Mycorrhizal symbiosis has contrasting effects on fitness components in Campanula rotundifolia

The benefits or costs of mycorrhizal symbiosis for the host plants are most frequently estimated in terms of positive or negative effects on plant growth and biomass. The effect of mycorrhizal symbiosis on evolutionarily important fitness traits, such as reproduction, and effects on the next generation are poorly known. In a set of glasshouse experiments involving two plant generations we investigated the impact of mycorrhiza on fitness traits in perennial hand-pollinated Campanula rotundifolia. Mycorrhiza reduced plant growth and flower production, but increased root-shoot ratio, and shoot phosphorus (P) concentration. Mycorrhiza had no effect on seed number per capsule, mean seed weight or germination rate, but increased seed P concentration. Seedlings from mycorrhizal plants had a higher relative growth rate (RGR) than seedlings from nonmycorrhizal plants. Self-pollinations yielded fewer seeds per capsule that had inferior germination than seeds from cross-pollinations. Mycorrhizal infection was associated with a cost for the reproductive output of the C. rotundifolia host plant, whose progeny, however, were better than those from the nonmycorrhizal plants. Mycorrhiza also showed the potential to affect the plant mating system by increasing self-incompatibility and inbreeding depression expressed during seedling growth.