We aimed to quantify the separate effects of photosynthetic and postphotosynthetic carbon isotope discrimination on δ13C of the fast-turn-over carbon pool (water soluble organic carbon and CO2 emitted from heterotrophic tissues), including their diel variation, along the pathway of carbon transport from the foliage to the base of the stem.
For that purpose, we determined δ13C in total and water-soluble organic matter of the foliage plus δ13C and δ18O in phloem organic matter of twigs and at three heights along the stem of Pinus sylvestris over a nine-day period, including four measurements per day. These data were related to meteorological and photosynthesis parameters and to the δ13C of stem-emitted CO2.
In the canopy (foliage and twigs), the δ13C of soluble organic matter varied diurnally with amplitudes of up to 1.9‰. The greatest 13C enrichment was recorded during the night/early morning, indicating a strong influence of starch storage and remobilization on the carbon isotope signatures of sugars exported from the leaves. 13C enrichment of soluble organic matter from the leaves to the twig phloem and further on to the phloem of the stem was supposed to be a result of carbon isotope fractionation associated with metabolic processes in the source and sink tissues. CO2 emitted from the stem was enriched by 2.3–5.2‰ compared with phloem organic matter. When day-to-day variation was addressed, water-soluble leaf δ13C and twig phloem δ18O were strongly influenced by ci/ca and stomatal conductance (Gs), respectively.
These results show that both photosynthetic and postphotosynthetic carbon isotope fractionation influence δ13C of organic matter over time, and over the length of the basipetal transport pathway. Clearly, these influences on the δ13C of respired CO2 must be considered when using the latter for partitioning of ecosystem CO2 fluxes or when the assessment of δ13C in organic matter is applied to estimate environmental effects in ci/ca.