Studying in vivo dynamics of xylem-transported 11CO2 using positron emission tomography.

Respired CO₂ in woody tissues can build up in the xylem and dissolve in the sap solution to be transported through the plant. From the sap, a fraction of the CO₂ can either be radially diffuse to the atmosphere or be assimilated in chloroplasts present in woody tissues. These processes occur simultaneously in stems and branches, making it difficult to study their specific dynamics. Therefore, an ¹¹C-enriched aqueous solution was administered to young branches of Populus tremula L. which were subsequently imaged by positron emission tomography (PET). This approach allows in vivo visualization of the internal movement of CO₂ inside branches at high spatial and temporal resolution, and enables direct measurement of the transport speed of xylem-transported CO₂ (v _CO2). Through compartmental modeling of the dynamic data obtained from the PET images, we (i) quantified v _CO2 and (ii) proposed a new method to assess the fate of xylem-transported ¹¹CO₂ within the branches. It was found that a fraction of 0.49 min⁻¹ of CO₂ present in the xylem was transported upwards. A fraction of 0.38 min⁻¹ diffused radially from the sap to the surrounding parenchyma and apoplastic spaces (CO_2,PA) to be assimilated by woody tissue photosynthesis. Another 0.12 min⁻¹ of the xylem-transported CO₂ diffused to the atmosphere via efflux. The remaining CO₂ (i.e. 0.01 min⁻¹) was stored as CO_2,PA, representing the build-up within parenchyma and apoplastic spaces to be assimilated or directed to the atmosphere. Here, we demonstrate the outstanding potential of ¹¹CO₂-based plant-PET in combination with compartmental modeling to advance our understanding of internal CO₂ movement and the respiratory physiology within woody tissues. [ABSTRACT FROM AUTHOR]