Effect of the proanthocyanidin fraction from Medemia argun on the in vitro growth and activity of selected soil microorganisms

Many substances of plant origin, e.g. alkyloresorcinols, benzoxazinoids, essential oils, isothiocyanates or saponins, have been tested as potential antimicrobial agents to control various pathogenic microorganisms. Medemia agrun is a mysterious, not well-known species of the fan palm from the Nubian Desert Oases of Southern Egypt and Northern Sudan. Nuts of M. argun have been found to be rich (about 5%) in proanthocyanidins, consisting of afzelechin, catechin and gallocatechin as the main subunits of these polyphenolic compounds. The aim of this work was to assess effects of the proanthocyanidin (PAC) fraction obtained from M. agrun nuts on the in vitro growth and activity of two soil-borne fungal pathogens of cereals (Gaeumannomyces graminis var. tritici and Cephalosporium gramineum) and a beneficial soil bacterium, Azotobacter chroococcum, known to fix atmospheric nitrogen. The fungi were grown on PDA medium and A. chroococcum was cultured on Burk’s N-free medium supplemented with different concentrations of the PAC fraction. This fraction applied at the highest concentration tested (200 μg cm-3) did not affect the mycelial growth of the fungus Cephalosporium gramineum on PDA medium but significantly reduced (by 20%) that of Gaeumannomyces graminis var. tritici. Proliferation and N2-fixation by Azotobacter chroococcum in N-free liquid medium containing 500 μg cm-3 of the PAC fraction were more intensive than those in the medium with an equivalent concentration of glucose. It has been shown for the first time that this bacterium can use the PAC fraction as the only source of C and energy for N2-fixation.