The use of positron emission tomography for studies of long‐distance transport in plants: uptake and transport of ¹⁸F

Positron emission tomography (PET) has been utilized to obtain dynamic images of long distance nutrient translocation in plants. Positron emitting 18 F, produced by a Van de Graaff accelerator using the reaction 18 O(p,n) 18 F, was fed in solution to excised stems of Glycine max positioned vertically in a large‐aperture PET detector system. Images of tracer activity were recorded with a time resolution of 0.5 min and a spatial resolution of 4 mm. Maximum tracer activities at stem sites were obtained within 3 min of the pulse feed. A model is presented enabling evaluation of regional values for tracer flow, tracer binding, flow speed and flow volume. Analysis of data for one stem position yielded a flow volume of 2.1mm 3 min −1 and a flow speed of 36cm min −1 . Comparison with the distribution of 14 C‐inulin, which was simultaneously fed to the cut stems, indicates the 18 F is suitable for use as an apoplastic tracer; 92% of the tracer activity accumulated in the leaves. The fraction of 18 F that remained bound was most concentrated at stem nodal regions, an observation consistent with the existence of transfer cells at these sites. Advantages and limitations of PET applied to plant physiological investigations are discussed.