Direct transfer of carbon between plants connected by vesicular-arbuscular mycorrhizal mycelium

Mycorrhizal infection of plants in many natural vegetation systems arises when uninfected roots make contact with mycelia spreading from infected roots^1,2. The result of the infection is that connections are present between the roots of individual plants. It has been shown by autoradiography that these interconnections provide a direct pathway for the transfer of carbon between ecto-mycorrhizal plants^2,3. Inter-plant transfer of carbon⁴ and phosphorus^5-7 has been reported in plants with vesicular-arbuscular (VA) mycorrhizas, but in the absence of comparable evidence it remains a possibility that in these cases the pathway is indirect, uptake by the fungus of the `receiver' plant occurring only after leakage of the nutrients from roots of the `donor'. We now show using autoradiography that transfer of carbon between plants connected by VA mycorrhizal mycelium occurs primarily by the direct hyphal pathway. It is further shown that the magnitude of the transfer is strongly influenced by shading of `receiver' plants indicating that movement is governed by source-sink relationships.