Cyclic electron flow around Photosystem II in vivo

Ondrej Prasil, Zbigniew Kolber, Joseph A. Berry, Paul G. Falkowski

Research output: Contribution to journalArticlepeer-review

116 Scopus citations


The oxygen flash yield (Y(O2)) and photochemical yield of PS II (Φ(PS II)) were simultaneously detected in intact Chlorella cells on a bare platinum oxygen rate electrode. The two yields were measured as a function of background irradiance in the steady-state and following a transition from light to darkness. During steady-state illumination at moderate irradiance levels, Y(O2) and Φ(PS II) followed each other, suggesting a close coupling between the oxidation of water and Q(A) reduction (Falkowski et al. (1988) Biochim. Biophys. Acta 933: 432-443). Following a light-to-dark transition, however, the relationship between Q(A) reduction and the fraction of PS II reaction centers capable of evolving O2 became temporarily uncoupled. Φ(PS II) recovered to the preillumination levels within 5-10 s, while the Y(O2) required up to 60 s to recover under aerobic conditions. The recovery of Y(O2) was independent of the redox state of Q(A), but was accompanied by a 30% increase in the functional absorption cross-section of PS II (σ(PS II)). The hysteresis between Y(O2) and the reduction of Q(A) during the light-to-dark transition was dependent upon the reduction level of the plastoquinone pool and does not appear to be due to a direct radiative charge back-reaction, but rather is a consequence of a transient cyclic electron flow around PS II. The cycle is engaged in vivo only when the plastoquinone pool is reduced. Hence, the plastoquinone pool can act as a clutch that disconnects the oxygen evolution from photochemical charge separation in PS II.

Original languageEnglish (US)
Pages (from-to)395-410
Number of pages16
JournalPhotosynthesis Research
Issue number3
StatePublished - 1996
Externally publishedYes

ASJC Scopus subject areas

  • Biochemistry
  • Plant Science
  • Cell Biology


  • Photosystem II
  • chlorophyll fluorescence
  • cyclic electron transport
  • oxygen evolution
  • quantum yield


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