Cold hardiness and dormancy are critical physiological adaptations enabling woody perennials to endure low temperatures in winter and initiate growth in spring. Hardiness in apple (Malus domestica) is genotype dependent, however maximum hardiness level gained by the buds is strongly regulated by environment. Understanding the relationship between dormancy progression and cold hardiness is essential for predicting plant responses to winter and spring freeze events, as well as optimizing breeding efforts for resilience and orchard decisions for frost management. Information on hardiness mechanism is becoming more crucial given the impact of global climate change in apple growing regions where earlier bloom dates have inherently greater risk of crop loss due to episodic spring frost events. The existing lethal temperature chart for apple floral buds is limited to the visible development stages and lacks critical temperature thresholds for the freeze resistance of floral buds during their transition out of ecodormancy. Further, there is no information distinguishing positional effects of individual florets of the apple cyme inflorescence with respect to their hardiness status. Because the ‘King’ floret is developmentally advanced compared to laterals and has the highest fruit growth and economic potential, the development of lethal temperature limits to inform its protection seems prudent. Thus, we evaluated the difference in LT50 between ‘King’ and ‘Lateral’ florets in ‘Gala’ on ‘Bud9’ rootstock during the transition phase from ecodormacy to growth resumption in 2022 and 2023. Our results demonstrated that cold hardiness is lost at early stages of, or transitions out of, ecodormancy; notably, this occurs prior to visible changes in bud phenology. Moreover, the ‘King’ florets are markedly less hardy than the ‘Laterals’ by approximately 6°C. The magnitude of difference in lethal temperatures between ‘King’ and ‘Lateral’ florets differed between years but remained significant. Increasing susceptibility to freeze mortality at higher temperatures was commensurate with an increase in the relative water content (RWC) of the buds and a marked decrease in sorbitol and sucrose concentration. In both years, an RWC of approximately 60% coincided with the visible green stage of bud burst. Our data indicate that decreasing sorbitol concentration can act as a potential biochemical marker for cold hardiness loss and inform protection measures of the ‘King’ fruit. Further study incorporating multiple commercially important apple cultivars will be beneficial in understanding the cold hardiness in apples.
