Introduction

1. Wyoming big sagebrush screens UV radiation more effectively at higher altitudesMichael J. Dyslin, John D. Krenz and Christopher T. Ruhland Department of Biological Sciences, Minnesota State University, Mankato • Results and Discussion • Epidermal transmittance of UV increased at lower elevations; mean adaxial UV-transmittance values ranged from 10.2% (low elevation) to 2.3% (high elevation; Figure 1A). • Concentrations of UV-absorbing compounds increased with elevation and ranged from 0.64 to 2.25 A300 cm-2and 0.43 to 1.35 A365 cm-2(Figure 1B, 1C). • Adaxial leaf-hair density increased from a mean of 14,400 cm-2at low elevation to a mean of 22,500 cm-2at high elevation (Figure 2). • Based upon the observed epidermal transmittance of UV, we suspect that foliage of A. tridentata is protected against potentially damaging UV radiation. • Furthermore, the flux of biologically-effective ultraviolet radiation reaching the Earth’s surface diminishes at lower elevations which may cause physiological and morphological phenotypic differences within plant populations. • Because the distance along the elevation gradient was only 18 km, gene flow likely prevents ecotypic differentiation; the ultimate cause of the cline in screening effectiveness is likely the evolution of phenotypic plasticity in both biochemical and anatomical properties of leaves in response to UV stimuli. Introduction We examined adaxial epidermal UV-screening effectiveness, concentrations of bulk-soluble sun-screen compounds, and leaf-hair densities of Artemisia tridentata ssp. wyomingensis (Wyoming big sagebrush) along an 800-m elevation gradient in central Wyoming. Some phenolic compounds (flavonoids and hydroxycinnamic acids) accumulate in the epidermis and absorb biologically effective UV (280-400 nm) which shields underlying photosynthetic tissues from potential damage (Burchard et al., 2000). Additionally, increasing adaxial leaf-hair densities may be induced by UV-radiation (Karabourniotis et al., 1992), particularly at high elevations. These results are relevant because of the recent increase in UV-radiation due to anthropogenic stratospheric ozone depletion (Zerefos et al., 1995). Methods We measured UV-screening effectiveness of foliage at 14 elevations along the 800-m gradient (n=10 per site) with a pulse-amplitude modulated UVA fluorometer (UVA PAM; Gademann Instruments, Würzburg, Germany). We estimated the concentration of screening compounds across the gradient (n=180 plants) and counted adaxial leaf hairs in a subsample (n=25). Concentrations of bulk-soluble UV-absorbing compounds were measured spectrophotometrically by extracting them in a heated MeOH:HCl:H2O (90:1:1 v/v) solution. Concentrations were measured at λ=300 and 365 nm and corrected for leaf area. We counted leaf hairs using an epifluorescence microscope and corrected for leaf area. References Burchard, P., Bilger, W. & Weissenbock, G. 2000. Contribution of hydroxycinnamates and flavonoids to epidermal shielding on UV-A and UV-B radiation in developing rye primary leaves as assessed by ultraviolet-induced chlorophyll fluorescence measurements. Plant, Cell and Environment 23: 1373 -1380. Karabourniotis, G., Papadopoulos, K., Papamarkou, M., Manetas, Y., 1992. Ultraviolet-B radiation absorbing capacity of leaf hairs. Physiologia Plantarum 86: 414-418. Zerefos, CS., Meleti, C., Bias, AF., Lambros, A., 1995. The recent UVB variability over southeastern Europe. Journal of Photobiology (B:Biology) 31: 15-19. Acknowledgements We thank the Undergraduate Research Conference of Minnesota State University. Brooks Kennedy, Erika Kendall and Bailey Jarvis assisted in measuring epidermal transmittance. Karl H. Krenz and Tyler Grupa collected and assisted in UV-absorbing compound estimates. Tensleep Preserve (The Nature Conservancy) provided logistical support.