## 3. Asxorbic Acid Accumulation and Metabolism in Tomato  
Considering the broad range of functions of AsA in fruit tissues, it is of great importance to understand why this molecule accumulates at high concentrations in some particular species and cultivars, and not in some others, but also what is the functional significance of this variability. As a general rule, concentrations in fruit AsA content tend to be high in wild accessions and lower in cultivated species due to the domestication process [47]. In tomato fruit, concentrations may range from 10 to 40 mg 100 g\({}^{-1}\) in cultivated species [47; 48; 49; 14], whilst wild accessions, such as _S. pennellii_ or _S. pimpinellifolium_, have nearly five-times and three-times higher levels, respectively [50]. The range of natural diversity found within the tomato germplasm can be considered as moderate, compared to other fruit species, such as strawberry, apple, and kiwifruit (Table 1). Since the dawn of plant breeding, cultivar selection was based on a few key genes exerting a large phenotypic effect on desirable traits such as fruit size, shape, and color, as well as disease resistance [51]. As tomatoes are grown under more controlled environmental conditions, devoid of stress, it is evident that a low selective pressure to keep alleles conferring enhanced AsA levels may have taken place during the domestication process [6; 48]. Tomato genetic resources include wild, landscapes (heirlooms and local cultivars), modern cultivars, and breeding populations, offering untapped genetic and phenotypic diversity, with over 20,000 accessions being kept in gene banks over the world [51]. As AsA content is under the fine regulation of multiple genomic regions, the existence of great intra-species diversity within the tomato germplasm is of paramount importance for breeding not only due to its high nutritional value but also because of its potential implication in plant abiotic stress tolerance. Towards this end, the use of different populations derived from crosses between modern varieties and wild species serves as a valuable resource for genomic regions regulating AsA content in the tomato fruit. Stable chromosome regions targeting chromosomes (Chr) 2, 8, 9, 10, and 12 have been implicated in AsA accumulation in Quantitative Trait Locus (QTL) studies [50; 55]. For instance, within the stable QTL regions of Chr9, orthologue copies of key genes of the AsA metabolic pathways such as _GME_, _GMP_, and _MDHAR_ were detected. Further studies confirmed the role of MDHAR in governing the tomato fruit AsA pool through the ripening process [14], or for extending postharvest life [55], whilst Genome Wide Association Studies (GWAS) revealed several key single-nucleotide polymorphisms (SNPs) associated with _MDHAR_[56]. Introgression lines (ILs) derived from the cross between the wild _S. pennellii_ and M82 cultivar can serve as a valuable mapping source for QTL analysis [57]. In particular, a sub-line of the region 12-4 (IL 12-4-SL) harboring a QTL for enhanced AsA content at the bottom of chromosome 12 has been previously identified [58]. The accumulation of AsA in this line is approximately 40% higher compared to M82, probably as a result of the enhanced metabolic flux through the D-galacturonate pathway, which is driven by cell wall and pectin degradation triggered by ethylene during the ripening process [58; 59]. Additionally, in the high AsA content IL, the expression of an orthologue of ascorbate peroxidase (_APX_) was significantly down-regulated, suggesting that AsA degradation may also contribute to the enhancement of the AsA pool in this line. Supporting this notion,