Plant growth inhibitors in turmeric (Curcuma longa) and their effects on Bidens pilosa

Turmeric (Curcuma spp.) has numerous biological activities, including anticancer, antibacterial, antifungal and insecticidal properties. Here, we evaluated the plant growth‐inhibitory activities of two cultivars of Curcuma longa (C. longa; Ryudai gold and Okinawa ukon) against radish, cress, lettuce and Bidens pilosa (B. pilosa). The methanol extracts of both turmeric varieties inhibited the seed germination and seedling growth of all the tested plants. Ryudai gold had a significantly higher inhibitory effect on the seed germination and root and shoot growth of the plants than Okinawa ukon. Therefore, Ryudai gold was chosen for the isolation of plant growth‐inhibitory compounds using a silica gel column and high‐performance liquid chromatography. The structural identification of the compounds was carried out using 1H NMR, 13C NMR and liquid chromatography–tandem mass spectrometry. The growth inhibitors were identified as four curcuminoids; dihydrobisdemethoxycurcumin (1), bisdemethoxycurcumin (2), demethoxycurcumin (3) and curcumin (4). The IC50 of the curcuminoids against the root and shoot growth of B. pilosa ranged from 8.7 ± 1.7 to 12.9 ± 1.8 and from 15.5 ± 1.8 to 38.9 ± 2.8 μmol L−1, respectively. Compound 1 showed the lowest IC50 against the root and shoot growth of B. pilosa. These results suggested that Ryudai gold has a potential growth‐inhibitory effect against B. pilosa.