Xylem-transported respiratory CO2 as a substrate for carbon assimilation in trees

Background/Question/Methods

The CO₂ concentration [CO₂] in the xylem of trees exceeds atmospheric [CO₂] by up to 3 orders of magnitude. Internal CO₂ originates from respiration of below- and aboveground tissues. Some of this respired CO₂ fluxes to the atmosphere and some dissolves in sap and moves upward by mass flow in the transpiration stream. Previous investigations demonstrated that leaves and green woody tissues assimilate internally transported CO₂, but measurements were made on detached branches. In this study, we examined the internal transport and fate of CO₂ originating at the base of intact tree stems. We infused ¹³C-labeled solution at two concentrations (0.012 and 0.001 mol L^-1; high and low label, respectively) into the xylem stream at the base of Populus deltoides trees for two days. Trees were harvested four days after the start of infusion. Woody parts (stem and branches) were separated into xylem and cortex; leaves were separated into petiole, vein, and mesophyll. Biomass was quantified for each tissue and samples were analyzed for ¹³C enrichment compared to unlabeled controls. For each tissue, ¹³C enrichment results were scaled with biomass to determine whole-tree assimilation of the label.

Results/Conclusions

On average, 6.8 g (high label) and 0.7 g (low label) of dissolved ¹³C was infused into the trees via the xylem stream. The label was subsequently found throughout the trees. High-labeled trees assimilated 6% (0.38 g) and low-labeled trees assimilated 16% (0.12 g) of the internally-transported ¹³C. The remaining label (84-94%) was assumed to have fluxed to the atmosphere. Diffusion of the label to the atmosphere was confirmed by ¹³C analysis of gas flux samples. In high-labeled trees, ¹³C enrichment (the rate of assimilation integrated over the four-day experimental period) was highest in the cortex, intermediate in the xylem, and lowest in the leaves. Enrichment followed similar trends in low-labeled trees but differences were not significant. However, when enrichment data were scaled by tissue to the whole tree, the amount of ¹³C assimilated followed a different pattern: branch xylem> branch cortex> stem xylem> stem cortex> leaves. Previous studies reported that woody tissue photosynthesis occurs in the cortex, but here we showed that on a whole-tree basis, assimilation by woody tissue was dominated by the xylem. Our results indicate that trees can supplement their carbon gain by recycling respired CO₂. We also suggest that estimates of root and stem respiration that do not account for internal transport of respired CO₂ may be inaccurate.