Extracellular ascorbic acid and enzyme activities related to ascorbic acid metabolism in Sedum album L. leaves after ozone exposure

Ascorbic acid (AA) and glutathione (GSH) contents and enzymatic activities of dehydroascorbate (DHA) reductase and glutathione reductase were measured in the apoplast and whole leaves of Sedum album L. plants after a 2-hr exposure to different ozone (O3) concentrations (0.2, 0.4 and 0.6 μl/l O3). Although the reduced AA level decreased in the apoplast of exposed plants, the total amount of ascorbic acid (AA+DHA) increased in that compartment following O3 exposure. The increase depended on the O3 concentration and could reach twice the control values. These results suggested the existence of a continuous supply of AA from cells in response to oxidant exposure. AA and GSH levels were depleted in the whole leaves during the exposure to O3 but rapidly recovered after exposure. Neither dehydroascorbate reductase nor glutathione reductase activities could be detected in the apoplast compartment. In cell extracts, dehydroascorbate reductase was activated and glutathione reductase was not affected by O3 exposure. These results are discussed in the context of a cycle of reactions involving ascorbate peroxidase, dehydroascorbate reductase, glutathione reductase and glucose 6-P dehydrogenase, which would operate in the apoplast and in the interior of the cell. The rapid recovery of AA and GSH levels after O3 exposure suggests a prominent role for these enzymes in cell protection against oxidative damage.