The oxygen flash yield (Y
O2) and photochemical yield of PS II (
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
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 O
2 became temporarily uncoupled.
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 (
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.Abbreviations ADRY
acceleration of the deactivation reactions of the water-splitting enzyme (agents)
- Chl
chlorophyll
- cyt
cytochrome
- DCMU
3-(3,4-dichlorophenyl)-1,1-dimethylurea
- F
O
minimum fluorescence yield in the dark-adapted state
- F
I
minimum fluorescence yield under ambient irradiance or during transition from the light-adapted state
- F
M
maximum fluorescence yield in the dark-adapted state
- F
M
maximum fluorescence yield under ambient irradiance or during transition from light-adapted state
- F
V, F
V
variable fluorescence (F
V=F
M–F
O ; F
V=F
M–F
I)
- FRR
fast repetition rate (fluorometer)
-
PS II
quantum yield of Q
A reduction (
PS II=(F
M – F
O)/F
M or
PS II)=(F
M= – F
I=)/F
M=)
- LHCII
Chl
a/b light harvesting complexes of Photosystem II
- OEC
oxygen evolving complex of PS II
- P680
reaction center chlorophyll of PS II
- PQ
plastoquinone
- POH
2
plastoquinol
- PS I
Photosystem I
- PS II
Photosystem II
- RC II
reaction centers of Photosystem II
- PS II
the effective absorption cross-section of PHotosystem II
- TL
thermoluminescence
- Y
O2
oxygen flash yield
The US Government right to retain a non-exclusive, royalty free licence in and to any copyright is acknowledged.
相似文献