Uptake of nitrogen and carbon from double-labelled (15N and 13C) glycine by mycorrhizal pine seedlings

While it is accepted that many ectomycorrhizal fungi can assimilate organic substrates and facilitate transfer of their elemental components to plants, the fate of the carbon contained in these materials remains uncertain. Here we investigate the compartmentation of carbon and nitrogen in ectomycorrhizal seedlings of Pinus sylvestris fed with double-labelled (N-15 and C-13) glycine as their sole N source. Using isotope ratio mass spectrometry, the quantities of N and C derived from this glycine were determined in sequentially harvested samples of mycorrhizas, roots and shoots. Whereas considerable quantities of N-15 were observed in the mycorrhizal tips, roots and shoots, comparable amounts of C-13 were observed only in mycorrhizal tips and roots. It is clearly important to resolve the role of compound specificity as a factor determining the extent of amino-acid C transfer from roots to shoots. However, from the standpoint of the C budget of the whole plant, wherever heterotrophically acquired C is available as an energy source it will reduce demands on photosynthetically fixed sources of the element.