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Hydrogen stable isotope abundance patterns provide further insight into organic matter exchange in orchid mycorrhiza

Gerhard Gebauer1, Katja Preiss1, Iris Schmiedinger1
1 BayCEER - Labor für Isotopen-Biogeochemie, Universität Bayreuth

O 2.6 in Biodiversität: Verbreitung, Funktion und Erhaltung

11.10.2012, 11:15-11:30, H8

Water is the ultimate hydrogen source of autotrophic plants. In photosynthesis terrestrial plants use water taken up from the soil together with atmospheric CO2 to form carbohydrates. Carbohydrates are the precursors of all other organic compounds later on synthesized in plant metabolism. The isotopic composition of water and isotope fractionations in plant metabolism determine the hydrogen isotope signature in the tissue of autotrophic plants. In addition to the majority of autotrophic plants also heterotrophic plants have repeatedly evolved during evolution. Heterotrophic higher plants have lost the ability to photosynthesize and therefore cover their carbohydrate demand either through parasitism on other plants (holoparasites) or through parasitism on fungi (mycoheterotrophs). This tapping on alternative organic carbon sources is expected to affect the isotopic patterns of heterotrophic plants and provides insight into mechanisms of their organic matter gain. Unique hydrogen isotope signatures of holoparasites (Ziegler, 1994) and unique carbon and nitrogen isotope signatures of mycoheterotrophic plants (Gebauer & Meyer, 2003) have already been reported. Mycoheterotrophy is most widespread among representatives of the orchid family. For some green orchids from dark forest habitats also a unique mixed carbon nutrition through photosynthesis and simultaneous tapping on the fungal carbon source (partial mycoheterotrophy) has been reported (Gebauer & Meyer, 2003).

Here we report for the first time on hydrogen and carbon isotope signatures of fully mycoheterotrophic orchids, green orchids including species that have already been identified as partial mycoheterotrophs, fungi forming ectomycorrhizas, saprotrophic fungi and autotrophic reference plants from a temperate broadleaf forest in NE Bavaria, Germany. Hydrogen isotope abundances were analyzed from dried tissues by TC-IRMS coupling. Identical tissue samples were used for carbon isotope abundance analysis by EA-IRMS.

Reference plants were depleted in 13C and 2H in comparison to precipitation water and atmospheric CO2 in a manner as to be expected for autotrophic plants following the C3 pathway of photosynthesis. Compared to the reference plants fully mycoheterotrophic plant were enriched in 13C by 8 ‰ and in 2H by as much as 69 ‰ and matched the C and H isotope signatures of the investigated fungi. C and H isotope signatures of green orchids were positioned between autotrophic reference plants and full mycoheterotrophs and therefore indicate a mixed carbohydrate gain from two sources: photosynthesis and tapping on fungi.

The 13C enrichment found for fully and partially mycoheterotrophic orchids and for fungi is explained by a carbohydrate transfer from autotrophic plants through mycorrhizal fungi mycelia to fully or partially mycoheterotrophic plants. Carbohydrates are known to be enriched in 13C in comparison to bulk autotrophic plant tissue. The simultaneous 2H enrichment found for mycoheterotrophic orchids and fungi is more complex, because hydrogen incorporated into photosynthetic products during primary reduction steps is highly depleted in 2H (Yakir, 1992). We therefore hypothesize that carbohydrates transferred from autotrophic plants through fungal mycelia to mycoheterotrophic plants has been affected by a series of heterotrophic metabolism steps accompanied by a replacement of primary hydrogen atoms with water, and thus leading to the unique 2H enrichment as observed here.

 

Gebauer, G. & Meyer, M. 2003. 15N and 13C natural abundance of autotrophic and myco-heterotrophic orchids provide insight into nitrogen and carbon gain from fungal association. New Phytologist 160:209-233.

Yakir, D. 1992. Variations in the natural abundance of oxygen-18 and deuterium in plant carbohydrates. Plant, Cell and Environment 15:1005-1020.

Ziegler, H. 1994. Deuterium content in organic material of hosts and their parasites. In: Schulze, E.-D. & Caldwell, M.M. (eds.) Ecophysiology of photosynthesis. Ecological Studies Vol. 100. Springer, Berlin, p. 393-408.



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Letzte Änderung 21.09.2012