoids, phenylpropanoids, polyphenols, flavonoids, and sterols. However, there has been limited information available on the allelopathic properties and allelopathic substances of O. fragrans. We investigated the allelopathy and allelopathic substances of O. fragrans and Osmanthus heterophyllus (G.Don) P.S. Green, as well as Osmanthus × fortunei Carrière, which is the hybrid species between O. fragrans and O. heterophyllus. The leaf extracts of O. fragrans, O. heterophyllus, and O. × fortunei suppressed the growth of cress (Lepidium sativum L.), alfalfa (Medicago sativa L.), Lolium multiflorum Lam., and Vulpia myuros (L.) C.C.Gmel with the extract concentration dependently. The extract of the hybrid species O. × fortune was the most active among the extracts. The main allelopathic substances of O. × fortunei and O. fragrans were isolated and identified as (+)-pinoresinol and 10-acetoxyligustroside, respectively. (+)-Pinoresinol was also found in the fallen leaves of O. × fortunei. Both compounds showed an allelopathic activity on the growth of cress and L. multiflorum. On the other hand, several allelopathic substances including (+)-pinoresinol may be involved in the allelopathy of O. heterophyllus. O. fragrans, O. heterophyllus, and O. × fortunei are evergreen trees. but their senescent leaves fall and cover the soil under the trees. It is possible that those allelopathic substances are liberated through the decomposition process of the leaves into their rhizosphere soil, and that they accumulate in the soil and provide a competitive advantage to the species through the inhibition of the growth of the neighboring competing plants. Therefore, the leaves of these Osmanthus species are allelopathic and potentially useful for weed management options in some agriculture settings to reduce commercial herbicide dependency for the developing sustainable agriculture systems.

The phytotoxic potential of plants and the effect of their active components on another plant species is being explored as a potential alternative to synthetic herbicides for weed control. In the current study, we investigated the phytotoxic potential of the leaves of Dregea volubilis (L.f.) Benth. ex Hook.f. against four test plants [timothy (Phleum pratense L.), barnyard grass (Echinochloa crus-galli (L.) P. Beauv), lettuce (Lactuca sativa L.), and alfalfa (Medicago sativa L.)] and observed significant growth inhibition on those plants at concentrations >3 mg D.W. equivalent extract mL−1. A bioassay-governed purification of the D. volubilis extracts using different chromatography phases produced two growth inhibitory compounds, 3-hydroxy-α-ionone (compound 1) and 5-hydroxy-3,4-dimethyl-5-pentylfuran-2(5H)-one (compound 2). The compounds retarded the growth of barnyard grass and cress (Lepidium sativum L.) with I50 (concentration required for 50% growth suppression) values ranging from 0.098 to 0.450 mM for 3-hydroxy-α-ionone and 0.029 to 0.420 mM for 5-hydroxy-3,4-dimethyl-5-pentylfuran-2(5H)-one. Thus, the extracts and identified compounds may have the possibility to be utilized as bioagents for weed control.

Phytotoxic substances in plants that may serve as alternative natural herbicides for controlling weeds are required for sustainable agriculture. We explored the phytotoxic activities of aqueous methanol extracts of Senna garrettiana (Craib) Irwin & Barneby leaves and the active substances they contain. The results revealed that the S. garrettiana leaf extracts had significant phytotoxic effects on three dicotyledons (Lepidium sativum L., Lactuca sativa L., and Medicago sativa L.) and two monocotyledons (Phleum pratense L. and Lolium multiflorum Lam.). An bioassay-guided isolation process yielded three active substances; caffeic acid, methyl caffeate, and (S)-6-hydroxymellein inhibited the seed germination, seedling growth, and biomass accumulation of L. sativum in a concentration-dependent manner. Based on the concentration required for 50% growth inhibition (IC50), (S)-6-hydroxymellein had the highest inhibitory effects on L. sativum in all test parameters, followed by methyl caffeate and caffeic acid. The L. sativum roots were the most susceptible to (S)-6-hydroxymellein (IC50 = 383 µM) and caffeic acid (IC50 = 2627 µM), whereas methyl caffeate (IC50 = 1361 µM) had the greatest effect on the L. sativum shoots. Thus, three isolated compounds may explain the phytotoxic effects of the S. garrettiana extracts. Consequently, caffeic acid, methyl caffeate, and (S)-6-hydroxymellein could be potential candidates for the future production of bioherbicides.

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.

Medicinal plants are rich sources of bioactive substances that can be used to develop environmentally friendly weed control alternatives. Marsdenia tenacissima (Roxb.) Moon is a traditional medicinal plant well known for its pharmacological activities and several bioactive compounds. However, its allelopathy and related substances have not been reported. Hence, the present study was conducted to explore the allelopathic potential and substances from M. tenacissima leaves. Aqueous methanol extracts of M. tenacissima showed significant inhibitory activities against the growth of cress (Lepidium sativum L.) and Italian ryegrass (Lolium multiflorum Lam.). The extracts were purified through various chromatography steps, and two allelopathic substances were isolated and determined by spectral data to be steroidal glycoside 1 (5,6-dihydrogen-11α-O-acetyl-12β-O-tigloyl-17β-marsdenin), a novel compound, and steroidal glycoside 2 (5,6-dihydrogen-11α,12β-di-O-tigloyl-17β-marsdenin). Both compounds significantly inhibited the growth of cress seedlings. Steroidal glycoside 1 showed 1.6- and 4-times greater growth inhibitory potential against the cress shoots and roots than steroidal glycoside 2. The concentrations needed for 50% growth inhibition of the cress seedling shoots and roots were 0.46 and 0.03 mM for steroidal glycoside 1, respectively, and 0.74 and 0.12 mM for steroidal glycoside 2, respectively. Therefore, these results suggest that steroidal glycosides 1 and 2 may be responsible for the allelopathy of the M. tenacissima leaves.

Senna garrettiana (Craib) Irwin & Barneby (Fabaceae) is a medicinal plant known to be rich in biologically active compounds that could be exploited to produce bioherbicides. The present study was conducted to explore the allelopathic potential and phytotoxic substances of S. garrettiana. Extracts of S. garrettiana leaves were found to significantly inhibit the growth of Lepidium sativum L. and Echinochloa crus-galli (L.) P. Beauv. (p ≤ 0.05). The phytotoxic substances were isolated and identified as vanillic acid and ferulic acid by bioassay-directed fractionation and spectral data analysis. The two compounds were shown to significantly inhibit the seed germination, seedling growth, and dry biomass of L. sativum. Based on the concentration required for 50% growth inhibition (defined as IC50), the roots of L. sativum were the most sensitive to the compounds, and the inhibitory effect of ferulic acid (IC50 = 0.62 mM) was >1.3 times more potent than that of vanillic acid (IC50 = 0.82 mM). In addition, a mixture of the two compounds (0.3 mM) resulted in synergistic inhibitory activity against the L. sativum roots compared with the individual compounds. These results suggest that the extracts of S. garrettiana leaves and their phytotoxic compounds have potential as candidate natural herbicides.

Allelopathic plants and their related phytotoxic chemicals have been explored as alternatives for environmentally friendly weed management. Dregea volubilis (L.f.) Benth. ex Hook.f., a woody vine, is well-known for its uses in various therapeutic treatments, and several bioactive compounds have been isolated from this species. However, to date, no phytotoxic substances from D. volubilis have been reported. Hence, in this research, we aimed to investigate the allelopathic effect of D. volubilis and characterize its phytotoxic substances. Extracts of D. volubilis exhibited significant inhibitory activity against the seedling growth of Italian ryegrass (Lolium multiflorum Lam.) and cress (Lepidium sativum L.). The extracts were purified through several chromatography steps, yielding two phytotoxic substances, and using spectroscopy, their structures were determined as dehydrovomifoliol and loliolide. The compounds inhibited the Italian ryegrass and cress seedlings with the I50 (concentrations needed for 50% inhibition) values ranging from 0.022 to 0.102 mM, for loliolide and 3.24–4.60 mM, for dehydrovomifoliol. These results suggest that both phytotoxic compounds might be responsible for the allelopathy of D. volubilis, and they may be released into the soil through the decomposition of the D. volubilis leaf and probably act as allelopathic active substances. Therefore, the extracts of D. volubilis and its related phytotoxic compounds may be applied as biological agents for controlling weeds.

Metasequoia glyptostroboides Hu et W.C. Cheng is one of the oldest living conifer species, and it has remained unchanged for millions of years compared to its fossils from the Cretaceous period. The species are cultivated in the parks, gardens, and roadsides in many countries. We investigated the allelopathy and allelopathic substances in fallen leaves of M. glyptostroboides. An aqueous methanol extract of the fallen leaves inhibited the growth of cress (Lepidium sativum L.), lettuce (Lactuca sativa L.), alfalfa (Medicago sativa L.), Lolium multiflorum Lam., Phleum pretense L., and Vulpia myuros (L.) C.C.Gmel. The extract was then purified by several chromatographic steps, and two allelopathic substances were isolated and determined by spectral data to be (+)-rhododendrol and 9-epi-blumenol C. The compound inhibited the growth of cress and L. multiflorum. M. glyptostroboides is a deciduous perennial tree, and accumulation of its fallen leaves occur on the soil under the trees. Therefore, those allelopathic substances in the fallen leaves may be liberated into the rhizosphere soil during the decomposition process of fallen leaves and provide a competitive advantage for M. glyptostrob through the growth inhibition of competing plant species nearby. Therefore, M. glyptostroboides is allelopathic, and (+)-rhododendrol and 9-epi-blumenol C may be contribute to the allelopathy.