Molecular Plant Breeding 2025, Vol.16, No.4, 241-249 http://genbreedpublisher.com/index.php/mpb 243 4.3 Regulation of enzymatic activity in sugar biosynthesis under different humidity levels Changes in humidity can also affect the activity of enzymes related to sugar metabolism. Studies have found that in melons planted in spring (with low humidity), the contents of soluble sugar, sucrose phosphate synthase (SPS), and sucrose synthase (SS) are all higher than those in autumn (with high humidity), indicating that a low-humidity environment is more favorable for the activities of these enzymes and conducive to sugar synthesis and accumulation (Diao et al., 2022). As the fruit ripens, the activities of enzymes such as acid invertase, neutral invertase and sucrose synthase usually increase first and then decrease. Humidity affects the amount of sugar in the fruit indirectly by influencing their activity (Wu et al., 2020). 5 Humidity Control Technologies in Protected Melon Cultivation 5.1 Passive vs. active ventilation systems Passive ventilation mainly relies on the structure of the greenhouse, such as the roof and side Windows, to allow air to circulate naturally, thereby regulating temperature and humidity. Active ventilation uses equipment such as fans to forcibly drive the air, achieving a faster and more obvious regulatory effect. Seo et al. (2022) pointed out that relying solely on rooftops and side Windows for ventilation has very limited effects in summer, as the temperature cannot be lowered and the humidity is difficult to control. If active ventilation is added, such as using circulating fans, the situation will improve a lot. Seo et al. (2022) also found that when the atomization system is used in conjunction with the circulating fan, it can reduce the temperature in the greenhouse by 3.2°C to 7.0°C and increase the humidity by 12% to 28%. This is beneficial for the melon to grow taller and can also enhance its sweetness and texture. Using fans for ventilation in high temperatures also helps reduce pests and diseases, allowing plants to grow better. 5.2 Fogging, misting, and dehumidification devices Fogging and misting devices can increase air humidity and counteract the negative impacts brought by high temperatures and droughts. Experiments show that this method can help melons grow faster and have better fruit quality. However, if the humidity is too high, the fruit weight and sugar content of melons will decrease instead. Therefore, the humidity should be controlled within an appropriate range (Jeenprasom et al., 2019). When the humidity is too high, using dehumidification equipment can lower it, prevent diseases from occurring, and also protect the quality of the fruits. Different humidity control methods will all have an impact on the growth time, fruit weight and stress resistance of melons. Reasonable humidity regulation can not only make the fruits grow better, but also prolong the harvest time and increase the total yield (Adinegara et al., 2017). 5.3 Sensor-based smart control systems for humidity optimization Nowadays, many melon greenhouses have begun to adopt intelligent control systems that combine the Internet of Things (IoT) and various sensors. These systems can monitor temperature, humidity and light in real time, and then automatically control devices such as fans, atomizers and heaters to achieve relatively precise regulation (Figure 1) (Parenreng et al., 2024; Supriyanto et al., 2025). Furthermore, machine learning algorithms such as XGBoost and VAR have also been used to predict environmental changes and make regulatory responses in advance, which helps melons accumulate sugar and improve quality (Jeon et al., 2024). Some low-cost and easy-to-operate intelligent systems have also begun to be promoted, enabling small and medium-sized farmers to use scientific humidity management solutions. 6 Genotypic Variation in Melon Response to Humidity 6.1 Differential sugar accumulation under varying humidity among cultivars Different melon varieties show significantly different performances under changes in humidity. When the humidity is high, some varieties grow larger fruits and have a longer harvest time. However, some varieties perform better in drought or low-humidity environments, not only being drought-resistant but also having a relatively high yield (Adinegara et al., 2017). The sugar content is greatly influenced by the interaction between environmental humidity and varieties. Neto et al. (2025) demonstrated that different melon hybrids respond differently to humidity, with some being particularly sensitive to it and others having strong adaptability.
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