Acquired thermotolerance of jack pine, white spruce and black spruce seedlings.

The acquired thermotolerance of first-year seedlings of jack pine (Pinus banksiana Lamb.) hardened at 36, 38, 40 or 42 degrees C for 90, 180 or 360 minutes and of black spruce (Picea mariana (Mill.) B.S.P.) hardened at 34, 36, 38 or 40 degrees C for 30, 90, 180 or 360 minutes was determined by comparison of needle damage to that of non-hardened seedlings (25 degrees C) following exposure to temperatures of 49 and 47.5 degrees C, respectively. Compared to seedlings kept at 25 degrees C, heat injury sustained from exposure to high temperatures was markedly reduced following hardening for 180 minutes at 36 and 38 degrees C in jack pine and black spruce, respectively. Increasing the exposure time at 36 degrees C in jack pine, and at 36 to 40 degrees C in black spruce, also reduced needle damage. The duration of increased thermotolerance was investigated in jack pine, black spruce and white spruce (Picea glauca (Moench) Voss) by comparing heat injury from high temperatures in non-hardened seedlings and in seedlings hardened at 38 degrees C for 180 minutes a day for either 1, 3 or 6 days. In all three species, the duration of acquired thermotolerance increased with the number of days of heat hardening. For jack pine and white spruce seedlings hardened at 38 degrees C for 6 days, increased thermotolerance persisted for at least 14 and 10 days, respectively, after the end of the hardening treatment. In contrast, the thermotolerance of black spruce seedlings hardened at 38 degrees C for 6 days remained elevated for only 4 days.