Influence of photo- and thermoperiod on the initial stages of frost hardening and dehardening of phytotron-grown seedlings of Scots pine (Pinus silvestris L.) and Norway spruce (Picea abies (L.) Karst.)

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.