ORIGINAL ARTICLE Nikolai G. Bukhov Æ Sridharan Govindachary Subramanyam Rajagopal Æ David Joly Robert Carpentier Enhanced rates of P700 + dark-reduction in leaves of Cucumis sativus L. photoinhibited at chilling temperature Received: 19 July 2003 / Accepted: 29 October 2003 / Published online: 18 December 2003 Ó Springer-Verlag 2003 Abstract The changes in electron transport within pho- tosystem I (PSI) were studied in detached leaves of Cucumis sativus L. during the course of irradiation with moderate white light (300 lmol photons m )2 s )1 ) at 4°C. When intact leaves were exposed to the combina- tion of moderate light and low temperature, the ampli- tude of far-red light-induced P700 absorbance changes at 820 nm (DA 820 ), a relative measure of PSI, progres- sively decreased as the light treatment time increased. Almost no oxidation of P700 was noticeable after 5 h. Methyl viologen accelerated the oxidation of P700 to a steady-state level and also increased the magnitudes of DA 820 changes in photoinhibited leaves, reflecting the rapid removal of electrons from native carriers. Pho- toinhibition under moderate light and chilling tempera- ture also accelerated the rate of P700 + reduction after far-red light excitation as the half-times of the two exponential components of P700 + decay curves de- creased relative to the control ones. A detailed analysis of the kinetics of P700 + reduction using diuron alone or the combination of diuron and methyl viologen strongly favours an increased rate of electron donation from stromal reductants to PSI through the plastoquinone pool following photoinhibitory treatment. Importantly, the marked acceleration of P700 + re-reduction is the consequence of the irradiation of leaf segments at low temperature and not caused by chilling stress alone. Keywords Chilling stress Æ Cucumis Æ P700 Æ Photoinhibition Æ Photosystem I Abbreviations A 0 and A 1 Primary acceptor chlorophyll and secondary electron acceptor phylloquinone Æ FR Far-red light Æ F X , F A , and F B Iron–sulfur centers Æ MT Multiple-turnover flash Æ MV Methyl viologen Æ Ndh NAD(P)H-dehydrogenase Æ PQ Plastoquinone Æ PS Photosystem Æ P700 Reaction-center chlorophyll of PSI Æ ST Single-turnover flash Introduction Light energy provides the driving force for photosyn- thesis. Paradoxically, light is also harmful to oxygenic photosynthetic organisms as excessive irradiation gen- erally leads to over-saturation of photosynthetic light reactions, which causes photoinhibitory damage. Under these conditions, photosynthetic electron transport generates reactive oxygen species (Asada 1994) that are capable of damaging the components of the photosyn- thetic machinery itself as well as a variety of other enzymic reactions at multiple sites. Under conditions favourable for photosynthesis, various protective mechanisms operate in the chlorop- lasts to minimize overreduction of electron transport intermediates. Among these mechanisms, reversible regulation of light harvesting in photosystem II (PSII) that decreases the flow of electrons into the electron transport chain and reversible activation of electron transport on the acceptor side of photosystem I (PSI) gain more relevance (Foyer et al. 1992; Horton et al. 1996). These protective mechanisms are effective under normal conditions. However, this is not the case when leaves experience chilling stress. Particularly, the com- bination of low temperature with weak or moderate light increases the severity of photoinhibition as it reduces linear electron transport, carbon reduction, and sinks for absorbed excitation energy (Allen and Ort 2001). PSI appears to be the primary target of photoin- hibitory damage at temperatures as low as 4–6°C (Havaux and Davaud 1994). At these chilling Planta (2004) 218: 852–861 DOI 10.1007/s00425-003-1165-6 N. G. Bukhov Æ S. Govindachary Æ S. Rajagopal Æ D. Joly R. Carpentier (&) Groupe de Recherche en E ´ nergie et Information Biomole´culaires, Universite´ du Que´bec a` Trois-Rivie`res, Que´bec, G9A 5H7, Canada E-mail: robert_carpentier@uqtr.ca Fax: +1-819-3765057 N. G. Bukhov K.A. Timiriazev Institute of Plant Physiology, Russian Academy of Sciences, Botanicheskaya 35, 127276 Moscow, Russia