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The fruits of Piper retrofractum Vahl. contain a variety of compounds, but no phytotoxic compound with plant growth inhibitory activity has been reported. Therefore, the phytotoxic activity of P. retrofractum fruit extracts was investigated and found the reduction of seedling growth of cress, lettuce, alfalfa, barnyard grass, Italian ryegrass, and jungle rice. Seven bioactive compounds were isolated from the fruit extracts and identified as 3-phenylpropanoic acid (1), (2E,4E)-methyl piperate (2), (2E,4Z)-methyl piperate (3), piperlonguminine (4), dihydropiperine (5), isochavicine (6), and piperine (7). There have been no reports for their phytotoxic activity. The activity of these compounds was determined against the germination and seedling growth of cress and barnyard grass. The total germination percentage (GP) of cress was significantly inhibited by 1, 2, and 6 (40–100% inhibition), while its germination index or progressive total of cumulative germination (GI) was the most sensitive to 1, leading to the longest time required for 50% germination (T50) at 57 h behind control. However, the seedling growth of cress was highly inhibited by 6 with I50 values of 11 µM. The compounds 1 and 5–7 showed significant inhibition on GP of barnyard grass by 55–47%, while 5 and 1 effectively reduced GI and extended T50 to 42–48 h behind control, and 7 showed the most growth inhibition with I50 values of 37 µM. The inhibitory effects of the seven compounds indicate the phytotoxic property of P. retrofractum fruits, which 1, 5–7 may be the main compounds responsible for the phytotoxicity.

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 (T 50) 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 T 50 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.