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 belowground C input, the spatial and temporal distribution of C, and the interactions between plants and soil microorganisms. Based on 14 C 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 14 CO 2 atmosphere, and herbariums of roots and shoots were prepared 6 h, 2 d, and 11 d after the 14 C pulse. The 14 C 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 14 C specific activity expressed as digital light units (DLU mm –2 ) of the imaging plates. Areas with high relative 14 C activity were classified as hotspots. Strong 14 C hotspots were detected mainly at the root tips already 6 h after the 14 C assimilation, and they remained active for at least 2 d. Eleven days after the 14 C assimilation, the hotspots at the root tips disappeared and the 14 C distribution was much more even than after 6 h or after 2 d. 14 C 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.