Ice Formation and the Affects of Cold Acclimation on Cold Hardiness in a Subtropical Fruit Species

Abstract

Fruit production in the southeastern United States is highly vulnerable to freezes. Understanding the mechanism of damage in various fruit crops leads to methods for mitigation of damage through direct protection, cultural practices, and genetic modification to enhance cold hardiness. The current study was conducted to assess the pattern of ice formation in stems and/or leaf tissue of Satsuma mandarins (Citrus unshiu (Marc.) ‘Owari’) at various stages of acclimation. Plants, were subjected to subfreezing temperatures. Ice formation was measured in stems using differential thermal analysis. The first exotherm was bulk xylem water freezing, with secondary exotherms, when present, associated with tissue death. The temperature differential of the first exotherm varied with air temperature and acclimation treatment, indicating a differential rate and amount of ice formation. Watersoaking and electrolyte leakage of leaves usually corresponded with the peak of the first stem exotherm, with the rate of electrolyte leakage being a function of acclimation and temperature treatment. For some treatments, leaf electrolyte leakage leveled off at 50%, which was attributed to a differential response of leaf tissues to treatments. Leaf death occurred when electrolyte leakage was greater than 50%.