Emir. J. Food Agric. 2012. 24 (6): 546-556 http://ejfa.info/ 546 REVIEW ARTICLE Impact of UV-B radiation on photosynthesis – an overview Fernando J. C. Lidon 1* , Fernando H. Reboredo 1 , António E. Leitão 2 , Maria Manuela A. Silva 3 , Maria Paula Duarte 1 and José C. Ramalho 2 1 Departamento de Ciências e Tecnologia da Biomassa, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Quinta da Torre, 2829-514 Caparica, Portugal 2 Unidade de Ecofisiologia, Bioquímica e Biotecnologia Vegetal (Eco-Bio), Centro de Ambiente, Agricultura e Desenvolvimento (BioTrop), Instituto de Investigação Científica Tropical (IICT), Quinta do Marquês, Av. da República, 2784-505 Oeiras, Portugal 3 ESE Almeida Garrett, Grupo Universidade Lusófona, COFAC, Largo do Sequeira, 1100-587 Lisboa, Portugal Abstract Ultraviolet-B (UV-B) radiation constitutes a minor part of the solar spectrum, being most of this solar radiation absorbed by the UV-screening stratospheric ozone layer. Yet, a global depletion of the ozone layer, largely due to the release of chlorofluorocarbons caused by human activities, has resulted in an increase of solar UV-B radiation at the earth’s surface. Accordingly, in the temperate latitudes, such ozone decrease reached ca. 3% and 6% in the North and South hemispheres, respectively, between 2002 and 2005 (as compared to the 1970s). Despite the uncertainty of long-term predictions, it is also estimated an UV-B increase of 5-10% over temperate latitudes within the next 15 years In this context, this work aim at to present an overview of plants sensitivity/tolerance to UV-B irradiation mostly considering the key photosynthetic metabolism. Key words: Ozone depletion, Photosynthesis, Reactive oxygen species, Ultraviolet-B radiation Introduction Ozone depletion and UV-B radiation on Earth’s surface Most of UV radiation does not reach Earth’s surface due to its interaction to the atmospheric components. In fact, UV-C radiation might be completely absorbed by the atmospheric gases, UV- B radiation is absorbed by the stratospheric ozone layer, whereas UV-A radiation is hardly absorbed by this layer. The ozone layer can be depleted by free radical catalysts, including nitric oxide, nitrous oxide and hydroxyl, as well as atomic chlorine and bromine. Although there are natural sources for all of these species, the concentrations of chlorine and bromine have increased markedly in recent years due to the release of large quantities of man-made organohalogen compounds, especially chlorofluorocarbons, with a half-life ranging from 50 to 150 years (Madronich et al., 1998). These highly stable compounds are capable of surviving the rise to the stratosphere, where Cl and Br radicals are produced by the action of UV radiation. Each radical is then able to initiate and catalyze a chain reaction capable of breaking down over 100,000 ozone molecules. Such breakdown of ozone molecules in the stratosphere can result in a decrease of the effectiveness of UV radiation absorption and therefore more radiation reaches the Earth, with each 1% reduction in ozone causing an increase of 1.3- 1.8% in UV-B radiation reaching the biosphere (Caldwell and Flint, 1994; McKenzie et al., 2003). Nowadays ozone levels over the northern hemisphere have been dropping by 4% per decade. Much higher seasonal declines have been measured in approximately 5% of the Earth's surface, around the north and south poles, constituting the so called ozone-holes. Current stratospheric ozone levels are at the lowest point since measurements began in 1970s and global terrestrial UV-B radiation levels range between 0 and 12 kJm −2 on a given day, with near Equator and mid-latitudes receiving higher doses (McKenzie et al., 2011). The changes in ozone and UV-B are not uniform over the Earth’s surface (Figure 1). The ozone concentrations in the high latitudes (comprising Antarctic and Arctic regions) are 40–50% lower than the pre-1980 values; in the mid-latitudes (35–60ºN and 35–60ºS) are 3–6% lower than the pre-1980 values; and, at the Equator, show minimum changes (Forster et al., 2011). Received 22 May 2012; Revised 12 June 2012; Accepted 13 July 2012 *Corresponding Author Fernando J. C. Lidon Departamento de Ciências e Tecnologia da Biomassa, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Quinta da Torre, 2829-514 Caparica, Portugal Email: fjl@fct.unl.pt