Citronella grass (Cymbopogon nardus) is a perennial herb. Many researchers have studied the exploitation of the active compounds in this plant for medicinal purposes, but there have been no studies on its phytotoxic compounds. Therefore, we investigated the phytotoxic active compounds in the roots of C. nardus. A significant inhibition was observed on the germination of cress (Lepidium sativum L.), lettuce (Lactuca sativa L.), barnyard grass (Echinochloa crus-galli L.), and Italian ryegrass (Lolium multiflorum Lam.) by the aqueous methanol extract of C. nardus roots. The extract decreased the total germination percentage (GP) and germination index (GI, progressive total of daily cumulative germination), and extended the time required for 50% germination (T50) of all test species. A phytotoxic compound of the extract was isolated and identified as N-octanoyl tyramine by spectral analysis. This compound showed the inhibition on the germination of cress and barnyard grass. The decline in GP, GI, and T50 was observed at concentrations of 30–2000 μM of N-octanoyl tyramine. The compound also showed the significant inhibition on the growth of cress and barnyard grass seedlings at concentrations greater than 100 and 300 μM, respectively. This study suggests that C. nardus could produce phytotoxic compounds, and N-octanoyl tyramine may be responsible for its phytotoxic activity.
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