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.