Chinese Bulletin of Botany ›› 2018, Vol. 53 ›› Issue (2): 203-211.DOI: 10.11983/CBB17026

• EXPERIMENTAL COMMUNICATIONS • Previous Articles     Next Articles

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

Ruifeng Yu, Jianjun Zhu*   

  1. College of Life Sciences, Ludong University, Yantai 264025, China
  • Received:2017-02-18 Revised:2017-05-04 Accepted:2017-05-09 Online:2018-03-01 Published:2017-06-06
  • Contact: Jianjun Zhu

Abstract: 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.