Abstract
Piper methysticum (kava) root is known to possess promising weed suppressing activity. The present study was conducted to search for potent plant growth inhibitors from the root of this medicinal pepper plant. The ethyl acetate (EtOAc) extract exhibited the strongest reduction on growth of Raphanus sativus (radish) (IC50 shoot and root growth = 172.00 and 51.31 µg/mL respectively) among solvent extracts. From this active extract, nine potent growth inhibitors involved in the inhibitory activities of P. methysticum root were isolated, purified and characterized by column chromatography (CC), gas chromatography-mass spectrometry (GC-MS), electrospray ionization-mass spectrometry (ESI-MS) and nuclear magnetic resonance (NMR). The six fractions purified by CC included two flavanones: 5-hydroxy-4′,7-dimethoxyflavanone (C1) and 5,7-dihydroxy-4′-methoxy-6,8-dimethylflavanone (matteucinol, C2) and six kavalactones: 5,6-dehydro-kavain (C3), a mixture of kavain and yagonin (C4), yagonin (C5) and dihydro-5,6-dehydrokavain, 7,8-dihydrokavain, dihydromethysticin and methysticin (C6). The amounts of 5-hydroxy-4′,7-dimethoxyflavanone, matteucinol, 5,6-dehydrokavain and yangonin were 0.76, 2.50, 2.75 and 2.09 mg/g dry weight (DW), respectively. The two flavanones C1 and C2 exhibited the strongest inhibition on shoot elongation (IC50 = 120.22 and 248.03 µg/mL, respectively), whilst the two kavalactone mixtures C4 and C6 showed the highest suppression on root growth of R. sativus (IC50 = 7.70 and 15.67 µg/mL, respectively). This study was the first to report the purification and inhibitory activities of the two flavanones 5-hydroxy-4′,7-dimethoxyflavanone and matteucinol in P. methysticum root. The isolated constituents from P. methysticum root including the flavanones C1 and C2 and the mixtures C4 and C6 may possess distinct modes of action on plant growth. Findings of this study highlighted that the combinations of hexane-ethyl acetate by 9:1 and 8:2 ratios successfully purified flavanones and kavalactones in P. methysticum root.
Highlights
Allelopathy is described as a phenomenon by which a plant possesses a natural ability to either inhibit or stimulate the growth of other plants in its vicinity by its biological toxins [1]
Findings of this study highlighted that the combinations of hexane-ethyl acetate by 9:1 and 8:2 ratios successfully purified flavanones and kavalactones in P. methysticum root
Eight phenolic compounds including gallic acid, protocatechuic acid, p-hydroxybenzoic acid, p-coumaric acid, ferulic acid, salicylic acid, trans-o-coumaric acid and trans-cinamic acid potentially involved in the weed suppressing ability of P. methysticum root were identified by high performance-liquid chromatography (HPLC) and thin layer chromatography (TLC)
Summary
Allelopathy is described as a phenomenon by which a plant possesses a natural ability to either inhibit or stimulate the growth of other plants in its vicinity by its biological toxins [1]. Taking advantages of this antagonistic plant–plant interaction, growth inhibitors from many plants have been examined for weed management. Since synthetic herbicides have negative impacts on environment and on human health, natural-based herbicide application is considered as an alternative tool to manage weeds in crop cultivation and establish a sustainable agriculture [2]. The plant has long been reputed as an indispensable beverage for ceremonial and Molecules 2018, 23, 1907; doi:10.3390/molecules23081907 www.mdpi.com/journal/molecules
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