Identification of four allelopathic compounds including a novel compound from Elaeocarpus floribundus Blume and determination of their allelopathic activity

Isolation and identification of allelochemicals and their activities and functions

Allelopathy is the interaction between donor plants and receiver plants through allelochemicals. According to a great number of publications, allelopathy may be involved in several ecological aspects such as the formation of monospecific stands and sparse understory vegetation for certain plant species. Allelopathy also contributes to the naturalization of invasive plant species in introduced ranges. Autotoxicity is a particular type of allelopathy involving certain compounds. Many medicinal plants have been reported to show relatively high allelopathic activity. We selected plant species that show high allelopathic activity and isolated allelochemicals through the bioassay-guided purification process. More than 100 allelochemicals, including novel compounds have been identified in some medicinal and invasive plants, plants forming monospecific stands, plants with sparse understory vegetation, and plants showing autotoxicity. The allelopathic activity of benzoxazinones and related compounds was also determined.

Norsesquiterpenes from the Latex of Euphorbia dentata and Their Chemical Defense Mechanisms against Helicoverpa armigera

Euphorbia dentata (Euphorbiaceae), an invasive weed, is rarely eaten by herbivorous insects and could secrete a large amount of white latex, causing a serious threat to local natural vegetation, agricultural production and human health. In order to prevent this plant from causing more negative effects on humans, it is necessary to understand and utilize the chemical relationships between the latex of E. dentata and herbivorous insects. In this study, three new norsesquiterpenes (1-3), together with seven known analogues (4-10), were isolated and identified from the latex of E. dentata. All norsesquiterpenes (1-10) showed antifeedant and growth-inhibitory effects on H. armigera with varying levels, especially compounds 1 and 2. In addition, the action mechanisms of active compounds (1-3) were revealed by detoxifying enzyme (AchE, CarE, GST and MFO) activities and corresponding molecular docking analyses. Our findings provide a new idea for the development and utilization of the latex of E. dentata, as well as a potential application of norsesquiterpenes in botanical insecticides.

Phytotoxicity and Phytotoxic Substances in Calamus tenuis Roxb

Calamus tenuis is a shrub species distributed across South Asia. It grows well in diversified habitats and tends to dominate plants in the surrounding environment. The phytotoxicity of C. tenuis and the action of its phytochemicals against other plant species could explain its dominant behavior. Compounds with phytotoxic activity are in high demand as prospective sources of ecofriendly bioherbicides. Therefore, we investigated the phytotoxicity of C. tenuis. Aqueous methanol extracts of this plant species significantly limited the growth of four test plant species, two monocots (barnyard grass and timothy), and two dicots (alfalfa and cress), in a dose- and species-dependent manner. Bio-directed chromatographic isolation of the C. tenuis extracts yielded two major active substances: a novel compound, calamulactone {(S)-methyl 8-(5-oxo-2,5-dihydrofuran-2-yl) octanoate}, and 3-oxo-α-ionone. Both of the identified compounds exerted strong growth inhibitory effects on cress and timothy seedlings. The concentrations of 3-oxo-α-ionone and calamulactone required to limit the growth of the cress seedlings by 50% (I50) were 281.6-199.5 and 141.1-105.5 µM, respectively, indicating that the effect of calamulactone was stronger with lower I50 values. Similarly, the seedlings of timothy also showed a considerably higher sensitivity to calamulactone (I50: 40.5-84.4 µM) than to 3-oxo-α-ionone (I50: 107.8-144.7 µM). The findings indicated that the leaves of C. tenuis have marked growth-inhibitory potential, and could affect surrounding plants to exert dominance over the surrounding plant community. Moreover, the two identified phytotoxic substances might play a key role in the phytotoxicity of C. tenuis, and could be a template for bioherbicide development. This paper was the first to report calamulactone and its phytotoxicity.

Allelopathic Activity of a Novel Compound, Two Known Sesquiterpenes, and a C13Nor-Isopenoid from the Leave of Croton oblongifolius Roxb. for Weed Control

Investigation of allelopathic substances from herbal plants may lead to the development of allelochemical-based natural herbicides. Croton oblongifolius (Roxb.) is a well-known herbal plant with a long history of being used for traditional medicines and for being the source of a diverse range of bioactive compounds. This plant has been reported to have allelopathic potential; however, its allelopathic-related substances have not yet been described. Therefore, we conducted this investigation to explore the allelopathic substances from the leaves of C. oblongifolius. Aqueous methanol extracts of C. oblongifolius leaves exhibited significant growth inhibitory potential against four test plants (monocot barnyard grass and timothy, and dicot cress and lettuce). The leaf extracts were purified in various chromatographic steps and yielded four active compounds identified as (3R,6R,7E)-3-hydroxy-4-7-megastigmadien-9-one (I), 2-hydroxy alpinolide (a novel compound) (II), alpinolide (III), and epialpinolide (IV) via an analysis of the spectral data. These identified compounds significantly restricted the seedling growth of cress. The concentration necessary for 50% growth reduction of the cress seedlings varied from 0.15 to 0.24 mM for (3R,6R,7E)-3-hydroxy-4-7-megastigmadien-9-one, 0.04 to 0.11 mM for 2-hydroxy alpinolide, 0.07 to 0.12 mM for alpinolide, and 0.09 to 0.16 mM for epialpinolide. Therefore, the leaf extracts of C. oblongifolius and the characterized compounds have the potential to be used as weed-suppressive resources for natural weed control.

Allelopathic Potential of Marsdenia tenacissima (Roxb.) Moon against Four Test Plants and the Biological Activity of Its Allelopathic Novel Compound, 8-Dehydroxy-11β-O-Acetyl-12β-O-Tigloyl-17β-Marsdenin

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.