## 1 Introduction  
Tarocco blood orange (_Citrus sinensis_ (L) [PERSON]) is rich in anthocyanins, flavonoids, and other phenolic substances that contribute to the specific color or taste of the fruit [1]. These substances are also of interest in the human diet, due to their antioxidant activities [2]. Consumers are primarily looking for the inner color of the fruit in blood oranges because the pigment is associated with health benefits [3]. However, the quality of blood oranges has degraded over long-term cultivation in some countries, resulting in a less sweet and more sour taste, as well as decreased blood color [4; 5]. Compared with some high-quality sweet oranges, the flavor, quality, market price, and economic benefits of blood orange fruit are low. Therefore, improving blood orange fruit quality and increasing economic income through chemical methods are worth further research.  
Nitric oxide (NO) is a highly active substance that acts as an antioxidant and biological messenger in plant systems. NO is a signal molecule with multiple functions, such as regulating flowering transition [6], promoting photosynthesis [7], and participating in fruit ripening [8; 9]. The fruit ripening process is related to reactive oxygen species (ROS) production and increased lipid peroxidation. Exogenous NO prevents the elevation of H\({}_{2}\)O\({}_{2}\) levels and lipid peroxidation, and it combines with 1-aminocyclopropane-1-carboxylate (ACC) oxidase (ACO) to form a binary complex (ACO-NO), which further combines with ACC to produce a stable ACC-ACO-NO complex. ACC-ACO-NO inhibits the increase in abscisic acid (ABA) and ethylene levels [10; 11], delays fruit senescence, inhibits the activity of cell wall softening enzymes (polygalacturonase, pectin methylesterase, and spectate lyase), delays pectin degradation, and maintains fruit firmness [8].  
The main components of fruit sweetness come from fructose and sucrose. It was previously reported that NO inhibited the activity of cellular glycolytic enzymes and ATP synthase through S-nitrosylation modification and reduced the activities of acetyl coenzyme A, ATP, ADP-glucose, and UDP-glucose. This ultimately led to the inhibition of polysaccharide biosynthesis, the accumulation of monosaccharides, and the increased sweetness of Arabidopsis plants [12]. NO also increased the concentration of sugars in pear and papaya [8].  
Anthocyanins are important for the color development of mature fruit. It has been reported that NO could inhibit anthocyanin degradation by changing the normal structure of prophenoloxidase (PPO) active sites in fruit, inhibiting PPO activity, increasing the activities of enzymes in the anthocyanin synthesis pathway, and promoting the production of flavonoids and the accumulation of anthocyanins [13; 14].  
Currently, NO treatment is widely used to maintain postharvest fruit quality. NO could delay the quality reduction of postharvest mango [15], Japanese plum [16], and grape [17]. However, whether it can be further applied to fruit trees to improve fruit quality is still unknown. Nitric oxide is a byproduct of nitrate reduction [6; 7]. In addition to direct NO treatments, nitrate treatments on different fruits have been also documented. For example, pre-harvest nitrate application provided adequate storage N to apple trees with no detriment to fruit quality [18]. The optimal mixed nitrogen nitrate and ammonium treatments increased fructose levels in grape berries [19].  
NO treatment has been shown to improve sugar accumulation and limit anthocyanin degradation [8; 12; 13; 14], which supports our strategy to improve the quality of blood orange fruit by NO treatments on living blood orange trees. In addition, the mechanisms by which NO improves fruit flavors were further explored in this study.