Limits of tolerance to high temperatures causing direct and indirect damage to black spruce

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 degrees 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 degrees 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.