Tolerance of butternut embryonic axes to low and ultra-low temperatures

Butternut (Juglans cinerea L.) survival is threatened in North America by the fungus Sirococus clavigignenti-juglandacearum. To due, there is no control for this fungal disease and long-term seed storage (i.e. longer than 2-3y), to ensure survival of the species, is not a viable option. Initially, low (0, -5, -10, -15, -40 degrees C) and ultra-low (190 degrees C) temperature tolerance of butternut embryonic axes isolated from nuts collected from one tree was examined. Embryonic axes with approximately 3 mm of cotyledonary tissue attached to the hypocoty area germinated after exposure to 0, -5, -10, -15 and -40 degrees C for 4 h and to -196 degrees C for 24 h. Percentage germinations after exposure to 0 and -5 degrees C were 87 and 82% respectively and after -10 and -15 degrees C were 29 and 27% respectively. Thirty-two percent of axes germinated after -40 degrees C and 36% germinated after exposure to -196 degrees C. Tolerance to -196 degrees C was examined in the embryonic axes isolated from the nuts of 13 other trees. Significant tree-to-tree variation was found in the embryonic axes tolerance to low temperature. It was also found that increasing the duration of seed storage prior to excision of the embryonic axes resulted in a significantly increased tolerance to low and ultra-low temperatures. These results suggest that low and ultra-low temperature storage of embryonic axes may be a viable method for the ex situ conservation of butternut.