Résumé :
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Induced resistance is one of the most attractive scientific domains in plant pathology today. By clarifying the mode of operation and the detection of better inducers, the way of this concept to practical plane protection should be smoothed. We found that not only the disease severity but also detrimental effects on yield could be reduced by suitable inducers and, therefore, not only resistance but also tolerance could be induced. Consequently, the use of induced tolerance as means of plant protection will be easier, if the effects of tolerance-inducing substances could be recognized in juvenile planes already. On the one hand, tolerance is measured as yield of plants with similar degrees of infection, on the other hand, tolerance in juvenile plants can be estimated by relative sucrose distributions between the sink of the youngest leaf of the growing plant and the 'pathological' sink that may be caused by infected tissue. Considering the juvenile plane's assimilate metabolism, induced tolerance can be recognized by the uncoupling of the relationship between disease and plant performance as measured by the source content of the sink leaf. Among the tested pathogens Puccinia hordei, Uromyces appendiculatus, Rhopalosiphum, Colletotrichum lagenarium, Cochliobolus sativus and Tobacco mosaik virus on their hose plants barley, bean, wheat, cucumber and tobacco, respectively, only the two rust fungi and the aphids produced a 'pathological' sink after inoculation or infestation. Application of our inducer 'B 50' onto plants infected with biotrophic fungi or aphids resulted in a decrease of pathological sink-activity for the benefit of the sink-activity of the youngest leaf as compared to untreated control plants of similar infection density. That implies an uncoupling of the relationship between disease and damage, which is a characteristic feature of induced tolerance. Using the host-parasite-system wheat Rhopalosiphum padi induced tolerance was demonstrated by an increase in biomass production of the aphid-free third leaf. Possible causes of the altered sink-source-relationship were characterized by the photosynthesis output of plant's sink organ. Since there was no evidence of an increased chlorophyll fluorescence or gas exchange of the third leaf, the cause of increased biomass production should be the import of assimilates from lower parts of the plant. The transferability of altered sink-source-relationship in juvenile plants to the generative phase is discussed in relation to rile possibility of using this effect in finding suitable inducers of tolerance or resistance.
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