Environmental and Experimental Botany 73 (2011) 10–18 Contents lists available at SciVerse ScienceDirect Environmental and Experimental Botany journal homepage: www.elsevier.com/locate/envexpbot Ambient UV-B radiation reduces PSII performance and net photosynthesis in high Arctic Salix arctica  Kristian R. Albert a,∗ , Teis N. Mikkelsen a , Helge Ro-Poulsen b , Marie F. Arndal c , Anders Michelsen b a Biosystems Department, Risø DTU, P.O. Box 49, Frederiksborgvej 399, DK-4000 Roskilde, Denmark b Department of Biology, University of Copenhagen, Øster Farimagsgade 2D, DK-1353 Copenhagen K, Denmark c Division of Forest and Landscape Ecology, University of Copenhagen, Hørsholm Kongevej 11, DK-2970 Hørsholm, Denmark article info Keywords: JIP test Photoinhibition Jmax UV-B exclusion Vcmax plant stress Stomatal conductance abstract Ambient ultraviolet-B (UV-B) radiation potentially impacts the photosynthetic performance of high Arctic plants. We conducted an UV-B exclusion experiment in a dwarf shrub heath in NE Greenland (74 ◦ N), with open control, filter control, UV-B filtering and UV-AB filtering, all in combination with leaf angle control. Two sites with natural leaf positions had ground angles of 0 ◦ (‘level site’) and 45 ◦ (‘sloping site’), while at a third site the leaves were fixed in an angle of 45 ◦ to homogenize the irradiance dose (‘fixed leaf angle site’). The photosynthetic performance of the leaves was characterized by simultaneous gas exchange and chlorophyll fluorescence measurements and the PSII performance through the growing season was investigated with fluorescence measurements. Leaf harvest towards the end of the growing season was done to determine the specific leaf area and the content of carbon, nitrogen and UV-B absorbing com- pounds. Compared to a 60% reduced UV-B irradiance, the ambient solar UV-B reduced net photosynthesis in Salix arctica leaves fixed in the 45 ◦ position which exposed leaves to maximum natural irradiance. Also a reduced Calvin Cycle capacity was found, i.e. the maximum rate of electron transport (J max ) and the maximum carboxylation rate of Rubisco (V cmax ), and the PSII performance showed a decreased quantum yield and increased energy dissipation. A parallel response pattern and reduced PSII performance at all three sites indicate that these responses take place in all leaves across position in the vegetation. These findings add to the evidence that the ambient solar UV-B currently is a significant stress factor for plants in high Arctic Greenland. © 2010 Elsevier B.V. All rights reserved. DOI of original article: 10.1016/j.envexpbot.2010.07.001. Abbreviations: FO, minimal fluorescence in dark adapted leaves; FJ , fluores- cence at J-step; FI , fluorescence at I step; FM, maximal fluorescence in dark adapted leaves; Fv, variable fluorescence; FV/FM = TRo/ABS, maximum quantum yield of pri- mary photochemistry; Fs ′ , steady state fluorescence; FO ′ , minimal fluorescence in light; FM ′ , maximal fluorescence in light; FV ′ /FM ′ , quantum yield of primary photochemistry in light; qP and qN, photochemical and non photochemical quench- ing; NPQ, Stern–Volmer non photochemical quenching; ETR, electron transport in light; ETO/TRO, probability that a trapped exciton moves an electron into the electron transport chain beyond Q A - ; REO/ETO, efficiency with which an electron can move from the reduced intersystem electron acceptors to the PSI end accep- tors; ABS/CSX, absorption per exited leaf cross section e.g. approximated by FO or FM; TRO/CSXDIO/CSX, ETO/CSX and REO/CSX, approximated energy flux trapping, energy flux dissipation, flux of electron transport and reduction of PSI end acceptors per leaf cross section; RC/CSX, active PSII RCs per leaf cross section; RC/ABS, approx- imated ratio of reaction center chlorophyll to antenna chlorophyll; TRO/RCDIO/RC, ETO/RC and REO/ABS, approximated energy flux trapping, energy flux dissipation, flux of electron transport and reduction of PSI end acceptors per leaf cross sec- tion; PI abs , performance index; PI total , total performance index; PICSx, performance index per leaf cross section; ‘F’, filter control; ‘C’, open control, ‘UV-B’UV-B exclu- sion; ‘UV-AB’, UV-AB exclusion; PS, photosystem; RC, reaction center; Jmax, maximal 1. Introduction The stratospheric ozone layer reduces the solar ultraviolet-B (UV-B) radiation (280–315 nm) reaching the surface of the earth (Kerr, 1993; McKenzie et al., 2007). The ozone layer over the Arctic is projected to recover to pre-1960s levels around 2050 (Shindell et al., 1998; Taalas et al., 2000; McKenzie et al., 2007). Strato- sphere cooling by greenhouse gasses is increasing the formation of polar stratospheric clouds, which are the sites of ozone destruc- tion reactions. This is one of the reasons that elevated UV-B levels velocity of RuBP regeneration; Vcmax, maximal velocity of Rubisco carboxylation; SLA, specific leaf area; UV-B abs.comp , leaf content of UV-B absorbing compounds; PAR, photosynthetic active radiation; Pn, net photosynthesis; gs , stomatal conductance; C i , intercellular CO2 concentration; MED, mean erythremal dose.  The publisher regrets that this article was previously published in an issue of Environmental and Experimental Botany 72/3 (2011) 439–447, for citation purposes please use original publication details. ∗ Corresponding author. Tel.: +45 4677 4100; fax: +45 4677 4109. E-mail address: kria@risoe.dtu.dk (K.R. Albert). 0098-8472/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.envexpbot.2011.08.003