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Lipids are the major form of carbon storage in arbuscular-mycorrhizal fungi. We studied fatty acid synthesis by Glomus intraradices and Gigaspora rosea. Our data strongly suggest that the fatty acid synthase activity of arbuscular-mycorrhizal fungi is expressed exclusively in the intraradical mycelium and indicate that fatty acid metabolism may play a major role in the obligate biotrophism of arbuscular-mycorrhizal fungi.

The variations in the numbers of propagules and in the colonization rates of vesicular–arbuscular mycorrhizal fungi in soils of two sugar maple forests, one near Waterloo in southern Ontario, the other near Lacolle in southern Quebec, were investigated. Both forests comprised similar plant communities, and in each case the most numerous mycorrhizal spores were identified as those of Glomus macrocarpum, Glomus geosporum, and Acaulospora foveata. In both forests, root colonization and length of hyphae in soil peaked in spring and again in autumn, and spore densities were highest in autumn and gradually decreased during the rest of the year. Mean percent root colonization at Lacolle was twice as high as at Waterloo, and mean spore densities were almost 10 times higher. In contrast, average total length of hyphae in the soil was approximately 10 times higher at Waterloo than at Lacolle. Sporulation by G. macrocarpum was positively correlated with soil organic matter content, and sporulation by A. foveata was negatively correlated with increasing pH. Root colonization was not correlated with spore densities but was positively correlated with the total length of hyphae in the soil. Relationships among root colonization, production of external hyphae, and spore abundance in natural ecosystems are discussed. Key words: vesicular–arbuscular mycorrhizae, Acer saccharum, environmental influences.