ORIGINAL ARTICLE Sun Mi Choi Æ Suk Won Jeong Æ Won Joong Jeong Suk Youn Kwon Æ Wah Soon Chow Æ Youn-Il Park Chloroplast Cu/Zn-superoxide dismutase is a highly sensitive site in cucumber leaves chilled in the light Received: 20 October 2001 / Accepted: 15 February 2002 / Published online: 10 August 2002 Ó Springer-Verlag 2002 Abstract Light-chilling stress, the combination of low- light illumination and low temperature, preferentially inactivated photosystem I (PSI) of cucumber (Cucumis sativus L.) leaves, resulting in the photoinhibition of photosynthesis. The extent of PSI photoinhibition, de- termined in vivo by monitoring absorption changes around 810 nm (induced by far-red light), was closely correlated with the redox state of the PSII electron ac- ceptor Q A , measured as the chlorophyll fluorescence parameter, 1–qP, where qP is a photochemical quench- ing coefficient. In contrast, the decrease in the far-red- induced leaf absorptance signal was not well correlated with the limited fragmentation of the PsaA/B gene products in the PSI reaction center after the light-chill- ing stress. Amongst various enzymes involved in the photooxidative damage such as superoxide dismutase (SOD), ascorbate peroxidase, and NAD(P)H dehydro- genase, only SOD was inhibited by light-chilling treat- ment. Further, an approximately 3-fold increase in the leaf content of H 2 O 2 , a potent inhibitor of Cu/Zn-SOD, was observed after light-chilling stress. From these re- sults, we suggest that Cu/Zn-SOD is the primary target of the light-chilling stress, followed by subsequent inactivation of PSI by reactive oxygen species. Keywords Cucumis Æ Cu/Zn-superoxide dismutase Æ Light-chilling Æ Photosystem I Æ Photoinhibition Abbreviations APX: ascorbate peroxidase Æ D1 protein: psbA gene product Æ F: chlorophyll (Chl) fluorescence yield during illumination Æ F o , F m : minimum and maxi- mum Chl fluorescence yield corresponding to open and closed PSII, respectively, after dark pre-treatment; F v =F m –F o Æ NDH: NAD(P)H dehydrogenase Æ NPQ: non-photochemical quenching parameter Æ P700: special Chl pair in the PSI reaction center Æ PS: photo- system Æ Q A : primary quinone acceptor in PSII Æ qP: photochemical quenching coefficient Æ ROS: reactive oxygen species Æ SOD: superoxide dismutase. Introduction Photosynthesis in higher plants comprises assimilatory and non-assimilatory electron transport pathways. While assimilatory pathways include photosynthetic carbon (C), nitrogen (N) and sulfur (S) reduction, the non-assimilatory encompass photorespiration and the Mehler reaction (Heldt 1997). The Mehler reaction is a reduction of diatomic oxygen to the superoxide radical on the reducing side of PSI, inevitably occurring in all oxygenic photosynthetic organisms, and it constitutes at least 10–15% of the total electron flux through PSII even under a saturating concentration of NADP + (Asada and Takahashi 1987) and of CO 2 (Park et al. 1996a). The balance between the influx of excitation energy, and the utilization of reducing power (NAD- PH) and chemical energy (ATP) in C-, N- and S-as- similation in the chloroplast determines the extent of electron flux to oxygen, and hence varies with envi- ronmental stresses. Under conditions where various abiotic stresses hin- der the consumption of reducing power, the fraction of electron flux to oxygen is increased. For instance, water stress decreases the CO 2 supply for CO 2 fixation, thereby suppressing the consumption of energy for CO 2 -assimi- lation. When these unfavorable conditions are combined with high light, the production of superoxide ( Æ O 2 – ), and of H 2 O 2 catalyzed by superoxide dismutase (SOD), is Planta (2002) 216: 315–324 DOI 10.1007/s00425-002-0852-z S.M. Choi Æ S.W. Jeong Æ Y.-I. Park (&) Department of Biology, Chungnam National University, Daejeon 305-764, Korea E-mail: yipark@cnu.ac.kr Fax: +82-42-8229690 S.W. Jeong Æ W.J. Jeong Æ S.Y. Kwon Korea Research Institute for Bioscience and Biotechnology, Plant Cell Biotechnology Laboratory, Daejeon 305-333, Korea W.S. Chow Photobioenergetics Group, Research School of Biological Sciences, Australian National University, GPO Box 475, Canberra, ACT 2601, Australia