Plant Science 167 (2004) 925–935 Biochemical and ultrastructural changes in leaves of potato plants grown under supplementary UV-B radiation Isabel Santos ∗ , Fernanda Fidalgo, José M. Almeida, Roberto Salema Departamento de Bot ˆ anica and Instituto de Biologia Molecular e Celular (IBMC), Universidade do Porto, Rua do Campo Alegre 823, 4150-180 Porto, Portugal Received 29 January 2004; received in revised form 16 April 2004; accepted 25 May 2004 Available online 19 June 2004 Abstract As tolerance to UV-B radiation involves many mechanisms, changes in several parameters associated to plant protection against UV-B radiation were studied in leaves of potato plants, exposed to this radiation. UV-B exposure increased constitutive flavonoids and two new types were induced. Chlorophyll amount, as well as, total protein content, was slightly decreased. However, the synthesis of a 34kDa polypeptide was induced. Leaf area diminished, leaf dry weight and leaf thickness increased, but the gross anatomy was not changed, neither was the structural integrity of the cells. All sub-cellular structures maintained their integrity although some changes were detected. In guard cells, the fractional volume of both plastids and starch was reduced, whereas thylakoids increased. The appearance of paracrystalline inclusions in peroxisomes in both epidermal and palisade cells was conspicuous. The fractional volume of both starch and chloroplasts in palisade cells decreased. The activity of the antioxidant enzymes catalase, ascorbate peroxidase and guaiacol peroxidase increased associated with the induction of a new catalase isoform and three new guaiacol isoperoxidases. Our results show that potato plants activate several defence systems, altogether contributing to the preservation of cell structural integrity, suggesting that this encompassing defence response enables plants to cope with UV-B aggression. © 2004 Elsevier Ireland Ltd. All rights reserved. Keywords: Catalase; Flavonoids; Peroxidase; Potato; Ultrastructure; UV-B radiation 1. Introduction The stratospheric ozone layer is the principal agent ab- sorbing ultraviolet radiation in the Earth’s atmosphere and the thinning of this layer has led to an increase in solar UV-B radiation (280–320 nm) reaching the Earth’s surface. The impact of UV-B radiation on growth, development, biomass accumulation, yield and metabolism of plants has been stud- ied by several research groups (references in [1,2]). Studies have revealed considerable variation in sensitivity to UV-B within, as well as between species [3–5]. Measurements of some physiological parameters such as the level of UV-B absorbing compounds and chlorophyll content have proved useful indicators of UV-B tolerance or sensitivity [6]. The differential sensitivity of plants is partially explained by ∗ Corresponding author. Tel.: +351 22 607 4900; fax: +351 22 609 9157. E-mail address: isantos@ibmc.up.pt (I. Santos). their ability to respond to UV-B through the induction of de- fensive pathways plus increases in protective pigments and in leaf thickness [2,7–10]. There is evidence that flavonoids reduce damage from UV-B radiation because they act as UV filters, reducing the penetration of potentially damaging UV-B radiation. Mutants of Arabidopsis lacking flavonoid production are hypersensitive to UV-B radiation whereas an Arabidopsis mutant possessing constitutive elevated ac- cumulation of flavonoids and other phenolics is tolerant to lethal UV-B level [9,11]. Plant capability to accumulate UV-B absorbing compounds and the readiness of this ac- cumulation has been correlated with UV-B tolerance [12]. Several recent studies have shown that UV-B radiation cause increase in the level of cellular reactive oxygen species generating oxidative stress and it is generally accepted that the mechanism of UV-B toxicity involves oxidative damage (review in [13,14]). Superoxide dismutase, ascorbate perox- idase and catalase are important enzymes that protect plants against oxidative damage. The behaviour of this antioxidant 0168-9452/$ – see front matter © 2004 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.plantsci.2004.05.035