Ascorbic Acid-Dependent Regulation of Redox Levels of Chlorogenic Acid and its Isomers in the Apoplast of Leaves of Nicotiana tabacum L.

There is a question whether ascorbic acid (AA) can control redoxlevels of phenolics in the apoplast. The present study was designedto answer this question. AA, dehydroascorbic acid (DHA), chlorogenicacid (CGA) and its two structural isomers were present in theapoplast of leaves of tobacco (Nicotiana tabacum L. cv. BelW3).The levels of AA plus DHA (AA + DHA) and the ratios of AA to(AA + DHA) decreased while the levels of CGA plus its isomersincreased during leaf aging. o-Quinones of CGA plus its isomerswere found in the apoplast only in aged leaves of which apoplasticlevel of AA was nearly zero. In addition, activity of apoplasticperoxidase that could oxidize CGA and its isomers increasedduring leaf aging. From the observations, it is concluded thatAA can regulate the accumulation of the o-quinones of CGA andits isomers in the apoplast. Based on the conclusion, it isproposed that soluble peroxidase in the apoplast has two functions,namely, (i) scavenging of H₂O₂ and/or regulation of the levelof apoplastic H₂O₂ in the presence of AA, and (ii) accumulationof oxidation products of the phenolics in the absence of AA. (Received January 30, 1998; Accepted April 7, 1998)