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- Ensminger, Ingo, et al.
(author)
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Intermittent low temperatures constrain spring recovery of photosynthesis in boreal Scots pine forests
- 2004
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In: Global Change Biology. - : Wiley. - 1354-1013. ; 10:6, s. 995-1008
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Journal article (peer-reviewed)abstract
- During winter and early spring, evergreen boreal conifers are severely stressed because light energy cannot be used when photosynthesis is pre-empted by low ambient temperatures. To study photosynthetic performance dynamics in a severe boreal climate, seasonal changes in photosynthetic pigments, chloroplast proteins and photochemical efficiency were studied in a Scots pine forest near Zotino, Central Siberia. In winter, downregulation of photosynthesis involved loss of chlorophylls, a twofold increase in xanthophyll cycle pigments and sustained high levels of the light stress-induced zeaxanthin pigment. The highest levels of xanthophylls and zeaxanthin did not occur during the coldest winter period, but rather in April when light was increasing, indicating an increased capacity for thermal dissipation of excitation energy at that time. Concomitantly, in early spring the D1 protein of the photosystem II (PSII) reaction centre and the light-harvesting complex of PSII dropped to their lowest annual levels. In April and May, recovery of PSII activity, chloroplast protein synthesis and rearrangements of pigments were observed as air temperatures increased above 0°C. Nevertheless, severe intermittent low-temperature episodes during this period not only halted but actually reversed the physiological recovery. During these spring low-temperature episodes, protective processes involved a complementary function of the PsbS and early light-induced protein thylakoid proteins. Full recovery of photosynthesis did not occur until the end of May. Our results show that even after winter cold hardening, photosynthetic activity in evergreens responds opportunistically to environmental change throughout the cold season. Therefore, climate change effects potentially improve the sink capacity of boreal forests for atmospheric carbon. However, earlier photosynthesis in spring in response to warmer temperatures is strongly constrained by environmental variation, counteracting the positive effects of an early recovery process.
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| 2. |
- Sveshnikov, Dmitry, et al.
(author)
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Excitation energy partitioning and quenching during cold acclimation in Scots pine.
- 2006
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In: Tree Physiology. - 0829-318X. ; 26:3, s. 325-36
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Journal article (peer-reviewed)abstract
- 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.
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