## 1 Introduction  
In recent decades, the link between nutrition and health has gained universal acceptance, and therefore, increasing importance has been given to dietary regimes based on vegetables rich in antioxidants [1]. The health-promoting properties of antioxidants correlate to the ability to counteract the effect of oxidative stress and the over-production of free radicals in cells, thus providing protection against aging and several degenerative cellular conditions [2]. The pepper (_Capsicum annuum_ L.) is the most economically important species of its genus, widely recognized as a valuable source of bioactive compounds [3; 4]. Sweet peppers have long been considered a main dietary source of antioxidants, such as vitamin C (ascorbic acid, AsA) [4]. AsA is the primary water-soluble antioxidant in plants, and it is also implicated in plant development and hormone signaling [5; 3]. Peppers are also rich in polyphenols, another important class of antioxidants that can provide several health benefits [6]. Phenolic compounds are a large group of secondary metabolites whose accumulation in plants is often linked to stress response [1; 7]. The amount and type of polyphenols in edible products can have a strong effect on taste (e.g., determining bitterness and astringency) and on the appearance of the product (i.e. color) [8]. The level of bioactive compounds in pepper, as well as in other horticultural products, is affected by the maturity stage of the fruit [9; 10; 11]. Peppers are harvested and consumed at different ripening stages, from immature green to fully ripe. Variations occurring during maturation do not only have well-known agronomical implications (e.g., taste, color, size, post-harvesting properties, etc. ), but are relevant to determine fruit quality and use too. Differences in AsA accumulation have also a genetic base. Different studies indicated that the accumulation of AsA in fruits may vary among pepper cultivars [10; 12; 13], although not all aspects of this phenomenon are fully elucidated (e.g., correlation of AsA content with other organs and prevalence of the different biosynthetic pathways, also at different stages) [14; 15]. AsA accumulation in plants is a finely tuned process that results from the balance of de novo biosynthesis, recycling, and degradation [16; 17]. In addition, AsA accumulation also depends on the relation between different source- and sink-organs [18]. The de novo synthesis of AsA in higher plants relies mainly on the L-galactose pathway, also known as the Smirnoff-Wheeler pathway [19], which, in a number of steps, converts simple sugars into galactose and then into ascorbic acid. Additional pathways for AsA biosynthesis are those based on the conversion of 1-D-myo-inositol, D-galacturonic acid or L-glucose, although the L-glucose pathway is also considered a branch of the L-galactose pathway [20]. Besides synthesis, the pool of AsA in cells is also maintained by recycling [16; 21]. In a simplistic way, AsA is oxidised by reactive oxygen species (ROS) into monodehydroascorbate (MDHA), a short-lived intermediate that can be converted into dehydroascorbate (DHA) or to ascorbate. Genes involved in AsA biosynthesis, recycling and degradation have been cloned and characterized in different horticultural species [22; 23]. While some genes have been highlighted as important determinants of AsA content, the molecular mechanisms that regulate AsA accumulation and dynamics remain not entirely clarified. It is generally accepted that L-galactose pathway has the largest influence on AsA metabolism [23; 24]. Nonetheless, the genetic regulation as well as the importance of the expression of the AsA-biosynthetic genes are not well specified, also because AsA-dependent negative feedback are involved in the regulation of the biosynthesis [22; 25]. The study of the accumulation of main antioxidant compounds during maturation is getting increasing interest in plant science because their content is considered an important parameter concerning the quality of vegetables [26; 27].