Photoassimilate allocation and dynamics of hotspots in roots visualized by 14C phosphor imaging

Understanding photoassimilate allocation into the roots and the release of organic substances from the roots into the rhizosphere is an important prerequisite for characterizing the below-ground C input, the spatial and temporal distribution of C, and the interactions between plants and soil microorganisms. Based on C-14 phosphor imaging, we visualized the allocation of assimilates into Lolium perenne roots and estimated the life time of hotspots at the root tips. Lolium shoots were labeled in a (CO2)-C-14 atmosphere, and herbariums of roots and shoots were prepared 6 h, 2 d, and 11 d after the C-14 pulse. The C-14 distribution in roots and leaves revealed that pulse labeling does not yield homogeneously labeled plant material. The spatial distribution of assimilate allocation was evaluated based on the C-14 specific activity expressed as digital light units (DLU mm(-2)) of the imaging plates. Areas with high relative C-14 activity were classified as hotspots. Strong C-14 hotspots were detected mainly at the root tips already 6 h after the C-14 assimilation, and they remained active for at least 2 d. Eleven days after the C-14 assimilation, the hotspots at the root tips disappeared and the C-14 distribution was much more even than after 6 h or after 2 d. C-14 phosphor imaging proved to be a promising tool to visualize the allocation of photoassimilates into the roots and the rhizosphere and can be used to identify hotspots and their dynamics.