Diploma Thesis

Mode of Nutrition of Mediterranean Orchids

Heiko T. Liebel (06/2007-11/2007)

Support: Gerhard Gebauer, Carl Beierkuhnlein

In the first stages of their development, all orchids have a heterotrophic phase and rely on symbiotic fungi for C and N supply (mycoheterotrophy). Some orchids remain achlorophyllous in the adult phase (e.g. Neottia nidus-avis) and they are highly specialized exploiters of ectomycorrhizal (ECM) fungi. Mycoheterotrophic plants show a significant enrichment in the heavy isotopes 15N and 13C in comparison to non-orchids from the same habitats. This typical isotope signature results from the organic nutrient gain from the mycorrhizal association.

The isotope signatures of some green orchid species from shaded forest sites in Central Europe have been shown to be intermediate between autotrophic and mycoheterotrophic plants. Thus, these plants are apparently partially mycoheterotrophic and their isotope signature can be used to estimate the N and C gain from the fungus. These orchids are also associated with fungi forming ectomycorrhizae with trees.

The Mediterranean region is much richer in orchids than Central Europe. However, until now little is known about the nutritional constraints of these orchids. To compare the current knowledge on the mode of nutrition of Central uropean orchids with the same species appearing in the Mediterranean region and to increase the knowledge on not yet investigated orchid genera and orchid species, a wide screening was carried out in continental Italy and Sardinia.

Orchid leaf and root samples were collected in spring 2007 of Aceras, Barlia, Cephalanthera, Epipactis, Gennaria, Limodorum, Listera, Neottia, Ophrys, Orchis, Serapias and Spiranthes species. Molecular analyses on the root samples revealed fungal partners of the orchids and with the help of stable isotope analyses (EA-IRMS) it was possible to estimate the C and N gain from the fungi.
Partial mycoheterotrophy is present also in the Mediterranean region at least in Limodorum abortivum and L. trabutianum, Cephalanthera longifolia and C. damasonium, Epipactis helleborine and Listera ovata. All of these orchid species thrive in dark forests and live associated with ectomycorrhizal fungi.
At open habitats the isotopic signatures of Barlia robertiana and Orchis purpurea suggest a weak partially
mycoheterotrophic living. All other orchids of open habitat show a slight enrichment in 15N only. The molecular determination of mycorrhizal partners suggests a nutrients gain from saprotrophic fungi to the orchids which is less effective than a supply via ECM fungi in non-tropical climates.
Investigated orchids with a Central European and Mediterranean distribution (e.g. Epipactis helleborine, Orchis and Ophrys species) behave in a similar way in their mode of nutrition. They live together with the same functional fungal groups and show similar isotope signatures as the representatives in Central Europe.