Abstract
Factor (s) associated with the degree of resistance of tomato cultivars to the fruit rot caused by Fusarium oxysporum f. sp. lycopersici race J3 (J3: currently proposed to be revised as F. oxysporum f. sp. radicis-lycopersici) were examined by physiological and microscopical means. No significant differences in hyphal growth were found when the fungus was inoculated on the cut surfaces of fruits or in juice of fruits of a susceptible (Mie First) and a resistant cultivar (Zuiken). These results suggest that components of fruits might not be involved in the resistance of the resistant cultivars. Light microscopic observations demonstrated that hyphae grew in styles of both cultivars similarly for the first 4 days after inoculation. Their growth, however, stopped in styles of Zuiken thereafter, but not in Mie First. Thus, some factor (s) in styles of Zuiken might affect the growth of hyphae, leading to the resistance of this cultivar. Electron microscopy revealed that hyphae grew in middle lamellae of conducting tissues of styles. In Mie First, middle lamellae in infected areas of styles were degenerated almost completely by 6 days after inoculation, and thus the remaining cell walls were left arranging side by side. In contrast, middle lamellae and cell walls in infected areas of Zuiken swelled but were not completely degenerated by 6 days after inoculation. These results suggest a possibility that differences of structural and chemical components in styles might account for varied degrees of resistance of both cultivars.
