Lintunen Anna, Mayr Stefan, Salmon Yann, Cochard Hervé, Hölttä Teemu
Department of Forest Sciences University of Helsinki Helsinki Finland.
Department of Physics University of Helsinki Helsinki Finland.
Ecol Evol. 2017 Nov 28;8(1):333-343. doi: 10.1002/ece3.3665. eCollection 2018 Jan.
It is not well understood what determines the degree of supercooling of apoplastic sap in trees, although it determines the number and duration of annual freeze-thaw cycles in a given environment. We studied the linkage between apoplastic ice nucleation temperature, tree water status, and conduit size. We used branches of 10 gymnosperms and 16 angiosperms collected from an arboretum in Helsinki (Finland) in winter and spring. Branches with lower relative water content froze at lower temperatures, and branch water content was lower in winter than in spring. A bench drying experiment with confirmed that decreasing branch water potential decreases apoplastic ice nucleation temperature. The studied angiosperms froze on average 2.0 and 1.8°C closer to zero Celsius than the studied gymnosperms during winter and spring, respectively. This was caused by higher relative water content in angiosperms; when branches were saturated with water, apoplastic ice nucleation temperature of gymnosperms increased to slightly higher temperature than that of angiosperms. Apoplastic ice nucleation temperature in sampled branches was positively correlated with xylem conduit diameter as shown before, but saturating the branches removed the correlation. Decrease in ice nucleation temperature decreased the duration of freezing, which could have an effect on winter embolism formation via the time available for gas escape during ice propagation. The apoplastic ice nucleation temperature varied not only between branches but also within a branch between consecutive freeze-thaw cycles demonstrating the stochastic nature of ice nucleation.
尽管质外体汁液的过冷程度决定了给定环境中每年冻融循环的次数和持续时间，但目前人们对其影响因素还了解甚少。我们研究了质外体冰核形成温度、树木水分状况和导管大小之间的联系。我们使用了冬季和春季从芬兰赫尔辛基一个植物园采集的10种裸子植物和16种被子植物的枝条。相对含水量较低的枝条在较低温度下结冰，且冬季枝条的含水量低于春季。一项台架干燥实验证实，降低枝条水势会降低质外体冰核形成温度。在冬季和春季，所研究的被子植物平均比裸子植物更接近零摄氏度2.0和1.8°C结冰。这是由于被子植物相对含水量较高；当枝条被水饱和时，裸子植物的质外体冰核形成温度升高到略高于被子植物的温度。如前所示，采样枝条中的质外体冰核形成温度与木质部导管直径呈正相关，但枝条饱和后这种相关性消失。冰核形成温度的降低缩短了冻结持续时间，这可能通过冰扩展过程中气体逸出的可用时间对冬季栓塞形成产生影响。质外体冰核形成温度不仅在不同枝条之间变化，而且在同一枝条内连续的冻融循环之间也变化，这表明冰核形成具有随机性。
