¹⁵N and ¹³C natural abundance of autotrophic and myco‐heterotrophic orchids provides insight into nitrogen and carbon gain from fungal association

Abstract

Summary• Whereas mycorrhizal fungi are acknowledged to be the sources of nitrogen (N) and carbon (C) in achlorophyllous (myco-heterotrophic) orchids, the sources of these elements in autotrophic orchids are unknown. We have determined the stable isotope abundance of N and C to quantify their gain from different sources in these two functional groups and in non-orchids of distinctive mycorrhizal types.• Leaves of each plant were collected from four forest and four grassland sites in Europe. The N and C isotope abundance, and total N concentrations of their tissues and of associated soils were determined.• Myco-heterotrophic orchids were significantly more enriched in 15 N ( ε MHO-R = 11.5‰) and 13 C ( ε MHO-R = 8.4‰) than co-occurring non-orchids. δ 15 N and δ 13 C signatures of autotrophic orchids ranged from values typical of non-orchids to those more representative of myco-heterotrophic orchids.• Utilization of fungi-derived N and C probably explains the relative 15 N and 13 C enrichment in the myco-heterotrophs. A linear two-source isotopic mixing model was used to estimate N and C gain of autotrophic orchids from their fungal associates. Of the putatively autotrophic species, Cephalanthera damasonium obtained the most N and C by the fungal route, but several other species also fell into the partially myco-heterotrophic category.

Stratégie saprophyte ou symbiotique durant le développement d’ascocarpes de truffes dans une truffière à chêne vert. Une réponse basée sur l’abondance naturelle du 13C et du 15N

“…Moreover, this fractioning process does not occur in all symbiotic associations. In mycoheterotrophic orchids, the host plant gain most of its nitrogen from their associated ectomycorrhizal fungi and display the same isotope signature as their fungal partner (Bidartondo et al, 2004;Gebauer and Meyer, 2003). In our nursery experiment with trees and ectomycorrhizal fungi, it is clear that the fractioning process of nitrogen started through T. melanosporum mycorrhizae.…”

Further advances in orchid mycorrhizal research

“…Identical fungal ITS sequences in orchid roots and ECM of surrounding trees indicate epiparasitic interactions, although fulfilment of Koch's postulates, remain (Taylor and Bruns 1997;Selosse et al 2002a;Selosse et al 2004;Bidartondo et al 2004;Girlanda et al 2006;Abadie et al 2006). In the second form of experiment, stable isotope ratios of carbon and nitrogen within orchids match those of local ECM fungi (Gebauer and Meyer 2003;Trudell et al 2003;Bidartondo et al 2004;Whitridge and Southworth 2005;Julou et al 2005;Abadie et al 2006) indicating common pools of nutrients. The common mycelium linking orchids and trees ) has major conservation implications (Girlanda et al 2006).…”

“…Recent evidence shows that a third orchid nutritional mode exists -mixotrophy (Julou et al 2005). Such orchids are photosynthetic at the adult stage but augment their carbon requirements via mycorrhizal fungi (Gebauer and Meyer 2003;Bidartondo et al 2004;Selosse et al 2004;Julou et al 2005). Mixotrophic orchids may be an evolutionary step between photosynthetic and MH orchids (Julou et al 2005).…”

Gene expression in mycorrhizal orchid protocorms suggests a friendly plant–fungus relationship

“…The work by Cameron et al (2006Cameron et al ( , 2008 suggests, in contrast to earlier studies (Hadley and Purves 1974;Alexander and Hadley 1985), that adult photosynthetic orchids can transfer carbon to their symbiotic fungus. However, some species remain fully achlorophyllous as adults, or with an inefficient photosynthesis due to adaptation to shaded environments (Gebauer and Meyer 2003;Selosse and Roy 2009). Thus, the nature of the symbiotic orchid-fungus relationship in the adult stage can be rather complex, depending on the trophic strategy of the host plant species and on the environmental conditions.…”