δC of organic matter transported from the leaves to the roots in Eucalyptus delegatensis: short-term variations and relation to respired CO.

Post-photosynthetic carbon isotope fractionation might alter the isotopic signal imprinted on organic matter (OM) during primary carbon fixation by Rubisco. To characterise the influence of post-photosynthetic processes, we investigated the effect of starch storage and remobilisation on the stable carbon isotope signature (δC) of different carbon pools in the Eucalyptus delegatensis R. T. Baker leaf and the potential carbon isotope fractionation associated with phloem transport and respiration. Twig phloem exudate and leaf water-soluble OM showed diel variations in δC of up to 2.5 and 2‰, respectively, with C enrichment during the night and depletion during the day. Damped diel variation was also evident in bulk lipids of the leaf and in the leaf wax fraction. δC of nocturnal phloem exudate OM corresponded with the δC of carbon released from starch. There was no change in δC of phloem carbon along the trunk. CO emitted from trunks and roots was C enriched compared with the potential organic substrate, and depleted compared with soil-emitted CO. The results are consistent with transitory starch accumulation and remobilisation governing the diel rhythm of δC in phloem-transported OM and fragmentation fractionation occurring during respiration. When using δC of OM or CO for assessing ecosystem processes or plant reactions towards environmental constraints, post-photosynthetic discrimination should be considered.