Plant and Cell Physiology Advance Access originally published online on October 20, 2005
Plant and Cell Physiology 2006 47(1):22-31; doi:10.1093/pcp/pci219
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Distinct Roles of the Cytochrome Pathway and Alternative Oxidase in Leaf Photosynthesis
Department of Biology, Graduate School of Science, Osaka University, 1–1 Machikaneyama-cho, Toyonaka, Osaka, 560-0043 Japan
1 Corresponding author: E-mail, kyoshi80{at}bio.sci.osaka-u.ac.jp; Fax, +81-6-6850-5808.
In illuminated leaves, mitochondria are thought to play roles in optimizing photosynthesis. However, the roles of the cytochrome pathway (CP) and alternative oxidase (AOX) in photosynthesis, in particular in the redox state of the photosynthetic electron transport chain, are not separately characterized. We examined the effects of specific inhibition of two respiratory pathways, CP and AOX, on photosynthetic oxygen evolution and the redox state of the photosynthetic electron transport chain in broad bean (Vicia faba L.) leaves under various light intensities. Under saturating photosynthetic photon flux density (PPFD; 700 µmol photon m–2 s–1), inhibition of either pathway caused a decrease in the steady-state levels of the photosynthetic O2 evolution rate and the PSII operating efficiency, 
II. Because these inhibitors, at the concentrations applied to the leaves, had little effect on photosynthesis in the intact chloroplasts, two respiratory pathways are essential for maintenance of high photosynthetic rates at saturating PPFD. CP or AOX inhibition affected to Chl fluorescence parameters (e.g. photochemical quenching and non-photochemical quenching) differently, suggesting that CP and AOX contribute to photosynthesis in different ways. At low PPFD (100 µmol photon m–2 s–1), only the inhibition of AOX, not CP, lowered the photosynthetic rate and II. AOX inhibition also decreased the II/I ratio even at low PPFD levels. These data suggest that AOX inhibition caused the over-reduction of the photosynthetic electron transport chain and induced the cyclic electron flow around PSI (CEF-PSI) even at the low PPFD. Based on these results, we discuss possible roles for CP and AOX in the light.
(Received July 9, 2005; Accepted October 17, 2005)
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