| Five broadleaf tree species, Nuttall oak, Chinese tallow, Chinese elm, green ash and Southern magnolia, were used for localization of UV absorbing compounds with fluorescence microscope using fluorescence-inducing reagents like Naturstoffreagenz A (diphenylboric acid 2-aminoethyl ester). All five broadleaf tree species showed that UV absorbing compounds presented dominantly in the leaf upper and lower epidermises and leaf petiole epidermises, and sparsely in vascular bundles and palisade tissues. The epidermal layers thus may play an important role in attenuation of harmful UV-B radiation in these species. Green ash leaves showed the most increase of UV absorbing compounds in both leaf and petiole vascular bundle and epidermises compared to other species, from young to mature. Green ash and Southern magnolia also showed UV absorbing compounds in their leaf trichomes (leaf hairs). A reverse-phase high-performance liquid chromatographic (RP-HPLC) method based on isocratic elution was developed and validated for the simultaneous quantification of flavonols (keaempferol, kaempferol-3-O-glucoside, kaempferol-3-O-rutinoside, myricetin, quercetin hydrate and rutin), flavone (apigenin), flavonone (naringenin) and phenolic acids (chlorogenic and gallic). All these compounds were found to be important constituents of UV-B absorbing compounds in leaves. The wavelengths monitored for the detection of flavonols, flavone, flavonone and phenolic acids to achieve the emax were 254, 280, 310, 350 and 370 nm, respectfully. The above eight flavonoids and two phenolic acids were quantified in mature leaves of all twelve tree species with five replications each. The results showed all twelve broadleaf tree species possessed the ten UV absorbing compounds except Nuttall oak. Myricetin was not present in Nuttall oak. The following is a summary of the most and the least dominant compounds for each species, according to HPLC quantifications expressed as percentage dry weight (% dry wt) values. Rutin and chlorogenic acid were dominant in American elm and kaempferol was least dominant. Chlorogenic acid was the dominant UV absorbing compound in Chinese elm and gallic acid was the least dominant. Kaempferol-3-O-glucoside was the dominant UV absorbing compound in Chinese tallow and chlorogenic acid was the least dominant. Rutin was the dominant UV absorbing compound in green ash and gallic acid was the least dominant. Myricetin, rutin, gallic acid, k-3-O glucoside and quercetin were the dominant UV absorbing compounds in live oak and naringenin was the least dominant. Chlorogenic acid and gallic acid were the dominant UV absorbing compounds in Nuttall oak and rutin and K-3-O-rutinoside were the least dominant. Kaempferol-3-O-rutinoside and kaempferol-3-O-glucoside were the dominant UV absorbing compounds in pecan and kaempferol was the least dominant. Kaempferol-3-O-glucoside was the dominant UV absorbing compound in river birch and rutin was the least dominant. Kaempferol-3-O-glucoside was the dominant UV absorbing compound in Shumard oak and rutin was the least dominant. Rutin and kaempferol-3-O-rutinoside were the dominant UV absorbing compounds in Southern magnolia and chlorogenic acid was the least dominant. Rutin was the dominant UV absorbing compound in Southern red oak and kaempferol-3-O-rutinoside was the least dominant. Rutin was the dominant UV absorbing compound in willow oak and chlorogenic acid was the least dominant.;Comparison of total ten combined compounds among the twelve species ranked from high to low is as follows: green ash, Chinese elm, American elm, Chinese tallow, Shumard oak, Nuttall oak, pecan, river birch, Southern red oak, willow oak, Southern magnolia and live oak.;In essence, plants use photosynthetically active radiation (PAR, 400-700 nm) to carry out their photosynthesis in the visible spectrum. At the same time, plants utilize UV radiation to synthesize secondary metabolites (flavonoids / phenolic acids) as a defense mechanism for survival. However, the absolute amounts of constitutive and UV-B induced flavonoids vary greatly from species to species. |
