Responses of nine Trifolium repens L. populations to ultraviolet-B radiation:: Differential flavonol glycoside accumulation and biomass production

This study aimed to quantify and identify flavonoids involved in the response of nine populations of white clover (Trifolium repens L.) to ultraviolet-B radiation (UV-B). Plants were grown for 12 weeks in controlled environment rooms with or without supplemental UV-B radiation of 13.3 kJ m(-2) d(-1). Methanol-water extractable flavonoids were quantified using high performance liquid chromatography (HPLC). Two major peaks showed significant enhancement in the HPLC chromatogram in response to supplemental UV-B. The structures of the compounds responsible were identified by H-1 and C-13 nuclear magnetic resonance (NMR) spectroscopy to be the flavonols quercetin-3-O-beta-D-xylopyranosyl-(1 --> 2)-beta-D-galactopyranoside and kaempferol-3-O-beta-D-xylopyranosyl-(1 --> 2)-beta-D-galactopyranoside. With supplemental UV-B, quercetin glycoside levels increased on average by 200 % while the kaempferol glycoside response was much smaller. Significant differences in flavonol accumulation were found among T. repens populations, both constitutively and in response to UV-B. Stress-adapted populations displayed particularly high flavonol levels under UV-B. There was an inverse correlation between plant productivity and quercetin accumulation. Furthermore, higher quercetin accumulation under UV-B was correlated with tolerance against UV-B-induced growth reduction. In conclusion, within-species comparisons in T. repens lend support to a distinct role for ortho-dihydroxylated flavonoids in the adaptation to UV-B stress and suggest particular advantages in this UV-B-induced biochemical adaptation for populations characterized by low habitat and plant productivity. (C) 2000 Annals of Botany Company.