High temporal resolution 13C tracing to link xylem – phloem pathways of carbon in oak trees

Carbon (C) assimilated in the canopy of trees is transported downwards via phloem to fuel metabolic processes, such as respiration of above- and below-ground tissues.  Part of the respired CO₂ can be dissolved into xylem water and transported along the trunk up to the canopy, causing a CO₂ efflux which is dislocated from the site of respiration. While the individual processes of C transport in the phloem and xylem in trees have been comparatively well described, little is known on the linkage of xylem–phloem pathways of C and the potential of re-assimilation of root-respired C in the canopy.

In this study, we randomly assigned a set of five-year-old oak trees (Quercus rubra) to stem infusion of ¹³CO₂ dissolved water (n=4) or ¹³CO₂ canopy labeling treatment (n=3), thereby labeling xylem C flow and phloem C flow, respectively. Using high temporal resolution isotope ratio measurement by laser spectroscopy, we monitored ¹³C in tissue and in respiratory CO₂ efflux resulting from phloem- and xylem-transported C to trace the fate of C from photosynthesis through the phloem and xylem to respiration and re-assimilation. We observed that CO₂ efflux was related to both phloem and xylem transport of ¹³C and that a quick lateral transport of sugars occurred from phloem to xylem. Furthermore, we found evidence for re-assimilation of CO₂ transported through the xylem in branches and leaf petioles. The re-assimilated ¹³C transported by the xylem was also found in stem tissues at various heights, 24 to 96 hours after labeling. Moreover, stem ¹³CO₂ efflux showed a diurnal variation, suggesting a potential incorporation of recycled C into respiratory substrate at different stem heights shortly upon re-assimilation. Our results demonstrate a phloem–xylem C linkage leading to repeated coupling of assimilation and respiration, with consequences for whole tree C dynamics.