Fungi growing on stored soybeans and their significance in lipid breakdown*

US Grade 2 soybeans contain about 20% cracked and split seeds, 3% damaged seeds and 2% foreign material. Examination by scanning electron microscopy showed that intact seeds were externally almost free of microorganisms. In cracked seed coats and in damaged areas on broken soybeans, profusely developing fungal mycelia were frequently observed. Fungi penetrated the beans through these cracks and through the micropyle. Penetration through the hilum was observed only in cases where the seed coats showed pronounced signs of damage. No penetration through pore-like structures in the seed coat could be observed. Light microscope observations of sections of naturally infested soybeans showed little penetration into the underlying cell layers. Sections of soybeans kept at 99% r.h. and 26d?C for 4 days revealed profusely developing fungi colonising both seed coat tissue and cotyledons. In the cotyledonous tissue hyphae did not penetrate into the cells but developed only in the intercellular spaces. In inoculated, detached seed coats, fungi developed better on the inner than on the outer side. On cotyledons free of seed coats, fungal development was better than in the seed coat itself. In broken soybeans of commercial stock origin, free fatty acids (FFA), mould count and respiration rate increased 3.5-, 3.5-and 5.0-fold, respectively as compared with intact ones. When broken and intact soybeans were mixed there was no increase in the rate of deterioration. It is concluded that fungi are involved in the increase of FFA in soybeans during storage and that this process is significantly faster in the broken bean fraction.     

Scanning Electron Microscopy of the Infection of Beans by Rhizoctonia solani Propagules

Germination, hyphal spread, and formation of infection cushionsand lesions on bean hypocotyls by infective large propagulesof Rhizoctonia solani were studied with the scanning electronmicroscope. The surface of old, but not of young, hyphae appearedtypically wrinkled and shrunken. Infection cushions appearedas a network of several layers of crossed anastomosing hyphae.These features could not be observed under the light microscope.No differences in the surfaces of infective and non-infectivesmall propagules could be observed. Non-infective propagulesgerminating on the hypocotyls gave rise to spreading hyphaebut did not develop infection cushions.