Estimating the Viability of Vesicular-Arbuscular Mycorrhizae Fungal Spores Using Tetrazolium Salts as Vital Stains

Abstract

The tetrazolium salts 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide and 2-(p-iodophenyl)-3-(p-nitrophenyl)-5-phenyl-2H-tetrazolium chloride were evaluated as vital stains for estimating the viability of fungal spores of Glomus mosseae and Glomus clarum. Respiring fungal spores accumulated tetrazolium bromide-formazan and tetrazolium chloride-formazan in the cytoplasm as a visually distinct color reaction. Estimates, however, of fungal spore viability based on tetrazolium bromide staining were inconsistent with viability estimates based on a bioassay using single spores of G. clarum. Inconsistencies were due largely to the ambiguous interpretation of the significance of black spores. In contrast, color reactions in tetrazolium chloride were relatively stable and consistent with bioassay estimates of viability. Generally, incubation in 1 mg ml-1 tetrazolium chloride for 48 to 72 h was sufficient to ensure maximum color reaction. Viable spores were distinguishable from those which were unable to colonize a host plant on the basis of the intensity of color reaction in tetrazolium chloride. We conclude that tetrazolium chloride staining can be used as a rapid indicator of vesicular-arbuscular mycorrhizae fungal spore viability.