Growth and biochemical responses of kale to supplementary irradiation with different peak wavelengths of UV-A light-emitting diodes

UV-A light has different effects on the growth and bioactive compounds of vegetables, medicinal plants, and other crops. The purpose of this study was to determine the effects of short-term irradiation with UV-A light-emitting diodes (LEDs) on the growth and bioactive substances of kale ( Brassica oleracea var. acephala). Two-week-old kale seedlings were cultivated for 3 weeks in a plant factory illuminated with LEDs (red:white:blue = 8:1:1) of 150 μmol m −2 s −1 photosynthetic photon flux density. Then, the plants were continuously exposed to five peak wavelengths (365, 375, 385, 395, and 405 nm) of UV-A LEDs with an energy of 30 W m −2 in addition to the background lighting for 7 days. Treatments with 395 and 405 nm wavelengths increased most of the assessed growth characteristics and photosynthetic rates compared to the control after 7 days of treatment. The maximum quantum efficiency of photosystem II (Fv/Fm) value started to decrease after 1 day of treatment, and after 5 days, we detected an increase in the concentration of reactive oxygen species with a decrease in the wavelength of the UV-A light treatment. There were increases in the total phenolic and flavonoid contents and antioxidant levels of kale plants subjected to all of the UV-A LEDs compared with control plants after 7 days. Our observations indicated that phenylalanine ammonia lyase (PAL) and chalcone synthase gene expression and PAL enzyme activity were upregulated by the UV-A LED treatments, although no significant differences among treatments were detected. Collectively, our results indicate that kale biomass and bioactive compounds can be enhanced through supplementary UV-A radiation, with 405-nm LEDs having the best effects, thereby suggesting that it would be beneficial to conduct additional research on the spectral threshold between UV-A and deep-blue light.