Experiments were conducted to determine if dormant buds of Forsythia taxa exhibit the deep supercooling characteristic. Specimens were collected from thirteen Forsythia taxa including: F. suspensa (Thunb.) Vahl, F. x intermedia cv. Spectabilis (Koehne), F. x intermedia cv. Lynwood (G.E. Peterson), F. europaea (Degen and Baldacci), F. giraldiana (Lingelsh), F. japonica (Makino) var. saxatilis (Nakai), F. mandshurica (Uyeki), F. ovata (Nakai), F. suspensa var. fortunei (Lindl.), F. viridissima (Lindl.), F. x intermedia cv. Arnold Giant (Sax), F. cv. Arnold's Dwarf, and F. cv. Meadowlark (Flint). Buds and attached stem segments, were cooled at 2C (3.6F) per hour, and the temperature at which freezing occurred was determined by thermal analysis. Typically, two distinct freezing events were detected within Forsythia buds. The first freezing event, or high temperature exotherm, occurred just below 0C (32F), while the second freezing event, or low temperature exotherm, occurred between −16C (3.2F) and −28C (−18.4F). The low temperature exotherm corresponded to the freezing of supercooled water within dormant buds, and the detection of low temperature exotherms in buds of all 13 Forsythia taxa indicated that deep supercooling is common among members of this genus. In nine of the 13 Forsythia taxa, the temperature of the low temperature exotherm was an accurate indicator of bud freeze-tolerance (LT50), as determined by a laboratory freeze-stress protocol. The discrepancies noted in the other four taxa were apparently due to the occurrence of field freezing injury prior to conducting these laboratory studies. Evidence indicated a relationship between the extent of supercooling and the size of the pistil in dormant Forsythia buds.
The authors would like to thank Vicki Stirm and Dr. Cyrille Precetti for technical assistance, Dr. Tom Ward of the Arnold Arboretum for collaboration on plant materials, Judy Santini and Dr. Wyman Nyquist for statistical assistance, and Drs. Robert Joly and Mike Foley for reviewing the manuscript. Partial funding was provided by The Horticultural Research Institute, 1250 I St. N. W., Washington, DC 20005. Journal paper No. 14280 of the Purdue University Agricultural Experiment Station.
2Graduate Assistant and Professor, respectively.