Isotopic evidence of arbuscular mycorrhizal cheating in a grassland gentian species

All orchids and pyroloids are mycoheterotrophic at least in the early stage. Many species are predisposed to mycoheterotrophic nutrition even in the adult stage, due to the initial mycoheterotrophy during germination. Although other green plants, such as gentian species, also produce numerous minute seeds, whose germination may depend on fungal associations to meet C demands, physiological evidence for partial mycoheterotrophy in the adult stage is lacking for most candidate taxa. Here, we compared the natural abundances of C-13 and N-15 isotopes in the AM-associated gentian species Pterygocalyx volubilis growing in high-light-intensity habitats with those of co-occurring autotrophic C-3 and C-4 plants and AM fungal spores. We found that P. volubilis was significantly enriched in C-13 compared with the surrounding C-3 plants, which suggests the transfer of some C from the surrounding autotrophic plants through shared AM networks. In addition, the intermediate delta N-15 values of P. volubilis, between those of autotrophic plants and AM fungal spores, provide further evidence for partial mycoheterotrophy in P. volubilis. Although it is often considered that light deficiency selects partial mycoheterotrophy, we show that partial mycoheterotrophy in AM-forming plants can evolve even under light-saturated conditions. The fact that there have been relatively few descriptions of partial mycoheterotrophy in AM plants may not necessarily reflect the rarity of such associations. In conclusion, partial mycoheterotrophy in AM plants may be more common than hitherto believed.