Changes in the level of endogenous formaldehyde (HCHO), some N-methylated compounds (choline and trigonelline) and peroxidase activity were examined in the leaves of bean genotypes (Phaseolus vulgaris L.) with different disease-sensitivity during ontogenesis in the stressfree condition and after natural infection by Pseudomonas savastanoi pv. phaseolicola (until the appearance of lesions). HCHO, as its dimedone adduct, and fully N-methylated compounds were determined by overpressured layer chromatography (OPLC) in different developmental stages and in the infected leaves/leaf discs. Peroxidase activity was measured by a spectrophotometric method. HCHO level decreased with agetng of the primary leaf and accordingly in the leaves at different developmental stages, then increased again in both cases due to the demethylation and methylation processes. Concentration of choline and trigonelline as potential HCHO generators des creased considerably while peroxidase activity increased with agetng of the plants. Comparing the symptomless and the Pseudomonas infected leaf discs (with watersoaked lesions) we found a decrease in the level of HCHO, choline and trigonelline and there was det ectable increase in the peroxidase activity in the infected leaftissues. Our findings are in accordance with previously published results that peroxidases play an important role in oxidative demethylation processes. Our hypothesis is that the high level of HCHO in the old leaves can originate from methylated components as the result of peroxidase activity and this high level may lead to the old leaf betng resistant to pathogen. This conclusion is supported by the fact that the leaves of susceptible bean genotypes became resistant to Pseudomonas while growing older.
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