Molecular Plant Breeding 2025, Vol.16, No.4, 241-249 http://genbreedpublisher.com/index.php/mpb 242 2.3 Temporal pattern of sugar composition during ripening During the process of fruit ripening, the sugar content inside will also gradually change. At the beginning, reducing sugar and starch were mainly used. When the fruit was nearly ripe, the activity of sucrose synthase and sugar transporter became stronger, and sucrose would increase rapidly and become the main sugar. The signal transduction of ethylene and some transcription factors (such as CmMYB44, CmERFI-2) can regulate the key genes of sucrose synthesis, thereby facilitating sucrose accumulation (Gao et al., 2023). In addition, environmental factors such as low temperature can also affect ethylene production and the activity of glucose metabolism enzymes, thereby changing the rate and total amount of sugar accumulation (Yang et al., 2025). 3 Environmental Factors Affecting Sugar Accumulation in Protected Cultivation 3.1 Overview of microclimatic variables: temperature, light, CO2, and humidity Temperature can directly affect the activity of enzymes that control sugar synthesis, regulate ethylene production, and influence sucrose accumulation and fruit ripening (Gao et al., 2023; Yang et al., 2025). Light provides energy and is the foundation of photosynthesis. The sugar produced by the leaves is transported to the fruits by it. Ren et al. (2023) thought that the concentration of carbon dioxide affects the rate of photosynthesis and indirectly influences the synthesis and transportation of sugar. Humidity affects the opening and closing of stomata, the intensity of transpiration and the flow of water, ultimately influencing whether sugar can be smoothly transported into the fruit. 3.2 Unique challenges of humidity control under enclosed conditions In facilities like greenhouses and sheds, humidity regulation can be more troublesome. Due to the closed space and poor air circulation, it is very easy for the humidity to be too high or too low. These changes will affect the accumulation of sugar. When the humidity is too high, transpiration weakens and sugar is not easily transported away from the leaves. When the humidity is too low and there is insufficient moisture, the fruit cells may not function properly (Ren et al., 2023). Moreover, humidity is also related to the occurrence of diseases and the improvement of fruit quality, making its management more complicated. 3.3 Interactions between humidity and other abiotic factors At low temperatures, if the humidity changes, it may affect the production of ethylene, influence some genes that regulate sugar, and thereby affect the sucrose content and the flavor of melons (Gao et al., 2023; Yang et al., 2025). Humidity may also alter the activity of glucose metabolism enzymes and glucose transport proteins, which can affect how sugar is distributed and stored in the fruit (Ren et al., 2023). Buczkowska et al.’s research in 2023 found that humidity also plays a role in conjunction with irrigation. When the two work well together, they can significantly increase the total sugar and reducing sugar content in melon fruits. 4 Role of Humidity in Melon Sugar Metabolism 4.1 Influence on transpiration, stomatal conductance, and plant water status Environmental humidity directly affects the transpiration rate and stomatal conductance of melons. When the air humidity is high, it will weaken evaporation and reduce water loss. However, in this case, the stomatal conductance of the leaves may also decrease, and the efficiency of water and nutrient transport will be affected. Jeenprasom et al. (2019) found that under high humidity conditions, the plant height and leaf area of melons were slightly smaller than those under normal humidity conditions, indicating that appropriate transpiration is actually beneficial for maintaining water balance and normal growth. 4.2 Impact on sugar transport via phloem loading and unloading Humidity can also affect the accumulation of sugar in melon fruits, as it can alter the efficiency of sugar transport in the phloem. Under high humidity, the TSS content of the fruit will significantly decrease, indicating that high humidity may cause the sugar in the leaves to be unable to transport or transport slowly, resulting in less sugar in the fruit (Jeenprasom et al., 2019). Appropriate humidity can maintain the pressure gradient in the phloem, which is crucial for the smooth flow of sugar to the fruit.
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