Characteristics of UVB Radiation Tolerance in Broadleaf Trees in Southern USA

Yadong Qi , Gordon M. Heisler , Wei Gao , Thomas C. Vogelmann , and Shuju Bai

1 Urban Forestry Program, P.O. Box 11288, Southern University Baton Rouge, LA 70813, USA E-mail: [email protected]

2 USDA Forest Service Northeastern Research Station, c/o SUNY ESF,

5 Moon Library, Syracuse, NY 13210, USA E-mail: [email protected] 3 USDA UV-B Radiation Monitoring Program, and Senior Research Scientist, Natural Resource Ecology Laboratory, Colorado State University Fort Collins, CO 80523-1499, USA E-mail: [email protected] 4 Botany Department, University of Vermont, Burlington, VT 05405, USA E-mail: [email protected] 5 Department of Computer Science, Southern University Baton Rouge, LA 70813, USA E-mail: [email protected]

Abstract Research has indicated that the ozone layer in the earth's stratosphere has decreased significantly in the last two decades. Such a reduction has led to an increase in solar ultraviolet-B (UV-B) radiation (280 nm - 315 nm) striking the earth's surface. Nearly two-thirds of the 400 plant species and cultivars tested to date appear to be UV-B sensitive, one-third of which show certain tolerant characteristics. However, the majority of plants evaluated thus far have been annual agricultural species. Very few studies have been conducted on tree species which account for more than 80% of the global net primary production. Scientists from Southern University, the USDA Forest Service Northeastern Research Station, the USDA UV-B Monitoring and Research Program, and the University of Vermont, have recently completed a five-year program of collaborative research to assess UV-B (280 nm - 315 nm) radiation tolerance characteristics of more than 30 common broadleaf tree species in the southern US. The project has established a database of leaf optical properties, depth of UV-B penetration into leaves, concentration of UV-B absorbing compounds, and leaf anatomy. The study concluded that on a whole leaf basis, the tree leaves absorb 91% - 95%, reflect 5% - 9%, and transmit very little ( < 1%) incident UV-B radiation. At the tissue level, the upper leaf epidermis appears to be the main site absorbing the most UV-B radiation. The study has identified 23 broadleaf tree species that possess strong epidermal UV-B screening functions and attenuate 92% - 99% of the UV-B through their epidermal layers. These species include 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. The epidermal attenuation is shown to be the dominant UV-B screening characteristic in most of the species studied. Thus, the effectiveness of the epidermal function of UV-B screening underlines the important aspect of the UV-B protection mechanism in the broadleaf trees. Within the species, there are cumulative increases in leaf thickness, leaf total concentration of UV-B absorbing compounds, and leaf chlorophyll content with an increase in UV-B radiation during the period of leaf growth and development (from April to August). The increased concentration in leaf UV-B absorbing compounds may help protect plants against the enhanced UV-B level during the growing season. Comparisons among the species, however, showed that large inter-specific variations exist in the leaf total UV-B absorbing compound concentration, leaf epidermal thickness, leaf total thickness, and depth of UV-B penetration, indicating the individualistic nature of the species. The knowledge of UV-B absorbing compounds and their strategic locations within leaf tissues may increase our current understanding of UV-B tolerance characteristics. Further research is necessary to identify and localize these compounds in leaf tissues. Such information may help us define the biochemical aspects of UV-B protection in broadleaf trees.

Keywords Leaf optical property, UV-B penetration, concentration of total UV-B absorbing compounds, UV-B tolerance, broadleaf tree species

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