Key messageThis paper shows that germinating Norway spruce somatic embryos are dependent on the carbon and nitrogen supplied in the medium, and that supplied glutamine accounts for 50 % of assimilated nitrogen during germination.AbstractThe female megagametophyte, which provides the zygotic embryo with nitrogen (N), carbon (C) and energy during germination, is not present in Norway spruce (Picea abies) mature somatic embryos. Therefore, somatic embryos presumably rely on nutrients supplied in the germination medium in addition to their storage compounds accumulated during maturation. However, to what extent stored versus supplied compounds contribute to a somatic embryo germination is unclear. In this 24-day study, we addressed the above question by monitoring the biomass changes and the N and C budget during somatic embryo germination, under low-intensity red light. We found that the C and N storage reserves, accumulated during the maturation phase, were not sufficient to support the growth of the germinating somatic embryos, rather they were dependent on the medium components. In addition, in a previous study it has been found that glutamine (Gln) supplied in the medium was crucial for maintaining the primary amino acid (AA) metabolism and growth of the proliferating embryogenic cultures of Norway spruce (Carlsson et al., PLoS One 12(8):e0181785, 2017). Therefore, we hypothesised that Gln would be required as a significant source of N also during somatic embryo germination. By tracing the uptake of isotopically labelled N-sources from the medium and further into primary N assimilation, we found that Gln was the preferred source of N for the germinating somatic embryos, accounting for 50% of assimilated N. As the amounts of both arginine (Arg) and Gln were increased in the germinating somatic embryos, it also suggested that germination in low-intensity red light promoted N storage, similar to what has been observed in the zygotic embryo maturation in conifers (King, Gifford, Plant Physiol 113:1125-1135, 1997).