Short-Term Ultraviolet (UV)-A Light-Emitting Diode (LED) Radiation Improves Biomass and Bioactive Compounds of Kale

Abstract

The aim of this study was to determine the influence of two types of UV-A LEDs on the growth and accumulation of phytochemicals in kale (Brassica oleracea var. acephala). Fourteen-day-old kale seedlings were transferred to a growth chamber and cultivated for 3 weeks. The kale plants were subsequently subjected to two types of UV-A LEDs (370 and 385 nm) of 30 W/m² for 5 days. Growth characteristics were all significantly increased in plants exposed to UV-A LEDs, especially at the 385 nm level, for which dry weight of shoots and roots were significantly increased by 2.22 and 2.5 times, respectively, at 5 days of treatment. Maximum quantum efficiency of photosystem II photochemistry (Fv/Fm ratio) began to decrease after 3 h of treatment compared to the control. The total phenolic content of plants exposed to the two types of UV-A LEDs increased by 25% at 370 nm and 42% at 385 nm at 5 days of treatment, and antioxidant capacity also increased. The two types of UV-A LEDs also induced increasing contents of caffeic acid, ferulic acid, and kaempferol. The reactive oxygen species (ROS) temporarily increased in plants exposed to the two types of UV-A LEDs after 3 h of treatment. Moreover, transcript levels of phenylalanine ammonia-lyase (PAL), chalcone synthase (CHS), and flavanone 3-hydroxylase (F3H) genes and PAL enzyme activity were higher in plants treated with UV-A LEDs. Our results suggested that short-term UV-A LEDs were effective in increasing growth and improving antioxidant phenolic compounds in kale, thereby representing a potentially effective strategy for enhancing the production of phytochemicals.

Keywords: UV-A LEDs; antioxidant capacity; kale; phenolic compound; reactive oxygen species; transcript level.

Figure 1

Relative spectral distribution of the fluorescent lamp (A) and the two types of UV-A LEDs with 370 or 385 nm peak wavelength used (B).

Figure 2

Photosynthetic rate of kale subjected to two types of UV-LED lights (370 and 385 nm) at 4 days (A) and maximum quantum efficiency of photosystem II (Fv/Fm) of kale subjected to two types of UV-LED lights (370 and 385 nm) for 5 days (B). The vertical bars indicate standard errors (n = 5). Different letters indicate significant differences, as assessed by ANOVA (p < 0.05).

Figure 3

Total phenolic content (A) and antioxidant capacity (B) of kale subjected to two types of UV-A LED lights for 5 days. The vertical bars indicate standard errors (n = 4). Different letters indicate significant differences, as assessed by ANOVA (p < 0.05).

Figure 4

Individual phenolic compounds [caffeic acid; (A) ferulic acid; (B) and kaempferol; (C)] of kale subjected to two types of UV-A LED lights for 5 days. The vertical bars indicate standard errors (n = 4). Different letters indicate significant differences, as assessed by ANOVA (p < 0.05).

Figure 5

Hydrogen peroxide (A) and superoxide radical (B) of kale subjected to two types of UV-A LED lights for 5 days. The vertical bars indicate standard errors (n = 4). Different letters indicate significant differences, as assessed by ANOVA (p < 0.05).

Figure 6

Phenylalanine ammonia-lyase (PAL) activity (A) and transcript levels of PAL (B), chalcone synthase (CHS) (C), and flavonoid 3-hydroxylase (F3H) genes (D) of kale subjected to two types of UV-A LED lights for 5 days. Gene expression levels were normalized to that of actin gene, a housekeeping gene. The vertical bars indicate standard errors (n = 4). Different letters indicate significant differences, as assessed by ANOVA (p < 0.05).