| This research used'Dangshansu'pear as material, leaves as explant and the culture filtrate of Venturia nashicola as selective agent. We studied leaf culture and plantlet regeneration of pear scion cultivar'Dangshansu', and selection of toxin-promoted conditions from Venturia nashicola and toxicological detection, then we studied effect of crude toxin isolated from Venturia nashicola on physiological characteristic of in vitro leaves of pear varieties with different resistance, Finally, the callus resistant to Venturia nashicola was selected by using one-step and multi-step screening method at different concentration of the crude toxin, the disease resistant mutant selection system was established and several mutant plants were obtained. The results as follows:1. Pear scion cultivar'Dangshansu'was chosen as material. The factors which include medium, plant growth regulators and carbon sources, dark culture period and concentration of agar that influence adventitious buds regeneration from leaf were examined systematically. The results showed that the optimal combination for leaf regeneration of pear cultivar'Dangshansu'was NN69+6-BA 2.5mg·L-1+IBA 0.3mg·L-1+sorbitol 20g·L-1+agar 8.0 g·L-1, and dark period of 20days at the beginning of culture. The highest regeneration frequency of pear cultivar'Dangshansu'was 83%, and the mean bud number per leaf was 2.17. The regenerated shoots were rooted on 1/4MS+IBA 0.5mg·L-1+NAA 0.3mg·L-1+sucrose 20mg·L-1.2. The conditions of producing toxin from Verturia noshicola were systematically studied on following apsects: kind of liquid culture medium,light time,culture temperture,culture time, pH value of culture medium and oscillation frequency.Then, we detected germinate rate of `Dangshansu` pear leaves spot size, plant height and root length in test tube plantlet after the treatment with toxin. The results showed, the optimum toxin producing condition was PDA liquid media with 6.0 pH value, in 22℃,with 110 r·min-1 oscillation frequence in the dark cultured 22d. We can detecte toxicity using four methods.3. The different disease resistant pear cultivars were inoculated with crude toxin isolated from Venturia nashicola. The result showed that the peroxidase (POD) and phenylalanine ammonialyase (PAL) activities, the malondialdehyde (MDA) content and electrical conductivity of susceptible and resistant pear cultivars displayed ascending trend. Meanwhile, they had one or two peak value. The POD and PAL activities of resistant cultivars in vitro leaves were higher than susceptible cultivar, and resistant cultivars in vitro leaves showed a lower increasing rate than susceptible cultivar in MDA content and electrical conductivity. In addition, chlorophyll and carotenoid content in leaves decreased. There was more injured degree in susceptible cultivar than in resistant cultivar. To sum up, the variation of POD and PAL activities had positive correlation with disease resistant ability, and the variation of MDA content and electrical conductivity had negative correlation with disease resistant ability in different cultivars. This proved that the latter have a better ability in pear scab resistance.4. The regenerated plantlets were obtained and their resistance was detected. The result showed that the maxium permissible concentration of crude toxin isolated from Venturia nashicola with'Dangshansu'pear was 50%, and it had remarkable inhibitory effects on callus induction and differention. As the toxin concentration increased, this inhibitory effects rised. According to the resistance detection, mutant plantlets showed resistant to the disease of Venturia nashicola compared to the'Dangshansu'pear.
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