Fate of xylem-transported 11C- and 13C-labeled CO2 in leaves of poplar

In recent studies, assimilation of xylem-transported CO2 has gained considerable attention as a means of recycling respired CO2 in trees. However, we still lack a clear and detailed picture on the magnitude of xylem-transported CO2 assimilation, in particular within leaf tissues. To this end, detached poplar leaves (Populusxcanadensis Moench Robusta') were allowed to take up a dissolved (CO2)-C-13 label serving as a proxy of xylem-transported CO2 entering the leaf from the branch. The uptake rate of the C-13 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 C-13 diffused from the labeled leaves and was fixed in the mesophyll of the non-labeled leaves. However, C-13 leaf tissue enrichment analysis with elemental analysis coupled to isotope ratio mass spectrometry was limited in spatial resolution at the leaf tissue level. Therefore, C-11-based CO2 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 C-13, C-11-based autoradiography can be used to study the fate of xylem-transported CO2 at leaf level, allowing the acquisition of data at a yet unprecedented resolution.