Solar UVB Radiation and Global Dimming Effects on Plant Growth and UVShielding

11 2 Ronald J. Ryel , Stephan D. Flint , and Paul W. Barnes

'Department of Wildland Resources and the Ecology Center, Utah State University Logan, UT 84322-5230, USA E-mail: [email protected] 2Department of Biological Sciences, Loyola University New Orleans, LA 70118, USA E-mail: [email protected]

Abstract Atmospheric aerosols and other particulates have been linked to reductions in shortwave radiation (global dimming). While much research has focused on the effects on plants caused by ambient and elevated UV-B (290 nm - 320 nm), the direct effects of global dimming, and those mediated by associated changes in UV-B radiation, have not been effectively assessed. We conducted an experiment in the high-UV environment of Mauna Kea, Hawaii to compare the effects of UV-B reduction with a simulation of global dimming (accomplished with 13% shading). Using fava beans, (Vicia faba), we found that structural differences due to the treatments were minimal in this high light environment. Most surprising was the minimal effect of UV-B on plant growth given the high UV-B environment. However, both UV-B and shading significantly influenced epidermal UV transmittance, suggesting that changes that occur in secondary chemistry can affect epidermal transmittance for UV. Additional experiments suggest that the change in epidermal transmittance due to shading would most likely occur with foliage in high-shade environments. Such changes in secondary chemistry have the potential to affect herbivory, nutrient cycling, and plant response to pathogens. While there are few experimental studies that specifically address possible reductions in radiation due to global dimming, many UV reduction experiments have been conducted in recent decades. Attempts to generalize latitudinal responses from these studies, however, are frustrated by a number of factors. Chief among them are different methodologies (reduction of different UV wavebands in different experiments) and a lack of reporting UV irradiance levels.

Keywords global dimming, UV-A epidermal transmittance, UV reduction experiment, leaf optical properties, UV-A and UV-B absorbing pigments, UVA-PAM, Vicia faba

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