Efeitos de compostos orgânicos sobre o crescimento micelial de Gigaspora gigantea in vitro

Abstract

The beneficiai effects of vesicular-arbuscular mycorrhizal fungi (VAM) on plant mineral nutrition and development are well known. However, their application in large scale is still restricted because of the difficulties found in inoculum production, for the VAM fungi are obligate biotrophs and have not been cultured in laboratory yet. The difficulties imposed by the biotrophisn will only be overcome when their nutritional requeriments are determined and culture media able to support their mycelial growth In vitro developed. Therefore, the present study was undertaken to characterize the mycelia growth of the VAM fungus Gjgaspora giganta, establish its growth curve, and determine the effects of carbohydrates and organic acids on its mycelial growth in vitro. In the first part of the study, asseptic pre-germinated spores were incubated for various periods of time in a liquid nutrient médium, suplemented or not with sucrose in order to determine the mycelial growth curves in the presence and absence of carbon. Latter, the effects of different concentrations of the carbohydrates glucose, fructose, arabinose, sucrose, trealose, mannitol, starch and pectin and of piruvic, citric, oxalic and tartaric acids on the mycelia growth of the fungus were studied. The effects of different periods of pre-growth in nutrient médium on the subsequent response to sucrose were also studied. In the end of the incubation periods, the spores were ob6erved under dissecting microscope to determine the morphological feature6 of the mycelium formed. The mycelial growth was assessed by a hyphal intersection method modified from HEPPER & JAKOBSEN (1983). The mycelial growth curve of (L_ gigantea was similar to that of other filamentous fungi and was not affected by the carbon availability in the médium. For the carbohydrates, stimulatory effects were obtained in low concentrations of pectin and inhibition in low and high concentrations of trealose. Stimulatory effects for sucrose were obtained when this carbohydrate was added after 10 or 20 days of incubation of the spores in the nutrient médium. The organic acids had marked effects on growth: 70 and 54% stimulation was obtained for the tartaric and citric acids, and 42 and 69% inhibition for the piruvic and oxalic acids, respectively. The organic compounds did not cause any changes in hyphal morphology however, a tendency for a relation of proportionality between mycelial growth and number of auxiliary cells formed was observed.