## 4 Discussion  
The L-galactose pathway is the main pathway for the synthesis of ascorbic acid (VC) in plants. L-Galactose dehydrogenase galactose1-dehydrogenase (NADP\({}^{+}\)) catalyzes the oxidation of L-galactose to form L-galactono-1,4-lactone (GaIL), while L-galacturonic acid-1,4-lactone dehydrogenase (GaILDH) catalyzes GaIL, resulting in the synthesis of VC [47]. It has been reported that the content of VC in pepper fruit treated with exogenous NO was closely related to the activity of l-galactose-1,4-lactone dehydrogenase, in which the increased GaILDH activity promoted the accumulation of VC in the fruit [48]. In this study, NO treatment significantly promoted the contents of soluble sugar, fructose, sucrose, and the sugar-acid ratio. This is consistent with the results of [PERSON] et al. [49]. Glucose is the final product of starch hydrolysis, but starch in fruit is hydrolyzed to glucose with increasing maturity. In this study, the glucose content in pulp decreased after NO treatment, compared with the control. This is consistent with the reduction in starch granules in shoot and root starch in Arabidopsis seedlings after NO treatment [12]. Polysaccharides are the main components of plant cell walls, including cellulose, hemicellulose, and pectin [50]. Fruit softening is regulated by the degradation of the cell wall, and cell wall-modifying enzymes are responsible for regulating the fruit ripening and softening process, including pectin esterase (PE), polygalacturonase (PG), and cellulase [51]. In this study, NO treatment decreased the activities of PG and PE, but increased the activity of cellulose, which inhibited the reduction of polysaccharides in the cell wall and the degradation of protopectin. [PERSON] et al. [52] indicated that NO inhibited the demethyl esterification of homogalacturonic acid catalyzed by PE, and delayed the further hydrolysis of the cell wall by PG. [PERSON] et al. [53] believed that the undesirable taste and texture of citrus fruits were due to the high cellulose content in citrus pulp. Amino acids are important indicators of fruit flavor and nutritional value, and they are closely related to human taste perception [54]. The aromatic amino acids include phenylalanine, tyrosine, and tryptophan, which are the precursors of many phenolic compounds [55]. Aspartate and glutamate contribute to the umami taste, while alanine, glycine, and serine contribute to the sweet taste [56]. In this study, the amino acid with the highest content in blood orange fruits was proline. However, NO treatment significantly reduced the content of proline and increased the contents of aromatic amino acids (phenylalanine, tyrosine, and tryptophan), umami amino acids (aspartic acid and glutamate), and sweet amino acids (glycine and serine). This may be because NO treatment increased pyruvate dehydrogenase (PDH) activity, promoted the degradation of proline to glutamate [57], and activated the shikimic acid pathway, which catalyzes the formation of shikimic acid and aromatic amino acids [55]. Organic acids are important components of fruit flavor and nutritional quality, and they have important nutritional value for the human body [58]. Citric acid, malic acid, and succinic acid were the main organic acids affecting the flavor characteristics of citrus juice [59]. In this study, the content of citric acid in blood orange fruit was highest, followed by malic acid and succinic acid. Bagging and NO treatments both improved the content of organic acids, and NO treatment had the most obvious effect. It has been reported that a high-concentration SNP treatment increased the activities of citrate synthase (CS), NAD-malate dehydrogenase (NAD-MDH), and phosphoenolpyruvate phosphatase (PEPP) and promoted the accumulation of citric acid in citrus roots with aluminum deficiency [58]. SNP treatment also increased the toxicity of succinic acid, propionic acid, butyric acid, oxalic acid, formic acid, malic acid, malonic acid, and benzoic acid under a boron stress in wheat buds and roots [60].