THE INFLUENCE OF CULTURE CONDITIONS ON MYCORRHIZA FORMATION BETWEEN THE ECTOMYCORRHIZAL FUNGUS PISOLITHUS SP AND AFZELIA-AFRICANA SM SEEDLINGS

Interactions between an isolate of the ectomycorrhizal fungus Pisolith us sp. and Afzelia africana Sm. seedlings were studied at the structur al and ultrastructural levels. Several different conditions were teste d with or without sugar and in a sterile or nonsterile medium. In the growth cabinet, the A. africana/Pisolithus sp. interactions did not pr oduce ectomycorrhizas. A fungal sheath was formed but no Hartig net, a nd an unusual host epidermal cell wall was observed. Hyphae of Pisolit hus sp. induced modifications of epidermal cells of 15-day-old A. afri cana seedlings indicative of non-mycorrhizal interactions, such as wal l thickening, wall ingrowth, papillae formation, degraded host wall ma terial and the presence of intracellular hyphae. Wall ingrowth consist ed of depositions of host cell wall materials giving a positive reacti on for polysaccharides; however, wall thickenings and papillae showed no homogeneous reactions for polysaccharides. In glasshouse conditions , inocula of Pisolithus sp. in the form of spores or mycelia entrapped in peat-vermiculite added to sterilized soil produced typical ectomyc orrhizae only with 6-month-old A. africana seedlings. Under these cond itions, no conspicuous cell wall reactions occurred on A. africana roo ts. The results demonstrate that the establishment of an association b etween an ectomycorrhizal fungus and a potential host plant is strongl y influenced by seedling age and/or environmental conditions. Therefor e, in vitro synthesis is not a conclusive demonstration of a symbiotic relationship.