© 1992 Heron Publishing—Victoria, Canada
Limits of tolerance to high temperatures causing direct and indirect damage to black spruce
S. J. Colombo (1) and V. R. Timmer (2)
1. Ontario Forest Research Institute, Ontario Ministry of Natural Resources, P.O. Box 969, 1235 Queen Street, Sault Ste. Marie,
Ontario P6A 5N5, Canada / 2. Faculty of Forestry, University of Toronto, 33 Willcocks Street, Toronto, Ontario M5S 3B3, Canada / Received November 21, 1991
Summary
The heat tolerance of actively growing 13–15-week-old black spruce (Picea mariana (Mill.) B.S.P.) was determined by exposing seedlings to temperatures of 40 to 60 °C for durations of 5 seconds to 3 hours
by direct immersion in a hot water bath. Direct and indirect heat damage to needles were differentiated by assessing damage
5 minutes (direct) and 3 weeks (indirect) after exposure to high temperature. Both direct and indirect damage increased exponentially
with the duration of exposure to high temperatures. However, indirect damage occurred at lower temperatures and with shorter
periods of exposure than direct damage. Arrhenius plots of length of exposure versus exposure temperature revealed that the
energy of activation for indirect damage was 384 kJ mol–1, 36% higher than for direct damage. Both direct and indirect damage were less in seedlings preconditioned by a heat shock
treatment (3 hours of exposure to an air temperature of 38 °C on each of 6 days prior to immersion in the water bath). Preconditioned
seedlings withstood higher temperatures and longer durations of high temperature exposure than seedlings not preconditioned
by heat shock.
