Evidence for the Involvement of a Specific Cell Wall Layer in Regulation of Deep Supercooling of Xylem Parenchyma

Current theory indicates that the structure of the cell wall is integral to the ability of a tissue to exhibit deep supercooling. Our previous work has indicated that the structure of the pit membrane and/or amorphous layer (protective layer), rather than the entire cell wall, may play a major role in deep supercooling (21, 22). The present study indicates a shift in the low-temperature-exotherm of current year shoots of peach can be induced by soaking twigs in water over 3 to 10 days. Alternatively, these shifts can be inhibited by exposing tissues to 10(-4) molar cycloheximide. Ultrastructural observations indicated a marked alteration of the amorphous layer in xylem parenchyma of water-soaked tissue. Alterations consisted of an apparent loosening or partial dissolution of portions of the amorphous layer. Changes in the density or uniformity of the amorphous layer in cycloheximide-treated tissues were not as readily apparent. The appearance of the protoplast in tissue soaked in water for up to 10 days was characteristic of deacclimated cells. However, in tissue soaked in cycloheximide for the same period these changes were not evident. These observations further support our contention that the structure of the amorphous layer may play a key role in establishing and regulating the ability of a cell to exhibit deep supercooling.