Mutants affected in flavonoid (t/4) or sinapate ester (fahl) biosynthesis were used to assess the relative importance of these phenolic UV photoprotectants in Arabidopsis. Flavonoid and sinapate ester absorption was more specific for UV-B than major nonphenolic chromophores in crude extracts. A new method of evaluating phenolic UV-B attenuation was developed using fluorescence analysis. When excited by UV-B, sinapate ester containing leaves and cotyledons had enhanced sinapate ester fluorescence and reduced chlorophyll fluorescence relative to those without sinapate esters. Although fluorescence analysis gave no evidence of UV-B attenuation by f1avonoids, enhanced chlorophyll and protein loss were observed upon UV-B exposure in flavonoid-deficient leaves, suggesting they have another mechanism of UV-B protection. The hydroxycinnamates have been largely ignored as UV-B attenuating pigments, and the results indicate that greater attention should be paid to their role in attenuating UV-B.

Mutants affected in flavonoid (tt4) or sinapate ester (fah1) biosynthesis were used to assess the relative importance of these phenolic UV photoprotectants in Arabidopsis. Flavonoid and sinapate ester absorption was more specific for UV‐B than major nonphenolic chromophores in crude extracts. A new method of evaluating phenolic UV‐B attenuation was developed using fluorescence analysis. When excited by UV‐B, sinapate ester containing leaves and cotyledons had enhanced sinapate ester fluorescence and reduced chlorophyll fluorescence relative to those without sinapate esters. Although fluorescence analysis gave no evidence of UV‐B attenuation by flavonoids, enhanced chlorophyll and protein loss were observed upon UV‐B exposure in flavonoid‐deficient leaves, suggesting they have another mechanism of UV‐B protection. The hydroxycinnamates have been largely ignored as UV‐B attenuating pigments, and the results indicate that greater attention should be paid to their role in attenuating UV‐B.

No abstract

No abstract

The orthodihydroxy structure may mediate flavonoid coloration or antioxidant activity, making it of interest to floral or health food industries, respectively. In the first of two companion papers, the authors demonstrate a model system to study the orthodihydroxy structure in colorless flavonoids of Arabidopsis, with tools similar to those used upon the colored flavonoids. The system includes a fluorescent staining procedure to visualize colorless orthodihydroxy flavonoids at a subcellular level and to screen seedlings for rare genetic recombinants. The system also includes mutant lines to study the synthesis of colorless orthodihydroxy flavonoids, their potential role as antioxidants, and their potential risk as oxidative mutagens.