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Photosynthetic electron transport activity in heat-treated barley leaves: The role of internal alternative electron donors to photosystem II |
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| Authors: | Szilvia Z Tóth Gert Schansker Reto J Strasser |
| Institution: | a Laboratory of Bioenergetics, University of Geneva, Chemin des Embrouchis 10, CH-1254 Jussy, Geneva, Switzerland b Institute of Plant Biology, Biological Research Center of the Hungarian Academy of Sciences, PO Box 521, H-6701 Szeged, Hungary |
| Abstract: | Electron transport processes were investigated in barley leaves in which the oxygen-evolution was fully inhibited by a heat pulse (48 °C, 40 s). Under these circumstances, the K peak (∼ F400 μs) appears in the chl a fluorescence (OJIP) transient reflecting partial QA reduction, which is due to a stable charge separation resulting from the donation of one electron by tyrozine Z. Following the K peak additional fluorescence increase (indicating QA− accumulation) occurs in the 0.2-2 s time range. Using simultaneous chl a fluorescence and 820 nm transmission measurements it is demonstrated that this QA− accumulation is due to naturally occurring alternative electron sources that donate electrons to the donor side of photosystem II. Chl a fluorescence data obtained with 5-ms light pulses (double flashes spaced 2.3-500 ms apart, and trains of several hundred flashes spaced by 100 or 200 ms) show that the electron donation occurs from a large pool with t1/2 ∼ 30 ms. This alternative electron donor is most probably ascorbate. |
| Keywords: | Asc ascorbate DCMU 3-(3&prime 4&prime -dichlorophenyl)-1 1-dimethylurea Fd ferredoxin F0 initial fluorescence (measured at 20 μs) all PSII RCs are open FJ fluorescence intensity at &sim 3 ms (at 3000 μmol photons m&minus 2 s&minus 1) FM maximum fluorescence all PSII RCs are closed FNR ferredoxin-NADP+-reductase FP maximum measured fluorescence intensity FR far-red light I820nm transmission at 820 nm I step in the OJIP transient at 30 ms J step in the OJIP transient at &sim 3 ms (at 3000 μmol photons m&minus 2 s&minus 1) K peak in the OJIP transient at &sim 400 μs (at 3000 μmol photons m&minus 2 s&minus 1) MV methylviologen 1 1&prime -dimethyl-4 4&prime -bipyridinium-dichloride OEC oxygen evolving complex OJIP chl a fluorescence transient defined by the names of its intermediate steps PpBQ phenyl-p-benzoquinone P680 primary electron donor of PSII P700 primary electron donor of PSI PC plastocyanin PQ plastoquinone PSI photosystem I PSII photosystem II QA primary electron acceptor quinone of PSII QB secondary electron acceptor quinone of PSII TyrZ tyrozine Z φPo maximum quantum yield for primary photochemistry |
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