Comparative EPR and thermoluminescence study of anoxic photoinhibition in Photosystem II particles |
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Authors: | Sándor Demeter Jonathan H A Nugent László Kovács Gábor Bernát Michael C W Evans |
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Institution: | (1) Institute of Plant Biology, Biological Research Center, Hungarian Academy of Sciences, P.O.Box 521, H-6701 Szeged, Hungary;(2) Department of Biology (Darwin Building), University College London, London, UK |
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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.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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Keywords: | Photoinhibition Photosystem II quinone-iron complex electron paramagnetic resonance (EPR) thermoluminescence (TL) |
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