The effects of nutrients and temperature on germination and viability of the ascospores of Chaetomium rectopilium [Fungi]

Ascospores of C. rectopilium germinated poorly (8% or less) on a synthetic agar medium containing all of the major nutrients and on media containing a single nutrient (minerals, nitrogen source, vitamins, glucose). Various concentrations of potassium acetate (optimum concentration was 2.5 g per liter) markedly stimulated germination at 46 C. increasing germination 8-fold and shortening the latent period from 48 to 12 hr. Moderate stimulation was observed at 42 C. At 35 C and below KAc had no stimulating effect. An additive stimulation by a short exposure to a supramaximal temperature to that of KAc stimulation was observed at 55, but not at 60 C. Malt extract agar allowed even better germination (89%) than KAc. A vesicle emerged through the single germ pore and quickly protruded one or two germ tubes. Occasionally, perithecial hairs which had escaped removal by the washing procedure, were observed to protrude a hyaline hypha from the fractured part. Germination was observed at from 16 to 48 C. The cardinal temperatures for ascospore germination (determined on KAc agar) were found to be: minimum, 12-16 C; optimum, 46 C; maximum, 48-50 C. A high percentage of ascospores germinated on malt extract agar at pH 4.9 to 7, but none germinated at pH 3.5. Very low germination was observed on Ionagar with no added nutrients at pH 3.5 to 7.3. Potassium acetate in agar repressed germination at pH levels below 6.1. The optimum pH on the three media was 5.7 to 6.4. The thermal death point was determined to be 60-62 C. The ascospores are not very thermoduric, surviving 8 hr exposure to 55 C, but not 12 hr; surviving 20 min exposure to 60 C, but not 30 min.