Utilizing diverse propagation approaches to advance Norway spruce breeding and reforestation

Abstract Norway spruce, an ecologically and economically important conifer species, requires efficient propagation methods for mass production and for use in breeding programs. This review explores several propagation methods, including seed-based and vegetative approaches, with a particular emphasis on the cutting method. It examines key factors affecting rooting success, such as donor tree age, seasonal sampling effects, sample position within the crown, and surrounding rooting conditions. Unlike seed-based propagation, which faces major limitations, including long maturation times, irregular seeding year, and genetic variability, vegetative propagation methods can overcome the mentioned challenges. Vegetative propagation using the cutting method offers advantages such as genetic uniformity, higher genetic gain, and faster regeneration. Nonetheless, compared to seed-based propagation, its higher cost and obstacles, such as reduced rooting success in older trees and plagiotropism of cuttings must be considered. Hedging, serial propagation, and selecting the optimal sampling position within the crown can help overcome these constraints, and enhance rooting success. Achieving an acceptable rooting success rate even in older Norway spruce trees, and even without applying auxin hormones, presents a unique opportunity for propagating mature trees in breeding programs, especially for traits influenced by both additive and non-additive genetic effects. In addition, both additive and non-additive genetic effects can also be utilized through seed-based propagation methods, such as a complete diallel cross, which involves crossing all parental trees in every possible two-way combination and testing their progenies, leading to enhanced genetic gain. Somatic embryogenesis is an alternative propagation method that enables the long-term cryopreservation of cell lines and their mass propagation after evaluating the regenerated seedlings. By integrating different propagation methods, including cuttings, somatic embryogenesis, and seeds, Norway spruce breeding programs can be accelerated, enabling the efficient production and deployment of high-quality planting stock for both reforestation and breeding purposes.