Light limitation and partial mycoheterotrophy in rhizoctonia-associated orchids
Partially mycoheterotrophic (PMH) plants obtain organic molecules from their mycorrhizal fungi in addition to carbon (C) fixed by photosynthesis. Some PMH orchids associated with ectomycorrhizal fungi have been shown to flexibly adjust the proportion of organic molecules obtained from fungi according to the habitat’s light level. We hypothesise that Neottia ovata and Ophrys insectifera, two orchids associated with saprotrophic rhizoctonia fungi, are also able to increase uptake of organic molecules from fungi as irradiance levels decrease. We continuously measured light availability for individuals of N. ovata and O. insectifera at a grassland and a forest during orchid flowering and fruiting. We repeatedly sampled leaves of N. ovata, O. insectifera and autotrophic reference species for stable isotope natural abundances (δ13C, δ15N, δ2H, δ18O) and C and N concentrations. We found significant 13C enrichment in both orchids relative to autotrophic references at the forest but not the grassland, and significant 2H enrichment at both sites. The 13C enrichment in O. insectifera was linearly correlated with the habitat’s irradiance levels. We conclude that both species can be considered as PMH and at least in O. insectifera, the degree of partial mycoheterotrophy can be fine-tuned according to light availability. However, exploitation of mycorrhizal fungi appears less flexible in saprotroph-associated orchids than in orchids associated with ectomycorrhizal fungi.
KeywordsNeottia ovata Ophrys insectifera Orchidaceae Stable isotopes Mycoheterotrophy Mycorrhiza
The authors thank Christine Tiroch and Petra Eckert (BayCEER—Laboratory of Isotope Biogeochemistry) for skilful technical assistance with stable isotope abundance measurements. We also thank the Regierung von Oberfranken for authorisation to collect the orchid samples.
Author contribution statement
GG conceived this study. JM-IS and CK conducted fieldwork and prepared samples for stable isotope analyses. JM-IS and MIB generated sequencing data and conducted molecular analyses, JM-IS performed the statistical analyses and drafted the manuscript. All authors contributed to the manuscript.
The study was funded by the German Research Foundation (DFG Ge 565/7-2).
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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