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 i ts redox state, either at intracellular or extracellular level. O-3 exposur e induced a rise in free extracellular peroxidase (POD) activity, assayed b y syringaldazine as electron donor, as well as in the ionically and covalen tly cell wall bound PODs. On the contrary, the activity of both extracellul ar and intracellular guaiacol-POD did not show significant changes as a con sequence of the pollutant exposure. The stimulation of syringaldazine-POD a ctivities may be related to the effect of ozone on the growth of the cells, inducing an early senescence through the activation or acceleration of lig nification processes. Beside: the reduced plasticity of the cell wall may o ppose an unspecific mechanical resistance against the abiotic stress induce d by the ozone exposure. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.