Estimation of mycelial biomass of arbuscular mycorrhizal fungi associated with the annual legume Kummerowia striata by ergosterol analysis

The biomass of internal and external mycelia of an arbuscular mycorrhizal (AM) fungus, Gigaspora margarita Becker & Hall, symbiotic with the annual legume, Kummerowia striata (Thunb.) Schindler, was estimated in a sterile culture experiment. When ergosterol, which is a component of fungal cell membranes, was measured in the mycorrhizal roots and soil at 20, 40, 60 and 80 days after inoculation with the AM fungus, the content of ergosterol in the roots increased from 0.036 g per plant (at 20 days) to 1.85 g per plant (at 80 days). Ergosterol content in the soil also increased with time, but the ratio of external to internal mycelial biomass decreased from 24.7 at 40 days to 5.6 at 80 days after sowing. The average ergosterol concentration in the external mycelia of G. margarita was 0.63 mg g^–1. It was estimated that at 80 days after inoculation, the biomass of internal and external mycelia of the AM fungus accounted for approximately 16 and 92% of root biomass, respectively. For comparison, ergosterol content in the roots of K. striata growing in the field was also measured. The results suggest that AM fungi can be a large sink of the carbon that is assimilated by the host plants.