Determination of the Rate of Photoreduction of O_2 in the Water-Water Cycle in Watermelon Leaves and Enhancement of the Rate by Limitation of Photosynthesis
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概要
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A study was performed to determine how the electron fluxes for the photosynthetic carbon reduction(PCR) and the photorespiratory carbon oxidation(PCO) cycles affect the photoreduction of O_2 at PSI, which is the limiting step in the water-water cycle. Simultaneous measurements were made of CO_2-gas exchange, transpiration and quantum yield of PSII [Φ(PSII)] using leaves of watermelon(Citrullus lanatus). The total electron flux in PSII [Je(PSII)], as estimated from Φ(PSII), was always larger than the total electron flux required for the PCR and PCO cycles at various partial pressures of CO_2 and O_2 and 1, 100 μmol photons m^<-2>s^<-1>. This observation suggested the existence of an alternative electron flux(Ja). Ja was divided into O_2-dependent [Ja(O_2-depend)] and O_2-independent [Ja(O_2-independ)] components. The magnitude of half Ja(O_2-depend), 7.5 to 9.5 μmol e^-m^<-2>s<-1> and its apparent K_m for O_2, about 8.0 kPa, could be accounted for by the photoreduction of O_2 at PSI either mediated by ferredoxin or catalyzed by monodehydroascorbate reductase. The results indicated that Ja(O_2-depend)was driven by the water-water cycle. A decrease in the intercellular partial pressure of CO_2 from 23 to 5.0 Pa at 21 kPa O_2 enhanced Ja(O_2-depend)by a factor of 1.3. Saturation of the activities of both the PCR and PCO cycles by increasing the photon flux density induced Ja. These results indicate the electron flux in PSII that exceeds the flux required for the PCR and PCO cycles induces the photoreduction of O_2 in the water-water cycle.
- 日本植物生理学会の論文
著者
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Miyake Chikahiro
Graduate School of Biological Science, Nara Institute of Science and Technology
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Yokota Akiho
Graduate School Of Biological Sciences Nara Institute Of Science And Technology
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Miyake Chikahiro
Graduate School Of Biological Sciences Nara Institute Of Science And Technology
-
Yokota Akiho
Graduate School Of Biological Science Nara Institute Of Science And Technology
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Miyake Chikahiro
Graduate School of Biological Sciences, Nara Institute of Science and Technology
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