Evaluating shoot-tip regrowth of 25 Rubus L. species and hybrids after 15 to 20 years of cryopreserved storage.

Rubus L. species are pan-global in their distribution and used as food throughout the world. Their fruits, collectively called brambles, come in a variety of colors from black, through various shades of red and yellow to white. The USDA-ARS, National Plant Germplasm System maintains >2250 accession seed- and screenhouse-based Rubus collection in Corvallis, OR and the USDA-ARS, National Laboratory for Genetic Resources Preservation in Fort Collins, CO has cryopreserved and maintains 200 accessions in liquid nitrogen (LN). We investigated regrowth of 25 selected Rubus accessions cryopreserved by droplet vitrification and stored cryogenically for 15-20 years, to determine the need for possible repropagation and reprocessing. The initial regrowth of the stored accessions after 24 h in LN ranged from 50 to 100 %. After long-term LN storage, overall regrowth was significantly lower than the initial regrowth (on the average for the 25 accessions by 8.7 %). However, except for one accession with recovery of 23 % (R. laciniatus PI 553651), the regrowth of the other 24 accessions ranged from 40 to 100 %, with 16 accessions at >73 % regrowth. High initial viability was important for recovering large numbers of living shoots after long-term storage. Extending regrowth of five accessions from six to 12 wk did not significantly increase the regrowth, suggesting that a 6-wk cultivation is sufficient for regrowth assessment. Knowledge of changes in viability over time in cryopreserved collections is vital information for curators to determine what repropagation intervals are needed for specific germplasm collections. These studies also provide large-scale information on changes that might be needed in the cryopreservation protocol for the next storage cycle.