Mixotrophy in orchids: insights from a comparative study of green individuals and nonphotosynthetic individuals of Cephalanthera damasonium

Abstract

Summary• Some green orchids obtain carbon (C) from their mycorrhizal fungi and photosynthesis. This mixotrophy may represent an evolutionary step towards mycoheterotrophic plants fully feeding on fungal C. Here, we report on nonphotosynthetic individuals (albinos) of the green Cephalanthera damasonium that likely represent another evolutionary step.• Albino and green individuals from a French population were compared for morphology and fertility, photosynthetic abilities, fungal partners (using microscopy and molecular tools), and nutrient sources (as characterized by 15 N and 13 C abundances).• Albinos did not differ significantly from green individuals in morphology and fertility, but tended to be smaller. They harboured similar fungi, with Thelephoraceae and Cortinariaceae as mycorrhizal partners and few rhizoctonias. Albinos were nonphotosynthetic, fully mycoheterotrophic. Green individuals carried out photosynthesis at compensation point and received almost 50% of their C from fungi. Orchid fungi also colonized surrounding tree roots, likely to be the ultimate C source.• Transition to mycoheterotrophy may require several simultaneous adaptations; albinos, by lacking some of them, may have reduced ecological success. This may limit the appearance of cheaters in mycorrhizal networks.

Enigmatic Sebacinales

“…In study sites of France, 78 % of investigated root pieces were colonized by Tuber spp., but other associated fungi also included Sebacinales. Another example of mixotrophy of Cephalanthera damasonium from France, involving Sebacinales among other taxa, has been described by Julou et al (2005). In a comparative study of mixotrophic green and albino Cephalanthera longifolia from Estonia, Abadie et al (2006) also found Sebacina in a chlorophyllous plant.…”

Interactions of fungi with other organisms

“…A third nutritional mode, in which fungi partly subsidize photosynthetic orchids with both organic and inorganic compounds, is termed "partial mycoheterotrophy" or "mixotrophy" (Selosse et al 2004) and accounts for a significant number of forest species (Julou et al 2005;Girlanda et al 2006;Tedersoo et al 2007). Mixotrophy allows plants to colonize shaded habitat like dense forests and to be more or less independent of irradiance .…”

Biological control of the terrestrial carbon sink

“…According to Read (Schulze, 2003) soil respiration decreases about 30% if roots and mycorrhizae are excluded. Julou et al (2005) showed that there are plants that make use of the carbohydrate flow from the root to the mycorrhizae and tap this flow as parasites. Orchids seem to be especially capable of doing this as a basis for survival in the shade.…”