CRYOPRESERVATION OF CITRUS FOR LONG-TERM CONSERVATION

More than 850 varieties of Citrus, Fortunella, and Citrus-related species are maintained within the USDA-ARS National Plant Germplasm System and the University of California Citrus Variety Collection. These genetic resources are held within duplicated field, screenhouse, and greenhouse collections and are at risk of disease infestations, abiotic stresses, and natural disasters. We have developed a cryopreservation method by which we can back up citrus genetic resources for the long term at liquid nitrogen temperatures. We excise shoot tips directly from greenhouse or screenhouse source plants, surface sterilize, and then treat with cryoprotectants. These solutions dehydrate and allow the shoot tips to survive liquid nitrogen exposure. Plants are recovered by micrografting thawed shoot tips onto ‘Carrizo’ seedling rootstocks. Experiments that compared survival after freezing in cryovials vs. on aluminum foil strips reveal higher levels and more uniform survival when foil strips were utilized. High viability was maintained for at least 1 year. These data further support the use of shoot tips to conserve diverse citrus cultivars in genebank collections.