## 3 Discussion  
_Citrus_ fruit are well known for being excellent sources of vitamin C for humans, and its contents in the pulp and juice of different _Citrus_ species has been thoroughly studied [17; 33; 34]. Although environmental and cultural conditions can affect AsA content in _Citrus_ fruit [22], variability appears to be mainly genetically determined and higher AsA contents are usually found in oranges and lemons followed by grapefruits and mandarins [18; 33; 35]. Moreover, the availability of the _Citrus sinensis_ genome sequence [36; 37] facilitated the analysis of the transcriptional regulation of many processes in _Citrus_, including AsA metabolism in fruits [19; 21; 23]. Nonetheless, the molecular regulation of AsA metabolism in other plant organs and tissues of _Citrus_ remains poorly understood, and information is mostly limited to the quantification of AsA degrading and recycling enzymes under stress conditions [24; 25; 26; 27; 28]. However, seasonal variations in AsA accumulation and transcriptional regulation in leaves have been recently assessed [21]. Thus, the aim of the present work was to shed light on the transcriptional regulation of AsA metabolism in the  
Figure 9: Illustration of transcription factors binding site located in the promoter region of the _Citrus sinensis_ genes involved in ascorbic acid (AsA) metabolism. Transcription factors binding sites were obtained comparing the 600 bp regions upstream of the start codon (ATG) with the MatInspector database of matrices for plant transcription binding sites (version 9.4), using a \(p\)-value \(\leq\) 0.05 as cut-off criteria. Abbreviations: ABRE, ABA response elements; ASRC, AS1/AS2 repressor complex; ATAF, ATAF-like NAC domain containing proteins; FLO2, Floral homeotic protein APETALA 2; GTBX, GT-box elements; HMGF, high mobility group factors; IDDF, ID domain factors; SEF4, soybean embryo factor 4; STKM, storekeeper motif; SURE, sugar response elements and SURE-like sequences in plant promoters. flavedo and pulp of fruit during the whole reproductive development and also to conduct a comparative analysis in different organs of orange, namely: petals and leaves. To that end, we selected the sweet orange (_Citrus sinensis_) cultivar Valencia Late, which is cultivated worldwide and characterized by a long period of fruit developmental and late ripening, prolonging the harvest time as late as April and May in Northern hemisphere countries. The analysis of AsA and DHA concentrations revealed that petals were the tissue with the highest levels of total AsA (up to 632 mg/100 g FW at anthesis), which were 1.5-times higher than the maximum detected in leaves and in the fruit flavedo, and 10-times higher than those in the pulp (Figures 2 and 3). Interestingly, petals accumulated not only high amounts of AsA but also important concentrations of DHA. Furthermore, each organ or tissue seemed to have a distinct pattern of accumulation during development. In petals and leaves, AsA remained nearly constant during development, whereas a near 4-fold increase in DHA levels was detected during petal development (Figure 2B,D). In other plant species, AsA content decreased during development of petals of harvested flowers, while it increased or remained constant in flowers remaining in the plant [38; 39]. The physiological reason for the high accumulation of vitamin C in petals is not well understood, but it can be reasoned that the short period of petal development during flower expansion and opening may require a high antioxidant status. Moreover, these organs, which usually abscise few days after anthesis, may be a good alternative source of this antioxidant for the industries of cosmetics or dairy supplements, among others. The concentration of AsA detected in leaves of Valencia oranges revealed that these organs are also rich in AsA and DHA (\(\sim\)400 mg/100 g FW), although with contents lower than petals and comparable to the maximum detected in the flavedo of fruits in December (Figure 3C).