Isotope evidence for partial mycoheterotrophy and trophic flexibility in the arbuscular mycorrhizal green plant <i>Gentiana zollingeri</i>

Arbuscular mycorrhizal (AM) associations are central to terrestrial ecosystems, typically facilitating the exchange of organic carbon from plants for mineral nutrients from fungi. Full mycoheterotrophy, in which plants derive all their carbon from AM fungi, is widespread among many achlorophyllous plant taxa such as Voyria and Voyriella (Gentianaceae). However, the extent and ecological significance of partial mycoheterotrophy, where green AM plants supplement photosynthesis with fungal carbon, remain under debate. This study investigates the nutritional mode of Gentiana zollingeri, a photosynthetic Gentianaceae species, using ¹³C and ¹⁵N stable isotope analysis, focusing on the relationship between isotopic enrichment and leaf ratio (leaf biomass relative to total shoot biomass). Gentiana zollingeri exhibited significantly higher ¹³C and ¹⁵N than autotrophic reference plants, suggesting a reliance on fungal-derived carbon and nitrogen. A negative correlation between ¹³C enrichment and leaf ratio indicates that the enrichment reflects fungal dependence rather than alternative physiological traits. Notably, the underground stem showed ¹³C enrichment comparable to that of fully mycoheterotrophic Gentianaceae, suggesting it is primarily composed of fungal carbon. These findings support the hypothesis that G. zollingeri employs a flexible nutrition strategy, adjusting fungal dependence according to photosynthetic capacity. Although modest ¹³C and ¹⁵N enrichment alone cannot confirm partial mycoheterotrophy, integrative approaches combining stable isotope data with ecological and physiological indicators (e.g., negative correlation between ¹³C enrichment and leaf ratio) strongly support fungal carbon acquisition in G. zollingeri. Similar frameworks will help to rigorously assess partial mycoheterotrophy in other Paris-type AM plants with subtle isotopic enrichment.