IMPROVED SURVIVAL OF IN VITRO-STORED RUBUS GERMPLASM

Medium-term in vitro cold storage of Rubus germplasm was investigated using various temperatures, photoperiods, and storage containers. Shoot cultures of several Rubus taxa were grown either in tissue-culture bags or 20 x 150-mm glass tubes. Cultures stored at 10C in darkness were in poor condition after 6 months. Overall survival and condition ratings were significantly better for bags than tubes when cultures were kept at 4C. Contamination was present in 14% of the tubes, but only 3% of the bags. Addition of a 12-hour photoperiod to 4C storage significantly improved both condition ratings and survival percentages of many individual genotypes. Evaluation of the 250-accession germplasm collection after 12 months at 4C (dark) showed 92% of accessions in bags and 85% in tubes in suitable condition to remain in storage. Storage of cold-sensitive genotypes in tissue-culture bags at 25C with a 16-hour daylength was extended to 9 months when the MS-medium nitrogen level was reduced to 25% of standard concentration. Survival of 'Mandarin' raspberry stored for 9 months improved from 40% at 4C (100% N) to 90% at 25C (25% N). Results of these studies suggest that most Rubus germplasm can be stored safely at 4C with 12 hours of light, Plastic tissue-culture bags are preferred over tubes due to higher survival and lower contamination rates. Storage at 25C on reduced-nitrogen medium is an alternative method for cold-sensitive genotypes.