The mycorrhizal status of Arbutus unedo in relation to compatible and incompatible fungi

Ectendomycorrhizae of Arbutus unedo L. were synthesized in the laboratory and were observed over the first 80 days of development. After 40 days some roots showed many hyphae running around enlarged tips, on the 60th day short roots had assumed a clavate form, and on the 80th day mycorrhizae could be observed with the naked eye. The laboratory-synthesized ectendomycorrhizae had a cruciform structure and were smooth, shiny, and pale orange-brown. They presented a loose sheath and a Hartig net. The first layer of cortical root cells was invaded by the fungus. Running hyphae and vesicles of Glomus microcarpum invaded A. unedo roots, but no root invasion was observed in the presence of five other vesicular-arbuscular mycorrhizal fungal species. The ericoid mycorrhizal fungus Hymenoscyphus ericae caused a darkening of cortical cells and root tips and the death of many seedlings. The correspondence between specific plant species and particular kinds of mycorrhizae and the possibility of intracellular colonization by nonspecific fungi are also discussed.