title: Are carbon and nitrogen exchange between fungi and the orchid Goodyera repens affected by irradiance?
abstract: OBJECTIVE
  The green orchid Goodyera repens has been shown to transfer carbon to its mycorrhizal partner, and this flux may therefore be affected by light availability. This study aimed to test whether the C and N exchange between plant and fungus is dependent on light availability, and in addition addressed the question of whether flowering and/or fruiting individuals of G. repens compensate for changes in leaf chlorophyll concentration with changes in C and N flows from fungus to plant.
  
  
  METHODS
  The natural abundances of stable isotopes of plant C and N were used to infer changes in fluxes between orchid and fungus across natural gradients of irradiance at five sites. Mycorrhizal fungi in the roots of G. repens were identified by molecular analyses. Chlorophyll concentrations in the leaves of the orchid and of reference plants were measured directly in the field.
  
  
  RESULTS
  Leaf δ(13)C values of G. repens responded to changes in light availability in a similar manner to autotrophic reference plants, and different mycorrhizal fungal associations also did not affect the isotope abundance patterns of the orchid. Flowering/fruiting individuals had lower leaf total N and chlorophyll concentrations, which is most probably explained by N investments to form flowers, seeds and shoot.
  
  
  CONCLUSIONS
  The results indicate that mycorrhizal physiology is relatively fixed in G. repens, and changes in the amount and direction of C flow between plant and fungus were not observed to depend on light availability. The orchid may instead react to low-light sites through increased clonal growth. The orchid does not compensate for low leaf total N and chlorophyll concentrations by using a (13)C- and (15)N-enriched fungal source.
authors.affiliation: Laboratory of Isotope Biogeochemistry, Bayreuth Center of Ecology and Environmental Research (BayCEER), University of Bayreuth, 95440 Bayreuth, Germany and Imperial College London and Royal Botanic Gardens, Kew TW9 3DS, UK.
authors.forename: Heiko T
authors.initials: HT
authors.lastname: Liebel
authors.affiliation: Laboratory of Isotope Biogeochemistry, Bayreuth Center of Ecology and Environmental Research (BayCEER), University of Bayreuth, 95440 Bayreuth, Germany and Imperial College London and Royal Botanic Gardens, Kew TW9 3DS, UK.
authors.forename: Martin I
authors.initials: MI
authors.lastname: Bidartondo
authors.affiliation: Laboratory of Isotope Biogeochemistry, Bayreuth Center of Ecology and Environmental Research (BayCEER), University of Bayreuth, 95440 Bayreuth, Germany and Imperial College London and Royal Botanic Gardens, Kew TW9 3DS, UK gerhard.gebauer@uni-bayreuth.de.
authors.forename: Gerhard
authors.initials: G
authors.lastname: Gebauer
mesh_terms: D002244:Carbon / Q000378:metabolism*; D002247:Carbon Isotopes / Q000032:analysis; D002734:Chlorophyll / Q000378:metabolism; D005656:Fungal Proteins / Q000235:genetics; D005658:Fungi / Q000502:physiology* / Q000528:radiation effects; D008969:Molecular Sequence Data; D038821:Mycorrhizae / Q000502:physiology* / Q000528:radiation effects*; D009584:Nitrogen / Q000378:metabolism*; D009587:Nitrogen Isotopes / Q000032:analysis; D029595:Orchidaceae / Q000502:physiology* / Q000528:radiation effects; D018515:Plant Leaves / Q000378:metabolism; D018517:Plant Roots / Q000378:metabolism / Q000382:microbiology; D017422:Sequence Analysis, DNA; D013472:Sunlight; D013559:Symbiosis
chemical_list: D002247:Carbon Isotopes; D005656:Fungal Proteins; D009587:Nitrogen Isotopes; D002734:Chlorophyll; D002244:Carbon; D009584:Nitrogen
keywords: 13C; 15N; Ceratobasidium; Goodyera repens; Tulasnella; chlorophyll concentration; light availability; mycoheterotrophy; mycorrhiza; nutrition; orchid; stable isotopes