Abstract

Seedlings of Pinus silvestris (L.) and Picea abies ((L.) Karst.), 16 weeks old, were hardened under various photo- and thermoperiods. Dehardening after a period of chilling was followed by exposure to various temperatures under long and short day conditions. The degree of frost hardiness of the seedlings was determined in freezing tests at -7°, - 11° and - 17°C. Damage caused by freezing was evaluated by measurement of conductivity. Short day conditions were of importance to hardening in both species, the most effective photoperiods being 6-12 hours. Shorter photoperiods (2-4 hours) were less effective. Low temperatures did not cause hardening under long-day conditions, but increased hardiness under short photoperiods, especially in pine. Night temperature had a greater influence on hardening in pine than did day temperature. Dehardenirng appeared to depend more on temperature than on photoperiod. Dehardening was a much faster process than hardening.