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 le gume, Kummerowia striata (Thunb.) Schindler, was estimated in a sterile cul ture experiment. When ergosterol, which is a component of fungal cell membr anes, was measured in the mycorrhizal roots and soil at 20, 40, 60 and 80 d ays after inoculation with the AM fungus, the content of ergosterol in the roots increased from 0.036 mu g per plant (at 20 days) to 1.85 mu g per pla nt (at 80 days). Ergosterol content in the soil also increased with time, b ut the ratio of external to internal mycelial biomass decreased from 24.7 a t 40 days to 5.6 at 80 days after sowing. The average ergosterol concentrat ion in the external mycelia of G. margarita was 0.63 mg g(-1). It was estim ated that at 80 days after inoculation, the biomass of internal and externa l mycelia of the AM fungus accounted for approximately 16 and 92% of root b iomass, respectively. For comparison, ergosterol content in the roots of K. striata growing in the field was also measured. The results suggest that A M fungi can be a large sink of the carbon that is assimilated by the host p lants.