Ex Situ Conservation of Birch Trees by Cryopreservation of Dormant Buds Adapted to Subzero Temperatures by Extracellular Freezing

The cryopreservation of organs, tissues and cells is a useful tool for long-term, ex situ conservation of forest tree species. Since Betula platyphylla, B. ermanii and B. maximowicziana are economically and ecologically important forestry trees in boreal areas of Japan, meaningful conservation efforts require the development of cryopreservation methods for the winter buds of Japanese birch genetic resources. This study investigated the cryopreservation of winter buds of Japanese birch by slow freezing. Regrowth rates of shoot primordia isolated from winter buds that were cryopreserved in liquid nitrogen (− 196 °C) after slow freezing from − 2 to − 40 °C at a rate of 0.2 °C/min were comparable with regrowth rates of shoot primordia that were not subjected to slow freezing and cryopreservation. Isolated shoot primordia of B. platyphylla showed regrowth rates of approximately 80