Abstract
Ultraviolet-B (UV-B; 280–315 nm) radiation induces the biosynthesis of secondary plant metabolites such as flavonoids. Flavonoids could also be enhanced by blue (420–490 nm) or green (490–585 nm) light. Flavonoids act as antioxidants and shielding components in the plant’s response to UV-B exposure. They are shown to quench singlet oxygen and to be reactive to hydroxyl radical. The aim was to determine whether treatment with blue or green light can alter flavonoid profiles after pre-exposure to UV-B and whether they cause corresponding biological effects in Brassicaceae sprouts. Based on their different flavonoid profiles, three vegetables from the Brassicaceae were selected. Sprouts were treated with five subsequent doses (equals 5 days) of moderate UV-B (0.23 kJ m–2 day–1 UV-BBE), which was followed with two subsequent (equals 2 days) doses of either blue (99 μmol m–2 s–1) or green (119 μmol m–2 s–1) light. In sprouts of kale, kohlrabi, and rocket salad, flavonoid glycosides were identified by HPLC-DAD-ESI-MSn. Both Brassica oleracea species, kale and kohlrabi, showed mainly acylated quercetin and kaempferol glycosides. In contrast, in rocket salad, the main flavonol glycosides were quercetin glycosides. Blue light treatment after the UV-B treatment showed that quercetin and kaempferol glycosides were increased in the B. oleracea species kale and kohlrabi while—contrary to this—in rocket salad, there were only quercetin glycosides increased. Blue light treatment in general stabilized the enhanced concentrations of flavonoid glycosides while green treatment did not have this effect. Blue light treatment following the UV-B exposure resulted in a trend of increased singlet oxygen scavenging for kale and rocket. The hydroxyl radical scavenging capacity was independent from the light quality except for kale where an exposure with UV-B followed by a blue light treatment led to a higher hydroxyl radical scavenging capacity. These results underline the importance of different light qualities for the biosynthesis of reactive oxygen species that intercept secondary plant metabolites, but also show a pronounced species-dependent reaction, which is of special interest for growers.
Highlights
Plants have photosensory mechanisms to detect ultraviolet-B (UV-B; 280–315 nm) radiation (Heijde and Ulm, 2012; Jenkins, 2014) and protect and repair sensitive targets from direct and indirect UV-induced injury (Britt, 1996; Jansen and Bornman, 2012)
Caffeoylquinic acid and feruloyl glucoside were increased by UV-B exposure and remain high after blue light treatment but not after green light treatment (Figure 2 and Supplementary Table 1)
Gentiobiosides acylated only with sinapoyl residues were found in high concentrations in kale plants, but mixtures of sinapoyl- and feruloyl-acylated gentiobiosides appeared in high concentrations in plants exposed to UV-B and subsequently to blue light treatment
Summary
Plants have photosensory mechanisms to detect ultraviolet-B (UV-B; 280–315 nm) radiation (Heijde and Ulm, 2012; Jenkins, 2014) and protect and repair sensitive targets from direct and indirect UV-induced injury (Britt, 1996; Jansen and Bornman, 2012). One line of defense is the increased biosynthesis of flavonoids and hydroxycinnamic acids (Jansen et al, 2008; Neugart and Schreiner, 2018). Changes in the biosynthesis of flavonoid glycosides and hydroxycinnamic acid derivatives in response to UV-B depends on the chemical structure of the compounds (Olsson et al, 1998; Neugart et al, 2012a, 2014; Harbaum-Piayda et al, 2016). Blue and red LEDs were the first choice for manufacturers as these wavelengths are efficiently absorbed by chlorophylls (Kaiser et al, 2019). It is a widespread but erroneous belief that plants prefer blue and red light and poorly absorb green light (Smith et al, 2017). Some basic principles need to be investigated for the later development of recipes
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