Journal of Plant Physiology 162 (2005) 181—194 Biophysical studies of photosystem II-related recovery processes after a heat pulse in barley seedlings (Hordeum vulgare L.) SzilviaZ.To´th a,b,à , Gert Schansker a , Judit Kissimon b , La ´szlo´Kova ´cs c , Gyo + zo + Garab c , Reto J. Strasser a a Laboratory of Bioenergetics, University of Geneva, Chemin des Embrouchis 10, CH-1254 Jussy, Switzerland b Faculty of Horticultural Science, Department of Molecular Plant Biology,Szent Istva´nUniversity,PO Box 53, Budapest H-1518, Hungary c Institute of Plant Biology, Biological Research Center of the Hungarian Academy of Sciences, PO Box 521, Szeged H-6701, Hungary Received 7 April 2004; accepted 25 June 2004 This paper is dedicated to Ga´bor Horva´th (1944–2000) Summary Leaves of 7-day-old barley seedlings were subjected to heat pulses at 50 1C for 20 or 40 s to inhibit partially or fully the oxygen evolution without inducing visible symptoms. By means of biophysical techniques, we investigated the time course and mechanism of photosystem II (PSII) recovery. After the heat treatment, the samples were characterized by typical heat stress symptoms: loss of oxygen evolution activity, strong decrease of F v /F m , induction of the K-step in the fluorescence induction transient, emergence of the A T -thermoluminescence-band and a dramatic increase in membrane permeability. In the first 4 h in the light following the heat pulse, the A T -band and the K-step disappeared in parallel, indicating the loss of this restricted activity of PSII. This phase was followed by a recovery period, during which PSII-activity was gradually restored in the light. In darkness, no recovery, except for the membrane permeability, was observed. A model is presented that accounts for (i) the damage induced by the heat ARTICLE IN PRESS www.elsevier.de/jplph KEYWORDS Chlorophyll-a fluorescence induction; Heat stress; Photosystem II; Recovery; 0176-1617/$ - see front matter & 2004 Elsevier GmbH. All rights reserved. doi:10.1016/j.jplph.2004.06.010 Abbreviations: DA 515 , flash-induced electrochromic absorbance transient; APX, ascorbate peroxidase; F 100ms , fluorescence intensity at 100 ms; GPX, guaiacol peroxidase; LHCII, light-harvesting chlorophyll a/b protein complex of PSII; MDA, malondialdehyde; P680 and P700, reaction center chlorophylls of PSII and PSI, respectively; PSI, photosystem I; PSII, photosystem II; Q A and Q B , primary and secondary electron acceptor quinones of PSII, respectively; SOD, superoxide dismutase; TBARS, thiobarbituric-acid reactive substances; TL, thermoluminescence à Corresponding author. Laboratory of Bioenergetics, University of Geneva, Chemin des Embrouchis 10, CH-1254 Jussy, Switzerland. Tel: +41-(0)22-759-9940; fax: +41-(0)22-759-9945. E-mail address: toth1@etu.unige.ch(S.Z.To´th).