Systemic acquired resistance, peroxidase activity and lesion size in tobacco reacting hypersensitively to tobacco mosaic virus

Abstract

Systemic acquired resistance of Nicotiana tabacum cv. Samsun NN to tobacco mosaic virus, expressed primarily as a decrease in lesion size, is associated with increased initial peroxidase and catalase activities. The increase in peroxidase activity was positively correlated with the lesion density on inoculated leaves, but the level of resistance attained was influenced little either by the initial inoculum concentration or by the leaf position on the stem. In non-infected tobacco plants peroxidase activity progressively increases with leaf age. In young, expanding leaves no correlation between peroxidase activity and lesion size was observed ( r = 0·05), whereas in mature, fully grown leaves an inverse logarithmic relationship exists ( r = 0·99). No definite proportionality between catalase activity and lesion size was apparent. Leaves showing systemic resistance had properties characteristic of older leaves, but the decrease in lesion size was larger than could be accounted for on the basis of peroxidase alone. Artificial ageing, trimming of plants, root inundation, drought or salt-stress stimulated peroxidase activity but, upon subsequent inoculation, also enhanced lesion enlargement. Injection of leaves with tobacco or horseradish peroxidase either had no effect on, or increased, lesion size. No correlations between changes in individual peroxidase isoenzymes and effects on lesion size were noted. Peroxidase activity thus seems to be a reflection of a physiological state rather than being directly responsible for regulating the rate of lesion enlargement.