Fate of xylem‐transported 11 C ‐ and 13 C ‐labeled CO 2 in leaves of poplar

In recent studies, assimilation of xylem‐transported CO 2 has gained considerable attention as a means of recycling respired CO 2 in trees. However, we still lack a clear and detailed picture on the magnitude of xylem‐transported CO 2 assimilation, in particular within leaf tissues. To this end, detached poplar leaves ( Populus × canadensis Moench ‘Robusta’) were allowed to take up a dissolved 13 CO 2 label serving as a proxy of xylem‐transported CO 2 entering the leaf from the branch. The uptake rate of the 13 C was manipulated by altering the vapor pressure deficit ( VPD ) (0.84, 1.29 and 1.83 kPa ). Highest tissue enrichments were observed under the highest VPD . Among tissues, highest enrichment was observed in the petiole and the veins, regardless of the VPD treatment. Analysis of non‐labeled leaves showed that some 13 C diffused from the labeled leaves and was fixed in the mesophyll of the non‐labeled leaves. However, 13 C leaf tissue enrichment analysis with elemental analysis coupled to isotope ratio mass spectrometry was limited in spatial resolution at the leaf tissue level. Therefore, 11 C ‐based CO 2 labeling combined with positron autoradiography was used and showed a more detailed spatial distribution within a single tissue, in particular in secondary veins. Therefore, in addition to 13 C , 11 C ‐based autoradiography can be used to study the fate of xylem‐transported CO 2 at leaf level, allowing the acquisition of data at a yet unprecedented resolution.