Cyclic electron flow within PSII functions in intact chloroplasts from spinach leaves.
Using thylakoid membranes, we previously demonstrated that accumulated electrons in the photosynthetic electron transport system induces the electron flow from the acceptor side of PSII to its donor side only in the presence of a pH gradient ((Delta)pH) across the thylakoid membranes. This electron flow has been referred to as cyclic electron flow within PSII (CEF-PSII) [Miyake and Yokota (2001) Plant Cell Physiol. 42: 508]. In the present study, we examined whether CEF-PSII operates in isolated intact chloroplasts from spinach leaves, by correlating the quantum yield of PSII [Phi(PSII)] with the activity of the linear electron flow [V(O(2))]. The addition of the protonophore nigericin to the intact chloroplasts decreased Phi(PSII), but increased V(O(2)), and relative electron flux in PSII [Phi(PSII) x PFD] and V(O(2)) were proportional to one another. Phi(PSII) x PFD at a given V(O(2)) was much higher in the presence of (Delta)pH than that in its absence. These effects of nigericin on the relationship between Phi(PSII) x PFD and V(O(2)) are consistent with those previously observed in thylakoid membranes, indicating the occurrence of CEF-PSII also in intact chloroplasts. In the presence of (Delta)pH, CEF-PSII accounted for the excess electron flux in PSII that could not be attributed to photosynthetic linear electron flow. The activity of CEF-PSII increased with increased light intensity and almost corresponded to that of the water-water cycle (WWC), implying that CEF-PSII can dissipate excess photon energy in cooperation with WWC to protect PSII from photoinhibition under limited photosynthesis conditions.
Graduate School of Bioresource and Bioenvironmental Sciences, Kyushu University, 6-10-1, Hakozaki, Higashi-ku, Fukuoka, 812-8581 Japan. cmiyake@brs.kyushu-u.ac.jp
Chikahiro
C
Miyake
Kuniaki
K
Yonekura
Yoshichika
Y
Kobayashi
Akiho
A
Yokota
D002734:Chlorophyll / Q000378:metabolism; D002736:Chloroplasts / Q000187:drug effects / Q000378:metabolism* / Q000528:radiation effects; D004579:Electron Transport / Q000187:drug effects / Q000528:radiation effects; D006863:Hydrogen-Ion Concentration; D007476:Ionophores / Q000494:pharmacology; D008027:Light; D045342:Light-Harvesting Protein Complexes; D009550:Nigericin / Q000494:pharmacology; D010100:Oxygen / Q000378:metabolism*; D010788:Photosynthesis / Q000187:drug effects / Q000502:physiology / Q000528:radiation effects; D045322:Photosynthetic Reaction Center Complex Proteins / Q000187:drug effects / Q000378:metabolism* / Q000528:radiation effects; D018515:Plant Leaves / Q000187:drug effects / Q000378:metabolism* / Q000528:radiation effects; D018724:Spinacia oleracea / Q000187:drug effects / Q000378:metabolism* / Q000528:radiation effects; D014867:Water / Q000494:pharmacology / Q000502:physiology
D007476:Ionophores; D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D014867:Water; D002734:Chlorophyll; D009550:Nigericin; D010100:Oxygen