## 1 Introduction
In particular, differences in AsA concentrations among species/cultivars have been associated with differences in the expression of _GPP-mannose pyrophosphylase_ (_GMP_), _GDP-L-galactose transferase_ (_GGP_), and L-galactose-1-phosphate phosphatase (_GPP_). Nonetheless, [PERSON] et al. [19] suggested the importance of the D-galacturonic acid and the _myo_-inositol pathways in the regulation of AsA accumulation in _Citrus_ fruit tissues, with the genes _D-galacturonic acid reductase12_ (_GalUR12_) and _myo_-inositol oxygenase (_MIOX_) playing a key role. More recently, [PERSON] et al. [21] associated the decrease in AsA content in the juice of blood oranges during maturation to the down-regulation of _MIOX_ and _GalUR12_, together with _GDP-mannose-3',5'-epimerase_ (_GME_) and _L-Galactose dehydrogenase_ (_GalDH_) of the L-galactose pathway. Interestingly, these authors also proposed that the contribution of the different biosynthetic pathways to the AsA pool varied during fruit development, with the L-galactose and _myo_-inositol being important at the early stages of fruit development and the L-glucose pathway at the end of ripening [21]. Moreover, AsA accumulation in _Citrus_ fruit seems to be regulated by complex mechanisms affected also by changes in the rate of degradation and recycling [19; 20; 21]. In particular, differences in APX enzymatic activity have been reported in fruits with contrasting AsA concentrations [20; 21], and a marked induction of the _APX2_ isoform was observed during development and ripening, suggesting that this enzyme could be crucial for the degradation of AsA in the pulp [19]. In addition, the expression of genes involved in AsA recycling have been correlated with changes in AsA concentrations in the flavedo and pulp during fruit ripening, and also with differences in AsA contents among species or cultivars [19; 21]. Furthermore, external factors such as environmental conditions or cultural practices during pre-harvest can also influence the final accumulation of AsA in _Citrus_ fruit [22]. In relation to environmental factors, the effect of light on the accumulation and regulation of AsA metabolism has been demonstrated in different _Citrus_ species, in which on-tree fruit bagging provoked a reduction of AsA concentrations in the flavedo, without altering the contents in the pulp. Moreover, the absence of light markedly repressed the expression of _GalUlR8_, and also _GalUR12_ in some species, concomitantly with the reduction of AsA concentrations in the flavedo [23]. These findings support the notion that the D-galacturonic acid plays an important role in AsA accumulation in _Citrus_ fruits [19; 21]. Information of AsA metabolism in _Citrus_ organs other than fruits is very limited and mainly restricted to the analysis of enzymatic activities of AsA degrading and recycling enzymes under stress conditions [24; 25; 26; 27; 28]. Nonetheless, the temporal accumulation of AsA in leaves of different orange cultivars has been recently reported [21]. These authors found that AsA contents in leaves varied during the year in a cultivar-dependent manner and suggested that AsA  
Figure 1: Schematic representation of ascorbic acid (AsA) metabolism, depicting the different biosynthesis pathways and the degradation and recycling pathways in plants. The enzymes catalyzing the reactions are: GDP-mannnose pyrophosphylase (GMP), GDP-mannose-3’-5’-epimerase (GME), GDP-L-galactose transferase (GGP), L-galactose-1-phosphate phosphatase (GPP), L-galactose dehydrogenase (GalDH), L-galactono-1,4-lactone dehydrogenase (GalLDH), D-galacturonic acid reductase (GalUR), \(m_{\text{20}}\)-inositol oxygenase (MIOX), L-gallono-1,4-lactone oxidase (GLOase), mono-hydroascorbate reductase (MDHAR), dehydrorosorbate reductase (DHAR), ascorbate oxidase (AO), ascorbate peroxidase (APX). accumulation in vegetative tissue seems to be regulated by the synergistic action of the L-galactose and D-galacturonic pathways [21].