Hierarchy of ozone scavenging reactions in the plant cell wall

To estimate protection of the plasmalemma against ozone by the cell apoplas t, the decomposition network of ozone in the mesophyll cell wall is analyse d in consideration of data on published bimolecular reaction rate constants and concentrations of the reactants involved. The effect of dimerization o f ascorbate free radicals (AFR) on the stoichiometric ratio of ozone reduct ion by ascorbate is quantified over the range of cell wall acidity, pH = 5. 0-6.5. As the disproportionation of AFR decreases sharply towards higher pH , the flow of AFR through dimerization is low over the pH range 5.5-6.5, al lowing abstraction of the second electron from AFR and formation of dehydro ascorbate with a nearly 1:1 stoichiometric ratio in relation to ozone. The direct reaction between ozone and ascorbate (AA) in cell walls 0.3-0.5 mu m thick and at an AA concentration of 0.5 mM is able to detoxify 50-70% of t he O-3 that impinges on the wall surface. Generation of singlet oxygen and the hydroxyl radical, which are more reactive to AA than O-3, decreases mar kedly the O-3 flow to the plasmalemma. The question is raised whether cell wail alkalinization under ozone may hasten the decomposition of the polluta nt due to the more rapid generation of hydroxyl radical by phenolic compoun ds.