International Journal of Horticulture, 2025, Vol.15, No.5, 234-241 http://hortherbpublisher.com/index.php/ijh 236 3.2 Major sugars and their metabolic pathways The major sugars of Pitaya are glucose, fructose and sucrose. The biochemical metabolism of the sugars involves some key enzymes like vacuole acid converting enzymes (VAI), neutral converting enzymes (NI), and sucrose synthase (SS). These enzymes' actions on sugar metabolism play a vital role in setting the storage of fruit sugar content, of which glucose is the most dominant sugar in mature Pitayas. Sugar degradation and synthesis are strictly regulated in order to keep the ideal equilibrium of sugar during fruit development and ripening (Ren et al., 2023). 3.3 Role of key sugar metabolic enzymes Enzymes such as VAI, NI and SS play a role in the metabolism of fructose in bird's nest. VAI and NI hydrolyze sucrose into glucose and fructose, which enables these sugars to accumulate in the fruit (Liu et al., 2024). SS not only involves itself in the process of sugar synthesis, but also in catalyzing the process of sugar degradation, regulating dynamically sugar levels during fruit growth. Their expression is strictly connected to the expression of specific genes, which are regulated by a broad range of transcription factors (Li et al., 2018). 3.4 Transcription factors and signaling pathways involved in sugar accumulation Transcription factors such as HpDof1.7, HpDof5.4 and HpWRKY3 were discovered to be the key controllers of fructose deposition in bird's nest by the study. The transcription factors can stimulate gene expression involved in sugar metabolism, such as HpSuSy1 and HpINV2, which play crucial roles in sugar conversion and deposition. Besides that, HpDof1.7 and HpDof5.4 also contribute to enhancing sugar transporter gene functions (such as HpTMT2 and HpSWEET14) and further increasing sugar accumulation. They are part of complex networks that control fine tuning of sugar metabolism, which ultimately affects the fruit quality (Jiang et al., 2023). 4 Antioxidant Metabolism During Pitaya Fruit Ripening 4.1 Accumulation and scavenging mechanisms of reactive oxygen species (ROS) In ripening of Pitayas, elimination and build-up of reactive oxygen species (ROS) plays a crucial role in cell homeostasis. ROS is a normal by-product of cell metabolism, but when not properly regulated, can lead to oxidative damage. During fruit ripening, the balance between the production and scavenging of ROS is relevant to the normal maturation process. ROS regulation is generally performed by non-enzymatic and enzymatic antioxidant systems that work together to counteract oxidative stress and ensure fruit stability and quality (Li et al., 2023). 4.2 Dynamic changes of key antioxidant enzymes Key antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) display dynamic variation in the ripening of the bird's nest. These enzymes play a role in the removal of ROS and protection of the fruit from oxidative injury. Overall, the activity of these enzymes is boosted at the early stage of fruit ripening to fight against rising ROS levels, which may decline when the fruit is fully ripening. This regulation of enzyme activity is crucial to oxidative balance and the formation of sensory qualities of the fruit (Qi et al., 2020). 4.3 Accumulation of non-enzymatic antioxidants Non-enzymatic antioxidants such as vitamin C, phenolic compounds, and flavonoids gradually accumulate during Pitaya fruit ripening. Not only do they enhance the fruit's antioxidant activity, but they also play an important role in the ROS neutralization. Its accumulation tends to be closely associated with the development stage of the fruit, with advanced maturity, higher antioxidant content (Peng et al., 2022). The improvement of these antioxidants not only defends the fruit against oxidative stress, but also improves the nutritional value and marketability of the fruit (Ding et al., 2024). 4.4 Key antioxidant genes and their regulatory mechanisms During Pitaya ripening, the regulation of major antioxidant genes is strictly regulated. They code for enzymatic and non-enzymatic antioxidants and are regulated by several transcription factors and environmental factors. For
RkJQdWJsaXNoZXIy MjQ4ODYzNA==