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Most plants exhibit the ability to supercool to some extent without freezing. The extent of supercooling, however, is limited by the action of intrinsic and extrinsic ice nucleating agents which initiate ice formation and propagation within a plant at relatively warm subzero temperatures (-1.5 to -3.5 °C). In herbaceous plants, extrinsic ice-nucleating agents (such as ice-nucleation bacteria, dew, and other good nucleating agents) significantly limit the ability to supercool below 0 °C. It is believed that with an absence of these extrinsic nucleating agents that plants could supercool to less than -4 °C. Other evidence indicates that intrinsic nucleating agents may also significantly limit the extent of supercooling. Questions also exist about nucleation in woody plants and especially the new growth (flowers, leaves, and shoots) present in spring. A better understanding of how freezing is initiated in plants has been limited by the inability to determine and visualize the initial site of ice nucleation and pattern of ice propagation. We have used infrared video thermography to study freezing in young tomato ( Lycopersicon esculentum ) plants and to determine if a hydrophobic barrier on the plant surface could prevent the action of extrinsic nucleating agents such as Ice + bacterial strain (Cit7) of  Pseudomonas syringae  from initiating freezing within a plant. Tomato plants were grown in a greenhouse in individual pots and used when they were 4 to 6 weeks old. Freezing tests were conducted in a programmable freezing chamber, and freezing was visualized and recorded on videotape using an infrared radiometer. Freezing of the plants was extrinsically induced by the application of droplets (5 μl) of water containing Cit7. To provide a barrier to the action of extrinsic ice-nucleating agents, an emulsion of hydrophobic kaolin was applied to the plant surface before applying an extrinsic nucleating agent. Results indicate that dry, young tomato plants can supercool to as low as -6 °C whereas plants having a single droplet of Cit7 would freeze at -1.5 to -2.5 °C. Applying the hydrophobic barrier blocked the effect of Cit7 and allowed the plants to also supercool to -6 °C, despite the presence of frozen droplets. Experiments under natural freezing conditions are in progress.

189 The Use of Hydrophobic Clay Films as a Barrier to Ice Nucleation in Plants