Postphotosynthetic Fractionation in Leaves, Phloem and Stem
Stable carbon isotope ratios (δ13C) in organic matter convey important integrated and (if assessed in the tree ring archive) dateable information on plant physiology and related environmental drivers. While the generation of the δ13C signal in the primary assimilates in the leaves via photosynthetic carbon isotope fractionation is well understood, we still lack detailed knowledge of the processes that determine the isotopic fractionation in downstream processes in the leaves and during the transport in the stem, which in turn affect δ13C in the tree-ring archive. We here provide an update on processes that drive post-carboxylation carbon isotope fractionation in the leaves, on potential changes in δ13C related to phloem loading and transport and we also discuss the role of stem CO2 fluxes (bark photosynthesis, stem respiration and CO2 fixation by phosphoenol pyruvate carboxylase). Moreover, we address the impact of carbon storage and remobilization on the intra-annual variation of δ13C in tree rings. Finally, we point to the potential importance of the intra-molecular carbon isotope distribution in carbohydrates for tree ring δ13C and its relation to shifts in metabolic pathways.