PARENT SESSION
Posters P6C Photosynthesis, respiration and alternative electron sinks. Abstracts (660-680)

Responses of photosynthesis in Vicia faba leaves to respiratory inhibitors. Keisuke Yoshida^*,1, Ko Noguchi¹, Ichiro Terashima¹, ¹ Department of Biology, Graduate School of Science, Osaka University, 1-1 Machikaneyama-cho Toyonaka Osaka 560-0043 Japan, Machikaneyama-cho Toyonaka, Osaka, Japan

ABSTRACT- Recent studies have been claiming that functions of the respiratory chain are essential to optimal photosynthesis. Two roles of the respiratory chain in light have been suggested: (1) supplying ATP to the cytosol for sucrose synthesis and (2) dissipation of excess redox equivalents produced in chloroplasts. However, these hypotheses have been put forward based on the experiments with mesophyll protoplasts. Researches employing intact leaves to elucidate in vivo mechanisms of interactions between photosynthesis and mitochondrial respiration have been awaited for a long time. Very recently, we have devised a liquid-phase leaf disc oxygen-electrode system for concurrent measurements of oxygen exchange and chlorophyll fluorescence in intact leaves. Using this system, we evaluated contributions of the respiration chain to photosynthesis. We used Vicia faba leaves. A buffer in the presence or absence of respiratory inhibitors was vacuum-infiltrated into a leaf disc without its lower epidermis and placed in the electrode cuvette. When n-propyl gallate, an inhibitor of the alternative oxidase (AOX), was added at the final conc. 1 mM, the photosynthetic rate was decreased by approximately 40% at a saturated light intensity. Photochemical quenching also decreased, whereas non-photochemical quenching increased. Further an addition of antimycin A, an inhibitor of the cytochrome pathway, caused these effects more strongly. These results showed that the photosynthetic electron transport chain was over-reduced by the inhibition of the respiratory electron flow, whereby the photosynthetic rate decreased. That is, the respiratory chain would dissipate the excess redox equivalents and contribute to maintain the photosynthetic electron transport chain in more oxidized state. Therefore, we conclude that the mitochondrial respiratory chain plays an essential role in optimizing photosynthesis in vivo as well as in vitro.

KEY WORDS: improved liquid-phase oxygen electrode, respiratory chain, chlorophyll fluorescence, photosynthetic electron transport