© 2003 Heron Publishing—Victoria, Canada
Freezing tolerance of conifer seeds and germinants
B. J. Hawkins (1, 2), H. J. Guest (1) and D. Kolotelo (3)
1. Centre for Forest Biology, University of Victoria, P.O. Box 3020, STN CSC, Victoria, B.C., V8W 3N5, Canada / 2. Author to whom correspondence should be addressed (bhawkins@uvic.ca) / 3. B.C. Ministry of Forests Tree Seed Centre, 18793 32nd Avenue, Surrey, B.C., V3S 0L5, Canada / Received December 4, 2002; accepted May 4, 2003; published online November 17, 2003
Summary
Survival after freezing was measured for seeds and germinants of four seedlots each of interior spruce (Picea glauca × engelmannii complex), lodgepole pine (Pinus contorta Dougl. ex Loud.), Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and western red cedar (Thuja plicata Donn ex D. Donn). Effects of eight seed treatments on post-freezing survival of seeds and germinants were tested: dry, imbibed
and stratified seed, and seed placed in a growth chamber for 2, 5, 10, 15, 20 or 30 days in a 16-h photoperiod and a 22/17
°C thermoperiod. Survival was related to the water content of seeds and germinants, germination rate and seedlot origin.
After freezing for 3 h at –196 °C, dry seed of most seedlots of interior spruce, Douglas-fir and western red cedar had 84–96%
germination, whereas lodgepole pine seedlots had 53–82% germination. Freezing tolerance declined significantly after imbibition
in lodgepole pine, Douglas-fir and interior spruce seed (western red cedar was not tested), and mean LT50 of imbibed seed of these species was –30, –24.5 and –20 °C, respectively. Freezing tolerance continued to decline to a minimum
LT50 of –4 to –7 °C after 10 days in a growth chamber for interior spruce, Douglas-fir and lodgepole pine, or after 15 days for
western red cedar. Minimum freezing tolerance was reached at the stage of rapid hypocotyl elongation. In all species, a slight
increase in freezing tolerance of germinants was observed once cotyledons emerged from the seed coat. The decrease in freezing
tolerance during the transition from dry to germinating seed correlated with increases in seed water content. Changes in freezing
tolerance between 10 and 30 days in the growth chamber were not correlated with seedling water content.
Within a species, seedlots differed significantly in freezing tolerance after 2 or 5 days in the growth chamber. Because all
seedlots of interior spruce and lodgepole pine germinated quickly, there was no correlation between seedlot hardiness and
rate of germination. Germination rate and freezing tolerance of Douglas-fir and western red cedar seedlots was negatively
correlated. There was a significant correlation between LT50 after 10 days in the growth chamber and minimum spring temperature at the location of seedlot origin for interior spruce
and three seedlots of western red cedar, but no relationship was apparent for lodgepole pine and Douglas-fir.
Keywords:
cold hardiness, Douglas-fir, frost hardiness, germination, interior spruce, lodgepole pine, seed, seedlings, western red cedar.
