Emir. J. Food Agric. 2012. 24 (6) http://ejfa.info/ EDITORIAL UV-B radiation: “When does the stressor cause stress?” Fernando J. C. Lidon 1 , Éva Hideg 2 and Marcel A. K. Jansen 3 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 Institute of Biology, University of Pécs, Ifjusag u. 6. H-7624 Pécs, Hungary 3 School of Biological, Earth and Environmental Sciences, University College Cork, North Mall, Cork, Ireland Exposure to UV-B radiation can cause stress in plants Solar UV-B radiation (280 - 315 nm) has long been recognized as being potentially damaging to living organisms. Indeed, the literature on plant UV-B radiation effects has for decades been dominated by reports on UV-B mediated stress, including growth retardation, macroscopic injuries and oxidative damage (Caldwell et al., 1994; Searles et al., 2001). These negative effects comprise damaging effects on genetic material (formation pre-mutagenic cyclobutane pyrimidine dimers [CPD] and pyrimidine [6-4] pyrimidinone dimers), photosynthetic performance (in-tandem degradation of D1-D2 core proteins of photosystem II, inactivation RUBISCO, altered stomatal function) and a range of other cellular targets (see Jansen et al., 1998; Searles et al., 2001; Jordan 2002; Rozema et al., 2005; Jenkins, 2009). In this issue Lidon et al. (2012a) review our current understanding of the deleterious effects of UV-B on photosynthesis. UV-B damage is paralleled by the formation of lipid peroxidation products such as malondialdehyde (MDA) (Hideg et al., 2003; Lidon and Ramalho, 2011) and increased oxidation of antioxidants such as glutathione (Kalbin et al., 1997), both of which reflect the oxidative character of the UV-caused stress conditions. Consistently, UV-induced ROS have been measured using EPR spin trap reporters in both leaves (Hideg and Vass, 1996) and in isolated thylakoids (Lidon et al., 2012b) exposed to high doses of UV-B. UV-B acclimation Many studies have failed to find substantial, negative effects when plants are grown for prolonged periods under realistic levels of UV-B (Ballaré et al., 2011). Consistently, in this issue, Costa et al. (2012) conclude that there is no evidence that increases in UV-B influence wheat production. A major factor responsible for this lack of UV-B damage is the capability of plants to acclimate to ambient levels of UV-B. UV-B acclimation refers to the physiological adjustments that generate tolerance to transitory stress conditions. In the case of UV-B exposure, key components of the acclimation response are the increased capability of photorepair and the accumulation of UV-B absorbing flavonoids and other phenolics. These pigments have long been thought to accumulate mostly in the vacuoles of epidermal cells and to protect underlying tissues by absorbing UV-B photons. More recently, it has been argued that the main protective role of these phenolics is associated with their antioxidative capabilities (Agati and Tattini, 2010), and this fits the observation that flavonoids can be found in tissues not directly exposed to UV-B and also in sub-cellular domains as far apart as chloroplasts, vacuoles and nuclei, and roots and leaves. The UV- B induced increase in antioxidative defenses is further demonstrated by increases in both the reduction state and pool-size for antioxidants such as ascorbate, glutathione, xanthophylls, and tocopherol (Jansen et al., 2008). Moreover, numerous studies have reported upregulation of enzymatic antioxidant activities, including Cu or Zn superoxide dismutase (SOD), ascorbate peroxidase (APX), dehydroascorbate reductase (DHR), glutathione peroxidase (GPX), glutathione reductase (GR) and catalase activities (Hideg et al., 2006; Agrawal and Rathore, 2007; Xu et al., 2008). In this issue, Pessoa (2012) further highlights a range of UV-induced biochemical protection responses in algae and aquatic macrophytes. Interestingly, UV-protection appears to be largely dependent on physiological UV-acclimation. Few studies have reported evidence for UV-B driven genetic adaptation. In this issue Biswas and Jansen (2012) report that adaptation of local Arabidopsis thaliana accessions comprises the altered regulation of UV acclimation, thus again emphasize the relative importance of induced, physiological processes for UV-B protection. Is the concept of “UV-B stress” still relevant? Because of effective acclimation responses, UV-B mediated stress is in many circumstances a