Photosystem II reaction centres stay intact during low temperature photoinhibition |
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Authors: | Christina Ottander Torill Hundal Bertil Andersson Norman P A Huner Gunnar Öquist |
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Institution: | (1) Department of Plant Physiology, University of Umeå, S-901 87 Umeå, Sweden;(2) Department of Biochemistry, Arrhenius Laboratories, University of Stockholm, S-106 91 Stockholm, Sweden;(3) Department of Plant Sciences, University of Western Ontario, N6A 5B7 London, Ontario, Canada |
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Abstract: | Photoinhibition of photosynthesis was studied in intact barley leaves at 5 and 20°C, to reveal if Photosystem II becomes predisposed to photoinhibition at low temperature by 1) creation of excessive excitation of Photosystem II or, 2) inhibition of the repair process of Photosystem II. The light and temperature dependence of the reduction state of QA was measured by modulated fluorescence. Photon flux densities giving 60% of QA in a reduced state at steady-state photosynthesis (300 mol m–2s–1 at 5°C and 1200 mol m–2s–1 at 20°C) resulted in a depression of the photochemical efficiency of Photosystem II (Fv/Fm) at both 5 and 20°C. Inhibition of Fv/Fm occurred with initially similar kinetics at the two temperatures. After 6h, Fv/Fm was inhibited by 30% and had reached steady-state at 20°C. However, at 5°C, Fv/Fm continued to decrease and after 10h, Fv/Fm was depressed to 55% of control. The light response of the reduction state of QA did not change during photoinhibition at 20°C, whereas after photoinhibition at 5°C, the proportion of closed reaction centres at a given photon flux density was 10–20% lower than before photoinhibition.Changes in the D1-content were measured by immunoblotting and by the atrazine binding capacity during photoinhibition at high and low temperatures, with and without the addition of chloramphenicol to block chloroplast encoded protein synthesis. At 20°C, there was a close correlation between the amount of D1-protein and the photochemical efficiency of photosystem II, both in the presence or in the absence of an active repair cycle. At 5°C, an accumulation of inactive reaction centres occurred, since the photochemical efficiency of Photosystem II was much more depressed than the loss of D1-protein. Furthermore, at 5°C the repair cycle was largely inhibited as concluded from the finding that blockage of chloroplast encoded protein synthesis did not enhance the susceptibility to photoinhibition at 5°C.It is concluded that, the kinetics of the initial decrease of Fv/Fm was determined by the reduction state of the primary electron acceptor QA, at both temperatures. However, the further suppression of Fv/Fm at 5°C after several hours of photoinhibition implies that the inhibited repair cycle started to have an effect in determining the photochemical efficiency of Photosystem II.Abbreviations CAP
D-threochloramphenicol
- F0 and F
0
fluorescence when all Photosystem II reaction centres are open in dark- and light-acclimated leaves, respectively
- Fm and F
m
fluorescence when all Photosystem II reaction centres are closed in dark- and light-acclimated leaves, respectively
- Fs
fluorescence at steady state
- QA
the primary, stable quinone acceptor of Photosystem II
- qN
non-photochemical quenching of fluorescence
- qP
photochemical quenching of fluorescence |
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Keywords: | chlorophyll fluorescence D1-protein fluorescence quenching low temperature photoinhibition photosynthesis Photosystem II |
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