Low Temperature Tolerance of Apple Cultivars of Different Ploidy at Different Times of the Winter

Artificial freezing was used to evaluate diploid and triploid apple cultivars from the All Russian Research Institute of Fruit Crop Breeding at Orel throughout three winters. The studied apple varieties were developed by breeder E. N. Sedov and cytological analysis was carried out by cytologist G. A. Sedysheva. In early winter, all cultivars exhibited high tolerance to cold. In mid-winter buds and wood were severely damaged, while bark was more resistant for most cultivars. Basic components of hardiness were estimated: component I - frost resistance at -25 °C in the beginning of winter; component II - maximum value of frost resistance at -40 °C developed by plants during hardening; component III - ability to retain the hardened condition at -25 °C after a period of three-day thaw at +2 °C; and component IV - the ability to restore frost resistance at -30 °C after repeated hardening and three-day thaw at +2 °C. During late-winter thaws, buds suffered from frosts, while the bark and wood retained frost hardiness. Late in winter all cultivars demonstrated high resistance to repeated frosts. Triploid cultivars exhibited the highest level of cold hardiness of vegetative buds, bark and wood of annual shoots throughout the winter; these cultivars included ‘Zhilinskoye’, ‘Vavilovskoye’, ‘Osipovskoye’, ‘Patriot’, ‘Sinap Orlovski’, ‘Spasskoye’, ‘Turgenevskoye’, and diploids ‘Bolotovskoye’, ‘Sokovinka’, and ‘’Ranneye Aloye’.