Water relations in lichens at subzero temperatures: structural changes and carbon dioxide exchange in the lichen Umbilicaria aprina from continental Antarctica
Water relations in lichens at subzero temperatures: structural changes and carbon dioxide exchange in the lichen Umbilicaria aprina from continental Antarctica
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DOI：
10.1111/j.1469-8137.1995.tb05729.x
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发表时间：
1995-10
期刊：
影响因子：
9.4
通讯作者：
B. Schroeter;C. Scheidegger
中科院分区：
文献类型：
--
作者：
B. Schroeter;C. Scheidegger
摘要在零度以下的温度下，光合活性和结构的变化进行了监测在叶状地衣Umbilicaria aprina Nyl。从南极洲大陆。二氧化碳气体交换的测量结果表明，净光合作用和暗呼吸发生在零度以下的温度下，无论是否与液态水饱和的地衣叶状体暴露于零度以下的温度，或者如果一个干燥的叶状体是再水合只有从雪在零度以下的温度。当U. aprina在零度以下缓慢冷却，在-5.4 °C时可检测到冰核活性，表明细胞外水冻结。使用低温扫描电子显微镜（LTSEM），它表明，细胞外冰的形成导致cytorrhysis的光生物细胞和空化的mycobiont细胞。这两个过程是可逆的，如果地衣叶状体被重新加热。当干燥的地衣菌体被雪覆盖在零度以下的温度下，大量的再水化从雪可以观察到在LTSEM显微照片和重量测定。最终菌体含水量强烈依赖于温度状况，并给出20%d. wt %和56%d.在-4 °C下暴露16小时后的重量。二氧化碳气体交换的测量结果表明，代谢活动开始在从雪在零度以下的温度再水化。有人提出，在零度以下的温度下从雪中吸收的水发生在气相中，并仅取决于与温度相关的差异，在细胞内容和周围的大气之间的水势与雪平衡。在南极洲等极端环境中，代谢活动受到水的供应和低温的严重限制，因此，零度以下雪的光合作用和再水合作用对于初级生产具有重要的生态意义。
SUMMARY Photosynthetic activity and structural changes at subzero temperatures were monitored in the foliose lichen Umbilicaria aprina Nyl. from continental Antarctica. Carbon dioxide gas exchange measurements revealed that net photosynthesis and dark respiration occurred at subzero temperatures regardless of whether a lichen thallus saturated with liquid water was exposed to subzero temperatures, or if a dry thallus was re-hydrated only from snow at subzero temperatures. When water-saturated thalli of U. aprina were slowly cooled at subzero temperatures ice nucleation activity could be detected at — 5·4 °C, indicating extracellular freezing of water. Using low-temperature scanning electron microscopy (LTSEM) it was demonstrated that extracellular ice formation leads to cytorrhysis in the photobiont cells and to cavitation in the mycobiont cells. Both processes were reversible if the lichen thallus was re-warmed. When dry lichen thalli were covered with snow at subzero temperatures a substantial re-hydration from snow could be observed in LTSEM micrographs and measured gravimetrically. The final thallus water content was strongly dependent on the temperature regime and gave water contents between 20% d. wt at — 21 °C and 56% d. wt at —4·5 °C after 16 h exposure. Carbon dioxide gas exchange measurements revealed that metabolic activity was initiated during re-hydration from snow at subzero temperatures. It is proposed that water uptake from snow at subzero temperatures occurs in the gaseous phase and depends only on the temperature-related differences in water potential between the cell contents and the surrounding atmosphere in equilibrium with snow. Photosynthetic activity and re-hydration from snow at subzero temperatures are of great ecological importance for primary production in extreme environments such as Antarctica where metabolic activity is severely limited by water availability and low temperatures.