Can 14 C-labeled photosynthetic products move between Pinus densiflora seedlings linked by ectomycorrhizal mycelia?

•  Movement of 14 C-labeled photosynthetic products via ectomycorrhizal mycelia linking seedlings of Pinus densiflora is investigated here, a process which may have the potential to be highly ecologically significant. • Paired seedlings linked by hyphae of either an unidentified ectomycorrhizal fungus designated T01 or Pisolithus tinctorius were prepared by arranging mycorrhizal and nonmycorrhizal seedlings in an observation rhizobox and allowing mycelial links to form. Shoots of the initially mycorrhizal seedling were photosynthetically labeled with 14 CO2 and movement of 14 C chased by time-course autoradiography. •  Within 3 d, 14 C was transferred to mycelia and mycorrhizas formed on the unlabeled seedling, but little 14 C was subsequently transferred to the roots and shoots. The reverse movement of label was also demonstrated. Mycelial fans of T01, with greater hyphal density, accumulated more 14 C than mycelia of P. tinctorius, whereas higher radioactivity was detected in hyphal strands than in the mycelial front of P. tinctorius. •  Ectomycorrhizas and mycelia are strong sinks for photosynthetic products, and C transfer can occur bi-directionally in the mycelia. However, as yet no direct evidence indicates any benefit to receiver plants and, thus, supplementary supply of C to understory individuals may not be ecologically significant.