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 mu mol 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.

Automated summary

UV-A light irradiation has significant effects on the growth and bioactive compounds of kale. The use of 405 nm LED can enhance kale biomass and increase the content of antioxidants. Further research on the spectral threshold between UV-A and deep-blue light is recommended.