The Concentration of Leaf UVB Absorbing Compounds

The inter-specific comparison of the total UV-B absorbing compound concentration per unit of leaf area (A280 nm-32o nm/cm , assessed by integrating the absorbance values from 280 nm - 320 nm at 1 nm intervals) in the mature leaves is presented in Fig. 18.8. Large variations exist among the species. These UV-B absorbing compounds possess strong absorbance to the UV radiation (Qi et al., 2002; Qi et al., 2003b, c). It is possible that those species with the strong epidermal UV-B screening abilities, such as the first 23 species in Fig. 18.7, may have accumulated more UV-B absorbing compounds in their epidermal layers than the rest of the species. Further research is necessary to localize the UV-B absorbing compounds in leaves. The accumulation of flavonoids in the epidermis has been shown to reduce epidermal transmittance of UV-B radiation (Robberecht and Caldwell, 1978; Tevini et al., 1991). Research by Karabourniotis and Fasseas (1996) and Karabourniotis et al. (1998) suggests that these flavonoids, and possibly other phenolics, are present throughout the leaf, but accumulate significantly in leaf trichomes and epidermal cells. Research has also demonstrated that the Arabidopsis mutants' lack of UV-B absorbing compounds (e.g., cinnnamic acid precursors and flavonoids) were hypersensitive to UV-B radiation (Li et al., 1993; Reuber et al., 1996). Those species with weak epidermal screening abilities, such as river birch, American chestnut, red bud, Chinese elm, and sugarberry (see Fig. 18.7), may be able to accumulate UV-B absorbing compounds throughout the leaf cross-sections in order to provide additional UV-B defense. Ultraviolet-absorbing compounds and specific leaf anatomical features are important in determining leaf screening efficiency (Bornman, 1999). The identifications and locations of flavonoids and related phenolics in leaf tissues have been advanced by many studies (Schnitzler et al., 1996; Hutzler et al., 1998; Laakso et al., 2000; Semerdjieva et al., 2003; Sullivan et al., 2005). Using fluorescence microscopy, Semerdjieva et al. (2003) discovered Vaccinium myrtillus contained the highest concentration of methanol-extractable UV-B absorbing compounds which were distributed throughout the leaf, and were particularly concentrated in chlorophyll-containing cells, while in Vaccinium vitisidaea, most phenolic compounds were cell wall-bound and concentrated in the walls of the epidermis. These two plants represent extreme forms of two divergent strategies for UV-B screening.

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Figure 18.8 Comparisons of UV-B absorbing-compound concentrations in mature leaves among the selected broadleaf tree species grown in Baton Rouge, LA. The value for each species was the mean over four month measurements from July to Oct with four samples per month from sun-exposed leaves. The error bar indicates + 1SE

Figure 18.8 Comparisons of UV-B absorbing-compound concentrations in mature leaves among the selected broadleaf tree species grown in Baton Rouge, LA. The value for each species was the mean over four month measurements from July to Oct with four samples per month from sun-exposed leaves. The error bar indicates + 1SE

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