Effect of light on sensitivity of apple and Japanese pear leaves to AM-toxin from A lternaria 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.

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A host-specific toxin (AM-toxin) produced by Alternaria alternata apple pathotype has two primary action sites: plasma membranes and chloroplasts where the toxin induces electrolyte loss and inhibition of photosynthetic CO2 fixation, respectively. AM-toxin I at a concentration of 10-8M caused an increase in electrolyte loss and an inhibition of CO2 fixation as well as necrosis in susceptible apple leaves. Electrolyte loss and necrosis in moderately resistant apple and Japanese pear leaves were induced at 10-5M, while CO2 fixation was significantly inhibited at 10-7-10-6M.Furthermore, CO2 fixation was affected at the concentrations of 10-6-10-5M in resistant and some non-host leaves, without causing electrolyte loss and necrosis. AM-toxin induced infection with avirulent spores in susceptible and moderately resistant leaves at 10-7 and 10-5M, respectively. However, 10-5M AM-toxin I did not show the infection-inducing activity in resistant and non-host leaves. SH-reagents such as iodoacetamide gave a remarkable protection against AMtoxin-induced electrolyte loss and necrosis formation in susceptible leaves, only when the leaves were treated before toxin exposure.On the contrary, the inhibition of photosynthetic CO2 fixation induced by the toxin was not affected by the reagent. When susceptible leaves pre-treated with the reagent were inoculated with virulent spores, the spores failed to invade the tissues. These results indicate that a role of host-specificity factor of AMtoxin I is dependent on toxin action to plasma membranes rather than chloroplasts, and hence, an accessibility of cells for invading fungi necessitates the plasma membrane dysfunction caused by the toxin.

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Breeding, NIAR. MAFF. Ohmiya-machi, Naka-gun, Ibaraki 319-22 2Tottori Horticult ural Experiment Station, Daiei-cho, Tohaku-gun, Tottori 689-22 Summary 'Osanijisseiki' , a self-compatible, spontaneous bud sport of the Japanease pear •eNijisseiki' is an excellent cultivar with a smooth skin. However, this cultivar is susceptible to Japnaese pear black spot disease caused by Alternaria alternata Japanese pear pathotype. IRB 502-18T, care beiing examined.