The effects of temperature, photoperiod and chilling on the leafing-out of Norway spruce, Picea abies (L.) Karst. were studied. High temperature promotes breakage of post-dormancy, long photoperiods having no such effect. Photoperiod and chilling cause the breakage of true dormancy. However, under field conditions, photoperiod will have no effect on leafing-out date in the spring.
By use of clonal material it was possible to show substantial genetic differences between individuals in response to temperature and photoperiod. When the effect of clones was accounted for, treatments could be compared more precisely. Differences between clones were apparent in heat-sum required for leafing-out, in rapidity of response to favorable post chilling conditions, and in chilling requirement This latter quantity was given a new definition, applicable when both chilling and post chilling temperatures are controlled and specified. This is that period beyond which a further 10 days of chilling accelerates leafing-out by less than one day, i.e. the point at which the slope of the line relating days till leafing-out to chilling period, is equal to minus 0.10.
Differences in leafing-out date were shown between provenances taken from throughout the range of Picea abies. These differences were related to latitude with provenances of high latitude leafing-out first.