The arbuscular mycorrhizal association – usually conceived as a mutualistic relationship between fungi and plants – has the potential to turn into a parasitic interaction for the plant under certain environmental circumstances. In this study, we tried to unravel conditions that may induce parasitism by arbuscular mycorrhizal fungi (AMF) for two species with different mycotrophic levels, assuming that the more AMF-dependent forb Hieracium pilosella would show greater decreases in mycorrhizal growth response under the parasitism-promoting conditions than the less mycotrophic grass species Corynephorus canescens. A multifactorial experiment was conducted with plants of the two species in single and competition treatments and under the parasitism-promoting conditions of low irradiance, additional P fertilization and the combined treatment of both factors. Comparing non-mycorrhizal (NM) and mycorrhizal (AM) plants allowed for assessment of growth responses and control plants were used to estimate the impact of the parasitism-promoting conditions. Plant and fungal growth parameters as well as C, N and P tissue and soil P concentrations were measured. No negative differences in AMF-induced growth response were exhibited by C. canescens, whereas H. pilosella showed decreased growth responses but no growth depressions demonstrating the extreme dependence of the forb. There was no additive effect of the two treatments combined. The competitive interaction between both plant species was clearly dominated by C. canescens, whereas growth of H. pilosella was suppressed. This relationship did not differ between NM and AM pairs under parasitism-promoting conditions, thus indicating that the AMF did not affect the plant-plant interaction under these circumstances. Our study clearly emphasizes the importance of the mycotrophy level of a plant species for its AMF-induced response to parasitism-promoting conditions and highlights the differences in potential mechanisms and abilities of the two species to control AMF root colonization in relation to their specific dependence on the AMF association. Future studies might address the use of multiple harvests as well as comparisons between intra- and interspecific competition and the set-up of semi-field or field experiments to study parasitism-promoting conditions.

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Verena Lauströer fertigte die Arbeit am Lehrstuhl für Experimentelle Ökologie und Ökosystembiologie in der Arbeitsgruppe von Dr. Stephan Unger an.