Abstract
Effect of light on sensitivity of apple and Japanese pear leaves to AM-toxin from Alternaria alternata apple pathotype was investigated. Continuous irradiation of light to leaves immediately after toxin exposure inhibited toxin-induced necrosis on susceptible apple and moderately resistant apple and pear leaves. When light irradiation was interrupted by inserting with darkness for specified times immediately after toxin exposure, a period of darkness longer than 5hr was required for necrosis development. However, the required period became shortened to about 3hr if lightcut-off started later than 2hr after toxin exposure. The action spectrum for the photoprotection was estimated to be 570-680nm; the most effective wavelength was near 602nm. The effective light was very specific to the necrosis induced by AM-toxin, and was not affected by treatment with photosynthetic inhibitors. Moreover, light did not affect toxin-induced electrolyte loss and reduction of photosynthetic CO2 fixation, early events in toxin action. When the leaves were inoculated with virulent spores or avirulent spores plus AM-toxin, light had no protective effect on fungal invasion, but inhibited the lesion development.
