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

Summary 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 δ 15 N values of glutamic acid (δ 15 N Glu ) and phenylalanine (δ 15 N Phe ) 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 15 N Glu enrichment ( c. 7–8‰ per trophic step) and minimal 15 N Phe enrichment ( c. 0–1‰). We found that mycoheterotrophic plants invert this pattern, showing minimal 15 N Glu enrichment (−1.2 to 3.3‰) but pronounced 15 N Phe enrichment (11.2 to 19.6‰) relative to their fungal partners. They also exhibited amino acid δ 15 N profiles distinct from autotrophs, consistent with uptake of fungal‐derived organic nitrogen. To our knowledge, such strong 15 N Phe enrichment relative to nutrient sources has not been reported in other organisms. Overall, amino acid δ 15 N profiles complement bulk δ 2 H, δ 13 C, and δ 15 N values for assessing fungal‐derived organic nutrient acquisition in green plants.