Poster: Photosynthesis

Abs # 309: Oxygenindispensable to start photosynthetic CO2-fixation in maize

Presenter:	Kozi, Asada , asada@bt.fubt.fukuyama-u.ac.jp
Authors	Kozi, Asada  (A)   Tomohiro, Ohwaki  (A)
Affiliations:	(A): Dept Biotech, Faculty Life Sci & Biotech, Fukuyama Univ Univ

To understand possible role of the water-water cycle in chloroplasts of maize leaves that do not require oxygen for photorespiration, the effect of oxygen concentrations on CO2-fixation was surveyed. In a range between 20.9% and 0.2%, little effect of oxygen concentrations was observed on the CO2-fixation at the steady state including its induction kinetics, stomata opening and chlorophyll fluorescence parameters. However, below 0.2% oxygen, the CO2-fixation and the stomata opening was suppressed to below 15% with little induction kinetics as compared with that in air, and the half inhibition of the CO2-fixation was found at 0.1% oxygen. In the low oxygen, the electron flux through PSII was very low, but that through PSI was maintained at some levels indicating the operation of the cyclic electron flow. This requirement of oxygen, however, was not observed when the gas phase was changed to anaerobic state after the steady state of CO2-fixation was attained. If the gas phase was changed to anaerobic state while the short dark time after attaining CO2-fixation at the steady state, the oxygen was again required to start CO2-fixation. These observations indicate that oxygen is necessary to start CO2-fixation in maize, and the oxygen-initiated electron flux through the water-water cycle in mesophyll chloroplasts participates in the activation of the CO2-fixation cycle. The present results do not indicate whether the operation of the water-water cycle is indispensable or not to keep the CO2-fixation at the steady state, but, if it is necessary, the oxygen required for the water-water cycle is would be supplied by photosystem II.