Effect of light intensity and photoperiod on frost-hardiness development in 2-year-old Douglas-fir seedlings was studied using an electrical conductivity method to assess stem hardiness. Hardiness increased with light intensity to 1000 ft-c at 7.5 °C, under both 8-h and 16-h photoperiods, after about 8 weeks. This response suggested photosynthesis was necessary for rapid hardening, as also did the more rapid hardening obtained under continuous light at 40 ft-c, compared with 8-h photoperiods of the same intensity. By contrast, short photoperiods enhanced the rate of hardening at a light intensity of 3000 ft-c and 500 ft-c (after 7 weeks), compared with long photoperiods. After 7 weeks plants under short photoperiods with supplementary red light hardened like plants under long photoperiods, but after 10 weeks this effect of red light was no longer apparent. Far-red light did not produce any effect. Evidently a certain amount of photosynthesis must be carried out by Douglas-fir seedlings for rapid development of hardiness, but, with sufficient light for this to occur, short photoperiods hasten hardening.
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R. van den Driessche (1970) studied this question.
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