© 1991 Heron Publishing—Victoria, Canada
Acquired thermotolerance of jack pine, white spruce and black spruce seedlings
R. S. Koppenaal (1), S. J. Colombo (1) and E. Blumwald (2)
1. Ontario Ministry of Natural Resources, Ontario Forest Research Institute, P.O. Box 969, Sault Ste. Marie, Ontario, Canada
P6A 5N5 / 2. Department of Botany, University of Toronto, Toronto, Ontario, Canada M5S 3B3 / Received February 8, 1990
Summary
The acquired thermotolerance of first-year seedlings of jack pine (Pinus banksiana Lamb.) hardened at 36, 38, 40 or 42 °C for 90, 180 or 360 minutes and of black spruce (Picea mariana (Mill.) B.S.P.) hardened at 34, 36, 38 or 40 °C for 30, 90, 180 or 360 minutes was determined by comparison of needle damage
to that of non-hardened seedlings (25 °C) following exposure to temperatures of 49 and 47.5 °C, respectively. Compared to
seedlings kept at 25 °C, heat injury sustained from exposure to high temperatures was markedly reduced following hardening
for 180 minutes at 36 and 38 °C in jack pine and black spruce, respectively. Increasing the exposure time at 36 °C in jack
pine, and at 36 to 40 °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
°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 °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 °C for 6 days remained elevated for only 4 days.
