Previous reports have shown that CO₂ dissolved in xylem sap in tree stems can move upward in the transpiration stream.  To address questions about the fate of this CO₂, we allowed detached young branches of sycamore (Platanus occidentalis) to transpire water enriched with a known quantity of ¹³CO₂ in sunlight, while simultaneously measuring leaf net photosynthesis and CO₂ efflux from woody tissue in the light and dark.  Subsequently, we analyzed the branch and leaf tissues for ¹³C content to determine the quantity of transported ¹³CO₂ that was fixed.

Results/Conclusions

We found that treatment branches assimilated an average of 34.8 % of the ¹³CO₂ taken up in the treatment water.  The majority was fixed in the woody tissue of the branches, with smaller amounts fixed in the leaves and petioles.  Overall, the rate of fixation of internally transported ¹³CO₂ by woody tissues was 6 % of the rate of assimilation of CO₂ from the atmosphere by the leaves.  These results showed unequivocally that CO₂ transported in xylem sap can be fixed in photosynthetic cells in the branches of sycamore trees, suggesting that recycling of xylem-transported CO₂ may be a means by which trees reduce the carbon cost of respiration.