Redox state and peroxidase system in sunflower plants exposed to ozone

Sunflower plants subjected to a short-term fumigation with O-3 (150 ppb for 4 h repeated for 4 days) exhibited an increase in total ascorbate content, accompanied by a marked oxidation of ascorbate, leading to a decrease in its redox state, either at intracellular or extracellular level. O-3 exposure induced a rise in free extracellular peroxidase (POD) activity, assayed by syringaldazine as electron donor, as well as in the ionically and covalently cell wall bound PODs. On the contrary, the activity of both extracellular and intracellular guaiacol-POD did not show significant changes as a consequence of the pollutant exposure. The stimulation of syringaldazine-POD activities may be related to the effect of ozone on the growth of the cells, inducing an early senescence through the activation or acceleration of lignification processes. Beside: the reduced plasticity of the cell wall may oppose an unspecific mechanical resistance against the abiotic stress induced by the ozone exposure. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.