Many ways to exploit mycorrhizas: the mycoheterotrophy continuum in orchids

Julienne Schiebold1, Martin. I. Bidartondo2, Gerhard Gebauer1
1 BayCEER-IBG, University of Bayreuth
2 Department of Life Sciences, Imperial College, London & Royal Botanic Gardens, Kew, United Kingdom

P 3.5 in Rhizosphere interactions: Biogeochemical processes at the root-soil interface

Mycorrhizal carbon flow is not only reversed in achlorophyllous, fully mycoheterotrophic (FMH) plants but also in putatively autotrophic plants participating in mycorrhizal symbioses. Partially mycoheterotrophic (PMH) plants utilise carbon both from photosynthesis and a fungal source.  All 28,000 species in the largest angiosperm family, the Orchidaceae, produce dust seeds and are initially mycoheterotrophic; they need a mycobiont for nutrient provision in early life stages.  Thus, orchids may be predisposed to a PMH nutrition due to their initial FMH protocorm stage.

Materials and Methods       
We investigated the ecophysiology of the heterotrophic nutrition of 30 orchid species native to Central Europe using stable C, N and H isotope analysis and molecular identification of orchid mycorrhizal fungi, and a further 40 PMH and FMH Ericaceae and Orchidaceae species from a database.

We found (i) a varying degree of PMH in rhizoctonia-associated meadow orchids; (ii) a correlation between local light climate and 13C enrichment in  rhizoctonia-associated orchid species; (iii) the identity of the fungal partner (rhizoctonia, ECM basidiomycete, ECM ascomycete) associated with Epipactis taxa to drive plant 15N enrichment; (iv) that FMH seedlings of mature green-leaved orchids associated with rhizoctonia fungi are useful to assess the organic matter gain derived from fungi; and (v) taxonomic patterns of mycoheterotrophy.

Our findings demonstrate that PMH in orchids is a trophic continuum between the extreme endpoints of autotrophy and FMH, ranging from marginal to pronounced PMH.

In rhizoctonia-associated orchids, PMH plays a far greater role than previously assumed, even in full light conditions.

Keywords: orchid mycorrhiza, partial mycoheterotrophy, rhizoctonia, stable isotopes, ITS sequencing