Effects of Sucrose Preculture on Cryopreservation by Droplet-Vitrification of Strawberry Cultivars and Morphological Stability of Cryopreserved Plants

The droplet-vitrification method was applied to shoot tips of micropropagated strawberry plants (Fragaria × ananassa DUCH. cvs. 'Senga Sengana', 'Korona' and 'Aroma'). Shoot tips of 2 - 3 mm in length were precultured in sucrose (0.1, 0.25, 0.5, 0.75, 1.0 M) enriched media for 24 and 48 h. Subsequently, they were transferred into 6 l droplets of PVS2 vitrification solution for 20 min and plunged into liquid nitrogen. Rapid rewarming was done in liquid medium at room temperature. The highest recovery rate in all cultivars (60%) was achieved after a preculture with 0.25 M sucrose for 24 h. From the recovered shoot tips not all continued with shoot development and multiplication. The number of off-types observed in the field was affected by the sucrose concentration. After preculture in 0.1 M sucrose no off-types were observed, while after preculture in high sucrose concentrations the number of off-types was much higher reaching 20% at the highest sucrose concentration.