Hierarchy of ozone scavenging reactions in the plant cell wall

To estimate protection of the plasmalemma against ozone by the cell apoplast, the decomposition network of ozone in the mesophyll cell wall is analysed in consideration of data on published bimolecular reaction rate constants and concentrations of the reactants involved. The effect of dimerization of ascorbate free radicals (AFR) on the stoichiometric ratio of ozone reduction 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, allowing abstraction of the second electron from AFR and formation of dehydroascorbate 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 µm thick and at an AA concentration of 0.5 mM is able to detoxify 50‐70% of the O3 that impinges on the wall surface. Generation of singlet oxygen and the hydroxyl radical, which are more reactive to AA than O3, decreases markedly the O3 flow to the plasmalemma. The question is raised whether cell wall alkalinization under ozone may hasten the decomposition of the pollutant due to the more rapid generation of hydroxyl radical by phenolic compounds.