Photosystem II inhibition by moderate light under low temperature in intact leaves of chilling-sensitive and -tolerant plants Sridharan Govindachary a , Nikolai G. Bukhov a,b , David Joly a and Robert Carpentier a, * a Groupe de Recherche en E ´ nergie et Information Biomole´culaires, Universite´ du Que ´bec a ` Trois-Rivie`res, Que´bec, G9A 5H7, Canada b K.A. Timiriazev Institute of Plant Physiology, Russian Academy of Sciences, Moscow, Russia *Corresponding author, e-mail: robert_carpentier@uqtr.ca Received 8 October 2003; revised 2 December 2003 Photosystem II (PSII) activity was examined in leaves of chilling-sensitive cucumber (Cucumis sativus L.), tomato (Lycopersicum esculentum L.), and maize (Zea mays L.), and in chilling-tolerant barley (Hordeum vulgare L.) illumi- nated with moderate white light (300 mmolm 2 s 1 )at4  C using chlorophyll a fluorescence measurements. PSII activity was inhibited in leaves of all the four plants as suggested by the decline in F v /F m , 1/F o  1/F m , and F v /F o values. The changes in initial fluorescence level (F o ), F v /F m , 1/F o  /1/ F m , and F v /F o ratios indicate a stronger PSII inhibition in cucumber, maize and tomato plants. The kinetics of chloro- phyll a fluorescence rise showed complex changes in the magnitudes and rise of O-J, J-I, and I-P phases caused by photoinhibition. The selective suppression of the J-I phase of fluorescence rise kinetics provides evidence for weakened electron donation from the oxidizing side, whereas the accumulation of reduced Q A suggests damage to the acceptor sideofPSII.Thesefindingsimplythattheprocessofchilling- inducedphotoinhibitioninvolvesdamagetomorethanonesite in the PSII complexes. Furthermore, comparative analyses of the decline in F v /F o and photooxidation of P700 explicitly show that the extent of photoinhibitory damage to PSII and photosystemIissimilarinleavesofcucumberplantsgrownat a low irradiance level. Introduction The light-induced decline in the photochemical activity of photosystem II (PSII) or photosystem I is known as photoinhibition (Aro et al. 1993a, Havaux and Davaud 1994, Terashima et al. 1994). It usually occurs when the light-dependent reactions of photosynthesis produce ATP and NADPH in excess of that can be consumed by the reactions of dark carbon metabolism. Over reduc- tion of the electron transport chain results in the gener- ation of large amounts of active oxygen species, such as superoxide anion radicals or/and singlet oxygen (Asada 1994). Healthy leaves growing under favourable condi- tions can experience intense light without extensive photodamage. However, when the environmental condi- tions do not promote carbon fixation, even weak or moderate light may become harmful (Havaux and Davaud 1994, Terashima et al. 1994, Kudoh and Sonoike 2002). Photosystem II is considered as the primary target for excessive light. Based on in vitro studies, two mechan- isms of photoinhibitory damage have been proposed (for a review, see Ohad et al. 2000). Acceptor side photoinhib- ition originates from the over-reduction of the primary quinone acceptor, Q A , that occurs when the electron flow between Q A – and Q B , the secondary quinone acceptor, is blocked. Once Q A is doubly reduced, it leaves its binding site located in the polypeptide D2. This results in an increased yield of recombination of the primary radical pair P680 1 Pheo – with the conse- quent formation of chlorophyll (Chl) triplets (Styring et al. 1990, Vass et al. 1992). The other mechanism of PHYSIOLOGIA PLANTARUM 121: 322–333. 2004 DOI: 10.1111/j.1399-3054.2004.00305.x Printed in Denmark – all rights reserved Copyright # Physiologia Plantarum 2004 Abbreviations – Chl, chlorophyll; DBMIB, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone; F o , initial level of chlorophyll fluorescence; F m (P), maximum level of chlorophyll fluorescence; FR, far-red; F v (¼ F m  F o ), variable fluorescence; MV, methyl viologen; NPQ, non- photochemical quenching; OEC, oxygen-evolving complex; PAM, pulse amplitude modulated fluorimeter; PEA, plant efficiency analyser; Pheo, pheophytin; P680, primary electron donor of photosystem II; P700, primary electron donor of photosystem I; PQ, plastoquinone; PS, photosystem; Q A and Q B , primary and secondary quinone acceptors of photosystem II. 322 Physiol. Plant. 121, 2004