Carbon transfer between plants and its control in networks of arbuscular mycorrhizas

1, Two studies using the stable-isotope C-13 have shown that large amounts of carbon can move between plants linked by arbuscular mycorrhizal fungi. Quantities comparable to the carbon cost of the symbiosis for an individual plant may be transferred. 2, We measured C transfer between linked plants of the grass Cynodon dactylon (C-4, delta(13)C approximate to - 14 parts per thousand) and the herb Plantago lanceolata (C-3, delta(13)C similar to - 28 parts per thousand). To test the hypothesis that the carbon transferred between plants remained in fungal structures at all times, plants were grown for two harvests; at the first harvest they were clipped to ground level, so that shoot re-growth required the transport of carbon from the roots. We also tested the influence of the direction of growth of the fungus, to determine whether C was transported out of or into a newly colonized root, and of growing plants in elevated CO2, to increase the availability of carbon compounds in the roots. 3, Large amounts of C were transferred between linked plants, more so into Plantago than into Cynodon roots. Transfer occurred whether root systems were separated by a 20 mu m mesh, that excluded roots but not hyphae, or a 0.45 mu m mesh, intended to act as a barrier to hyphae as well. We believe that the high root densities achieved in the experiment allowed hyphae to cross the finer mesh between the two dense root mars. 4. Clipping the plants did not result in any movement of C from roots to shoots, thus confining the prediction that all C transferred remains in fungal structures. 5, The direction of growth of the fungus did not affect the direction of transfer, nor did the CO2 concentration in which the plants were grown. 6, The amount of C transferred was a positive correlate of the frequency of vesicles in the roots but a negative correlate of the frequency of hyphae, if C were moving into developing colonization Units, thus effectively giving the plant a 'free' symbiosis, the correlation with internal hyphae should be positive. The positive correlation with vesicles suggests that C is moving into fungal storage structures. 7. We propose a mycocentric view of the phenomenon of interplant C transfer, in which the fungal colonies within roots are seen as parts of an extended mycelium between which the fungus moves resources depending on the dynamics of its own growth. We do not believe that the transfer has an impact on plant C budgets or fitness, but that it may be a major element in the understanding of fungal C budgets.