No abstract

Interactions of plants with arbuscular mycorrhizal fungi (AMF) may range along a broad continuum from strong mutualism to parasitism, with mycorrhizal benefits received by the plant being determined by climatic and edaphic conditions affecting the balance between carbon costs vs. nutritional benefits. Thus, environmental conditions promoting either parasitism or mutualism can influence the mycorrhizal growth dependency (MGD) of a plant and in consequence may play an important role in plant-plant interactions. In a multifactorial field experiment we aimed at disentangling the effects of environmental and edaphic conditions, namely the availability of light, phosphorus and nitrogen, and the implications for competitive interactions between Hieracium pilosella and Corynephorus canescens for the outcome of the AMF symbiosis. Both species were planted in single, intraspecific and interspecific combinations using a target-neighbor approach with six treatments distributed along a gradient simulating conditions for the interaction between plants and AMF ranking from mutualistic to parasitic. Across all treatments we found mycorrhizal association of H. pilosella being consistently mutualistic, while pronounced parasitism was observed in C. canescens, indicating that environmental and edaphic conditions did not markedly affect the cost:benefit ratio of the mycorrhizal symbiosis in both species. Competitive interactions between both species were strongly affected by AMF, with the impact of AMF on competition being modulated by colonization. Biomass in both species was lowest when grown in interspecific competition, with colonization being increased in the less mycotrophic C. canescens, while decreased in the obligate mycotrophic H. pilosella. Although parasitism-promoting conditions negatively affected MGD in C. canescens, these effects were small as compared to growth decreases related to increased colonization levels in this species. Thus, the lack of plant control over mycorrhizal colonization was identified as a possible key factor for the outcome of competition, while environmental and edaphic conditions affecting the mutualism-parasitism continuum appeared to be of minor importance.

Adaptive traits ensuring efficient nutrient acquisition, such as extensive fine root systems, are crucial for establishment of pioneer plants on bare sand. Some successful pioneer species of temperate, European sand ecosystems are characterised as obligate mycorrhizals, thus likely substituting fine roots with arbuscular mycorrhizal fungi (AMF). However, it is not clear whether AM fungal-mediated acquisition of scarce and immobile nutrients such as phosphorus (P) is an advantageous strategy on bare sand over foraging via roots. We compared the foraging performance of three obligately mycorrhizal forbs and two facultatively mycorrhizal grasses, regarding the influence of AMF on their capacity to acquire P from bare sand. Comparison of mycorrhizal and non-mycorrhizal individuals revealed a markedly higher AM fungal-dependency for P acquisition and growth in the forbs than in the grasses. Periodical soil core sampling, allowing for assessment of root and hyphal growth rates, revealed hyphal growth to markedly enlarge the total absorptive surface area (SA) in the forbs, but not in the grasses. Correlations between SA growth and P depletion suggest an AM fungal-induced enhanced capacity for rapid soil P exploitation in the forbs. Our study showed that AM fungal-mediated foraging may be an advantageous strategy over root-mediated foraging in sand pioneer plants.

No abstract

No abstract