Abstract
Photosystem II particles were exposed to 800 W m−2 white light at 20 °C under anoxic conditions. The Fo level of fluorescence was considerably enhanced indicating formation of stable-reduced forms of the primary quinone electron acceptor, QA. The Fm level of fluorescence declined only a little. The g=1.9 and g=1.82 EPR forms characteristic of the bicarbonate-bound and bicarbonate-depleted semiquinone-iron complex, QA −Fe2+, respectively, exhibited differential sensitivity against photoinhibition. The large g=1.9 signal was rapidly diminished but the small g=1.82 signal decreased more slowly. The S2-state multiline signal, the oxygen evolution and photooxidation of the high potential form of cytochrome b-559 were inhibited approximately with the same kinetics as the g=1.9 signal. The low potential form of oxidized cytochrome b-559 and Signal IIslow arising from TyrD + decreased considerably slower than the g=1.9 semiquinone-iron signal. The high potential form of oxidized cytochrome b-559 was diminished faster than the low potential form. Photoinhibition of the g=1.9 and g=1.82 forms of QA was accompanied with the appearance and gradual saturation of the spin-polarized triplet signal of P 680. The amplitude of the radical signal from photoreducible pheophytin remained constant during the 3 hour illumination period. In the thermoluminescence glow curves of particles the Q band (S2QA − charge recombination) was almost completely abolished. To the contrary, the C band (TyrD +QA − charge recombination) increased a little upon illumination. The EPR and thermoluminescence observations suggest that the Photosystem II reaction centers can be classified into two groups with different susceptibility against photoinhibition.
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Abbreviations
- C band:
-
thermoluminescence band associated with Tyr-D+Q a − charge recombination
- Chl:
-
chlorophyll
- DCMU:
-
3-(3,4-dichlorophenyl)-1,1-dimethylurea
- EPR:
-
electron paramagnetic resonance
- Fo :
-
initial fluorescence
- Fm :
-
maximum fluorescence
- Q band:
-
thermoluminescence band originating from S2Q a -charge recombination
- Q a :
-
the primary quinone electron acceptor of PS II
- P 680:
-
the primary electron donor chlorophyll of PS II
- S2 :
-
oxidation state of the water-splitting system
- Phe:
-
pheophytin
- TL:
-
thermoluminescence
- Tyr d :
-
redox active tyrosine-160 of the D2 protein
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Demeter, S., Nugent, J.H.A., Kovács, L. et al. Comparative EPR and thermoluminescence study of anoxic photoinhibition in Photosystem II particles. Photosynth Res 46, 213–218 (1995). https://doi.org/10.1007/BF00020433
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DOI: https://doi.org/10.1007/BF00020433