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

Abstract

tective compounds of the primary leaf. This function is increasingly replaced by the epidermal flavonoids during leaf development and acclimation. The application of chlorophyll fluorescence measurements has been proven to be a useful tool for estimating relative contents of these compounds in epidermal tissue.Key-words: Secale cereale L.; chlorophyll fluorescence; epidermal transmittance; flavonoids; hydroxycinnamic acid conjugates; leaf development; ultraviolet screening pigments. INTRODUCTIONAn unavoidable consequence of the exposure of plants to natural sunlight is the exposure to ultraviolet (UV) radiation. Radiation at wavelengths below 320 nm (UV-B, 290-320 nm) is principally damaging to DNA, proteins and other essential molecules, e.g. auxin (Jordan 1996;Taylor, Tobin & Bray 1997;Jansen, Gaba & Greenberg 1998).Plants have evolved two major strategies for resistance to UV-B radiation: (1) repair mechanisms (e.g. repair of DNA damages by excision repair or by repair of pyrimidine-dimers and other DNA photoproducts by specific enzymes such as photolyase, activated by UV-A and photosynthetically active radiation (PAR) (Taylor et al. 1997)); and (2) avoidance mechanisms (primarily epidermal screening of UV-B radiation to protect the mesophyll tissue of a leaf by the accumulation of UV-absorbing compounds in cell vacuoles and/or cell walls of the epidermis (Caldwell, Robberecht & Flint 1983;Hutzler et al. 1998)). It is widely accepted that flavonoids and hydroxycinnamic acid conjugates (HCAs) are the most efficient UV-B screening compounds because of their strong absorbance in this wavelength region and because their concentration in epidermal layers is significantly increased by UV-radiation (Tevini, Braun & Fieser 1991;Beggs & Wellmann 1994;Bornman et al. 1997) From the fourth to the tenth day after sowing, epidermally located flavonoids increased in an age-and irradiation-dependent manner, whereas mesophyll flavonoids and epidermal HCAs, mainly 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 assessed by chlorophyll fluorescence measurements and contents of epidermal flavonoids. However, HCAs showed an additional contribution to UV-B shielding. In contrast, mesophyll flavonoids did not seem to play a respective role. When 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. However, the earlier and more UV-exposed abaxial epidermis of young unrolling leaves showed a significantly higher absorption. It is shown that in early stages of development the epidermal HCAs are the dominant UV-B pro-

Enhanced light intensity increases flavonol and anthocyanin concentrations but reduces flavone levels in the cotyledons of common buckwheat seedlings

“…Those results validate the hypothesis stating that anthocyanins play an important role in protecting plants against excessive visible light or UV-B radiation (Burchard et al 2000;Gould 2004;Agati and Tattini 2010;Fallovo et al 2011).…”

“…centrations of mesophyll flavonoids and other phenolic compounds remain stable (Burchard et al 2000). It appears that in common buckwheat cotyledons, quercetin O-glycosides are occurred mainly in the epidermal layer, whereas apigenin and luteolin C-glucosides are localized in the mesophyll layer.…”

Enhanced biosynthesis of quercetin occurs as A photoprotective measure in Lycopersicon esculentum mill. under Acute UV-B exposure

“…These compounds are mostly phenolics, e.g., anthocyanins and other flavonoids, which show effective absorption in the UV spectral region and are said to be induced on exposure to UV (Lee and Lowry 1980;Reuber et al 1996;Jenkins et al 2009 andStracke et al 2010). There are several reports focused on the accumulation of flavonoids, including anthocyanins under enhanced UV-B levels, acting as UV-B screens by serving as active oxygen scavengers (Liu et al 1995;Grace 1998;Hoque and Remus 1999;Burchard et al 2000). They participate in the elimination of free radicals, thereby strongly combating the oxidative damage (Jenkinset al 2009).…”

G-Protein-Coupled Receptor 1, G-Protein Gα-Subunit 1, and Prephenate Dehydratase 1 Are Required for Blue Light-Induced Production of Phenylalanine in Etiolated Arabidopsis

“…The aromatic amino acid Phe is required for protein synthesis and also serves as the precursor to a large number of secondary metabolites critical to normal development and stress responses (Gilchrist and Kosuge, 1980;Chapple et al, 1994;Kliebenstein, 2004;Winkel, 2004), including UV screening and other pigments (Burchard et al, 2000;Ruegger and Chapple, 2001;Casati and Walbot, 2005). Despite the fact that 30% to 40% of the mass of an individual plant can be derived from aromatic amino acids L-Phe and L-Tyr, little is known about their biosynthetic pathway in plants ( Lewis and Yamamoto, 1989;Davin et al, 1992;Razal et al, 1994Razal et al, , 1996.…”