Contribution of hydroxycinnamates and flavonoids to epidermal shielding ofUV-A and UV-B radiation in developing rye primary leaves as assessed by ultraviolet-induced chlorophyll fluorescence measurements

Epidermally located ultraviolet (UV)-absorbing phenolic compounds, flavonoi ds and hydroxycinnamic acid esters (HCAs), can shield the underlying tissue s in plants against harmful UV-radiation. The relative importance of the tw o different classes of phenolic compounds for UV-screening was a matter of recent debate. Using a non-invasive method based on chlorophyll fluorescenc e measurements to estimate epidermal UV transmittance, the relationship bet ween epidermal UV shielding and the content of the two different groups of secondary phenolic compounds in the epidermal layers and the underlying pho tosynthetic mesophyll of developing rye primary leaves grown under suppleme ntary UV-B radiation was investigated. From the fourth to the tenth day aft er sowing, epidermally located flavonoids increased in an age- and irradiat ion-dependent manner, whereas mesophyll flavonoids and epidermal HCAs, main ly ferulic acid and p-coumaric acid esters, were constitutively present and did not vary in their contents over the observed time period. There was an excellent correlation between epidermal UV-A and UV-B absorbances as asses sed by chlorophyll fluorescence measurements and contents of epidermal flav onoids. However, HCAs showed an additional contribution to UV-B shielding. In contrast, mesophyll flavonoids did not seem to play a respective role. W hen absorbances of the abaxial and adaxial epidermal layers were compared, it became apparent that in fully expanded primary leaves epidermal tissues from both sides were equally effective in absorption of UV-radiation. Howev er, the earlier and more UV-exposed abaxial epidermis of young unrolling le aves showed a significantly higher absorption. It is shown that in early st ages of development the epidermal HCAs are the dominant UV-B protective com pounds of the primary leaf. This function is increasingly replaced by the e pidermal flavonoids during leaf development and acclimation. The applicatio n of chlorophyll fluorescence measurements has been proven to be a useful t ool for estimating relative contents of these compounds in epidermal tissue .