Summary — A peak of specific peroxidase activity (increase in enzyme activity followed by decrease) has generally been found in crude extracts of stem-or shoot-cuttings prior to root formation. Effectors

Abstract. Rubbing young internodes of Eryonia dioica results in a reduced elongation and an increased diameter of the internodes. In the present study activities of some enzymes involved in the lignification process and levels of lignification were compared in rubbed and non-rubbed internodes. Rubbing caused an increase in the activities of phenylalanine ammonia-lyase and soluble and ionically-and covalently-bound cell wall peroxidases. Sensitivity of the covalently-bound walJ peroxidase assay was markedly increased if syringaldazine was used as a substrate. Mechanical perturbation induced an increase in lignin, lignin monomer (sinapylic, coniferylic and p-coumarylic alcohols) content and the number of lignifying vessels. Conversely, rubbing resulted in a decrease in cellulose content. The hypothetical interpretation of the thigmomorphogenetic response through cell wall lignifrcation and hence rigidification is consistent with all the presented results. A comparison is possible between this accelerated lignification and induced lignification as a mechanism of disease resistance. The thigmomorphogenetic response in Bryonia dioica can be considered as a mechanism of resistance in order to withstand further environmental mechanical perturbation.

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Habituated sugarbeet callus examined by means of a light-microscope is characterized by absence of differentiated tracheary elements and of the reaction with syringaldazine. Habituated tissues also were found to exhibit low guaiacol- and syringaldazine-peroxidase activities, a deficiency of lignin as well as lower cellulose, dry mass and chlorophyll contents as compared to the normal auxin-requiring callus. These histological and cytological features led to consider this habituated callus as a vitrified tissue under stress. The question of a relationship between vitrification and habituation is posed