Cryopreservation of temperate woody-plant material by dormant buds is less expensive than using shoot tips isolated from tissue cultured plants; however currently, dormant buds are used only for preservation of selected temperate tree and shrub species. Using dormant buds could be an efficient strategy for long-term preservation of blueberry (Vaccinium L.) genetic resources. In this study, viability of V. hybrid 'Northsky' (PI 554943) dormant buds was evaluated at 30 harvest dates over three consecutive fall/winter seasons to determine the optimal harvest time that promotes high post cryopreservation viability. Twigs with dormant buds were cut into 70 mm segments containing at least two nodes, desiccated, slowly cooled, stored in liquid nitrogen vapor and tested for post-cryopreservation regrowth. The highest regrowth of cryopreserved dormant buds was observed for buds harvested in mid-December and during the first half of January. Pearson's correlation coefficients were computed to evaluate the association between bud characteristics and viability at harvest date and logistic regression models were fit to test the ability of twig characteristics and temperatures to predict post cryopreservation bud viability. Post-cryopreservation viability was negatively correlated (p < 0.05) with average minimum, maximum and daily mean temperature preceding the bud harvest but was not correlated with the dormant bud initial and end moisture content, twig diameter, the number of dormant buds/cm of twig length and the number of days in desiccation. Regression tree analysis suggested post-cryopreservation viability to be between 52 and 80% for dormant buds harvested after a 10 day average maximum air temperature of <11.2 °C. Pre-harvest air temperature was a significant indicator of optimal dormant bud harvest time to produce adequate viability for long term preservation of blueberry genetic resources.

Willow (Salix L.) species have good qualities for becoming a biomass crop for production of biofuels. They grow on marginal soils, produce high yields of wood in a short period of time, and are easily propagated. Salix is one of the few genera that may be cryopreserved as dormant winter buds (DBs) instead of using tissue culture techniques. The objective of our study was to evaluate selected factors that affect cryotolerance of nine Salix germplasm accessions in the USDA-ARS National Plant Germplasm System collection. One-year-old branches harvested in January over 3 yr were cut into either 6-to 7-or 10-cm segments containing at least two DBs. Segments were treated with a slow-cooling procedure and stored in vapor-phase liquid N (LNV). Control segments were stored at −5 °C until planting. The LNV-exposed and the −5 °C-stored nodal segments were warmed and directly planted in a cold greenhouse. Six weeks later, the material was evaluated for shoot and root development. Segments that developed a shoot were considered viable. Average viability varied among years (0-35.1%) even for the same accession, species (4.2-51.4%), and segment length. Dormant buds on 10-cm segments had higher average viability (82.2%) than did DBs on 6-to 7-cm segments (43.9%), suggesting higher suitability for cryopreservation. In the material studied, Salix DB viability was correlated with branch diameter, DB density, and the ability to develop shoots and roots prior to cryopreservation.

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