The purpose of this study was to identify the pathway and sink activity of photosynthate translocation in the extraradical mycelium (ERM) of a Pisolithus isolate. We labelled ectomycorrhizal (ECM) Pinus thunbergii seedlings with (14)CO2 and followed (14)C distribution within the ERM by autoradiography. (14)C photosynthate translocation in the ERM resulted in (14)C distribution in rhizomorphs throughout the ERM, with (14)C accumulation at the front. When most radial mycelial connections between ECM root tips and the ERM front were cut, the whole allocation of (14)C photosynthates to the ERM was reduced. However, the overall pattern of (14)C distribution in the ERM was maintained even in regions immediately above and below the cut, with no local (14)C depletion or accumulation. We inferred from this result that every portion in the ERM has a significant sink activity and a definite sink capacity for photosynthates and that photosynthates detour the cut and reach throughout the ERM by translocation in every direction. Next, we prepared paired ECM seedlings, ERMs of which had been connected with each other by hyphal fusion, alongside, labelled the left seedling with (14)CO2, and shaded none, one or both of them. (14)C photosynthates were acropetally and basipetally translocated from the left ERM to ECM root tips of the right seedling through rhizomorphs in the left and right ERMs, respectively. With the left seedling illuminated, (14)C translocation from the left to the right ERM increased by shading the right seedling. This result suggests that reduced photosynthate transfer from the host to its ERM increased sink activity of the ERM.
Keywords: Acropetal and basipetal; Common mycelial network; Ectomycorrhizal symbiosis; Extramatrical mycelium; Soil nutrients.