Species of arbuscular mycorrhizal fungi affect mycorrhizal responses to simulated herbivory

Herbivory is thought to influence the mycorrhizal associations between plants and their fungal symbionts by limiting the amount of photosynthate available to the fungus. So far, studies of the herbivory-mycorrhiza relationship have not yielded consistent results. This may be related to differences between species of fungi, or the frequency of the herbivory. This study tested the effects of arbuscular mycorrhizal fungal (AMF) species with varying clipping frequency on the phenology and colonization by AMF in the roots of Bromus inermis. The species of AMF significantly affected root and shoot biomass, as well as the degree of mycorrhizal colonization. Clipping frequency also influenced plant biomass and mycorrhizal phenology. High frequency clipping typically had the strongest effect on the variables measured. However, the extent of the clipping effect was dependent on which AMF species were associated with the plant, and which mycorrhizal structure was assessed. For example, intraradical hyphae and arbuscules were negatively affected by clipping, whereas the production of vesicles and spores was stimulated, albeit to different degrees in different AMF species. In contrast, extraradical hyphal length did not respond significantly to clipping. This AMF x clipping interaction indicates that it is difficult to generalize on the effects of herbivory on plant and fungal responses, even when dealing with the same plant species. (C) 2004 Elsevier B.V. All rights reserved.