© 2006 Heron Publishing—Victoria, Canada
Excitation energy partitioning and quenching during cold acclimation in Scots pine
Dmitry Sveshnikov (1, 2, 3), Ingo Ensminger (4, 5, 6), Alexander G. Ivanov (5), Douglas Campbell (7), Jon Lloyd (4, 8), Christiane Funk (2), Norman P. A. Hüner (5) and Gunnar Öquist (1)
1. Department of Plant Physiology, University of Umeå, S-901 87 Umeå, Sweden / 2. Department of Biochemistry, University of Umeå, S-901 87 Umeå, Sweden / 3. Corresponding author (dmitry.sveshnikov@plantphys.umu.se) / 4. Max-Planck-Institut für Biogeochemie, Postfach 100164, 07701 Jena, Germany / 5. Department of Biology and Biotron, University of Western Ontario, London ON, N6A 5B7, Canada / 6. Max-Planck-Institut für Molekulare Pflanzenphysiologie, 14424 Potsdam, Germany / 7. Mount Allison University, Sackville NB, E4L 1G7, Canada / 8. School of Geography, University of Leeds, West Yorkshire, LS2 9JT, U.K. / Received December 20, 2004; accepted August 5, 2005; published online December 15, 2005
Summary
We studied the influence of two irradiances on cold acclimation and recovery of photosynthesis in Scots pine (Pinus sylvestris L.) seedlings to assess mechanisms for quenching the excess energy captured by the photosynthetic apparatus. A shift in temperature
from 20 to 5 °C caused a greater decrease in photosynthetic activity, measured by chlorophyll fluorescence and oxygen evolution,
in plants exposed to moderate light (350 µmol m–2 s–1) than in shaded plants (50 µmol m–2 s–1). In response to the temperature shift, maximal photochemical efficiency of photosystem II (PSII), measured as the ratio
of variable to maximal chlorophyll fluorescence (Fv/Fm) of dark-adapted samples, decreased to 70% in exposed seedlings, whereas shaded seedlings maintained Fv/Fm close to initial values. After a further temperature decrease to –5 °C, only 8% of initial Fv/Fm remained in exposed plants, whereas shaded plants retained 40% of initial Fv/Fm. Seven days after transfer from –5 to 20 °C, recovery of photochemical efficiency was more complete in the shaded plants
than in the exposed plants (87 and 65% of the initial Fv/Fm value, respectively).
In response to cold stress, the estimated functional absorption cross section per remaining PSII reaction center increased
at both irradiances, but the increase was more pronounced in exposed seedlings. Estimates of energy partitioning in the needles
showed a much higher dissipative component in the expoesd seedlings at low temperatures, pointing to stronger development
of non-photochemical quenching at moderate irradiances. The de-epoxidation state of the xanthophyll cycle pigments increased
in exposed seedlings at 5 °C, contributing to the quenching capacity, whereas significant de-epoxidation in the shaded plants
was observed only when temperatures decreased to –5 °C. Thermoluminescence (TL) measurements of PSII revealed that charge
recombinations between the second oxidation state of Mn-cluster S2 and the semireduced secondary electron acceptor quinone QB–(S2QB–) were shifted to lower temperatures in cold-acclimated seedlings compared with control seedlings and this effect depended
on irradiance. Concomitant with this, cold-acclimated seedlings demonstrated a significant shift in the S2 recombination with primary acceptor QA– (S2QA–) characteristic TL emission peak to higher temperatures, thus narrowing the redox potential gap between S2QB– and S2QA–, which might result in increased probability for non-radiative radical pair recombination betweem the PSII reaction center
chlorophyll a (P680+) and QA– (P680+QA–) (reaction center quenching) in cold-acclimated seedlings. In Scots pine seedlings, mechanisms of quenching excess light
energy in winter therefore involve light-dependent regulation of reaction center content and both reaction center-based and
antenna-based quenching of excess light energy, enabling them to withstand high excitation pressure under northern winter
conditions.
Keywords:
antenna quenching, cold stress, electron transport, Pinus sylvestris, reaction center quenching.
