Abstract
Freezing process of plants is an important issue but only few analyses have been made and mechanisms involved remain unsolved. This is because freezing proceeds too fast and there have been no good methods to analyze the process. To address this problem, we utilized digital infra-red thermography and observed freezing events in wintering rabbit eye blueberry stems to characterize the initiation and spread of freezing. Referential images (raw images minus reference image) clearly visualized where the first freezing events occurred and how they spread in the stems. Differential images (differences between each neighboring images) more precisely visually detected where the first freezing occurred, in which direction and how fast the freezing front proceeded, how many times a portion of the stem froze and whether the heat release was from the surface or inside. Freezing curves for each spot on the stem extracted from the thermography images also allowed us to analyze freezing behaviors of the tissues. Differential images visualized the first freezing event (ice nucleation), which tended to be sharp and narrow temperature rise (probably from the surface) and slow in propagation. This was followed by the second freezing event, which tended to be vague and broad temperature rise (probably from inside) and fast in propagation. Freezing curve of the ice nucleation site clearly detected these two freezing events, the first small heat release and the second large heat release. Differential thermal analysis (DTA) revealed that excised bark had exotherm at much higher temperature than xylem and pith tissues. From these data, the following procedure of freezing in blueberry stems was hypothesized: 1) freezing initiates in the bark, 2) this slowly spreads in both directions or in one direction, 3) in some places this is transmitted to the xylem, 4) freezing in the xylem rapidly proceeds to cover the whole stem. A high ice nucleation activity in the bark seems to initiate spontaneous freezing of the stem.
