Differential thermal analysis on the freezing dynamics of Chimonanthus praecox and Photinia serrulata stems

At a cooling rate of (0.66 ±0.2)℃/min, the thermodynamic behavior of living and heat-killed (100℃ for 10 min) young stems of Photinia serrulata Lindl and Chimonanthus praecox (L.) Link. were investigated using high-resolution differential thermal analysis (DTA) during freezing from 0℃--20℃. The freezing features of the stems were also analyzed based on their morphological structures. The DTA curves from living stems of P. serrulata and C. praecox showed three exothermic peaks, whereas those from heat-killed stems showed one exothermic peak. Based on the dynamics of the exotherms, structures of the stems, and thermal conductance, the three exothermic peaks from living tissues likely represented the freezing process in the stems in the following order:freezing of apoplastic water, freezing dehydration of cambium and phloem, and finally freezing dehydration of pith cells. The simulation of uniform tissue sap using filter papers drenched with normal saline also showed a single exothermic peak, which was similar to the thermal behavior of the killed stems, indicating the freezing of a solution with no isolation of the cell membrane. These results showed that many details on heat release, freezing temperature, and dynamics of the freezing process in tissues can be revealed by DTA, which is thus suitable for analysis of freezing dynamics in plants.