Picea glauca (Moench) Voss shows typical dormancy of winter buds. To break dormancy the trees should be chilled 4 to 8 weeks at 36° to 40°F., depending on their developmental stage and age when the treatment is begun. Apparently, young plants require more chilling than older ones. Plants exposed to chilling in July require more chilling than plants beginning treatment in September. Long photoperiods (18-20 hours) compensate in part for lack of chilling, while photoperiods of 13 hours have no compensating effect. In young plants--at least to the age of 2 years--long-day condition also extends the growth period; many plants show indeterminate growth, others show the typical determinate growth, and still others are intermediate with rapidly alternating periods of rest and growth. Plants that have passed through 5 normal periods of growth lose the ability to grow indeterminately and rarely show alternating periods of growth. Long photoperiods simply result in greater elongation in previously formed internodes. Various growth promoter-inhibitor systems are discussed as possible explanations of the observed results. It is suggested that the period of dormancy is a sequence of growth stages--some developmental, some transitional, and some steady-states. The different phases are controlled by a complex system of regulators, rather than a simple inhibitor system.
Document Type: Journal Article
Principal Plant Geneticist, North Central Forest Expt. Sta., Forest Service, U.S. Dept. of Agric., Project Leader at the Station's Institute of Forest Genetics, Rhinelander, Wis.