Refixation of xylem sap CO2 in Populus deltoides

Vascular plants have respiring tissues which are perfused by the transpiration stream, allowing solubilization of respiratory CO₂ in the xylem sap. The transpiration stream could provide a conduit for the internal delivery of respiratory CO₂ to leaves. Trees have large amounts of respiring tissues in the root systems and stems, and may have elevated levels of CO₂ in the xylem sap which could be delivered to and refixed by the leaves. Xylem sap from the shoots of three Populus deltoides trees had mean dissolved inorganic carbon concentrations (CO₂+H₂CO₃+HCO^?₃) ranging from 0. 5 to 0. 9 mM. When excised leaves were allowed to transpire 1 mM¹⁴C]NaHCO₃, 99. 6% of the label was fixed in the light. Seventy-seven percent of the label was fixed in major veins and the remainder was fixed in the minor veins. Autoradiography confirmed that label was confined to the vasculature. In the dark, approximately 80% of the transpired label escaped the leaf, the remainder was fixed in the major veins, slightly elevating dark respiration measurements. This indicates that the vascular tissue in P. deltoides leaves is supplied with a carbon source distinct from the atmospheric source fixed by interveinal lamina. However, the contribution of CO₂ delivered to the leaves in the transpiration stream and fixed in the veins was only 0. 5% of atmospheric CO₂ uptake. In the light 90% of the label was found in sugar, starch and protein, a pattern similar to that found for atmospheric uptake of¹⁴C]CO₂. Compared with leaves labelled in the light, leaves labelled in the dark had more label in organic acid, amino acid and protein and less label in sugar and starch. After a 5-s pulse the majority of the label fed to petioles in both the light and the dark was found in malate. The majority of the label was found in malate at 120 s in the dark; only 2% of the label was found in phosphorylated compounds at 120 s. The proportion of label found in phosphorylated compounds increased from 17% at 5 s to 80% at 120 s in the light. This suggests that CO₂ delivered to leaves in the light via the transpiration stream is fixed in the veins, a small portion through dark fixation into malate, the remainder by C-3 photosynthesis.