Effect of Freezing Deep Supercooled Water on the Viability of Ray Cells

Matching 1-yr-old stem segments of Malus pumila, Carya ovata, Cercis canadensis, Fraxinus pennsylvanica, Castanea deniata, Syringa vulgaris, Quercus rubra, and Fagus sylvatica were cooled during differential thermal analysis to a range of temperatures at which different amounts of deep supercooled water froze. After storage at 5 C, the treated twigs were examined for browning; individual xylem rays were excised, vital stained in neutral red, and plasmolyzed in CaCl2. In all species, the death of ray cells coincided with the start of freezing of deep supercooled water. Death was defined by loss of stainability. In Malus, death of ray cells increased linearly over a 20 C span, with the percentage of water frozen When all the deep supercooled water had frozen, no cells were alive No recognizable anatomic characteristics distinguished those ray cells which died at higher temperatures The start of xylem browning also coincided with the start of the low temperature exotherm, and, in general, browning increased with freezing.