The adaptation of perennial species to winter freezing temperatures is crucial for their reproductive success and has led to the evolution of diverse survival strategies to mitigate freezing damage. Bud survival is essential for species reproduction and fruit production, as buds carry the dormant flower primordia that will bloom in the next growing season. We studied two freezing survival mechanisms: deep supercooling (DS) and extraorgan freezing (EOF). Deep supercooling involves physical or structural changes that prevent ice nucleation in florets and meristems by sequestering small amounts of water. When the critical nucleating temperature for this sequestered water is reached, ice propagation is rapid, and cellular damage is lethal. Extraorgan freezing causes a gradual dehydration of inner bud tissues, driven by the vapor pressure deficit from extracellular ice formed in bud scales. Despite existing knowledge, the survival benefits of species undergoing deep supercooling, considered a limited strategy compared to extraorgan or extracellular freezing, remain unclear. Similarly, how adaptation to freezing impacts reproductive traits in woody species is not well understood. We focused on the Prunus genus for its dual survival strategies and productive and ornamental value. This study, conducted on six Prunus species at the Arnold Arboretum in Boston, MA, spanned three developmental stages: leaf drop in fall, dormancy in winter, and pre-bud swell in spring. Data encompassed phenology, vascular tissue development, flower primordia size, differential thermal analysis, controlled freezing tests, and characteristics of flowers, fruits, and seeds. Results indicated that DS Prunus species delay vascular tissue development and grow larger flower primordia from fall compared to EOF species. Conversely, EOF species bloom later, producing more and smaller flowers and fruits in a shorter time than DS species. In summary, in the Prunus genus, DS species appear to trade a lower temperature threshold for pre-forming fewer, larger flower primordia per bud, enabling earlier blooming and more efficient use of the growing season to develop larger fruits in contrast to EOF species.
