MEDIATION OF COMPETITION BETWEEN 2 COLONIZING VA MYCORRHIZAL FUNGI BY THE HOST PLANT

In two experiments, we investigated the effect of Scutellospora calospora (Nicol. & Gerd.) Walker & Sanders on mycorrhiza formation between subterranean clover and Glomus sp. (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 determine whether the decline 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 th experiment, S. calospora present on one side of the split-root 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.