Laboratory of Plant Ecology, Department of Plants and Crops, Faculty of Bioscience Engineering, Ghent University, Coupure Links 653, 9000 Ghent, Belgium.
MEDISIP-INFINITY, Department of Electronics and Information Systems, Faculty of Engineering and Architecture, Ghent University, Corneel Heymanslaan 10, 9000 Ghent, Belgium.
Tree Physiol. 2020 Jul 30;40(8):1058-1070. doi: 10.1093/treephys/tpaa048.
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 (vCO2). Through compartmental modeling of the dynamic data obtained from the PET images, we (i) quantified vCO2 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 (CO2,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 CO2,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.
木质组织中的呼吸 CO2 可以在木质部中积累，并溶解在 sap 溶液中，通过植物进行运输。从 sap 中，一部分 CO2 可以径向扩散到大气中，或者被木质组织中存在的叶绿体同化。这些过程在茎和枝中同时发生，使得研究它们的特定动态变得困难。因此，向欧洲山杨（Populus tremula L.）的幼枝注入了富含 11C 的水溶液，随后通过正电子发射断层扫描（PET）对其进行成像。这种方法允许在体内以高时空分辨率可视化 CO2 在树枝内部的内部运动，并能够直接测量木质部运输的 CO2 的运输速度（vCO2）。通过对从 PET 图像获得的动态数据进行房室模型分析，我们（i）量化了 vCO2，（ii）提出了一种新方法来评估木质部运输的 11CO2 在树枝内的命运。结果发现，木质部中存在的 CO2 中有 0.49 min-1 的部分向上运输。0.38 min-1 的部分从 sap 径向扩散到周围的薄壁组织和质外体空间（CO2，PA），被木质组织光合作用同化。另外 0.12 min-1 的木质部运输的 CO2 通过外排扩散到大气中。剩余的 CO2（即 0.01 min-1）作为 CO2，PA 储存，代表在薄壁组织和质外体空间中的积累，以待同化或导向大气。在这里，我们展示了基于 11CO2 的植物-PET 与房室模型分析相结合的出色潜力，以推进我们对内部 CO2 运动和木质组织呼吸生理学的理解。
