Short-term variation in the isotopic composition of organic matter allocated from the leaves to the stem of Pinus sylvestris:: effects of photosynthetic and postphotosynthetic carbon isotope fractionation

We aimed to quantify the separate effects of photosynthetic and postphotosynthetic carbon isotope discrimination on delta C-13 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 delta C-13 in total and water-soluble organic matter of the foliage plus delta C-13 and delta O-18 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 delta C-13 of stem-emitted CO2. In the canopy (foliage and twigs), the delta C-13 of soluble organic matter varied diurnally with amplitudes of up to 1.9 parts per thousand. The greatest C-13 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. C-13 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 parts per thousand compared with phloem organic matter. When day-to-day variation was addressed, water-soluble leaf delta C-13 and twig phloem delta O-18 were strongly influenced by c(i)/c(a) and stomatal conductance (G(s)), respectively. These results show that both photosynthetic and postphotosynthetic carbon isotope fractionation influence delta C-13 of organic matter over time, and over the length of the basipetal transport pathway. Clearly, these influences on the delta C-13 of respired CO2 must be considered when using the latter for partitioning of ecosystem CO2 fluxes or when the assessment of delta C-13 in organic matter is applied to estimate environmental effects in c(i)/c(a).