The accumulation of anthocyanin in blood oranges depends on a large diurnal temperature difference. At present, blood oranges grown in tropical/subtropical climates are generally not colored well, which limits the geographical cultivation of blood oranges. And the low sugar-acid ratio of blood oranges also reduces their flavor. To explore the approaches to improve the accumulation of sugar and anthocyanin in maturing blood oranges, Tarocco blood oranges (230 days after flowering, 230 DAF) were treated with 1 g/L fulvic acid (FA), and then high-performance liquid chromatography (HPLC) was used to test the content of sugar and organic acid in Tarocco blood oranges at different maturity stages. The expression of genes involved in the related metabolic pathways was tested by quantitative real-time PCR (qRT-PCR). The results showed that preharvest application of fulvic acid could promote pulp coloring and significantly increase the content of anthocyanin, total sugar, fructose, glucose, sucrose in blood oranges. At 324 DAF, the anthocyanin content of FA treated group was 48.92 mg/L, increased by 153.7% compared to the CK group. After FA treatment, the expressions of 4CL, CHS, DFR, ANS, UFGT, GST were significantly up-regulated. Meanwhile, the total sugar, fructose, glucose, and sucrose content of FA treated group were 75.33, 20.87, 20.22, 33.84 mg/g respectively, which increased by 10.2%, 15.1%, 14.5%, 6.7% respectively compared to the CK group. Using FA treatment, the expression of AI was significantly up-regulated. However, FA treatment had no significant effect on the content of organic acid in maturing blood oranges. Moreover, the CS, AC, IDH had little effect on the accumulation of citric acid. These results suggested that preharvest application of fulvic acid after the veraison stage could activate the expressions of 4CL, CHS, DFR, ANS, UFGT, GST, AI, and promote sugar accumulation and fruit coloring in maturing blood oranges, and finally improves the fruit quality of blood oranges.
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