Polyamine metabolism was examined in tobacco (Nicotiana tabacum L.) exposed to a single ozone treatment (5 or 7 hours) and then postcultivated in pollutant-free air. The levels of free and conjugated putrescine were rapidly increased in the ozonetolerant cultivar Bel B and remained high for 3 days. This accumulation was preceded by a transient rise of L-arginine decarboxylase (ADC, EC 4.1.1.19) activity. The ozone-sensitive cultivar Bel W3 showed a rapid production of ethylene and high levels of 1-aminocyclopropane-1-carboxylic acid after 1 to 2 hours of exposure. Induction of putrescine levels and ADC activity was weak in this cultivar and was observed when necrotic lesions developed. Leaf injury occurred in both lines when the molar ratio of putrescine to 1-aminocyclopropane-1-carboxylic acid or ethylene fell short of a certain threshold value. Monocaffeoyl-putrescine, an effective scavenger for oxyradicals, was detected in the apoplastic fluid of the leaves of cv Bel B and increased upon exposure to ozone. This extracellular localization could allow scavenging of ozone-derived oxyradicals at the first site of their generation. Induction of either polyamine or ethylene pathways may represent a control mechanism for inhibition or promotion of lesion formation and thereby contribute to the disposition of plants for ozone tolerance.Ozone (03) Fabaceae and Solanaceae, which include many crop plants, seem to be particularly sensitive (1 1, 14). Leaf damage as the most obvious pollutant effect can be observed as pigmented lesions, bleaching, chlorosis, and bifacial necrosis, depending on 03 dose and plant species. Cigar wrapper varieties of tobacco were among the first plants for which 03 damage ('weather flecks') was demonstrated (11). 03-tolerant and -sensitive lines were selected in the late 1950s. A hypersensitive line (Bel W3) from these breeding programs has been used for over 30 years as a biomonitor plant for 03 and responds to ambient concentrations with necrotic flecks that occur predominantly on the middle-aged leaves (1 1).Exposure of plants to subacute levels of 03 is now known to induce many biochemical and physiological changes (1,13,25). Several of these reactions form part of the plant's defense system toward oxidative stress, e.g. ascorbic acid and peroxidases (13), phenolic compounds (25), and polyamines (3, 13). The diamine putrescine, as well as spermidine and spermine, all commonly termed polyamines, exert several functions which counteract 03 effects. They have been implicated in an inhibition of lipid peroxidation of membranes (27), an activation of membrane-bound ATPases (13), and a reduction of ethylene formation (6,26). Polyamines were observed to accumulate in 03-treated barley (24), wheat (23), and Norway spruce (4, 25). Ethylene, which appears to be antagonistic to polyamines in senescence, is induced by 03 in many plant species (5,11,29