Photosynthesis Research 78: 77–85, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands. 77 Regular paper Retardation of photo-induced changes in Photosystem I submembrane particles by glycinebetaine and sucrose Subramanyam Rajagopal & Robert Carpentier ∗ Groupe de Recherche en ´ Energie et Information Biomol´ eculaires, Universit´ e du Qu´ ebec ` a Trois-Rivi` eres, C.P. 500 Trois-Rivi` eres, Qu´ ebec, Canada, G9A 5H7; ∗ Author for correspondence (e-mail: Robert_Carpentier@uqtr.ca; fax: +1-819-376-5057) Received 28 January 2003; accepted in revised form 6 June 2003 Key words: chlorophyll protein complexes, co-solutes, photobleaching, photochemical activity, photoinhibition, Photosystem I Abstract The protective role of co-solutes (glycinebetaine and sucrose) against photodamage in isolated Photosystem (PS) I submembrane particles illuminated (2000 µEm −2 s −1 ) for various time periods at 4 ◦ C was studied. The photo- chemical activity of PS I in terms of electron transport measured as oxygen uptake and P700 photooxidation was significantly protected. A photoinduced enhancement of oxygen uptake observed during the first hours of strong light illumination attributed to denaturation or dissociation of membrane-bound superoxide dismutase [Rajagopal et al. (2003) Photochem. Photobiol 77: 284–291] was also retarded by glycinebetaine and sucrose. Chlorophyll photobleaching resulting in a decrease of absorbance and a blue-shift of the absorbance maximum in the red was greatly delayed in the presence of co-solutes. This phenomenon was also observed in the chlorophyll-protein (CP) complexes of PS I particles exposed to strong illumination separated on non-denaturing poly-acrylamide gels. In this case, a decrease in the absorbance of the CP1b band coinciding with an increase of CP1a during the course of illumination and ascribed to oxidative cross-linking (Rajagopal et al. 2003) was also retarded. Our results, thus, clearly demonstrated for the first time that co-solutes could minimize the alteration of photochemical activity and chlorophyll-protein complexes against photodamage of PS I submembranes particles. Abbreviations: Chl – chlorophyll; CP – chlorophyll protein; DCIP-2,6-dichloro phenol indophenol – ; Fe-S – iron sulphur centers; LDS-PAGE – lithium dodecyl sulphate-polyacrylamide gel electrophoresis; LHC I – light harvesting complex I; MV – methyl viologen; NaN 3 – sodium azide; P700 – primary electron donor; PS I and II – Photosystem I and II; WL – white light Introduction Light constitutes the driving force for photosynthesis. However, excess light causes the inactivation of the photosynthetic apparatus (Barber and Andersson 1992; Aro et al. 1993). Even though photosystem (PS II) was reported to be more susceptible to excess light (Powles 1984; Aro et al. 1993), several reports demon- strated that PS I was also equally photoinactivated in chloroplasts exposed to strong light (Satoh 1970a, b; Inoue et al. 1986, 1989). These authors emphasized that strong illumination damaged various sites of PS I. Further, under chilling temperatures, weak light in- duced the inactivation of PS I, whereas PS II was marginally photoinactivated in intact leaves (Sonoike and Terashima 1994; Terashima et al. 1994; Sonoike et al. 1995, Tjus et al. 1999). Under these conditions, inhibition of PS I was observed on both donor and acceptor sides (Sonoike et al. 1995,1997; Tjus et al. 1999). Inactivation of PS I-mediated electron flow was also reported in isolated PS I particles such as spinach