Aqueous methanol extracts of a traditional Bangladeshi rice cultivar (Oryza sativa L. cv. Kartikshail) inhibited root and shoot growth of cress (Lepidium sativum), lettuce (Lactuca sativa), alfalfa (Medicago sativa), timothy (Phleum pratense), crabgrass (Digitaria sanguinalis), Italian ryegrass (Lolium multiflorum), barnyardgrass (Echinochloa crus-galli) and jungle rice (Echinochloa colonum). The inhibition was increased with increasing the extract concentration, which suggests that cv. Kartikshail may have growth inhibitory substances and allelopathic potential. The aqueous methanol extract of cv. Kartikshail was purified and two main inhibitory substances were isolated and identified by spectral data as 3-hydroxy-β-ionone and 9-hydroxy-4-megastigmen-3-one. The concentrations required for 50% growth inhibition on cress roots and shoots, respectively, were 4.9 and 9.5 μM for 3-hydroxy-β-ionone, and 0.54 and 0.72 μM for 9-hydroxy-4-megastigmen-3-one. The concentrations required for 50% growth inhibition on barnyardgrass roots and shoots, respectively, were 160 and 310 μM for 3-hydroxy-β-ionone, and 53 and 140 μM for 9-hydroxy-4- megastigmen-3-one. The inhibitory activity of a mixture of the two compounds was much higher than that of the sum of the two compounds, suggesting that the two compounds may act synergistically to inhibit the growth of cress and barnyardgrass. The present research suggests that 3-hydroxy-β -ionone and 9-hydroxy-4-megastigmen-3-one may be responsible for the growth inhibitory effect of cv. Kartikshail and may play important roles in the allelopathy of cv. Kartikshail. The traditional Bangladeshi rice cultivar Kartikshail may be potentially useful for weed management as a weed suppressing agent when this rice cultivar is incorporated into the soil or included in rice-based cropping systems.
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