Data from: Stable isotope signatures of underground seedlings reveal the organic matter gained by adult orchids from mycorrhizal fungi

1.Orchids produce dust seeds dependent on the provision of organic carbon by mycorrhizal fungi for their early development stages. Hence, all chlorophyllous orchids experience a dramatic switch in trophic strategies from initial mycoheterotrophy to either autotrophy or partial mycoheterotrophy during ontogeny. Yet, the degree to which partially mycoheterotrophic orchids gain carbon from their mycorrhizal fungi is unclear based on existing approaches.

2.Here, we propose a novel approach to quantify the fungal-derived organic matter gain of chlorophyllous mature orchids mycorrhizal with rhizoctonia fungi using the stable isotope signatures of their fully mycoheterotrophic (FMH) seedlings in a linear two-source mixing model.

3.We conducted a field germination experiment with seven orchid species and measured carbon, nitrogen and hydrogen stable isotope natural abundances and nitrogen concentrations of mature orchids, underground seedlings and autotrophic references.

4.After in situ burial for 19 – 30 months, germination rates varied considerably among five orchid species and failed for two. On average, underground seedlings were enriched in 13C and 15N relative to mature orchids and had higher nitrogen concentrations. Using the mean enrichment factors ε13C and ε2H of seedlings as FMH endpoint, the organic matter gain derived by mature orchids from mycorrhizas was ca. 20%.

5.Chlorophyllous orchids mycorrhizal with rhizoctonias are predisposed to partially mycoheterotrophic nutrition due to their initially mycoheterotrophic seedling stage. We show that the carbon and hydrogen isotope abundances of underground seedlings can be used in an improved mixing-model to identify a significant proportion of fungal-derived organic matter in mature orchids.

1. 兰花会产生粉尘状种子，其早期发育阶段依赖菌根真菌（mycorrhizal fungi）提供有机碳。因此，所有具叶绿素的兰花在个体发育（ontogeny）过程中，营养策略会发生显著转变：从初始的菌异养（mycoheterotrophy）转变为自养（autotrophy）或部分菌异养（partial mycoheterotrophy）。然而，基于现有研究方法，部分菌异养兰花从菌根真菌中获取碳的程度仍不明确。
2. 本研究提出一种全新方法，借助完全菌异养（FMH）幼苗的稳定同位素特征（stable isotope signatures），利用线性双源混合模型（linear two-source mixing model），量化与丝核菌属真菌（rhizoctonia fungi）形成菌根的具叶绿素成熟兰花的真菌源有机质获取量。
3. 我们针对7种兰花开展了野外萌发实验（field germination experiment），并测定了成熟兰花、地下幼苗以及自养参照样本的碳、氮、氢稳定同位素自然丰度（stable isotope natural abundances）与氮浓度（nitrogen concentrations）。
4. 经原位埋藏（in situ burial）19至30个月后，5种兰花的萌发率（germination rates）差异显著，另有2种未萌发。平均而言，地下幼苗相较于成熟兰花，其¹³C与¹⁵N同位素丰度更高，且氮浓度也更高。以幼苗的碳同位素富集因子ε¹³C与氢同位素富集因子ε²H作为完全菌异养终点，成熟兰花从菌根中获取的有机质占比约为20%。
5. 与丝核菌属真菌形成菌根的具叶绿素兰花，因其幼苗初始阶段为菌异养，故倾向于采取部分菌异养的营养方式。本研究表明，地下幼苗的碳、氢同位素丰度可通过改进的混合模型，定量确定成熟兰花中真菌源有机质的显著占比。