Adjustment of pot culture nutrient solutions increased root colonization and sporulation of vesiculararbuscular mycorrhizal (VAM) fungi. Paspalum notatum Flugge and VAM fungi were grown in a sandy soil low in N and available P. Hoagland nutrient solution without P enhanced sporulation in soil and root colonization of Acaulospora longula, Scutellospora heterogama, Gigaspora margarita, and a wide range of other VAM fungi over levels produced by a tap water control or nutrient solutions containing

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... P. However, Glomus intraradices produced significantly more spores in plant roots in the tap water control treatment. The effect of the nutrient solutions was not due solely to N nutrition, because the addition of NH4NO3 decreased both colonization and sporulation by G. margarita relative to levels produced by Hoagland solution without P. Large numbers of spores are frequently necessary to conduct research with vesicular-arbuscular mycorrhizal (VAM) fungi. Large populations of homogeneous spores are required for spore germination and storage research. More important, application of glasshouse-conducted research to the field depends upon the production of large amounts of inoculum of VAM fungi, including spores. These factors demonstrate the need for a broadly applicable method to enhance sporulation of VAM fungi in pot culture. Experiments were conducted to test the hypothesis that sporulation of VAM fungi would be increased if nutrient regimens were manipulated to provide conditions conducive to the bilateral transfer of nutrients (P from fungus to host and C from host to fungus). The ratio and amount of N and P in nutrient solutions were varied. Low levels of available P maintain high levels of colonization and enhance the efficacy of fungal transfer of P to the host. High levels of other nutrients increase the amount of photosynthate available to meet the metabolic demand for sporulation. Application of nutrient solution without P consistently yielded the greatest colonization of roots and production of soilborne spores by VAM fungi.