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

Abstract

There is a question whether ascorbic acid (AA) can control redox levels of phenolics in the apoplast. The present study was designed to answer this question. AA, dehydroascorbic acid (DHA), chlorogenic acid (CGA) and its two structural isomers were present in the apoplast of leaves of tobacco (Nicotiana tabacum L. cv. BelW3). The levels of A A plus DHA (AA + DHA) and the ratios of AA to (AA + DHA) decreased while the levels of CGA plus its isomers increased during leaf aging. o-Quinones of CGA plus its isomers were found in the apoplast only in aged leaves of which apoplastic level of A A was nearly zero. In addition,'activity of apoplastic peroxidase that could oxidize CGA and its isomers increased during leaf aging. From the observations, it is concluded that AA can regulate the accumulation of the o-quinones of CGA and its isomers in the apoplast. Based on the conclusion, it is proposed that soluble peroxidase in the apoplast has two functions, namely, (i) scavenging of H2O2 and/or regulation of the level of apoplastic H2O2 in the presence of AA, and (ii) accumulation of oxidation products of the phenolics in the absence of AA.