Endodormancy release in Norway spruce grafts representing trees of different ages

Studies addressing endodormancy release in adult trees are usually carried out using twigs detached from the trees in the experiments. Potential problems caused by cutting the root-shoot connection when detaching the twigs can be avoided by using grafts as the experimental material. We studied the effects of chilling on the endodormancy release in Norway spruce (Picea abies (L.) Karst.) grafts where twigs of 16-, 32- and 80-year-old trees were used as the scions. The grafts were first exposed to chilling in natural conditions and then samples of them were transferred at intervals to a regrowth test in forcing conditions in a greenhouse. The bud burst percentage, BB%, in the forcing conditions generally increased from zero to near 100% with increasing previous chilling accumulation from mid-October until mid-November, indicating that endodormancy was released in almost all of the grafts by mid-November. The days to bud burst, DBB, decreased in the forcing conditions with successively later transfers until the next spring. Neither BB% nor DBB was dependent on the age of the scion. However, in the early phase of ecodormancy release, the microscopic internal development of the buds was more advanced in the grafts representing the 16-year-old than in those representing the 32- or 80-year-old trees. In conclusion, our findings suggest that no major change in the environmental regulation of endodormancy release in Norway spruce takes place when the trees get older. Taken together with earlier findings with Norway spruce seedlings, our results suggest that regardless of the seedling or tree age, the chilling requirement of endodormancy release is met in late autumn. The implications of our findings for Norway spruce phenology under climatic warming and the limitations of our novel method of using grafts as a proxy of trees of different ages are discussed.