Assimilation dynamics of xylem-transported CO₂ by woody tissue photosynthesis revealed with ¹¹C- and ¹³C-labelling

A considerable quantity of locally respired CO2 in stems can be assimilated via woody tissue photosynthesis (Pwt). Hence, Pwt locally provides carbon fueling radial stem growth and therefore plays a major role in the stem carbon balance. It is, however, not straightforward to quantify or visualize its dynamic behavior, especially on small spatial scales. To assess Pwt we traced xylem-transported 11CO2 in detached branches of Populus tremula L. and used positron autoradiography to visualize and quantify its dynamics on a submillimeter scale, while manipulating light exposure to the branch. Experiments using 18F-fluoride were conducted to study water transport. In light-exposed branch segments 11C was found to be highly retained whereas light-excluded parts solely indicated transport of 11CO2 by the sap. The latter results were confirmed by visualizing sap transport through 18F-labelling. Analogue experiments with 13CO2 unambiguously revealed assimilation of xylem-transported CO2 into fructose and glucose, and also suggest light-independent fixation in light-excluded parts. The combination of stable and unstable isotope experiments highlight that (i) Pwt is highly light-responsive in young branches; and (ii) xylem-transported CO2 is fixed by photosynthetic cells in bark and locally provides sugars for growth and energy to woody tissues.