Differential Thermal Analysis and Freezing Injury of Deacclimating Peach and Sweet Cherry Reproductive Organs

Abstract

Abstract The freezing patterns of flower buds in peach [Prunus persica L.) Batsch ‘Redhaven’] and sweet cherry (Prunus avium L. ‘Bing’) changed significantly during spring deacclimation. Nucleation temperature, measured by differential thermal analysis (DTA), and freezing injury were monitored for reproductive organs as affected by the presence and absence of vegetative tissue, surface moisture, and the ice nucleation-active (INA) bacterium Pseudomonas syringae van Hall. The flower buds retained the capacity to deep supercool until early bud swell, when the low temperature (LT) exotherm distribution widened and fewer LT exotherms were produced. Following the disappearance of the LT exotherms until the petals tips first appeared through the calyx, the nucleation temperature of the flower buds increased 1° to 3°C. During this period the presence of stem tissue had no effect on nucleation temperature, whereas surface moisture increased the nucleation temperature several degrees. After full bloom, flowers attached to the stem froze at higher temperatures than excised flowers whether wet or dry. From the time the flower buds lost the capacity to deep supercool until petal tip emergence, freezing injury was reduced significantly by increasing the ice nucleation temperature, either by wetting peach flower buds or by inoculating sweet cherry flower buds with INA P. syringae bacteria. Following petal tip emergence, the higher nucleation temperatures no longer reduced freezing injury. Apparently, the flower buds of these Prunus species avoid freezing during the winter by deep supercooling of the dormant flowers, yet tolerate freezing during the early stages of flower development in the spring.