On the role of flavonoids in the integrated mechanisms of response of Ligustrum vulgare and Phillyrea latifolia to high solar radiation

• The role of flavonoids in mechanisms of acclimation to high solar radiation was analysed in Ligustrum vulgare and Phillyrea latifolia, two Mediterranean shrubs that have the same flavonoid composition but differ strikingly in their leaf morphoanatomical traits. • In plants exposed to 12 or 100% solar radiation, measurements were made for surface morphology and leaf anatomy; optical properties, photosynthetic pigments, and photosystem II efficiency; antioxidant enzymes, lipid peroxidation and phenylalanine ammonia lyase; synthesis of hydroxycinnamates and flavonoids; and the tissue-specific distribution of flavonoid aglycones and ortho-dihydroxylated B-ring flavonoid glycosides. • A denser indumentum of glandular trichomes, coupled with both a thicker cuticle and a larger amount of cuticular flavonoids, allowed P. latifolia to prevent highly damaging solar wavelengths from reaching sensitive targets to a greater degree than L. vulgare. Antioxidant enzymes in P. latifolia were also more effective in countering light-induced oxidative load than those in L. vulgare. Consistently, light-induced accumulation of flavonoids in L. vulgare, particularly ortho-dihydroxylated flavonoids in the leaf mesophyll, greatly exceeded that in P. latifolia. • We conclude that the accumulation of flavonoid glycosides associated with high solar radiation-induced oxidative stress and, hence, biosynthesis of flavonoids appear to be unrelated to 'tolerance' to high solar radiation in the species examined.