Abstract
To study the effects of ultraviolet (UV) light on the essential oil concentration in Japanese mint (Mentha arvensis L. var. piperascens), we exposed the plants to different combinations of irradiation with a white fluorescent lamp (W), UV-A fluorescent lamp (UVA; peak wavelength, 360 nm), and UV-B fluorescent lamp (UVB; peak wavelength, 306 nm). Japanese mint transplants hydroponically grown from a rhizome in a controlled environment were used as the plant material. Young plants were cultivated in growth chambers [air temperature, 25/23°C; photosynthetic photon flux, 250 μmol·m−2·s−1; CO2 concentration, 1,000 μmol·mol−1] under the following light conditions: W, W+ UVA (2.0 mW·m−2, 315–400 nm), W+UVB (0.5 mW·m−2, 280–350 nm), and W+UVA+ UVB (2.5 mW·m−2, 280–400 nm). The UV irradiation period was 2 h per day during the light period (12 h). After 7 days of irradiation, the plants grown in different light conditions showed no difference in the number of leaves and leaf area. The l-menthol and limonene concentrations and the total antioxidant capacity (TAC) in the upper leaves of plants grown under the W+UVB and W+UVA+UVB conditions were significantly higher than those in the upper leaves of plants grown under the W condition. The upper leaves unfolded after the initiation of UV irradiation; further, supplemental UV-B irradiation seemed to increase the essential oil concentration and the TAC of the leaves. These results suggest that longer supplemental UV-B irradiation of Japanese mint plants may increase the yield of essential oils per plant by increasing the number of leaves that contain high concentrations of essential oils.
