Factors Influencing Freezing of Supercooled Water in Tender Plants'

Seedlings of beans (Phaseolus vulgaris), corn (Zea mays), and tomatoes (Lycopersicon esculentum) were grown in the greenhouse and then exposed to controlled freezing conditions in a growth chamber. Variables were adjusted to determine the influence of plant water potential, freezing time, and external dew formation on the seedlings' susceptibility to frost injury. Freezing, detected visually and by release of latent heat, progressed rapidly throughout plants with high water potential and was always lethal. Spreading of the ice phase was impeded in plants with low water potential. In this case, the freezing injury ap• peared as spots on the leaves which gradually enlarged to encompass the entire leaf as the exposure continued. In general, the plant water supercooled before freezing. Supercooled water within the plant appeared to be internally nucleated if the leaf temperature remained above th atmospheric dewpoint temperature. Under these conditions root temperature, plant water energy, and duration of the freezing period all influenced the stability of the supercooled water. On the other hand, external inoculation prevailed when the freezing temperatures were accompanied by condensation of water from the air and subsequent formation of ice crystals on the leaves. One important exception was noted when ice on corn seedling leaves failed to nucleate the supercooled internal plant water with a potential of —18 bars.