Phenylalanine ¹⁵N enrichment likely indicates fungal-derived organic nutrient acquisition in mycoheterotrophic plants across fungal guilds

・Beyond fully mycoheterotrophic plants, many green plants may also obtain carbon from fungal partners. However, bulk stable isotope analyses often lack sufficient resolution in arbuscular mycorrhizal and rhizoctonia-associated orchid systems, limiting inference of fungal-derived organic nutrient acquisition. ・We measured δ¹⁵N values of glutamic acid (δ¹⁵NGlu) and phenylalanine (δ¹⁵NPhe) in 10 fully mycoheterotrophic species spanning arbuscular mycorrhizal, ectomycorrhizal, and saprotrophic fungal guilds, as well as in three photosynthetic species with contrasting inferred nutritional modes, together with co-occurring autotrophic references and, where available, fungal material. This Glu-Phe framework was originally developed for animal food-web studies, in which trophic position is estimated from strong ¹⁵NGlu enrichment (c. 7–8‰ per trophic step) and minimal ¹⁵NPhe enrichment (c. 0–1‰). ・We found that mycoheterotrophic plants invert this pattern, showing minimal ¹⁵NGlu enrichment (−1.2 to 3.3‰) but pronounced ¹⁵NPhe enrichment (11.2 to 19.6‰) relative to their fungal partners. They also exhibited amino acid δ¹⁵N profiles distinct from autotrophs, consistent with uptake of fungal-derived organic nitrogen. ・To our knowledge, such strong ¹⁵NPhe enrichment relative to nutrient sources has not been reported in other organisms. Overall, amino acid δ¹⁵N profiles complement bulk δ2H, δ13C, and δ¹⁵N values for assessing fungal-derived organic nutrient acquisition in green plants.