PHOTOSYNTHETICA 41 (4): 571-578, 2003 Moderately elevated temperature eliminates resistance of rice plants with enhanced expression of glutathione reductase to intensive photooxidative stress R. KOUŘIL * , D. LAZÁR *,+ , H. LEE ** , J. JO **,++ , and J. NAUŠ * Laboratory of Biophysics, Faculty of Science, Palacký University, tř. Svobody 26, 771 46 Olomouc, Czech Republic * Department of Animal Science, Kyungpook National University, Daegu 702701, South Korea ** Abstract We developed transgenic rice plants (Oryza sativa L. cv. Daeribbyeo) overproducing cytosolic glutathione reductase (GR) using a GR gene from Brassica campestris and studied their response to photo-oxidative stress in the presence of methyl viologen (MV, 10 and 50 µM concentrations) under room (25 °C) and moderately elevated (35 °C) temperature by analysis of chlorophyll (Chl) a fluorescence parameters (F V /F M , q N , and q P ) and of Chl content. Elevated temperature enhanced and accelerated the photo-oxidative damage to photosynthetic apparatus expressed mainly by a fast decrease of q N . Higher temperature supported the protective reaction in transformed rice plants for lower MV concentration (10 µM) and eliminated the enhanced tolerance of photosystem 2 photochemistry to photooxidative stress for higher (50 µM) MV concentration. Different mechanisms and temperature dependence of oxidative and protective reactions ex- plain the results. Additional key words: active oxygen species; chlorophyll fluorescence; Oryza sativa; photochemical and non-photochemical fluores- cence quenching; transgenic plants. Introduction In general, exposure of plants to biotic or abiotic stress leads to the increased production of active oxygen species (AOS) which can induce a structural and functional da- mage of photosynthetic enzymes and components of pho- tosynthetic apparatus (for reviews see Foyer and Harbinson 1994, Foyer et al. 1994a, Foyer and Noctor 2000). In chloroplasts, several effective mechanisms that eliminate formation of AOS and protect photosynthetic machinery from oxidative damage were found. Involve- ment of β-carotene or zeaxanthin in quenching of excited state of singlet oxygen or triplet chlorophylls plays an im- portant role in protection of photosystem 2 (PS2) against oxidative stress under high irradiance (for reviews see Foyer et al. 1994a,b, Pospíšil 1997). On the reducing side of photosystem 1 (PS1), superoxide generated in Mehler reaction (Mehler 1951) is effectively scavenged by super- oxide dismutase (SOD), producing H 2 O 2 , which is con- sequently reduced to water through the action of ascor- bate peroxidase (APX). Regeneration of ascorbate can proceed via the reduction by ferredoxin, by the action of monodehydroascorbate reductase, or via the ascorbate- glutathione cycle, which includes the regeneration of glu- tathione catalysed by glutathione reductase (GR), at the expense of NADPH (for reviews see Asada 1999, 2000, Foyer and Noctor 2000). ——— Received 2 January 2003, accepted 11 August 2003. + To whom correspondence for photo-oxidative stress should be addressed; fax: +420 585225737, e-mail: lazard@seznam.cz ++ To whom correspondence for plant genetic engineering should be addressed; fax: 82-53-950-6750, e-mail: jkjo@knu.ac.kr Abbreviations: AOS, active oxygen species; APX, ascorbate peroxidase; Chl, chlorophyll; DHA, dehydroascorbate; F 0 , minimal Chl fluorescence intensity of the dark-adapted leaf segment; F M , maximal Chl fluorescence intensity of the dark-adapted leaf segment; F 0 ’ , minimal Chl fluorescence intensity of the light-adapted leaf segment; F M ', maximal Chl fluorescence intensity of the light-adapted leaf segment; F V /F M = (F M – F 0 )/F M , maximal quantum yield of photosystem 2 photochemistry; GR, glutathione reductase; MV, methyl viologen; PAR, photosynthetically active radiation; PS, photosystem; Q A , primary stable quinone acceptor of PS2; q P and q N , photochemical and non-photochemical Chl fluorescence quenching coefficients; SOD, superoxide dismutase; WT, wild type. Acknowledgement: This work was supported by the grant of the Ministry of Education of the Czech Republic No. MSM 153100010. D.L. thanks the Grant Agency of the Czech Republic (project 204/02/P071) for financial support. This work was also supported by Korea Research Foundation (2000-005-G00003). 571