Systemic suppression of mycorrhizal colonization of barley roots already colonized by AM fungi

In a split root system with barley plants, one half of the divided root system had been colonized by the arbuscular mycorrhizal fungi Glomus mosseae, G. intraradices or Gigaspora rosea. After extensive root colonization by the fungi, the other half of the divided root system was inoculated with G. mosseae. Prior colonization of one half of the split root system with any of the three fungi resulted in a clear suppression of colonization by G. mosseae in the other half of the root system. As carbohydrates are one of the principal elements of plant growth, AM colonization, which is supposed to present a carbon sink, on one half of the split root system, could affect root growth and thus root fresh weight, on the non-mycorrhizal half of the root system. Besides, the improved P-status of mycorrhizal plants has been suggested to affect further AM root colonization. Our experiment showed that neither the root fresh weight, nor the P-concentration differed significantly in split root systems of non-mycorrhizal control plants and in split root systems of half mycorrhizal plants. Thus the suppressional effect in prior AM colonized plants on subsequent AM root colonization seemed not to be linked to a competition of the AM fungi for carbohydrates or an altered P-status. A systemic suppression of further mycorrhization in mycorrhizal roots, which is not AM fungal species or genera specific is discussed.