Lindfors Lauri, Hölttä Teemu, Lintunen Anna, Porcar-Castell Albert, Nikinmaa Eero, Juurola Eija
Department of Forest Sciences, University of Helsinki, PO Box 27, FI-00014 Helsinki, Finland Department of Physics, University of Helsinki, PO Box 64, FI-00014 Helsinki, Finland
Department of Forest Sciences, University of Helsinki, PO Box 27, FI-00014 Helsinki, Finland.
Tree Physiol. 2015 Dec;35(12):1314-24. doi: 10.1093/treephys/tpv095. Epub 2015 Sep 30.
Boreal trees experience repeated freeze-thaw cycles annually. While freezing has been extensively studied in trees, the dynamic responses occurring during the freezing and thawing remain poorly understood. At freezing and thawing, rapid changes take place in the water relations of living cells in needles and in stem. While freezing is mostly limited to extracellular spaces, living cells dehydrate, shrink and their osmotic concentration increases. We studied how the freezing-thawing dynamics reflected on leaf gas exchange, chlorophyll fluorescence and xylem and living bark diameter changes of Scots pine (Pinus sylvestris L.) saplings in controlled experiments. Photosynthetic rate quickly declined following ice nucleation and extracellular freezing in xylem and needles, almost parallel to a rapid shrinking of xylem diameter, while that of living bark followed with a slightly longer delay. While xylem and living bark diameters responded well to decreasing temperature and water potential of ice, the relationship was less consistent in the case of increasing temperature. Xylem showed strong temporal swelling at thawing suggesting water movement from bark. After thawing xylem diameter recovered to a pre-freezing level but living bark remained shrunk. We found that freezing affected photosynthesis at multiple levels. The distinct dynamics of photosynthetic rate and stomatal conductance reveals that the decreased photosynthetic rate reflects impaired dark reactions rather than stomatal closure. Freezing also inhibited the capacity of the light reactions to dissipate excess energy as heat, via non-photochemical quenching, whereas photochemical quenching of excitation energy decreased gradually with temperature in agreement with the gas exchange data.
北方树木每年都会经历反复的冻融循环。虽然树木的结冰现象已得到广泛研究，但结冰和解冻过程中发生的动态响应仍知之甚少。在结冰和解冻时，针叶和茎中活细胞的水分关系会发生快速变化。虽然结冰大多局限于细胞外空间，但活细胞会脱水、收缩，其渗透浓度增加。我们在控制实验中研究了冻融动态如何反映在苏格兰松（Pinus sylvestris L.）幼树的叶片气体交换、叶绿素荧光以及木质部和活树皮直径变化上。在木质部和针叶中冰核形成和细胞外结冰后，光合速率迅速下降，几乎与木质部直径的快速收缩同步，而活树皮的光合速率下降则稍有延迟。虽然木质部和活树皮直径对温度降低和冰的水势变化反应良好，但在温度升高时这种关系不太一致。木质部在解冻时出现强烈的暂时肿胀，表明有水从树皮流入。解冻后，木质部直径恢复到结冰前的水平，但活树皮仍处于收缩状态。我们发现结冰在多个层面影响光合作用。光合速率和气孔导度的不同动态表明，光合速率下降反映的是暗反应受损而非气孔关闭。结冰还抑制了光反应通过非光化学猝灭以热的形式耗散多余能量的能力，而激发能的光化学猝灭则随温度逐渐降低，这与气体交换数据一致。
