The effect of host mycorrhizal status on host plant-parasitic plant interactions

Two pot experiments were conducted to examine three-level interactions betw een host plants, mycorrhizal fungi and parasitic plants. In a greenhouse ex periment, Poa annua plants were grown in the presence or absence of an AM f ungus (either Glomus lamellosum V43a or G. mosseae BEG29) and in the presen ce or absence of a root hemiparasitic plant (Odontites vulgaris). In a labo ratory experiment, mycorrhizal infection (Glomus claroideum BEG31) of Trifo lium pratense host plants (mycorrhizal versus non-mycorrhizal) was combined with hemiparasite infection (Rhinanthus serotinus) of the host (parasitize d versus non-parasitized). Infection with the two species of Glomus had no significant effect on the growth of P. annua, while hemiparasite infection caused a significant reduction in host biomass. Mycorrhizal status of P. an nua hosts (i.e. presence/absence of AM fungus) affected neither the biomass nor the number of flowers produced by the attached O. vulgaris plants. Inf ection with G, claroideum BEG31 greatly increased the biomass of T. pratens e, but hemiparasite infection had no effect. The hemiparasitic R. serotinus plants attached to mycorrhizal hosts had higher biomass and produced more flowers than plants growing with non-mycorrhizal hosts. Roots of T. pratens e were colonized by the ANI fungus to an extent independent of the presence or absence of the hemiparasite. Our results confirm earlier findings that the Mycorrhizal status of a host plant can affect the performance of an att ached root hemiparasite. However, improvement of the performance of the par asitic plant following, attachment to a mycorrhizal host depends on the ext ent to which the AM fungi is able to enhance the growth of the host.