Mediation of competition between two colonizing VA mycorrhizal fungi by the host plant



In two experiments, we investigated the effect of Scutellospora calosopora (Nicol. & Gerd.) Walker & Sanders on mycorrhiza formation between subterranean clover and Glossip. (WUM 10). The rate and extent of colonization by Glomus sp. in the first expt was negatively correlated with increasing quantities of inoculum of S. calospora. A split-root system was used to deter mine whether the declive in mycorrhiza formation was likely to be due to(1) a direct interaction (and/or an effect of‘prior occupancy'related to a higher number of infective hyphae present at higher inoculum levels) or (2) an indirect effect mediated by an effect of S. calospora on the plain itself. In this experiment, S. Cfralospora present on one side of the spllit-roor markedly reduced colonization (total, arbuscular and vesicular) by Glomus sp. on the other side. However, this effect was compounded by a difference between the length of roots formed in association with each fungus. These results indicate that, for these two fungi in association with subterranean clover, the mechanism of competition during mycorrhiza formation has a physiological basis and may involve the carbohydrate supply of the host.