We have used genotypic variation in birch (Betula pendula Roth) to investigate the roles of ozone (O 3 )-induced ethylene (ET), jasmonic acid, and salicylic acid in the regulation of tissue tolerance to O 3 . Of these hormones, ET evolution correlated best with O 3 -induced cell death. Disruption of ET perception by transformation of birch with the dominant negative mutant allele etr1-1 of the Arabidopsis ET receptor gene ETR1 or blocking of ET perception with 1-methylcyclopropene reduced but did not completely prevent the O 3 -induced cell death, when inhibition of ET biosynthesis with aminooxyacetic acid completely abolished O 3 lesion formation. This suggests the presence of an ET-signaling-independent but ET biosynthesis-dependent component in the ET-mediated stimulation of cell death in O 3 -exposed birch. Functional ET signaling was required for the O 3 induction of the gene encoding -cyanoalanine synthase, which catalyzes detoxification of the cyanide formed during ET biosynthesis. The results suggest that functional ET signaling is required to protect birch from the O 3 -induced cell death and that a decrease in ET sensitivity together with a simultaneous, high ET biosynthesis can potentially cause cell death through a deficient detoxification of cyanide.The concentration of tropospheric ozone (O 3 ) has increased during the past decades due to human activities, and it has been estimated that in year 2100, 50% of global forest area will be exposed to potentially phytotoxic O 3 concentrations (Fowler et al., 1999). In the leaves of O 3 -sensitive plants, symptoms of high O 3 are observed as rapid lesion formation. Traditionally, formation of reactive oxygen species (ROS), such as superoxide (O 2 ⅐ Ϫ ) and hydrogen peroxide (H 2 O 2 ) from the degradation of O 3 in the apoplast has been thought to alter the integrity of the plasma membrane and thus the integrity of the cell (Laisk et al., 1989; Heath, 1994). However, O 3 also induces an active and controlled apoplastic oxidative burst, the production of O 2 ⅐ Ϫ and H 2 O 2 in the leaves affected, which may initiate programmed cell death analogous to that induced by ROS in an incompatible plant-pathogen interaction (Schraudner et al., 1998;Pellinen et al., 1999Pellinen et al., , 2002Overmyer et al., 2000; Moeder et al., 2002; Wohlgemuth et al., 2002).Activation of ethylene (ET) biosynthesis by induction of the genes encoding 1-aminocyclopropane-1-carboxylate synthase (ACS) is one of the fastest and most obvious biochemical responses to O 3 and has been mechanistically linked to the regulation of O 3 lesion formation (Schlagnhaufer et al., 1997;Tuomainen et al., 1997;Vahala et al., 1998;Overmyer et al., 2000; Moeder et al., 2002). ET is perceived by a two-component His kinase receptor family (Chang et al., 1993a; Hua et al., 1995Hua et al., , 1998Sakai et al., 1998). The ET receptor can be pharmacologically blocked with a competitive inhibitor of ET action, norbornadiene, or with 1-methylcyclopropene (MCP; Serek et al., 1995), which prevents the binding ...