This study investigated the UV-B related biophysical, biochemical, and anatomical characteristics of more than 30 southern broadleaf tree species. It is concluded that leaves of the broadleaf trees generally reflect 4% - 9%, transmit 0% - 0.2%, and absorb 91% - 95% of the incident UV-B radiation. The main site of UV-B attenuation takes place within the upper leaf epidermis. The project has identified 23 broadleaf tree species (Arizona ash, chestnut oak, mocker nut hickory, pecan, American sycamore, bitternut hickory, green ash, sawtooth oak, American elm, blue Japanese oak, cherrybark oak, cottonwood, southern live oak, southern magnolia, shumard oak, sweetgum, American beech, white oak, Chinese tallow, water oak, yellow poplar, Bradford pear, and red maple) possessing a strong epidermal UV-B screening functions (attenuating over 90% UV-B). The effectiveness of the epidermal function of the UV-B screening underlines the major UV-B protection mechanism in most of the trees studied. Three species (sweet bay magnolia, red oak, and willow oak) possessed intermediate epidermal screening function, and five other species (river birch, American chestnut, red bud, Chinese elm, and sugarberry) showed somewhat weak epidermal screening functions. Considerable inter-specific variations exist in leaf UV-B absorbing compound concentrations, depth of UV-B penetration, and leaf anatomical features. In addition to the epidermal screening function, the leaf UV-B absorbing compounds may play an important role in providing additional defense against UV-B radiation penetration into mesophyll tissues. Further studies are necessary to focus on localization and identification of UV-B absorbing compounds in leaf tissues, to promote a better understanding of the UV-B tolerance mechanisms in trees.
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