Fully mycoheterotrophic (MH) plants not only obtain soil nutrients from fungal sources, as usual in mycorrhizal associations, but also carbon (C) that ultimately derives from interconnected, photosynthetically active plants. Studies revealed 13C and 15N enrichments as well as high nitrogen (N) concentrations in most MH plants compared to autotrophic reference plants. In this context, only five MH species from just two plant families engaging in arbuscular mycorrhiza, the most dominant mycorrhizal type, have been studied yet.
Material and Methods
In the present study, one orchid species associated with saprotrophic fungi and 13 fully MH plants of five families associated with arbuscular mycorrhizal (AM) fungi have been sampled together with autotrophic reference plants and soil in the tropics and the temperate zone of the southern hemisphere. Analyses of δ13C, δ15N, δ2H, δ18O and N concentrations were conducted, and DNA of reference plants and fungal hosts were sequenced.
We found significant 13C enrichments in most MH species, trending to be low in Thismiaceae sampled in the temperate zone, and 15N enrichments in several MH species except for Gentianaceae which exhibited the same N isotopic composition as autotrophic plants. Mycoheterotrophs associated with Rhizophagus irregularis fungi were stronger enriched in 15N, while associations with Non-Rhizophagus-Glomeraceae led to higher 13C enrichments. The N concentrations in MH plants were significantly higher than in green plants. AM mycoheterotrophs were also enriched in 2H.
In conclusion, 13C enrichments of 5.1 ± 2.2 ‰ can generally be attributed to AM mycoheterotrophs, while they are not compulsorily characterised by 15N enrichments. The differences between isotopic compositions of MH plants most likely result from family-specific physiological characteristics and the type of fungal hosts.