Studies on the physiology of conidial germination by Aspergillus flavus
In a phosphate-buffered (pH 5.7) inorganic salts solution (B), nearly full dependence en exogenous carbon was found for high (90 to 100%) germination of washed, Tween 20-treated conidia of Aspergillus flavus over a range of conidial densities. No exogenous carbon-independent germination was observed for settling tower-deposited dry conidia. At equivalent carbon, an amino acid mixture supported higher germination of washed conidia than a sugar-organic acid-alcohol mixture plus NH₄Cl; proline or alanine alone supported higher germination than several other single amino acids or single sugars plus NH₄Cl tested. Glucose plus NH₄Cl was the most stimulatory of the latter. "Amino acid carry-over" for conidial germination of A. flavus was not confirmed in these studies. When exogenous carbon was supplied as glucose, germination of washed conidia was more dependent, generally, on exogenous nitrogen, supplied as NH₄Cl, as the conidial density increased from 10³ to 10⁵ conidia/ml. At equivalent nitrogen, proline alone, or certain amino acid mixtures supported higher germination than NH₄Cl, KNO₃, or several other single amino acids tested in B solution plus glucose. Percentage germination decreased as the conidial density decreased from 10⁵ to 10³ conidia/nl when glucose plus NH₄Cl were supplied at each density at constant amounts/conidium. A light decrease in percentage germination was observed in glucose plus an amino acid mixture, and a moderate decrease was observed in glucose plus peptone. Percentage germination decreased as conidial density increased from 1 x 10³ to 1 x 10⁵ conidia/ml for each of three concentrations of glucose alone or glucose plus NH₄Cl examined. Removal of CO₂ from the germination medium almost completely inhibited germination and swelling in glucose plus NH₄Cl and suppressed germination in glucose plus an amino acid mixture. Two surfactants increased percentage germination of settling tower-deposited conidia in the presence of glucose plus NH₄Cl and supported low germination in B solution alone. Maximum germination occurred at 35°C in glucose plus NH₄Cl medium, while in glucose plus peptono medium maximum germination occurred at 30°C and 35°C. At 30°C the rate of germination was higher in glucose plus peptone medium than in glucose plus NH₄Cl medium. A broad pH optimum range (pH 3.0 to 7.5) was found for conidial germination using both citrate-phosphate and phosphate buffers containing glucose plus peptone. In glucose plus NH₄Cl a somewhat narrower pH optimum range, was observed. The possible relation of these findings to conidial germination in soil and in the geocarposphere of peanut fruit is discussed.