Assimilation of xylem-transported CO2: Effects of xylem CO2 concentration and transpiration rate

Background/Question/Methods

Prior reports have demonstrated that CO₂ released by respiration in roots and stems of trees can dissolve in xylem sap and move upward in the transpiration stream to the canopy, where it can be assimilated in woody tissues and leaves. However, the effect of transpiration rate on assimilation of xylem-transported CO₂ has not previously been quantified.  In this study, the cut ends of small detached branches of Populus deltoides were placed in beakers and allowed to take up ¹³CO₂-labeled solution at either high or low ¹³CO₂ concentration. The uptake of the labeled solutions served as a proxy for the internal transport of respired CO₂, while transpiration was controlled at either a high or low rate at the leaf level by altering the vapor pressure deficit of the air. Simultaneously, leaf gas exchange was measured. At the end of the solution uptake period, the xylem, cortex, and leaf tissue components of the branches were analyzed for ¹³C content to examine the effects of the treatments on the quantity of label that was assimilated.  Assimilation of ¹³CO₂ from the internal source was compared with leaf assimilation of atmospheric CO₂ to determine its relative contribution to the total carbon gain of the branches.

Results/Conclusions

The amount of solution taken up and the amount of ¹³C assimilated by the branches was greater under high transpiration compared with low transpiration. Label concentration did not affect solution uptake, but more ¹³C was assimilated under the high compared to the low label concentration treatment.  Among the tissue components, more ¹³C was assimilated in leaves than in woody tissue under the high label concentration treatment, while the opposite pattern was observed under the low label concentration treatment. The ratio of ¹³CO₂ label assimilation to atmospheric CO₂ assimilation was significantly greater under high transpiration compared to low transpiration, but was relatively small (maximum 1.9%) regardless of treatment combination. These results demonstrate that assimilation of internally supplied CO₂ by branch tissues is highly dependent on both transpiration rate and xylem sap CO₂ concentration, which is related to the rate of respiration. Therefore, we suggest that the contribution of internal CO₂ recycling to the whole-tree carbon budget strongly depends on factors controlling transpiration, respiration, and photosynthesis and that the assimilation of internally supplied CO₂ increases in importance under conditions of reduced leaf photosynthesis.