Regulation of Peroxidase-Dependent Oxidation of Phenolics in the Apoplast of Spinach Leaves by Ascorbate

The aqueous phase of the cell walls inside leaves (apoplast) of spinach contained ascorbate (AA) and dehydroascorbate (DHA). Ratios of AA to AA plus DHA were between 0.4 and 0.9, whereas those inside leaves were higher than 0.9. The amounts of AA plus DHA in the apoplast were between 15 and 60 nmol (g fr wt)−1 of leaves. If the volume of the apoplast is about 10% of total volume of leaf cells, the concentrations of AA plus DHA were between 0.15 and 0.6 mM. Apoplastic AA was oxidized by hydrogen peroxide, and the oxidation was stimulated by phenolics such as caffeic acid or ferulic acid by a factor of 10, suggesting the presence in apoplast of peroxidases which are different from AA peroxidase. The stimulation was due to the oxidation of AA by the primary oxidation products of phenolics with apoplastic peroxidase. Based on the data, the physiological significance of the occurrence of AA in the apoplast is discussed in relation to the regulation of the apoplastic oxidation of phenolics.