女贞和冬青卫矛叶片低温下胞外结冰模式的热力学新证据

低温条件下植物组织的结冰模式, 即胞外或胞内结冰, 直接决定着细胞的生死。目前胞外结冰的直接证据很少, 尤其缺乏热力学证据。用高分辨率差热扫描结合显微观察分析了女贞(Ligustrum lucidum)和冬青卫矛(Euonymus japonicus)叶片在降温过程中的结冰放热现象, 发现2种植物胞外结冰的热力学和组织结构变化的新证据。2种植物的活叶片在冷却过程中均呈现双放热峰, 即双相结冰的特征; 而预先冰冻杀死的叶片和叶片组织提取液浸润的滤纸片在同样冷却过程中仅有1个快速的大单放热峰, 即单相结冰的特征。显微观察也显示, 结冰过程中活组织细胞间隙中形成大量的白色冰晶, 且细胞虽然脱水收缩但细胞内的有色溶液没有流失, 表现出胞外结冰的特征。实验结果为深入揭示植物的冰冻伤害机制提供了新证据和研究方法。

New Evidence for the Mode of Extracellular Freezing in Leaves of Ligustrum lucidum and Euonymus japonicus under Low Temperatures

The freezing mode, namely intracellular or extracellular freezing, is vital to plants under low temperatures, but direct evidence of extracellular freezing, especially from thermodynamic experiments, are still very rare. In this paper, we examined exotherms from differential thermal analyses as well as microscopic observations in detached leaves from Ligustrum lucidum and Euonymus japonicus during cooling. We show new evidence of extracellular freezing during cooling. The living leaves of the two plants consistently exhibited biphasic freezing exotherms (i.e., an initial small exotherm followed by a slow but much larger exotherm). In contrast, the freeze-killed leaves, as well as the filter paper drenched with extracted tissue solutions from leaves of the two plants, exhibited just one fast and large exotherm typical for a single-phase freezing. Extensive colourless extracellular ice crystals were seen under the microscope, with cells dehydrated and shrunken but with no leakage of coloured cell solution, which agreed with the thermal analysis. The experiments presented a new approach and deeper insight into the mechanisms of freezing stress and injury in higher plants.