REST BREAK IN NORWAY SPRUCE SEEDLINGS - TEST OF A DYNAMIC TEMPERATURE RESPONSE HYPOTHESIS

A hypothesis concerning a dynamic temperature response of rate of rest break was tested using a chilling experiment with three northern provenances of 2-year-old seedlings of Norway spruce (Picea abies (L.) Karst.). According to the hypothesis, (i) within-population variation exists in the higher threshold for the rest-breaking temperature range and (ii) the higher threshold shifts towards lower temperatures as a result of chilling. The hypothesis was supported by experimental results. The results also indicated that at the beginning of the chilling period the higher threshold is a few degrees above 10-degrees-C in the examined seedling populations. Two conventional theories about the rest period of the trees were unable to explain the observed results. The results facilitate further development of simulation models of rest break and bud burst of the trees.