ORIGINAL ARTICLE Differential effects of severe water stress on linear and cyclic electron fluxes through Photosystem I in spinach leaf discs in CO 2 -enriched air Husen Jia Æ Riichi Oguchi Æ Alexander B. Hope Æ James Barber Æ Wah Soon Chow Received: 16 April 2008 / Accepted: 4 July 2008 / Published online: 18 July 2008 Ó Springer-Verlag 2008 Abstract Linear and cyclic electron fluxes through Pho- tosystem I in 1% CO 2 were quantified in spinach leaf tissue under severe water stress. Using actinic light with a peak at 697 nm for preferential light absorption by Photosystem I while also stimulating Photosystem II to improve redox poising, the cyclic electron flux after 60 s of illumination was a substantial proportion (33–44%) of the total electron flux through PSI at irradiances up to *1,070 lmol photons m -2 s -1 . At the maximum irradiance, the cyclic electron flux changed little with the progressive water loss from leaf tissue up to *60%; by contrast, the linear electron flux was approximately halved. A reason for this differential effect of water stress on the capacity for cyclic and linear electron flow could be the increased crowding of soluble proteins in the stroma due to chloroplast shrinkage. Indeed the con- finement of soluble proteins to a smaller chloroplast volume was indicated by cryo-scanning electron micros- copy. It is known that the diffusion coefficient of large proteins is decreased when the background concentration of small proteins is raised; by contrast, the diffusion coefficient of small proteins is not affected by increasing the concentration of a large protein (Muramatsu and Min- ton in Proc Natl Acad Sci USA 85:2984–2988, 1988). Therefore, we suggest that linear electron flow, being coupled to the Calvin-Benson cycle, is limited by the dif- fusion of large macromolecules, especially the ribulose 1, 5-bisphosphate carboxylase/oxygenase complex. By con- trast, cyclic electron flow, involving relatively small macromolecules such as ferredoxin, is less susceptible to inhibition by crowding in the stroma. Keywords Cyclic electron flow  Linear electron flow  Macromolecular crowding  Photosystem I  Water stress Abbreviations ATP Adenosine 5 0 -triphosphate Chl Chlorophyll DCMU 3-(3,4-Dichlorophenyl)-1,1-dimethylurea ETR Electron transport rate through PSII F and F 0 m The prevailing and maximum relative Chl a fluorescence yield in the light-acclimated state, respectively F 0 v =F 0 m Photochemical efficiency of open PSII reactions in the light-adapted state LED Light-emitting diode MV Methyl viologen NADP + Oxidized nicotinamide adenine dinucleotide phosphate / PSII The average quantum yield of PSII photochemistry P700 Primary electron donor in PSI PSI and PSII Photosystem I and Photosystem II, respectively H. Jia  R. Oguchi  W. S. Chow (&) Photobioenergetics Group, Research School of Biological Sciences, The Australian National University, Canberra, ACT 0200, Australia e-mail: chow@rsbs.anu.edu.au R. Oguchi Plant Sciences, Department of Biological Sciences, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, Japan A. B. Hope School of Biological Sciences, Flinders University, GPO Box 2100, Adelaide, SA 5001, Australia J. Barber Division of Biomolecular Sciences, Faculty of Science, Imperial College, London SW7 2AZ, UK 123 Planta (2008) 228:803–812 DOI 10.1007/s00425-008-0783-4