## 3 Discussion
The content of AsA in the pulp of Valencia Late oranges are in a similar range to those reported in the juice of Navel and blood oranges, but in these varieties, it decreased more remarkably towards fruit maturation [21]. This different pattern of accumulation may be related to the different plant material, juice vs. pulp, but also to genotypic differences [20; 21]. In fruits of other species, such as tomato [12] and pepper [46], AsA increased with maturation, whereas in apple, cherry, and peach it was higher at immature stages [47; 48; 49]. Transcriptional analysis of the main genes involved in AsA metabolism revealed that AsA accumulation in the different organs and tissues is regulated in a complex and specific manner, and seems to depend on the relative balance between the rate of biosynthesis and the mechanisms of AsA degradation and recycling (Figure 1) [3; 4]. In petals, the changes in AsA and DHA contents were accompanied by the upregulation of most biosynthetic and recycling genes, but also of those involved in degradation (Figure 4). A substantial induction (more than 4-times) was detected in the genes _GPP_ and some _GalUR_ isoforms, indicating the importance of the D-galacturonic pathway in the biosynthesis of AsA in this organ. Expression of _GPP_ has been correlated to AsA levels in different plant tissues, and is considered a pivotal step in AsA synthesis [3]. Interestingly, AsA contents during petal development remained constant and the increase was mainly in DHA (Figure 2B). Therefore, it is possible that the relative contribution of AsA degradation is favored over the recycling, probably by the stimulation of the expression of all the _APX_ isoforms and the low rate of DHA conversion to AsA (Figure 4B,C). Collectively, these results indicate that in petals of orange, a synchronized activation of the different pathways during flower opening is playing a major role in the maintenance of AsA homeostasis. In the leaves, the evolution of AsA during development was accompanied by a downregulation in the majority of the genes analyzed (Figure 5). All genes involved in the L-galactose biosynthesis pathway and most of the _GalUR_ isoforms were repressed during leaf development (Figure 5A). These results are in agreement with those of [PERSON] et al. [21] in leaves of Navel and blood oranges, suggesting the synergistic contribution of the L-galactose and D-galacturonic pathways to AsA synthesis in _Citrus_ leaves. Moreover, the expression of most AsA degradation genes was maintained (_APX1_, _APX2_, _APX4_), while most of the recycling genes were downregulated. The fact that the downregulation of _AO_ and _APX3_ did not produce an increase in AsA suggest, overall, a coordinated decrease in the expression of biosynthetic and recycling genes to maintain AsA accumulation. Significant differences in the transcriptional regulation of AsA were also detected between flavedo and pulp of Valencia orange fruits. In the flavedo, accumulation of AsA positively correlated with the expression of _GGP_ of the L-galactose pathway (Figures 6 and 8B). Expression of this gene increased from June to November-December when AsA contents were maximum in the flavedo, and then declined towards maturation in agreement with the decrease in contents (Figures 3C and 6A). The induction of _GPP_ after November may also help to maintain the synthesis of AsA at later stages of maturation (up to February). Moreover, the expression of _GMP_ and _GalDH_ positively correlated to DHA contents (Figure 8B). It has been previously indicated that AsA accumulation in the flavedo, and also in the pulp and juice, is correlated with the transcriptional profiling of some of the L-galactose pathway genes [19; 20; 21], and in particular with the expression of _GGP_[19]. These results reinforce the notion that _GGP_ is an important regulatory step of AsA content in _Citrus_ flavedo. Similarly, AsA levels have been correlated to _GGP_ and _GPP_ transcript levels in tomato fruit [12; 50] and _GGP_ in apple [51].