Is it better to give than to receive? A stable isotope perspective on orchid-fungal carbon transport in the green orchid species Goodyera repens and Goodyera oblongifolia.

0 0 1??? 4??? Letter Letters Letters Letters Is it better to give than to receive? A stable isotope perspective on orchid–fungal carbon transport in the green orchid species Goodyera repens and Goodyera oblongifolia In the field of orchid research, species within the tribe Crani-chideae have taken center stage as a result of the recent findings of Cameron et al. (2006, 2008), which demonstrated carbon transport from adult Goodyera repens (L.) R. Br. orchids to their mycorrhizal fungus Ceratobasidium cornigerum (Bourdot) D. P. Rogers. The dependence of orchids in their early stages of development on fungi is a long-recognized trait of the family (Bernard, 1909; Dearnaley, 2007). However, there has been much controversy over the potential for carbon 'repayment' to the fungi once the orchid has formed leaves and is capable of assimilating its own carbohydrates through photosynthesis Using 14 C-labeled carbon fed either to the mycelia of the orchid's fungal symbiont or to the plant as 14 CO 2 , Cameron et al. (2008) were able to quantify the carbon transport between the orchid and fungus over an 8-d period. Their findings were that the net transfer of carbon from G. repens to C. cornigerum was over five times greater than that of carbon transported from the fungus to the plant. While this extremely well-executed study provides the 'first full bidirectional C budget for any mycorrhizal association' (Cameron et al., 2008), there are some limitations of their model and methods that must be taken into account. As mentioned in their recent article and the commentary by Johnson (2008), the carbon allocation to fungal biomass within the orchid's roots cannot be separated from that to the roots alone; nor can carbon respiration from the plant versus that from the fungus. Furthermore, the use of radiocarbon labeling gives measurements of carbon flow within a system for only a relatively short period of time. Also, as many of these labeling experiments are carried out in the laboratory, it is difficult to relate results to any field setting. A complementary method that has been applied to examine carbon and nitrogen gains from fungi by partially and fully myco-heterotrophic plants associated with ectomycorrhizal (ECM) and litter-or wood-decaying saprotrophic (SAP) fungi is the use of naturally occurring stable isotopes of carbon and nitrogen (13 C: 12 C and Measured isotope abundances are denoted as δ values and are calculated according to the equation: δ 15 …