Data from: Exploiting mycorrhizas in broad daylight: partial mycoheterotrophy is a common nutritional strategy in meadow orchids.

Partial mycoheterotrophy (PMH) is a nutritional mode in which plants utilize organic matter, i.e. carbon, both from photosynthesis and a fungal source. The latter reverses the direction of plant-to-fungus carbon flow as usually assumed in mycorrhizal mutualisms. Based on significant enrichment in the heavy isotope 13C, a growing number of PMH orchid species have been identified. These PMH orchids are mostly associated with fungi simultaneously forming ectomycorrhizas with forest trees. In contrast, the much more common orchids that associate with rhizoctonia fungi, which are decomposers, have stable isotope profiles most often characterized by high 15N enrichment and high nitrogen concentrations but either an insignificant 13C enrichment or depletion relative to autotrophic plants. Using hydrogen stable isotope abundances recent investigations showed PMH in rhizoctonia-associated orchids growing under light-limited conditions. Hydrogen isotope abundances can be used as substitute for carbon isotope abundances in cases where autotrophic and heterotrophic carbon sources are insufficiently distinctive to indicate PMH.
To determine whether rhizoctonia-associated orchids growing in habitats with high irradiance feature PMH as a nutritional mode, we sampled 13 orchid species growing in montane meadows, four forest orchid species and 34 autotrophic reference species. We analysed δ2H, δ13C, δ15N and δ18O and determined nitrogen concentrations. Orchid mycorrhizal fungi were identified by DNA sequencing.
As expected, we found high enrichments in 2H, 13C, 15N and nitrogen concentrations in the ectomycorrhiza-associated forest orchids, and the rhizoctonia-associated Neottia cordata from a forest site was identified as PMH. Most orchids inhabiting sunny meadows lacked 13C enrichment or were even significantly depleted in 13C relative to autotrophic references. However, we infer PMH for the majority of these meadow orchids due to both significant 2H and 15N enrichment and high nitrogen concentrations. Pseudorchis albida was the sole autotrophic orchid in this study as it exhibited neither enrichment in any isotope nor a distinctive leaf nitrogen concentration.
Synthesis. Our findings demonstrate that partial mycoheterotrophy is a trophic continuum between the extreme endpoints of autotrophy and full mycoheterotrophy, ranging from marginal to pronounced. In rhizoctonia-associated orchids, partial mycoheterotrophy plays a far greater role than previously assumed, even in full light conditions.

部分菌异养（Partial mycoheterotrophy，PMH）是一类营养模式，指植物同时利用光合作用产生的有机质与真菌来源的碳基物质。这一过程逆转了菌根共生（mycorrhizal mutualisms）中通常认定的植物向真菌的碳流方向。基于重同位素¹³C的显著富集现象，已有越来越多的PMH型兰科物种被鉴定报道。这类PMH兰科植物大多与同时能与林木形成外生菌根（ectomycorrhizas）的真菌共生。与之相对，更为常见的与丝核菌属真菌（rhizoctonia fungi，一类分解者）共生的兰科植物，其稳定同位素谱通常以高¹⁵N富集与高氮浓度为典型特征，但相较于自养植物（autotrophic plants），其¹³C富集程度不显著，甚至出现耗竭。近期有研究通过氢稳定同位素丰度（hydrogen stable isotope abundances）分析，证实光照受限生境中生长的丝核菌共生兰科植物存在PMH现象。当自养与异养碳源的区分度不足以用于判定PMH时，氢同位素丰度可作为碳同位素丰度的替代指标。

为明确在高光照生境中生长的、与丝核菌共生的兰科植物是否具备PMH营养模式，我们采集了13种生于山地草甸的兰科植物、4种森林兰科植物以及34种自养对照物种。我们对样品的δ²H、δ¹³C、δ¹⁵N与δ¹⁸O进行了分析，并测定了氮浓度。兰科菌根真菌（orchid mycorrhizal fungi）通过DNA测序完成鉴定。

正如预期，我们在与外生菌根共生的森林兰科植物中检测到了²H、¹³C、¹⁵N的高富集现象以及高氮浓度，而采自森林生境的、与丝核菌共生的Neottia cordata被鉴定为PMH型植物。多数栖息于向阳草甸的兰科植物未出现¹³C富集，甚至相较于自养对照物种出现了显著的¹³C耗竭。但基于这些草甸兰科植物同时存在显著的²H与¹⁵N富集以及高氮浓度，我们推断其中多数物种具备PMH特性。本研究中唯一的自养型兰科植物为Pseudorchis albida，该物种未出现任何同位素富集，且叶氮浓度无特异性差异。

综合分析。本研究结果表明，部分菌异养是介于自养与完全菌异养（full mycoheterotrophy）两个极端端点之间的营养连续体，其表现形式从边缘性到显著性不等。在与丝核菌共生的兰科植物中，部分菌异养所发挥的作用远大于此前的假设，即便在全光照条件下亦是如此。