||Four seedlots of white spruce (Picea glauca (Moench) Voss) and three of Engelmann spruce (Picea engelmannii Parry), covering a range of 10 degrees of latitude and a range of altitudes, were sown in BC/ CFS Styroblocks and grown in a heated greenhouse and an unheated shadehouse, using incandescent light to provide a 19-h photoperiod. Four intensities of lighting were used: 0, 100,200, and 400 Ix. A second experiment with the same seedlots was conducted in growth rooms that were programmed to evaluate the effect of low night temperature on seedling shoot growth when the photoperiod was extended to 19 h, using a light intensity of 200 Ix.
Shoot length of white and Engelmann spruce seedlings grown under an extended daylength of 100 Ix were significantly taller than the control (0 Ix). There were no significant differences in shoot length or weight among the three intensities of light used to extend the photoperiod for all seedlots except the southern latitude-low elevation population of Engelmann spruce. The more northern populations of white spruce and the high altitude populations of Engelmann spruce did not require light intensities higher than 100 Ix to maintain apical growth. Low night temperature (7°C) did produce significantly smaller seedlings than the warm night (1SoC) regime. However, terminal resting buds of seedlings grown under the cool night regime did not form any sooner than on those seedlings grown under warm nights.