High-Resolution in vivo Imaging of Xylem-Transported CO2 in Leaves Based on Real-Time 11C-Tracing

Abstract

Plant studies using the short-lived isotope 11 C to label photosynthate via atmospheric carbon dioxide (CO 2 ), have greatly advanced our knowledge about the allocation of recent photosynthate from leaves to sinks. However, a second source for photosynthesis is CO 2 in the transpiration stream, coming from respiration in plant tissues. Here, we use in vivo tracing of xylem-transported 11 CO 2 to increase our knowledge on whole plant carbon cycling. We developed a new method for in vivo tracing of xylem-transported CO 2 in excised poplar leaves using 11 C in combination with positron emission tomography (PET) and autoradiography. To show the applicability of both measurement techniques in visualizing and quantifying CO 2 transport dynamics, we administered the tracer via the cut petiole and manipulated the transport by excluding light or preventing transpiration. Irrespective of manipulation, some tracer was found in main and secondary veins, little of it was fixed in minor veins or mesophyll, while most of it diffused out the leaf. Transpiration, phloem loading and CO 2 recycling were identified as mechanisms that could be responsible for the transport of internal CO 2 . Both 11 C-PET and autoradiography can be successfully applied to study xylem-transported CO 2 , toward better understanding of leaf and plant carbon cycling, and its importance in different growing conditions.

Guide to Plant-PET Imaging Using 11CO2

“…Smaller hospitals usually do not have a cyclotron, generally have a single PET scanner (typically combined with CT) and purchase their PET radiopharmaceuticals from commercial vendors that have a cyclotron facility. Larger hospitals and academic institutes have PET centers that can accommodate one or more cyclotrons, a radiochemistry laboratory and often several (multimodal) PET scanners, including laboratory animal (e.g., Alexoff et al, 2011;Hubeau et al, 2019b), clinical (e.g., Garbout et al, 2012;Karve et al, 2015), or self-designed imaging systems (e.g., Uchida et al, 2004;Jahnke et al, 2009;Weisenberger et al, 2012;Kurita et al, 2020). In these larger centers the integral multidisciplinary workflow (Figure 2) can be followed.…”

“…Depending on the target atom a certain radionuclide can be produced (Table 1). The most widely used positron-emitting nuclide in plant science is carbon-11 ( 11 C, half-life of 20.39 min), which is usually administered as gaseous 11 CO 2 to study long-distance transport of photosynthates (Minchin and Thorpe, 2003;Karve et al, 2015;Hubeau et al, 2018) or can also be administered in an aqueous solution to study xylem-transported CO 2 (Bloemen et al, 2015;Mincke et al, 2018Mincke et al, , 2020Hubeau et al, 2019b). 11 CO 2 is generally produced in two different ways depending on the target material, i.e., N 2 /O 2 (Karve et al, 2015) or N 2 /H 2 (Hubeau et al, 2018).…”

“…Because the probability of annihilation increases with thickness, escape fractions were lower in thicker leaf areas like the midrib (1 -2 mm) (Alexoff et al, 2011). When studying single leaves, an approach to increase the detection of positrons actually annihilating inside the plant material includes the use of thin acrylate plates that can be positioned parallel with the leaf blade, while ensuring not to limit air contact with the leaf (Alexoff et al, 2011;Hubeau et al, 2019b). An alternative is calculating the annihilation probability of positrons according to the thickness of the tissue in which the positrons were detected.…”

From the Outside in: An Overview of Positron Imaging of Plant and Soil Processes

“…The application of positron imaging to plant studies has increased significantly since the initial water transport analyses by McKay et al 2 While dynamics of water movement with PET in plants has been widely studied, 25 - 35 other topics related to PET have increasingly been investigated, including uptake and translocation of nutrients, 36 - 52 , 98 , 99 photoassimilate fixation/allocation, 15 , 22 , 23 , 53 - 80 sugar transport, 21 , 82 …”

Out of the blue: Phototropins of the leaf vascular bundle sheath mediate the regulation of leaf hydraulic conductance by blue light

“…xylem-transported CO2 was suggested to be a source for CO2 assimilated in the bundle sheath and mesophyll (Janacek et al, 2009;Hubeau et al, 2019). Thus, assuming the Kleaf increase with the first light of day parallels the increase of CO2 permeability of aquaporins in the BSCs, the passage to the mesophyll of CO2 from the xylem originating in roots respiration will be enhanced even before full stomatal opening.…”