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.
