The structure and histochemistry of sclerotia ofSclerotinia minor Jagger

Summary

The development and structure of sclerotia of the fungusSclerotinia minor Jagger, was studied by light, scanning and transmission electron microscopy. The sclerotia formed beneath a weft of overlying vegetative hyphae, that sometimes became enveloped as the sclerotia enlarged. Differentiation of the sclerotial hyphae into regions of rind, cortex and medulla, began only 12 to 24 hours after sclerotial initiation occurred. The cortex was the last region to become discernible. The rind cells rapidly became vacuolate, while their walls thickened and became pigmented. At maturity the rind consisted of a closely packed layer of cells around the sclerotium. The cortex was about three cells wide and was made up of pseudoparenchymatous tissue. The prosenchymatous medulla constituted the main part of the sclerotium. Cytoplasmic reserves, tentatively identified as polyphosphate granules and protein bodies, accumulated in large numbers in cortical and medullary hyphae. Extracellular material was laid down very rapidly around hyphae of the cortex and medulla, until at maturity, it almost completely filled any interhyphal spaces. The ultrastructure of young sclerotial hyphae was very similar to that of actively growing vegetative hyphae. The numbers of nuclei and profiles of mitochondria decreased at later stages of development but there was an increase in the number of profiles of endoplasmic reticulum cisternae. The cytoplasm had a granular appearance throughout differentiation. The general structure of mature sclerotia ofS. minor was similar to that reported for sclerotia of other species in the genusSclerotinia.