Identification of the Flavonoid Glycosides that Accumulate in Brassica napus L. cv. Topas Specifically in Response to Ultraviolet B Radiation

We have shown in previous work that Brassica napus synthesizes epidermal flavonoids in response to UVB radiation (290–320 nm) and that these compounds are effective at screening the leaf mesophyll from UVB (Wilson and Greenberg, Photochem. Photobiol . 57, 556–563, 1993). This route of acclimation is common to many plant species. However, flavonoids are a highly diverse group of compounds that vary greatly from species to species, and little is known about the specific flavonoids synthesized in response to UVB. To address this, we extracted flavonoids from the leaves of B. napus plants exposed to photosynthetically active radiation (PAR: 400–700 nm), PAR + UVA (320–400 nm) and PAR + UVA + UVB. The compounds were resolved by HPLC and their structures were elucidated. Twelve distinct flavonoid glucosides with quercetin and kaempferol backbones were found. In some cases, a hydroxycinnamic acid moiety was linked via an ester bridge to a glucose. Of the 12 compounds identified, the leaf concentrations of 6 increased in response to UVB: kaempferol-3- O -sophoroside (K2), kaempferol-3- O -sophoroside-7- O -glucoside (Q3), quercetin-3- O -sophoroside (Q2), quercetin-3- O -sophoroside-7- O -glucoside (Q3), K3-coumaroyl ester and Q3-caffeoyl ester. Of these six compounds, K2, K3, Q2 and Q3 accumulated to high enough concentrations to contribute to UVB screening. Interestingly, the extractable amounts of the other six compounds identified were lower in the plants exposed to UVA or UVA + UVB compared to plants exposed only to PAR. Thus, in B. napus UV exposure seems to cause a shift in the population of flavonoid glycosides, with four of the UVB-induced flavonoids being generated in high concentrations.