Freezing tolerance of apple flower buds

Flower buds are the most vulnerable parts of the dormant apple trees to frost injury. Especially during fall and spring when the processes associated with the transitional periods of hardening and de-hardening occur, tissues are more active and thus less cold hardy. The critical temperatures affecting the flower bud and the associated damage are variable and unknown for most of the current apple cultivars. The goal of this study was to identify the critical temperatures of flower buds of apple during different developmental stages from dormant to full bloom. Differential Thermal Analysis (DTA) and a controlled freezing exposure method were used to test the freezing tolerance of flower buds for three cultivars including Fuji, Gala, and Red Delicious during the late winter and early spring season of 2012, and from late fall 2013 until the early spring season of 2014. Differences in hardiness were found for the same cultivar as well as among cultivars and for different stages of flower bud development. The temperature at which the flower buds become injured using the DTA method is commonly related to the initiation of the low temperature exotherm (LTE). However, for flower buds of apples only the high temperature exotherms (HTEs) were observed within the evaluated temperature range of −44 °C to 4 °C. The inability to detect LTEs in apple buds indicates that apple flower bud hardiness cannot be estimated using the DTA method. A traditional freezing exposure method was used to determine the freezing tolerance of apples. The information generated in this study will provide a better understanding of the flower bud cold hardiness that is important for long term tree survival and orchard sustainability.