Plant and Cell Physiology, 2002, Vol. 43, No. 9 1017-1026
© 2002 Oxford University Press
Physiological Functions of the Water–Water Cycle (Mehler Reaction) and the Cyclic Electron Flow around PSI in Rice Leaves
1 Graduate School of Agricultural Sciences, Tohoku University, Tsutsumidori-Amamiyamachi, Aoba-ku, Sendai, 981-8555 Japan
2 Graduate School of Bioresource and Bioenvironmental Sciences, Kyusyu University, Hokozaki, Higashi-ku, Fukuoka, 812-8581 Japan
3 Graduate School of Biological Sciences, Nara Institute of Science and Technology, Ikoma, Nara, 630-0101 Japan
Changes in chlorophyll fluorescence, P700+-absorbance and gas exchange during the induction phase and steady state of photosynthesis were simultaneously examined in rice (Oryza sativa L.), including the rbcS antisense plants. The quantum yield of photosystem II (
PSII) increased more rapidly than CO2 assimilation in 20% O2. This rapid increase in PSII resulted from the electron flux through the water–water cycle (WWC) because of its dependency on O2. The electron flux of WWC reached a maximum just after illumination, and rapidly generated non-photochemical quenching (NPQ). With increasing CO2 assimilation, the electron flux of WWC and NPQ decreased. In 2% O2, WWC scarcely operated and PSI was always higher than PSII. This suggested that cyclic electron flow around PSI resulted in the formation of NPQ, which remained at higher levels in 2% O2. The electron flux of WWC in the rbcS antisense plants was lower, but these plants always showed a higher NPQ. This was also caused by the operation of the cyclic electron flow around PSI because of a higher ratio of PSI/PSII, irrespective of O2 concentration. The results indicate that WWC functions as a starter of photosynthesis by generating 
pH across thylakoid membranes for NPQ formation, supplying ATP for carbon assimilation. However, WWC does not act to maintain a high NPQ, and PSII is down-regulated by pH generated via the cyclic electron flow around PSI.
4 Corresponding author: E-mail, makino@biochem.tohoku.ac.jp; Fax: +81-22-717-8765.
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