Spore Germination and Microcycle Conidiation of Two Penicillia in Soil

Abstract

Laboratory-produced spores of two penicillia isolated from soil were placed onto PVC membrane filters, inserted into microholders and buried in the A horizon of an oak-birch forest. After various intervals of time incubation filters were retrieved, stained, and mounted on glass slides for examination. Characteristics of growth and germination of Penicillium daleae and P. bilaii in the laboratory and in the soil were then compared. Spores of these indigenous soil fungi, reinoculated into the soil, germinated slowly, requiring several days for swelling and germ tube protrusion. The most significant observation was the occurrence of a microcycle in which germ tube volume was reduced, mycelial growth was eliminated, and conidiation consisted of a diminution in complexity of reproductive structures by decreasing the number of phialides in the penicillus and reducing spore volume. The typical life cycle of imperfect fungi includes germination from a swollen conidium, formation of mycelium, production of reproductive structures, and sporulation. Under certain environmental conditions, however, this is shortened to a microcycle when the mycelial stage is eliminated. These cycles have been demonstrated in vitro for a variety of microorganisms including Aspergillus niger Micheli (Anderson and Smith, 1971), Penicillium urticae Bain (Sekiguchi et al, 1975) and Phomopsis viticola Sacc. (Pezet et al, 1983). Microcycle conidiation is shown in the present study to also occur in soil. The ecological importance of such a system has significant implications to the survival and activity of fungi in this habitat. The study of microcycle conidiation was based on a new method in which spores of two soil penicillia, Penicillium daleae Zaleski and P. bilaii Chalabuda, are reinoculated into the soil on membrane filters in specially designed micro? holders (Gochenaur and Sheehan, 1980). After various periods of incubation in the soil, filters are removed, stained to assess spore viability and examined under the light microscope. Information derived from such a study is critical for predicting the role of microfungi in secondary productivity (Heal and Maclean, 1975) and in estimating the rate of nutrient turnover and substrate mineralization in litter based ecosystems (Dickinson, 1974).