Structural effects on flax stems of three potential retting fungi

Rhizomucor pusillus and Fusarium lateritium, isolated from flax dew-retted in South Carolina, and Epicoccum nigrum, isolated from flax dew-retted in the Netherlands, are evaluated for structural modifications of various cells in their flax stems, particularly the effects on fiber walls and bundles as related to retting. All fungi are effective in degrading parenchyma tissues and isolating fibers, but F. lateritium and particularly E. nigrum also degrade the fiber cell walls, including the secondary layers, indicating the potential for over-retting and reduced fiber strength. In contrast, R. pusillus does not appear to attack the main portion of the secondary wall, but degrades the most central portion of the fiber wall, thus expanding the fiber lumen. Extracellular enzymes produced by this fungus appear to effectively degrade the middle lamellae, separating bundles into fibers. The lignified walls of the core cells are partially degraded by F. lateritium but not by R. pusillus or E. nigrum. R. pusillus and E. nigrum appear to partially degrade or alter the cuticular surface of the stem, possibly facilitating hyphal penetration to the internal tissues. These observations of structural alterations indicate that R. pusillus possesses the best retting abilities. Results are interpreted in light of previous information on the enzymatic profile of these fungi.