Effect of glucose on the development of Glomus fistulosum colonization and extraradical mycelium on maize roots

Various doses of glucose were added weekly to pots with maize growing in a mixture of soil and Perlite inoculated with the arbuscular mycorrhizal (AM) fungus Glomus fistulosum to define the effects of an additional carbon source on plant growth, mycorrhizae and other microbial features of the cultivation substratum. Higher doses of glucose (100 and 300 mg per pot) decreased plant growth and abundance of root hairs after 6 weeks of cultivation. Lower doses of glucose (10 and 30 mg per pot) had a positive effect on some characteristics of the development of the arbuscular fungus, e.g. root colonization, abundance of arbuscules, the length of extraradical mycelium associated with root surface, length of mycelium in the substratum as well as the length of both mycelia showing dehydrogenase activity as compared with variants not supplied with glucose solution. Glucose did not affect the number of spores of AM fungus. No effect of glucose was found on substratum respiration but glucose amendment increased microbial biomass and particularly the occurrence of saprophytic fungi. In a subsequent experiment focused on nonsymbiotic phase of fungal life cycle, the mycorrhizal root segments were incubated for 6 weeks in Petri dishes on membranes covered with a soil layer and supplied weekly with four glucose concentrations from 0.3 to 10 mg. Highest total length of hyphae associated with the root surface and the length of hyphae showing dehydrogenase activity was found when the lowest dose of 0.3 mg glucose was added to the soil weekly, whereas a 10 mg dose increased the length and activity of hyphae associated with a membrane. The possible mechanisms of the effects of additional labile carbon pool on the development of mycorrhizal fungus are discussed.