To protect our environment from the toxic effect of herbicides, it is necessary to reduce the application of synthetic herbicides for controlling weeds in crop fields as well as to increase the use of natural herbicides produced from plant sources. Phytotoxic compounds obtained from plant species have gained attention as an alternate biological method for managing weeds. Albizia richardiana was previously reported to contain a number of phytotoxic substances. Therefore, we carried out this study to investigate more allelopathic substances in Albizia richardiana leaves. Aqueous methanol extracts of Albizia richardiana leaves were significantly inhabited the seedling growth of the two test species (alfalfa and timothy) in a concentration-dependent manner. The two most active substances were separated through different chromatographic steps and characterized as a novel compound 3-hydroxy-4-oxo-β-dehydroionol [(E)-6-hydroxy-3-(3-hydroxybut-1-en-1-yl)-2,4,4-trimethylcyclohexa-2,5-dien-1-one], and compound 3-oxo-α-ionone [3,5,5-trimethyl-4-(3-oxo-1-butenyl)-2-cyclohexen-1-one]. The novel compound 3-hydroxy-4-oxo-β-dehydroionol showed relatively stronger growth inhibitory potential compare to the compound 3-oxo-α-ionone and significantly restricted the seedling growth of crees and timothy. The required concentration for 50% reduction in the growth (I50 values) of the cress and timothy seedlings varied from 63.78 to 141.61 μM for 3-hydroxy-4-oxo-β-dehydroionol and 1059.60 to 1616.10 μM for 3-oxo-α-ionone. The results indicate that these two allelopathic compounds may be responsible for the phytotoxicity of Albizia richardiana leaves.
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