Rapid Increase in Deep Supercooling of Xylem Parenchyma

Malus pumila Mill. twigs were collected from September through December and stored at 5°C until the low temperature exotherms of the xylem were determined by differential thermal analysis. During the differential thermal analysis, cooling was interrupted, and temperatures of 5 to -18°C were held for 0.4 to 10 hours before cooling to -50°C was resumed. Control twigs were cooled to -50°C without interruption. Holding the twigs at 1.3 to -5°C shifted the start of the low temperature exotherm from about -20 to -30°C. Slightly higher (2.6°C) and lower (-10°C) temperatures were occasionally effective. The shift began within 20 to 30 minutes and increased progressively to 150 minutes. The acclimation was reversibly inhibited by N2 atmosphere.