Allocation of freshly assimilated carbon into primary and secondary metabolites after in situ¹³C pulse labelling of Norway spruce (<i>Picea abies</i>)

Plants allocate carbon (C) to sink tissues depending on phenological, physiological or environmental factors. We still have little knowledge on C partitioning into various cellular compounds and metabolic pathways at various ecophysiological stages. We used compound-specific stable isotope analysis to investigate C partitioning of freshly assimilated C into tree compartments (needles, branches and stem) as well as into needle water-soluble organic C (WSOC), non-hydrolysable structural organic C (stOC) and individual chemical compound classes (amino acids, hemicellulose sugars, fatty acids and alkanes) of Norway spruce (Picea abies) following in situ 13C pulse labelling 15 days after bud break. The 13C allocation within the above-ground tree biomass demonstrated needles as a major C sink, accounting for 86% of the freshly assimilated C 6 h after labelling. In needles, the highest allocation occurred not only into the WSOC pool (44.1% of recovered needle 13C) but also into stOC (33.9%). Needle growth, however, also caused high 13C allocation into pathways not involved in the formation of structural compounds: (i) pathways in secondary metabolism, (ii) C-1 metabolism and (iii) amino acid synthesis from photorespiration. These pathways could be identified by a high 13C enrichment of their key amino acids. In addition, 13C was strongly allocated into the n-alkyl lipid fraction (0.3% of recovered 13C), whereby 13C allocation into cellular and cuticular exceeded that of epicuticular fatty acids. 13C allocation decreased along the lipid transformation and translocation pathways: the allocation was highest for precursor fatty acids, lower for elongated fatty acids and lowest for the decarbonylated n-alkanes. The combination of 13C pulse labelling with compound-specific 13C analysis of key metabolites enabled tracing relevant C allocation pathways under field conditions. Besides the primary metabolism synthesizing structural cell compounds, a complex network of pathways consumed the assimilated 13C and kept most of the assimilated C in the growing needles.