The ascorbic acid system in seeds: to protect and to serve

The ascorbic acid (ASC) system functions dynamically in seeds, although the strategies for ASC production and utilization may vary according to seed developmental and functional stages. In orthodox seeds, ASC content and ASC peroxidase activity increase during the early stages of development, then decrease during the desiccation stage, so that, at quiescence, seeds have neither ASC nor ASC peroxidase, but retain a small amount of dehydroascorbic acid (DHA) and significant activities of ASC recycling enzymes. ASC and ASC peroxidase activity re-start after a few hours from the onset of imbibition. In contrast, the ASC system is little affected during germination of recalcitrant seeds. Although the presence of the ASC system in seeds has often been considered only within the framework of seed antioxidant defences, ASC function in seeds is also likely to be related to its action as a specific co-substrate required for the activity of dioxygenases (e.g. 1-aminocyclopropane carboxylate oxidase, gibberellic acid hydroxylases and 9-cis-epoxycarotenoid dioxygenases) involved in the synthesis of ethylene, gibberellins and abscisic acid, respectively. The possible role of ASC in coordinating the activities of these key enzymes is discussed.