Spore Germination and Germ Tube Growth of a Vesicular-Arbuscular Mycorrhizal Fungus in Vitro

The effects of mineral nutrients, thiamine, pH, and organic substrates on germination and germ tube growth of a vesicular-arbuscular mycorrhizal fungus were examined. Surfacesterilized azygospores of Gigaspora margarita were plated on 1% water agar containing different components and incubated at 28 C up to 30 da. Maximum germination and germ tube growth occurred when 20 mg 1-1 CaH2PO4 was used, whereas 24 mg 1-~ NH4NO3 did not affect germination and depressed germ tube growth by 55%. Potassium or concentrations of soluble nutrient salts as high as those used to grow seedlings axenically had no effect on spore germination or germ tube growth. Thiamine at 0.1 mg 1-1 increased germination, and maximum germ tube growth occurred at 0.01 mg 1-1. The interaction between medium pH and medium composition was significant. Spore germination was not enhanced by any of the organic substrates. Low concentrations (4 g 1-1) of sucrose favored germ tube growth. Relation of these results to nutrition and root-colonizing ability of VA mycorrhizal spores is discussed.