Yuan  
2 Key Laboratory of Biology and Genetic Improvement of Horticultural Crops (South China), Ministry of Agriculture and Rural Affairs, College of Horticulture, South China Agricultural University, Guangzhou 510642, China; [EMAIL_ADDRESS] (C.C.C. ); [EMAIL_ADDRESS] (J.C.); yyy9740282006163.com (Y.Y. ); [EMAIL_ADDRESS] (Q.H.) Yuan  
2 Key Laboratory of Biology and Geneticnutritional characteristics. In general, sugar and organic acid metabolism processes are developed during fruit development and maturation with sugar accumulation and organic acid degradation. Besides, sugar and organic acids are crucial components of pitaya fruit quality, including taste, flavor, and pH, which influences the needs of consumers. During fruit development and ripening, several metabolite processes are presented, of which sugar and organic metabolism perform important roles in fruit quality formation. Starch, as the predominant storage carbohydrate in plants, mainly consists of linear amylose and branched amylopectin. Starch is an important index associated with fruit texture, for instance, the softness of guava [5], and smoothness and dry constancy of pumpkin [6]. Starch synthesis and degradation pathway are involved in the regulation of fruit quality [7; 8]. Phosphoglucomutase (PGM), ADP-glucose pyrophosphylase (AGPS) and starch synthase (StSy) are key regulatory enzymes responsible for starch biosynthesis [8; 9; 10]. During starch metabolism, the phosphorylation of amylopectin is an essential step which requires close collaboration of dikinases, the glucan, water dikinase (GWD), and phosphoglucnan, water dikinase (PWD) [7]. Simultaneously, starch is degraded while the sugars are accumulated during fruit development and ripening [3; 6]. The levels of soluble sugars such as glucose, fructose, and sucrose are responsible for fruit sweetness. In citrus, sucrose, fructose, and glucose are the three major carbohydrates which is the main components of total soluble solids [11]. Glucose and fructose are the most abundant molecules in pomegranate [12] and watermelon [13]. Sucrose is the main component of soluble sugars in apricot [14] compared with pear, which is fructose-dominant [15], while red-peel pitaya is glucose-dominant [3; 16]. Sucrose phosphate synthase (SPS) is the key enzyme responsible for sucrose synthesis while invertase (Ivr) and sucrose synthase (SuSy) are the main enzymes involved in sucrose catabolism [17; 18]. Moreover, only hexobilases (HXKs) and fructkinases (FRKs) are capable of catalyzing the essential irreversible phosphorylation of glucose and fructose in plants [19]. Glucose-6-phosphate isomerase (PGI) catalyzes the interconversion between D-glucose-6-phosphate and D-tructose-6-phosphate which performs important roles in glycolysis and gluconeogenesis [20]. The fruit sourness is principally determined by organic acids (mainly citric and malic acids). Citric acid and malic acid are the main organic acids in pomegranates [12]. Malic acid is dominant in red-peel pitaya [3], watermelon [13], and apple [21] while citric acid is dominant in citrus [11] and pear [22]. The glycolytic pathway is the oxidization of glucose to pyruvate, of which phosphoglycerate kinase (PGK), enolase (Eno), and pyruvate kinase (PK) are key enzymes for catalyzing 1,3-bisphosphoglycerate (1,3-BPGA) to produce pyruvate and ATP [23]. The cofactor acetyl coenzyme A, generated from pyruvate by pyruvate dehydrogenase (PDH), is required directly for the tricarboxylic acid (TCA) cycle and other biochemical reactions [24]. Citrate synthase (CS), aconitate hydratase (ACO), succinyl-CoA synthetase (SCS), 2-oxoglutate dehydrogenase (OGDH), dihydroployllysine-residue succinyltransferase (DLST), succinate dehydrogenase (SDH), fumarate hydratase (FUM), and malic dehydrogenase (MDH) are also involved in the TCA cycle [25; 26]. Ascorbic acid (also named ascorbate or vitamin C) is one of the ubiquitous water-soluble low molecular weight antioxidants in fruits, such as guava [5] and kiwifruit [27].