Bioscience Evidence 2024, Vol.14, No.6, 281-292 http://bioscipublisher.com/index.php/be 283 By understanding these morphological characteristics and structural adaptations, researchers and cultivators can better appreciate the resilience and versatility of dragon fruit, which contributes to its increasing popularity and cultivation in various regions around the world. 3 Photosynthetic Metabolic Characteristics of Dragon Fruit (Hylocereus spp.) 3.1 Overview of CAM photosynthesis in dragon fruit Dragon fruit (Hylocereus spp.) utilizes Crassulacean Acid Metabolism (CAM) photosynthesis, a specialized form of photosynthesis adapted to arid conditions (Xu et al., 2024). CAM plants, including dragon fruit, fix CO2 at night, which is a reversal of the typical daytime CO2 fixation seen in C3 and C4 plants. This nocturnal CO2 fixation helps minimize water loss, making CAM photosynthesis particularly advantageous in drought-prone environments (Yamori et al., 2014). The CAM pathway in dragon fruit involves a complex regulatory network of genes, including those associated with the circadian clock, which orchestrates the timing of metabolic processes to optimize photosynthetic efficiency (Ma et al., 2021). 3.2 Carbon fixation patterns In dragon fruit, the CAM pathway is characterized by distinct day and night phases of carbon fixation. At night, CO2 is fixed into organic acids, primarily malate, which is stored in vacuoles. During the day, the stored malate is decarboxylated to release CO2 for the Calvin cycle, enabling photosynthesis to proceed while stomata remain closed to conserve water (Wang et al., 2019). This diurnal rhythm is regulated by a suite of genes, including MALATE DEHYDROGENASE (MDH), MALIC ENZYMES (ME), and PYRUVATE PHOSPHATE DIKINASE (PPDK), which exhibit peak expression levels at specific times of the day and night, ensuring efficient carbon fixation and utilization (Ma et al., 2021). 3.3 Adaptation strategies under drought and high light conditions Dragon fruit (Hylocereus spp.) has evolved several adaptation strategies to thrive under drought and high light conditions. One of the most significant adaptations is the utilization of the Crassulacean Acid Metabolism (CAM) photosynthetic pathway, which minimizes water loss by opening stomata during the cooler night hours, thereby reducing transpiration (Wang et al., 2019; Lee et al., 2023). This pathway allows the plant to maintain photosynthetic activity even under extreme environmental stress. Additionally, dragon fruit exhibits morphological traits such as thick, waxy cladodes that reduce water loss and reflect excess light. The plant's ability to modulate the expression of CAM-related genes in response to environmental stress further enhances its resilience. For instance, genes involved in the CAM pathway show rhythmic expression patterns that are finely tuned to environmental cues, optimizing water use efficiency and photosynthetic performance under varying conditions (Abirami et al., 2021; Ma et al., 2021). 3.4 Comparison with photosynthesis in other fruit trees Compared to other fruit trees that primarily utilize C3 or C4 photosynthesis, dragon fruit's CAM pathway offers distinct advantages in arid environments. C3 plants, which include many temperate fruit trees, fix CO2 directly through the Calvin cycle during the day, leading to higher water loss through open stomata. C4 plants, such as maize, have a more efficient CO2 fixation mechanism that reduces photorespiration but still primarily operates during the day (Yamori et al., 2014). In contrast, the nocturnal CO2 fixation in CAM plants like dragon fruit significantly reduces water loss, making them better suited to hot, dry climates. This unique adaptation not only supports the survival of dragon fruit in challenging environments but also contributes to its ability to produce high yields of nutrient-rich fruit under conditions that would be detrimental to many other fruit trees (Abirami et al., 2021; Ma et al., 2021). 4 Growth Cycle and Developmental Physiology 4.1 Growth stages The seedling stage of dragon fruit (Hylocereus spp.) begins with germination, where seeds sprout and develop into young plants. This stage is crucial for establishing a strong root system and initial shoot growth. The early
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