Using infrared thermography to study freezing in plants

Factors that determine when and to what extent a plant will freeze are complex. Although thermocouples have served as the main method of monitoring the freezing process in plants, infrared (IR) thermography offers distinct advantages and the use of this latter technology has provided new insights on the processes of ice nucleation and propagation. This technology is based on the fact that freezing is an exothermic event. The temperature and spatial resolution of a high-resolution IR camera has enabled researchers to clearly define initial sites of nucleation as well as monitor the ice front as it spreads into surrounding tissues. Ice nucleation is induced by both extrinsic and intrinsic nucleators. Ice nucleation-active bacteria and moisture are two major extrinsic agents. In herbaceous plants, the influence of extrinsic ice nucleators on ice nucleation can be moderated by thick cuticles or the application of synthetic hydrophobic barriers. The situation in woody plants, however, is different. Woody plants appear to possess native, intrinsic nucleating agents that are as active as many extrinsic agents. However, the identity of the intrinsic nucleating agents in woody plants is not known. Despite the presence of intrinsic nucleating agents, barriers exist in woody plants that inhibit growth of ice from older stems into primary, lateral appendages. This is important because many tissues in woody plants that are frost-sensitive are flowers and primary, elongating shoot tissues that arise from buds attached to older stems. Pictures derived from video segments of the freezing process and data on the ability to block nucleation through the use of hydrophobic kaolin are provided.