Membrane damage caused by the non-specific fungal toxin fusaric acid was less on pretreated than on control leaves when tobacco leaves were pretreated with anti-senescence plant hormones, such as kinetin, benzyladenine or the anti-ozonant N-[2-(2-oxo-1-imidazolidinyl)]ethyl-N'-phenylurea. Similarly, the necrosis caused by mercuric chloride was reduced by the above anti-senescence agents. In addition, in in vitro tests, leaves from selected paraquat-tolerant tobacco plants were less sensitive to Alternaria alternata (Fr) Keissler infection than those of the control paraquat-sensitive tobacco leaves. Paraquat-tolerant Conyza canadensis (L) Cronq weeds naturally selected in vineyards in Hungary showed similar inhibition of senescence to paraquat-tolerant tobacco, expressed as more green leaves and slower development. In accordance with this, the paraquat-tolerant Conyza leaves remained almost symptomless, while paraquat-sensitive plants showed severe symptoms after infection with Botrytis cinerea Pers. Oxidative burst (accumulation of hydrogen peroxide) was attenuated in TMV-infected leaves of Xanthi-nc tobacco as a result of treatment with salicylic acid or in leaves where systemic acquired resistance (SAR) had been induced by a previous TMV infection. Accordingly, superoxide dismutase (SOD) activity was higher in Xanthi tobacco leaves with SAR than without SAR. However, in NahG tobacco, in which SAR cannot develop, there was no augmentation of SOD activity. All the above data support the significance of delayed senescence and antioxidants in the resistance of plants to biotic and abiotic necrosis-inducing agents.