Abstract
Neem (Azadirachta indica) has been widely used as a traditional medicine and several bioactive compounds have been isolated from this species, but to date no potent allelopathic active substance has been reported. Therefore, we investigated possible allelopathic property and phytotoxic substances with allelopathic activity in neem. An aqueous methanol extract of neem leaves inhibited the growth of roots and shoots of cress, lettuce, alfalfa, timothy, crabgrass, ryegrass, barnyard grass and jungle rice. The extracts were then purified by several chromatographic runs while monitoring the inhibitory activity and two phytotoxic substances were isolated. The chemical structures of the two substances were determined by spectral data to correspond to novel compounds, nimbolide B (1) and nimbic acid B (2). Nimbolide B inhibited the growth of cress and barnyard grass at concentrations greater than 0.1‒3.0 μM. Nimbic acid B inhibited the growth of cress and barnyard grass at concentrations greater than 0.3–1.0 μM. These results suggest that nimbolide B and nimbic acid B may contribute to the allelopathic effects caused by neem leaves.
Highlights
Neem (Azadirachta indica) belongs to the Meliaceas family and grows in the Indian subcontinent and Southeast Asia
The extract obtained from 0.1 g dry weight of neem leaves inhibited the root growth of cress, lettuce, alfalfa, timothy, crabgrass, ryegrass, barnyard grass and jungle rice to 7.0%, 0%, 5.2%, 0%, 2.0%, 10.6%, 0%, and 1.6% of control root growth, respectively, and inhibited the shoot growth of cress, lettuce, alfalfa, timothy, crabgrass, ryegrass, barnyard grass and jungle rice to 11.3%, 0%, 1.8%, 12.3%, 31.3%, 18.8%, 24.1% and 26.2% of the control shoot growth, respectively
The extracts of neem leaves had inhibitory effects on both dicotyledonous and monocotyledonous plants, including weed species. These results suggest that neem leaves have allelopathic properties and contain allelopathic substances
Summary
Neem (Azadirachta indica) belongs to the Meliaceas family and grows in the Indian subcontinent and Southeast Asia. It was reported that litter and extracts of neem plants have strong herbicidal or allelopathic activity [8,9]. Only a few phenolic compounds have been reported as allelopathic compounds of the plants [4,10]. Phenolic compounds are universal in many plant species and such allelopathic activity of neem cannot be distinguished from that of other plant species only by the phenolic compounds. There may be another allelopathic substances in neem. Allelopathic substances have potential as either herbicides or templates for new synthetic herbicide classes [11,12,13,14,15]. Natural compounds are considered to be more environmentally benign than most synthetic herbicides [14,15]
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