Cell death induced by ozone stress in the leaves of Populus deltoides × maximowiczii

When exposed to an acute ozone stress, cell death occurred in leaves of the O₃ sensitive Populus deltoides × maximowiczii clone Eridano. After treatment (5 h fumigation and 24 h recovery), the damaged areas covered more than 50 % of the leaf surface. At cellular level, these lesions were preceded by some apoptotic hallmarks and by biochemical and physiological alterations evoked by the apoplastic O₃ dissociation. The cell death pattern was highly localized and involved an increase of membrane permeability, externalization of phosphatidylserine, DNA fragmentation, callose accumulation, polyphenol production, and a biphasic oxidative burst accompanied by NO overproduction. These results indicate a process of programmed cell death that could have the biological significance of limiting the spreading the oxidative burst triggered by ozone dissociation in apoplastic environment. Moreover, materials derived from cell dismantling could be remobilized toward developing structures which can conclude their ontogenetic program after the stress.