Abstract
Sweet oranges are an important source of ascorbic acid (AsA). In this study, the content of AsA in the juice and leaves of four orange clonal selections, different in terms of maturity time and the presence/absence of anthocyanins, was correlated with the transcription levels of the main genes involved in the biosynthesis, recycling, and degradation pathways. Within each variety, differences in the above pathways and the AsA amount were found between the analysed tissues. Variations were also observed at different stages of fruit development and maturation. At the beginning of fruit development, AsA accumulation was attributable to the synergic action of l-galactose and Myo-inositol, while the l-gulose pathway was predominant between the end of fruit development and the beginning of ripening. In leaves, the l-galactose pathway appeared to play a major role in AsA accumulation, even though higher GalUr isoform expression suggests a synergistic contribution of both pathways in this tissue. In juice, the trend of the AsA content may be related to the decrease in the transcription levels of the GME, GDH, MyoOx, and GalUr12 genes. Newhall was the genotype that accumulated the most AsA. The difference between Newhall and the other varieties seems to be attributable to the GLDH, GalUr12, APX2, and DHAR3 genes.
Highlights
Ascorbic acid (AsA) acts as a powerful antioxidant
In an attempt to unravel the complex metabolic pathway of ascorbic acid in citrus, we investigated the involvement of different genes and/or different pathways (Figure 1) and studied the correlation between the AsA content and transcript levels during the whole fruit development and ripening process
The differences in AsA accumulation in juice and leaves suggest that diverse mechanisms involved in AsA metabolism may act or differently in varying plant tissues, indicating probable tissue specificity, as already described [65]
Summary
Ascorbic acid (AsA) acts as a powerful antioxidant. It works as a cofactor for enzymes [1,2] and, participates in epigenetic modifications [3]. AsA is essential for human health and is involved in many metabolic processes in the body: it protects from cancer, stimulates the immune system, and has many other important effects [9]. This molecule is widely used in vitamin supplements, in the food industry as a preservative to maintain plant tissues, and in derivatives (juices, vegetable pulp, etc.) with bright colours, preventing the oxidation of pigments [10,11]. The content of ascorbic acid in citrus varies according to the species and part of the fruit [16]. Czech et al [17] reported average values of ascorbic acid content, ranging from 50.71 ± 5.21 mg 100 g−1 to 30.33 ± 3.52 mg 100 g−1 in the pulp and in the peel of the fruit, respectively
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