Growth and hardening of four provenances of containerized white spruce (Picea glauca (Moench) Voss) seedlings in response to the duration of 16 h long-night treatments

Long-night treatments of 16 h were applied to four provenances of 10-week-old Picea glauca (Moench) Voss seedlings for durations of 0 (control), 4, 8, 12 and 16 d. Their effects on growth and on the development of frost hardiness were determined during the fall. Height growth cessation was hastened by long-night treatments, with 4, 8, 12 and 16 d treatment seedlings ceasing growth approximately 5, 10, 13 and 15 d earlier than control seedlings. The rate of hardening was affected by the duration of the long-night treatment while the level of hardening was affected by both treatment duration and the latitude of origin of the seedlings. Specifically, seedlings receiving the 8, 12 and 16 d treatments had achieved approximately the same level of hardiness by mid-October, which was greater than the 0 and 4 d treatments. However, hardiness levels of the 12 and 16 d treatments were higher in late September than with the 8 d treatment. The level of frost hardiness in late November increased with both increasing duration of long-night treatments (0 to 8 d) and increasing latitude of origin.