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Cyclic electron flow within PSII functions in intact chloroplasts from spinach leaves
Chikahiro Miyake1, Kuniaki Yonekura, Yoshichika Kobayashi
1Graduate School of Bioresource and Bioenvironmental Sciences, Kyushu University, 6-10-1, Hakozaki, Higashi-ku, Fukuoka, 812-8581 Japan. cmiyake@brs.kyushu-u.ac.jp
Plant & Cell Physiology
|August 29, 2002
Summary
Cyclic electron flow within PSII (CEF-PSII) operates in intact chloroplasts, utilizing pH gradients to redirect electron flow and protect photosynthesis. This process complements the water-water cycle in dissipating excess light energy.
Area of Science:
- Plant Physiology
- Photosynthesis Research
- Chloroplast Biology
Background:
- Accumulated electrons in the photosynthetic electron transport system can induce electron flow from the acceptor side of Photosystem II (PSII) to its donor side.
- This process, termed cyclic electron flow within PSII (CEF-PSII), was previously shown to require a pH gradient across thylakoid membranes.
Purpose of the Study:
- To investigate the operation of CEF-PSII in isolated intact chloroplasts from spinach leaves.
- To correlate the quantum yield of PSII (Phi(PSII)) with the activity of linear electron flow (V(O(2))) to understand CEF-PSII's role.
Main Methods:
- Utilized isolated intact spinach chloroplasts.
- Correlated PSII quantum yield (Phi(PSII)) with linear electron flow (V(O(2))) measurements.
- Applied the protonophore nigericin to manipulate pH gradients and observed effects on electron flow dynamics.
Main Results:
- Nigericin addition decreased Phi(PSII) but increased V(O(2)) in intact chloroplasts, mirroring observations in thylakoid membranes.
- The relationship between relative electron flux in PSII (Phi(PSII) x PFD) and V(O(2)) indicated CEF-PSII occurrence in intact chloroplasts.
- CEF-PSII activity increased with light intensity and, in the presence of a pH gradient, accounted for excess electron flux not explained by linear electron flow.
- CEF-PSII activity was comparable to the water-water cycle (WWC).
Conclusions:
- CEF-PSII operates in intact spinach chloroplasts, requiring a pH gradient for its activity.
- CEF-PSII functions to dissipate excess photon energy, potentially in conjunction with the water-water cycle, offering photoprotection under light stress conditions.