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Abstract

The development and nature of cold hardiness was examined in first-year seedlings of the three conifer species: western red cedar (Thuja plicata Donn) from southern Vancouver Island, yellow cedar (Chamaecyparis nootkatensis (D. Don) Spach) from northern Vancouver Island and white spruce (Picea glauCa (Moench) Voss) from northern interior of British Columbia. The relationship between free thiols and the development of cold hardiness, and the effects of mefluidide on the induction of hardiness in the three species were also examined. To permit an accurate estimation of cold hardiness, a reliable method of determining hardiness based on the electrolyte conductivity method (EC) was developed for the three species. Optimization of tissue preparation resulted in accurate and reliable estimates of cold hardiness in the three coniferous species. A comparison of visual assessment of shoot damage with the results obtained by the EC method indicated that the two methods were highly correlated although the EC method slightly overestimated the temperature at which 50% of the samples were killed (LT₅₀). The absolute lethal temperature (LT₁₀₀) estimated by the EC method was lower (3 to 7 °C) than that determined by the visual method. The development of hardiness in the three species was characterized by different mechanisms: in white spruce it was initiated by short photoperiod, in red and yellow cedar it was basically regulated by low temperature. Low temperature (7/3 °C day/night) increased hardiness in all three species but subfreezing temperature (2/-3 °C day/night) increased the rate of hardening only in the two cedars. Furthermore, white spruce seedlings were apparently able to attain extreme levels of hardiness (below -65 °C) without exposure to temperatures below 0 °C. The ability to deharden in white spruce was related to the satisfaction of chilling requirements while red and yellow cedar seedlings appeared to deharden only in response to warm temperatures. Mefluidide (0.1 and 0.4 mg l⁻¹ ) applied as a root drench did not increase cold hardiness in any of the three species. Stomatal conductance was however decreased, thus resulting in increased shoot water potentials. Net photosynthesis was reduced predominantly due to stomatal limitations. Mefluidide increased synthesis of ABA in shoots of seedlings of the three species. The level of tissue ABA, like the other variables, was dependent on mefluidide concentration. Low temperature (7/3 °C day and night, 9 h photoperiod, 250 μmoles m⁻² s⁻¹, 400 - 700 nm) induced an increase in free thiols (predominantly reduced glutathione) which was accompanied by an increase in hardiness. Although short photoperiod at a warm temperature (9 h, 20/15 °C day and night) increased hardiness in seedlings of white spruce, no significant increase in thiols was observed. Root application of buthionone sulfoximine and dichlormid affected tissue glutathione levels but these changes did not result in significant changes in freezing resistance. There appears to be no relationship between glutathione and freezing resistance.