1 Introduction

Pitaya, commonly known as dragon fruit, belongs to the genus Hylocereus and is native to Central America, such as Mexico, El Salvador, Guatemala, and Honduras. The primary species include Hylocereus costaricensis, H. undatus, and H. megalanthus, which are cultivated extensively in tropical and subtropical regions worldwide, including Brazil and Turkey (Constantino et al., 2021; Attar et al., 2022). The fruit is celebrated for its vibrant appearance, sweet taste, and high nutritional value, making it a popular choice for fresh consumption and industrial processing into beverages and desserts (Lin et al., 2021). The economic value of pitaya has surged in recent years due to its increasing popularity and demand in both local and international markets (Chapai et al., 2024).

The flavor and nutritional quality of pitaya are critical factors that influence its market competitiveness and consumer acceptance. The sweetness and overall taste of the fruit are primary determinants of consumer preference, which directly impacts market demand (Jiang et al., 2023). Additionally, the nutritional profile, including antioxidant capacity and phenolic content, enhances the fruit's appeal as a health-promoting food. High levels of antioxidants and phenolic compounds, particularly in red-fleshed varieties, contribute to the fruit's health benefits and market value (Attar et al., 2022). Therefore, understanding and improving these attributes are essential for maintaining and expanding pitaya's market presence.

Sugars and organic acids play pivotal roles in determining the flavor, texture, and nutritional value of pitaya. The primary sugars found in pitaya include glucose, fructose, and sucrose, which contribute to the fruit's sweetness and overall taste profile (Constantino et al., 2021; Xie et al., 2022). Organic acids, such as malic and citric acids, influence the fruit's acidity and flavor balance. The metabolic processes governing the accumulation of these compounds are complex and involve various genes and biochemical pathways (Xie et al., 2022; Jiang et al., 2023). For instance, the HuSWEET family of sugar transporters has been identified as crucial in regulating sugar accumulation during fruit development (Jiang et al., 2023). Additionally, the interplay between sugars, organic acids, and other metabolites, such as betalains, further affects the fruit's quality and visual appeal (Wu et al., 2019).

This study systematically explores the impact of sugar and organic acid metabolism on the flavor and nutritional quality of pitaya, focusing on the biochemical traits and metabolic pathways that determine its sensory and nutritional characteristics. The study intends to uncover key factors that enhance the sweetness, overall taste, and health benefits of pitaya. These findings are expected to provide valuable insights for future research and practical applications in pitaya cultivation and processing, ultimately improving its market competitiveness and consumer acceptance.

2 Composition and Metabolism of Sugars in Pitaya

2.1 Analysis of types and content of sugars

Pitaya exhibits a diverse composition of sugars that significantly influence its flavor and nutritional quality. The primary sugars identified in various pitaya species include glucose, fructose, and sucrose. For instance, in the yellow-peel pitaya species ‘Wucihuanglong’ (Hylocereus undatus) and ‘Youcihuanglong’ (Hylocereus megalanthus), glucose is the dominant sugar in ‘WCHL’, while ‘YCHL’ contains higher levels of sucrose, fructose, and glucose (Xie et al., 2022; Xu et al., 2024).

Hylocereus undatus presents a total sugar content of 9.83%, with glucose and fructose being the major components (Wei et al., 2019; Constantino et al., 2021). The study found that glucose is the primary sugar in mature fruit, followed by fructose, while sucrose content is relatively low and exhibits minimal variation. During the later stages of fruit development (30 to 40 days after artificial pollination), the levels of glucose and fructose significantly increased, reaching 12.2-fold and 6-fold of their initial levels, respectively (Figure 1). In contrast, H. megalanthus has a lower total sugar concentration of 5.93%, with a notable presence of sucrose. Compared to H. polyrhizus, H. undatus has lower total phenolic content and antioxidant potential but stands out in terms of sugar composition. The flesh of H. undatus, with its low acidity and high sugar content, is particularly suitable for making desserts and beverages (Arivalagan et al., 2021). These variations in sugar composition among different pitaya varieties are crucial for determining their sweetness and overall flavor profile.

Figure 1 Glucose is the main sugar in mature pitaya fruit. (A) Photograph of pitaya fruit at different developmental stages. (B) Changes of soluble sugars (glucose, fructose, and sucrose) contents during fruit maturation. Fruit at 16, 21, 26, 30, 35, and 40 days after artificial pollination (DAAP) was sampled for analysis. Data represent mean values from three biological replicates (±S.E.) (Adopted from Wei et al., 2019)

2.2 Sugar metabolism pathways and key enzymes

The metabolism of sugars in pitaya involves several biochemical pathways and key enzymes that regulate the synthesis, breakdown, and transport of sugars. Key enzymes such as sucrose synthase (HpSuSy1) and invertase (HpINV2) play pivotal roles in the hydrolysis of sucrose into glucose and fructose, which are the predominant sugars in pitaya (Wei et al., 2019; Luo et al., 2021). The expression of these enzymes is regulated by transcription factors like HpWRKY3, which activates the transcription of HpSuSy1 and HpINV2, thereby enhancing sugar accumulation during fruit maturation.

The SWEET family of sugar transporters, particularly HuSWEET12a and HuSWEET13d, are crucial for sugar transport and accumulation in pitaya fruits. These transporters facilitate the movement of sugars across cell membranes, contributing to the overall sugar content in the fruit (Jiang et al., 2023). The regulatory mechanisms involving Dof transcription factors (HpDof1.7 and HpDof5.4) further modulate the expression of sugar metabolism-related genes, enhancing the activities of HpSuSy1, HpINV2, and sugar transporter genes like HpTMT2 and HpSWEET14 (Mou et al., 2022; Zou and Sun, 2023).

2.3 Influence of sugar metabolism on flavor and sweetness

The metabolism of sugars in pitaya significantly impacts its flavor and sweetness, which are critical determinants of fruit quality and consumer preference (Gou et al., 2023). The accumulation of glucose and fructose, facilitated by the activities of HpSuSy1 and HpINV2, directly correlates with the sweetness of the fruit (Wei et al., 2019; Wang et al., 2024). The expression of HuSWEET12a and HuSWEET13d during fruit maturation also contributes to the increased sugar content, enhancing the sweetness and overall flavor profile of pitaya (Xie et al., 2022).

Furthermore, the regulatory role of Dof transcription factors in activating sugar metabolic pathway genes underscores the complex interplay between sugar metabolism and flavor development in pitaya (Mou et al., 2022). The balance between different sugars, such as the higher sucrose content in ‘YCHL’ pitaya and the dominant glucose in ‘WCHL’ pitaya, also influences the perceived sweetness and flavor nuances of different pitaya varieties. Thus, understanding the biochemical pathways and regulatory mechanisms of sugar metabolism is essential for improving the flavor and nutritional quality of pitaya fruits.

3 Organic Acid Metabolism in Pitaya

3.1 Types and content of organic acids in pitaya

Pitaya fruits contain several organic acids that significantly contribute to their flavor profile. The primary organic acids identified in various pitaya species include malic acid and citric acid. For instance, Hylocereus costaricensis, H. undatus, and H. megalanthus have been reported to contain malic acid concentrations of 0.83%, 0.71%, and 0.62%, respectively, and citric acid concentrations of 0.37%, 0.36%, and 0.40%, respectively (Constantino et al., 2021). Additionally, ascorbic acid (vitamin C) content varies among different pitaya species, with some studies noting its presence in significant amounts during certain developmental stages (Xie et al., 2022).

The content of organic acids in pitaya can vary significantly across different varieties and developmental stages. For example, ‘Wucihuanglong’ (Hylocereus undatus) and ‘Youcihuanglong’ (Hylocereus megalanthus) pitayas exhibit different profiles of organic acid accumulation during fruit maturation. 'Wucihuanglong' pitaya has higher ascorbic acid content compared to ‘Youcihuanglong’ pitaya (Xie et al., 2022). Furthermore, the concentration of malic and citric acids also differs, with ‘Wucihuanglong’ primarily accumulating malic acid and ‘Youcihuanglong’ accumulating citric acid. This variability is crucial for breeding programs aimed at enhancing specific flavor profiles and nutritional qualities.

3.2 Enzymatic pathways of organic acid metabolism

The metabolism of organic acids in pitaya involves several key enzymes that regulate their synthesis and degradation. Enzymes such as malate dehydrogenase and citrate synthase play pivotal roles in the tricarboxylic acid (TCA) cycle, which is central to organic acid metabolism. Transcriptome analyses have identified numerous genes associated with these metabolic pathways, highlighting their importance in the regulation of organic acid content in pitaya (Yin et al., 2015; Xie et al., 2022). Additionally, enzymes involved in the ascorbate-glutathione cycle, such as ascorbate peroxidase, are crucial for maintaining ascorbic acid levels in the fruit (Li et al., 2018).

The regulation of organic acid metabolism in pitaya is complex and involves multiple layers of control, including transcriptional regulation. Studies have shown that the expression of genes involved in organic acid metabolism is tightly regulated during fruit development (Schvartzman et al., 2018). For instance, the expression of genes encoding enzymes like malate dehydrogenase and citrate synthase is modulated in response to developmental cues and environmental factors. Additionally, transcription factors such as WRKY and Dof have been implicated in the regulation of sugar and organic acid metabolism, further highlighting the intricate regulatory networks governing these processes (Wei et al., 2019; Mou et al., 2022).

3.3 Role of organic acids in flavor and acidity

Organic acids are key contributors to the tartness and overall flavor profile of pitaya. Malic acid and citric acid, in particular, are responsible for the characteristic tartness of the fruit. The balance between these acids and sugars determines the perceived sweetness and acidity, which are critical factors influencing consumer preference (Constantino et al., 2021). The presence of ascorbic acid also adds a subtle tanginess, enhancing the fruit's flavor complexity (Xie et al., 2022).

The balance of organic acids in pitaya significantly affects consumer preference. Fruits with a higher ratio of sugars to organic acids are generally perceived as sweeter and more palatable. For instance, H. undatus, with its higher sugar content and lower acidity, is preferred for fresh consumption and in desserts and beverages (Fernandes et al., 2018; Arivalagan et al., 2021; Singh et al., 2023). Conversely, H. megalanthus, which has a more balanced sugar-acid profile, is recommended for individuals on carbohydrate-restricted diets due to its lower overall sugar content (Constantino et al., 2021). Understanding the interplay between organic acids and sugars is essential for developing pitaya varieties that meet diverse consumer preferences.

4 Interaction between Sugar and Organic Acid Metabolism

4.1 Balance between sugar and acid content

The sugar-to-acid ratio is a critical determinant of fruit taste, influencing consumer preferences and overall flavor perception. In pitaya, the balance between sugars such as glucose, fructose, and sucrose, and organic acids like malic and citric acids, plays a significant role in defining the fruit's taste profile (Constantino et al., 2021; Xie et al., 2022). Higher sugar content generally enhances sweetness, while organic acids contribute to tartness, creating a complex flavor profile that is appealing to consumers. The specific ratios of these components can vary significantly among different pitaya species and cultivars, affecting their marketability and consumer acceptance.

The balance between sugars and acids in pitaya changes dynamically during fruit ripening. Early stages of fruit development are characterized by higher starch accumulation, which is later converted into soluble sugars as the fruit matures. Concurrently, the concentration of organic acids tends to decrease, leading to a sweeter taste in fully ripened fruits (Xie et al., 2022). This shift in sugar-acid balance is crucial for achieving the desired flavor and nutritional quality in pitaya. For instance, fruits harvested at an advanced color stage exhibit higher soluble sugar concentrations and lower acidity compared to those harvested at the color-break stage, resulting in superior taste quality (Hua et al., 2018; Sobral et al., 2019).

4.2 Influence on pitaya’s overall flavor profile

The interaction between sugars and organic acids contributes significantly to the complexity of pitaya's flavor profile. Sugars such as glucose and fructose are predominant in mature pitaya, while malic and citric acids are the main organic acids present (Constantino et al., 2021; Xie et al., 2022). This combination of sweet and tart flavors creates a balanced and complex taste that is highly valued by consumers. The presence of other metabolites, such as phenolic compounds and flavonoids, further enhances the flavor complexity by adding additional layers of taste and aroma (Wu et al., 2019; Xie et al., 2022).

Consumer preference for pitaya is largely influenced by the sensory perception of its flavor, which is determined by the balance of sugars and acids. Studies have shown that consumers tend to prefer pitaya varieties with higher sugar content and moderate acidity, as these attributes contribute to a more pleasant and palatable taste (Wei et al., 2019; Constantino et al., 2021). The sensory attributes of pitaya, including sweetness, tartness, and overall flavor intensity, are key factors in determining its market acceptance and consumer satisfaction.

4.3 Regulatory mechanisms involved in sugar-acid interactions

The metabolism of sugars and organic acids in pitaya is regulated by a complex interplay of genetic and environmental factors. Specific genes, such as those encoding for sugar transporters and metabolic enzymes, play crucial roles in the accumulation and distribution of sugars and acids during fruit development (Wei et al., 2019; Mou et al., 2022; Jiang et al., 2023). Environmental conditions, including temperature and storage conditions, also significantly impact the sugar-acid balance in pitaya. For example, fruits stored at lower temperatures tend to have lower sugar concentrations and higher acidity, affecting their taste quality.

Hormonal regulation is another critical aspect of sugar and organic acid metabolism in pitaya. Hormones such as ethylene, auxins, and abscisic acid are known to influence fruit ripening and the associated metabolic pathways (Durán-Soria et al., 2020). These hormones interact with sugar signaling mechanisms to modulate the expression of genes involved in sugar and acid metabolism, thereby affecting the overall flavor and nutritional quality of the fruit (Li et al., 2018; Durán-Soria et al., 2020). For instance, methyl jasmonate (MeJA) treatment has been shown to enhance sugar metabolism and phenolic accumulation in pitaya, contributing to improved flavor and quality.

5 Factors Influencing Sugar and Organic Acid Metabolism

5.1 Genetic factors influencing metabolism

Genetic variability plays a crucial role in determining the sugar and organic acid profiles in pitaya. Different pitaya species exhibit distinct sugar and acid compositions, which are influenced by the expression of specific genes. For instance, in yellow-peel pitayas, glucose is the dominant sugar in 'WCHL' pitaya, while sucrose, fructose, and glucose are prevalent in ‘YCHL’ pitaya. Malic and citric acids are the main organic acids in 'WCHL' and 'YCHL' pitayas, respectively (Xie et al., 2022). This genetic diversity underscores the importance of genetic factors in shaping the metabolic profiles of pitaya fruits.

Several key genes have been identified that regulate sugar and organic acid metabolism in pitaya. The Dof transcription factors HpDof1.7 and HpDof5.4 are known to enhance the expression of sugar metabolism-related genes such as HpSuSy1 and HpINV2, as well as sugar transporter genes HpTMT2 and HpSWEET14, thereby facilitating sugar accumulation (Mou et al., 2022). Additionally, the WRKY transcription factor HpWRKY3 has been shown to activate the expression of HpINV2 and HpSuSy1, further contributing to sugar accumulation during fruit maturation (Wei et al., 2019). The SWEET family genes, particularly HuSWEET12a and HuSWEET13d, also play significant roles in sugar accumulation in pitaya fruits (Figure 2) (Jiang et al., 2023).

Figure 2  Expression patterns of HuSWEETs in pitaya pulps. (A) Expression analyses of 19 HuSWEET genes in pitaya pulps during fruit development. (B) RT-qPCR analyses of 19 HuSWEETs in mature fruits of pitaya. The data were shown as the means ± SDs of three independently biological replicates. The different letters above bars indicate significant differences at the p < 0.05 level according to Duncan’s multiple comparison tests (Adopted from Jiang et al., 2019) Image caption: The figure shows that most HuSWEET genes exhibit a downward expression trend during fruit development, especially HuSWEET7a, HuSWEET7b, and HuSWEET12b, while the expression of HuSWEET12a, HuSWEET12d, and HuSWEET13d increases significantly as the fruit matures, reaching the highest levels in 35-day mature fruits. Combined with sugar accumulation data, the significant positive correlation between the expression of HuSWEET12a and HuSWEET13d and sugar content suggests that these genes may play critical roles in sugar transport and accumulation (Adapted from Jiang et al., 2019)

5.2 Environmental conditions and cultivation practices

Environmental conditions such as temperature, light, and soil quality significantly influence the metabolism of sugars and organic acids in pitaya. Optimal temperature and light conditions are essential for the proper functioning of metabolic pathways that govern sugar and acid synthesis. Soil conditions, including nutrient availability and pH, also affect the metabolic processes. For example, the accumulation of betalains, which are associated with sugar and organic acid metabolism, is influenced by these environmental factors (Wu et al., 2019).

Irrigation and fertilization practices are critical in maintaining the balance between sugars and organic acids in pitaya fruits. Adequate irrigation ensures the proper hydration of the plant, which is necessary for the efficient transport and metabolism of sugars and acids. Fertilization provides essential nutrients that support the metabolic pathways involved in sugar and acid synthesis (Diógenes et al., 2022; Oliveira et al., 2022). The balance between these practices can significantly impact the flavor and nutritional quality of the fruit.

5.3 Post-harvest handling and its impact on metabolism

Post-harvest handling, particularly storage conditions, plays a vital role in maintaining the metabolic stability of sugars and organic acids in pitaya. Storage temperature and humidity levels can affect the rate of metabolic reactions, leading to changes in the sugar and acid content of the fruit. Proper storage conditions are essential to preserve the flavor and nutritional quality of pitaya (Wu et al., 2019).

During ripening and storage, significant changes occur in the sugar and acid content of pitaya fruits. The levels of glucose, fructose, and sucrose increase as the fruit matures, while the content of organic acids such as malic and citric acids may decrease. These changes are regulated by the expression of specific genes involved in sugar and acid metabolism. For instance, the expression of HpINV2 and HpSuSy1 increases during fruit maturation, correlating with the elevated accumulation of glucose and fructose (Wei et al., 2019). Understanding these changes is crucial for optimizing post-harvest handling practices to ensure the best possible flavor and nutritional quality of pitaya fruits.

6 Impact on Nutritional Quality

6.1 Contribution of sugars to nutritional value

Sugars are a primary source of energy in pitaya, contributing significantly to its caloric content. The predominant sugars in pitaya are glucose and fructose, with sucrose also present in some species (Mou et al., 2022; Xie et al., 2022). For instance, Hylocereus undatus contains approximately 9.83% total sugars, distributed as 4.75% glucose and 5.08% fructose, while Hylocereus megalanthus has 5.93% total sugars, including 0.99% glucose, 3.25% fructose, and 1.69% sucrose (Constantino et al., 2021). These natural sugars provide essential energy, making pitaya a valuable fruit for quick energy replenishment.

The sugar composition significantly impacts the flavor and palatability of pitaya. The sweetness of the fruit, primarily due to glucose and fructose, is a crucial determinant of consumer preference (Wei et al., 2019; Jiang et al., 2023). The balance of these sugars enhances the overall taste, making pitaya a desirable fruit for fresh consumption and in processed forms such as beverages and desserts (Constantino et al., 2021). The presence of sucrose in Hylocereus megalanthus, for example, contributes to its perceived sweetness, despite its lower total sugar content, making it suitable for carbohydrate-restricted diets while still being sensorially appealing.

6.2 Nutritional significance of organic acids

Organic acids in pitaya, such as malic and citric acids, play a vital role in digestion and overall health. These acids can enhance digestive processes by stimulating the production of digestive enzymes and improving nutrient absorption (Xie et al., 2022). Additionally, organic acids can help maintain the pH balance in the stomach, promoting a healthy digestive tract. Organic acids also exhibit antioxidant properties, contributing to metabolic health. For instance, malic acid and citric acid found in pitaya have been shown to possess antioxidant activities, which can help neutralize free radicals and reduce oxidative stress (Li et al., 2018; Constantino et al., 2021). This antioxidant capacity is crucial for preventing chronic diseases and supporting overall metabolic functions.

6.3 Effect on consumer health and preferences

The nutritional profile of pitaya, rich in natural sugars and organic acids, influences health-related consumer choices. Consumers seeking natural energy sources and digestive health benefits may prefer pitaya due to its balanced sugar content and the presence of beneficial organic acids. Moreover, the antioxidant properties of these components make pitaya an attractive option for health-conscious individuals (Li et al., 2018; Constantino et al., 2021).

The balance of sugars and organic acids in pitaya significantly affects its taste and marketability. A higher sugar content enhances sweetness, while the presence of organic acids adds a refreshing tartness, creating a complex flavor profile that appeals to a wide range of consumers (Xie et al., 2022; Jiang et al., 2023). This balance not only improves the sensory attributes of the fruit but also increases its market value, making it a popular choice in both fresh and processed forms (Constantino et al., 2021). The ability to maintain quality attributes through treatments like methyl jasmonate further enhances the marketability of pitaya by preserving its taste and nutritional benefits during storage and processing (Li et al., 2018).

7 Case Studies

7.1 Molecular regulatory mechanisms of sugar accumulation in pitaya

The sweetness of pitaya fruit is primarily determined by the content of soluble sugars, including glucose, fructose, and sucrose. The metabolism and accumulation of sugars are regulated by various enzymes and their corresponding genes, such as sucrose synthase (SuSy) and invertase (INV). Studies have shown that transcription factors play a critical role in regulating the expression of sugar metabolism-related genes (Wei et al., 2019; Mou et al., 2022).

Mou et al. (2022), through qPCR, dual-luciferase reporter assays, and gel mobility shift assays, revealed the functions of two Dof transcription factors in pitaya, HpDof1.7 and HpDof5.4, providing important insights into the regulatory mechanisms of sugar metabolism. These transcription factors bind to the promoters of sugar metabolism genes (HpSuSy1 and HpINV2) and sugar transporter genes (HpTMT2 and HpSWEET14), activating their expression and promoting the synthesis and accumulation of glucose and fructose (Figure 3). The results demonstrate a significant positive correlation between the expression of HpDof1.7 and HpDof5.4 and sugar accumulation during pitaya maturation, providing a theoretical foundation for enhancing fruit sweetness through molecular breeding.

Figure 3 Molecular characterization of HpDof1.7 and HpDof5.4. (A) Expression patterns of HpDof1.7 and HpDof5.4 during pitaya maturation. (B) Subcellular localization assay of HpDof1.7 and HpDof5.4. Bars=50 μm. (C) Transcriptional activation assay of HpDof1.7 and HpDof5.4 in yeast cells. (D) Schematic representation of reporter and effector vectors constructed for transcriptional activity analysis. (E) Transcriptional activity analysis of HpDof1.7 and HpDof5.4 in vivo. The data are normalized to a value of 1 for the 62SK-BD group. 62SK-BD-VP16 served as positive control. Each value represents means ± SE of six biological replicates (*P<0.05, **P<0.01, compared to 62SK-BD) (Adopted from Mou et al., 2022) Image caption: The figure shows that the expression levels of the transcription factors HpDof1.7 and HpDof5.4 gradually increase with fruit maturation and are primarily localized in the nucleus, indicating their role in transcriptional regulation. Yeast two-hybrid and dual-luciferase assays further validated the transcriptional activation activity of HpDof1.7 and HpDof5.4, demonstrating their ability to significantly enhance the activity of target gene promoters. Overall, the results confirm that these two factors act as positive regulators, directly activating the expression of sugar metabolism and transport genes, thereby promoting sugar accumulation in pitaya and providing crucial molecular support for its sweetness and quality formation (Adapted from Mou et al., 2022)

7.2 Molecular mechanism of citramalic acid metabolism in pitaya

Organic acids in pitaya fruit not only influence flavor but also play a critical role in its nutritional value and market acceptance. As an unconventional organic acid, citramalic acid dominates during the early stages of fruit development. A study focusing on four pitaya cultivars—'GHB,' 'GHH,' 'WCHL,' and 'YCHL'—explored the metabolic mechanism of citramalic acid and the roles of key associated genes through metabolomics, gene expression analysis, and subcellular localization experiments (Chen et al., 2022).

The results showed that citramalic acid content peaked during the early fruit development stages (S3–S4) and gradually decreased thereafter. The expression of the HuIPMS2 gene was highly correlated with citramalic acid levels, and its encoded protein was localized to the chloroplast. It is hypothesized that HuIPMS2 regulates the citramalic acid biosynthesis pathway, thereby influencing the early fruit flavor. Pitaya cultivars 'WCHL' and 'GHB' exhibited higher citramalic acid accumulation, consistent with the expression trend of HuIPMS2, suggesting that this gene may impact fruit flavor by modulating citramalic acid synthesis.

7.3 Research on enhancing pitaya flavor by optimizing sugar-acid balance

Yellow-peel pitaya is highly favored by consumers due to their unique appearance, sweet flavor, and rich nutritional content. A study focusing on two yellow-peel pitaya species, ‘WCHL’ (H. undatus) and ‘YCHL’ (H. megalanthus), utilized metabolite profiling and transcriptomic analyses to uncover the changes in sugars and organic acids during fruit development and their molecular regulatory mechanisms (Xie et al., 2022). The findings revealed that the dominant flavor components in ‘WCHL’ pitaya are glucose and malic acid, whereas sucrose, fructose, and citric acid predominate in ‘YCHL’. Starch accumulation was prominent during the early developmental stages, transitioning into soluble sugars during maturation.

Total phenols and flavonoids were abundant in the early stages, while ascorbic acid levels were notably higher in ‘WCHL’ (Figure 4). Transcriptomic analyses identified 27 key genes, such as PMI1 and PMI3, associated with ascorbic acid biosynthesis, and 18 critical genes, including SuSy and FRK, involved in sugar metabolism. Additionally, the expression of key genes related to the TCA cycle, such as PEPC3 and CMS1, significantly impacted organic acid metabolism.

Figure 4 Schematic representation of sugar and organic acid metabolites during fruit developmental stages in two yellow-peel pitayas. Phe: phenolic; Fla: flavonoid; AA: ascorbic acid; Glu: glucose; Suc: sucrose; Fru: fructose; Gal: galactose; Ino: inositol; Sor: sorbitol; Mal: malic; Cit: citric; CM: citromalic; Fum: fumaric (Adopted from Xie et al., 2022)

The results indicate that the flavor formation in yellow-peel pitayas is influenced not only by sugar and acid metabolism but also by dynamic changes in gene expression. These findings provide a scientific foundation for optimizing fruit quality and developing high-flavor pitaya cultivars.

8 Strategies to Enhance Pitaya Flavor and Nutritional Quality

8.1 Variety breeding

Variety breeding has shown significant progress in improving the sugar and organic acid content of pitaya, which are crucial for its flavor and nutritional quality. Research has identified key genes involved in sugar and organic acid metabolism, such as those encoding for sucrose-hydrolyzing enzymes and sugar transporters. For instance, the expression of genes like HpSuSy1 and HpINV2 has been linked to increased glucose and fructose levels in pitaya (Wei et al., 2019; Mou et al., 2022; Xie et al., 2022). Additionally, the identification of transcription factors such as HpDof1.7, HpDof5.4, and HpWRKY3, which regulate these metabolic pathways, provides new avenues for breeding programs aimed at enhancing fruit sweetness and overall quality. The development of new pitaya cultivars with optimized sugar and organic acid profiles can significantly improve the fruit's taste and consumer appeal (Constantino et al., 2021).

8.2 Cultivation management techniques

Cultivation management techniques play a pivotal role in regulating sugar and organic acid metabolism in pitaya. Fertilization, irrigation, light exposure, and temperature control are critical factors that influence these metabolic processes. For example, studies have found that the application of potassium fertilizer increased pitaya yield and improved fruit quality, particularly by enhancing the sugar-acid ratio and soluble solids content, indicating that potassium plays a promotive role in sugar and acid metabolism (Fernandes et al., 2018). The combination of organic and inorganic fertilizers significantly improved the sugar-acid ratio and soluble sugar content in pitaya, further highlighting the critical role of fertilization methods in regulating sugar and acid metabolism (de Jesus et al., 2022).

Moreover, The use of methyl jasmonate (MeJA) as a pre-treatment has been found to maintain ascorbic acid and organic acid levels while enhancing phenolic accumulation, which contributes to the fruit's nutritional quality and flavor (Li et al., 2018; Serna-Escolano et al., 2020). Effective management of these cultivation parameters can lead to improved sugar and organic acid balance, thereby enhancing the overall quality of pitaya fruits.

8.3 Post-ripening and storage techniques

Post-ripening and storage techniques significantly influence the sugar and organic acid content of pitaya, affecting its flavor and nutritional quality. Proper postharvest treatment and storage conditions can help maintain or even enhance these attributes. For instance, the use of controlled atmosphere storage and temperature management can prevent the degradation of sugars and organic acids, thereby preserving the fruit's taste and nutritional value (Li et al., 2018). The application of plant oil-based coatings (Sta-Fresh®) and carnauba wax coatings (Endura-Fresh™) has been shown to effectively reduce water loss and skin wrinkling in pitaya, extending its shelf life to 15 days. These treatments help maintain the appearance, texture, and nutritional attributes of pitaya, such as soluble solids and acidity, under low-temperature conditions (7 °C, 85% relative humidity) (Razali et al., 2021).

Additionally, treatments with compounds like p-Anisaldehyde significantly reduce fruit senescence and preserve higher antioxidant enzyme activity (Xu et al., 2021). Blue light treatment can markedly slow the reduction in respiration rate and acidity in pitaya, while increasing the levels of certain organic acids and soluble sugars. Gaining deeper insights into these postharvest technologies is crucial for optimizing the flavor and nutritional quality of pitaya during storage and transportation.

9 Concluding Remarks

Studies have shown that sugar and organic acid metabolism play a crucial role in determining the flavor and nutritional quality of pitaya fruit. Glucose, fructose, and sucrose are the primary sugars, while malic acid and citric acid are the predominant organic acids. The interactions among these compounds significantly affect the sweetness, sourness, and overall flavor balance of dragon fruit, which is crucial for consumer acceptance and market competitiveness. Key transcription factors such as HpDof1.7, HpDof5.4, and HpWRKY3 regulate sugar accumulation by activating sugar metabolic genes like HpSuSy1 and HpINV2. Additionally, the application of methyl jasmonate (MeJA) has been shown to enhance phenolic accumulation and maintain fruit quality under stress conditions. Nutritional analysis further indicates that red-fleshed pitaya varieties exhibit higher antioxidant capacities compared to white-fleshed ones.

The findings have several implications for pitaya cultivation and quality improvement. Understanding the genetic and biochemical pathways regulating sugar and organic acid metabolism can facilitate the development of new cultivars with enhanced flavor and nutritional quality. For instance, key transcription factors and sugar transporters (such as HuSWEET12a and HuSWEET13d) provide targets for genetic engineering to increase sugar content. Additionally, exploring the application of MeJA treatments can improve post-harvest quality and extend shelf life by maintaining higher levels of phenolic compounds and sugars. Optimizing cultivation techniques based on the nutritional characteristics of different pitaya varieties can also meet specific market demands, such as high-antioxidant varieties for health-conscious consumers or high-sugar varieties for the dessert industry.

Future research should continue to delve into the molecular mechanisms of sugar and organic acid metabolism, particularly the interactions between key transcription factors and metabolic genes, to uncover more precise regulatory networks. Simultaneously, the integration of multi-omics approaches (such as transcriptomics, metabolomics, and epigenomics) should be employed to explore the metabolic characteristics of different pitaya cultivars, enabling the development of specialty varieties tailored to diverse market demands. Additionally, innovative post-harvest treatments and storage technologies, such as bioactive coatings or light-quality treatments, are recommended to extend the shelf life of pitaya fruits and enhance storage quality. These studies have the potential to further improve the quality and market competitiveness of pitaya, providing new impetus for the high-quality development of the industry.

Acknowledgments

The author extends special thanks to Dr. X. Fang, Professor and Director of the Hainan Institute of Tropical Agricultural Resources, for reviewing the manuscript and providing valuable suggestions for revision. The author also sincerely appreciates the two anonymous peer reviewers for their comprehensive evaluation of the manuscript and constructive feedback.

Funding

This study was supported by Hainan Province Science and Technology Special Fund (Grant No: ZDYF2021XDNY120).

Conflict of Interest Disclosure

The authors affirm that this research was conducted without any commercial or financial relationships that could be construed as a potential conflict of interest.

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