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- Investigating the ecology of partial and full myco-heterotrophy among Orchidaceae and Ericaceae using the stable isotope natural abundance analysis approach (2009)
- The group of nonphotosynthetic (chlorophyll-lacking) plants consists of about 4.100 species that directly parasitize on autotrophic plants and more than 400 so-called myco-heterotrophic species that rely upon organic nutrient supplies from associated fungi. Comprising almost a quarter of all known myco-heterotrophic species, the Orchidaceae are the most successful family among myco-heterotrophic plants. All orchids are myco-heterotrophic during their juvenile belowground phases but most species develop green leaves as adults. A similar situation occurs in the tribe Pyroleae (Ericaceae), consisting of mostly photosynthetic species that, as like as orchids, produce ‘dust seeds’ without endosperm and thus rely upon fungal support during early development. The use of stable isotope natural abundance analyses recently revealed that some adult green and hence putatively autotrophic Orchidaceae and Pyroleae gain organic carbon and nitrogen from their fungal partners in addition to the carbon obtained through photosynthesis and the mineral nitrogen assimilated from the soil. Plants exhibiting this mixed nutritional mode have been referred to as being partially myco-heterotrophic. Understanding the complex and fascinating ecology of partially and fully myco-heterotrophic plants is the basis for successful conservation to protect these mostly endangered species and to maintain their unique habitats. Analyses of stable isotope natural abundances and molecular identification of mycorrhizal fungi are modern techniques that can give insight into the plants’ nutritional modes under natural conditions without experimental disturbance. Based on these two methods, additional analyses of chlorophyll contents, light climate measurements and an in situ 13C labeling experiment, this thesis provides some new fundamental knowledge on the intriguing way of nutrient acquisition exhibited by several Orchidaceae and Ericaceae. A methodological approach that allows meta-analyses and improves the quantitative estimate of nutrient gains by partially myco-heterotrophic plants is presented in the first chapter. Using this enhanced method, the hitherto disputed or in most cases completely unknown nutritional status of a range of species was analyzed. It is evidenced that the ericaceous tribe Pyroleae comprises a fully myco-heterotrophic species (Pyrola aphylla) and stated that the leafless, ectomycorrhizal specialist orchid Corallorhiza trifida (hitherto considered as completely relying upon mycorrhizal fungi) is capable to photosynthesize. Based on the broad dataset on isotope signatures and mycorrhizal associates of numerous orchid species, the existence of a nutritionally new group of orchids could be proved, concluding that at least four nutritional modes can be found among terrestrial members of the Orchidaceae: autotrophy, where green orchids have carbon isotope signatures indistinguishable from those of surrounding autotrophs and mainly associate with Rhizoctonia species; partial myco-heterotrophy, where green orchids have carbon isotope signatures intermediate between those of autotrophs and myco-heterotrophs and associate with ectomycorrhizal fungi; full myco-heterotrophy, where orchids have lost the ability to photosynthesize, are specialized on either ectomycorrhizal or saprotrophic fungi and are enriched in 13C similar to their host fungi; and an additional strategy found in green orchids which mainly associate with ceratobasidioid and tulasnelloid fungi and are depleted in 13C compared to surrounding autotrophs - possibly due to a net plant-to-fungus transfer of 13C enriched carbon compounds. Studies on nutritional modes of orchids from Macaronesia and the Mediterranean region suggested that the availability of suited ectomycorrhizal fungi constrains the occurrence of partially and fully myco-heterotrophic species. Furthermore, we found a general pattern showing that high degrees of myco-heterotrophy in orchids are related to certain taxonomic groups and to the light-limited understory of forest sites while net plant-to-fungus carbon fluxes seem to be coupled to open light-saturated habitats. In a subsequent study on green Cephalanthera spp. from temperate forests, the effect of the prevalent micro-scale light climate on the degree of myco-heterotrophy was investigated more explicitly. It could be demonstrated that higher irradiances successively drive the orchids towards full autotrophy and that partial myco-heterotrophy thus is not a static nutritional mode but a surprisingly flexible mechanism allowing a well balanced utilization of carbon resources available in nature. Although many questions in this broad and novel scientific field remain to be answered, results of this thesis substantially contribute to our knowledge on myco-heterotrophy and the mechanisms behind. The presented findings allow drawing conclusions on habitat requirements and raise new aspects for species conservation.