Succinate dehydrogenase activity of external and internal hyphae of a vesicular-arbuscular mycorrhizal fungus, Glomus mosseae (Nicol. & Gerd.) Gerdmann and Trappe, during mycorrhizal colonization of roots of leek (Allium porrum L.), as revealed by in situ histochemical staining

The succinate dehydrogenase (SDH) activity of hyphae of the vesicular-arbuscular (VA) mycorrhizal fungus Glomus mosseae (Nicol. & Gerd.) Gerdmann and Trappe, in symbiotic association with leek (Allium porrum L.) roots, was investigated by histochemical staining in situ. Leek seedlings were transplanted to sand culture and inoculated with spores of G. mosseae placed just below the base of the stem. At intervals (14, 25, 35 and 60 days) after transplanting, the growth medium of seedlings was flooded with nitro blue tetrazolium chloride solution, thereby displacing the nutrient solution. This allowed sites of SDH activity of external and internal fungal structures of the mycorrhizas to be stained without physically disturbing the symbiotic system. After counterstaining harvested roots and mycelium with acid fuchsin, it was possible to differentiate clearly metabolically active and inactive regions of the fungus. The lengths of external hyphae and infected root both increased nearly exponentially, and were in constant proportion (1.4 m hyphae per cm of infected root) for up to 60 days. The percentage length of external hyphae with SDH activity remained almost constant (80%). In each infected length of root there was a gradation of SDH activity from inactive distal (older) hyphae to uniformly active proximal (younger) hyphae. These findings are discussed in relation to the symbiotic activity of the mycobiont.