Medium- and long-term storage of the Pycnanthemum (mountain mint) germplasm collection

The United States of America collection of mountain mint (Pycnanthemum Michx.) is held at the USDA-ARS National Clonal Germplasm Repository (NCGR) in Corvallis, Oregon as seed, potted plants and tissue cultures and a long-term storage collection is preserved at the USDA-ARS National Center for Genetic Resources Preservation (NCGRP) in Fort Collins, Colorado. The clonal collection is comprised of 34 accessions as potted plants that are duplicated with 31 accessions stored as in vitro cultures at 4 degrees C in tissue culture bags for medium-term storage at NCGR and as cryopreserved shoot tips in liquid nitrogen at NCGRP for long-term storage. This study reports on these two models of preservation of mountain mint at the U.S. National Plant Germplasm System. In vitro plants required 2 to 7 months for propagation on MS medium without growth regulators before storage at 4 degrees C. Plants remained in storage with good vigour in bags on 1/2x nitrogen MS medium without growth regulators for a mean of 2.08 y. An encapsulation-dehydration protocol was successful for cryopreservation of shoot tips from cold acclimated in vitro plants. Post-cryo viability, indicated by shoot tips with developed leaves and roots, ranged from 60 to 100 % for 27 accessions and 40 to 50 % for the other four. The encapsulation-dehydration cryopreservation method proved suitable for long-term preservation of the 31 Pycnanthemum accessions. These alternative storage forms allow for active use of the collection as well as base storage for clonally propagated accessions.