Assessment of watermelon (Citrullus lanatus) fruit photosynthetic performance based on chlorophyll fluorescence parameter and its impact on yield and quality

• Watermelon fruits retain active photosynthesis until harvest. • Shading reduces chlorophyll, carotenoids, and Rubisco gene expression. • Fruit photosynthesis enhances size, sweetness, color, and yield. • Full shading suppresses yield and quality by up to 62 %. Photosynthetically active nonfoliar organs have garnered increasing attention as supplementary carbon sources in plants. In watermelon ( Citrullus lanatus ), fruits retain their green coloration until harvest, suggesting a potential role in active photosynthesis. However, the specific contribution of fruit photosynthesis to overall yield and quality remains poorly understood. To address this gap, this study aimed to evaluate the effects of different fruit shading treatments on photosynthetic parameters and assess the contribution of fruit photosynthesis to yield and quality. A completely randomized design was used, with three shading levels applied to fruits: no shading (control), half shading, and full shading. The results showed that chlorophyll a , chlorophyll b , and carotenoid contents were significantly higher in control fruits, indicating enhanced photosynthetic capacity. Fluorescence parameters, including effective photochemical yield of photosystem II and electron transport rate, were also markedly higher in fruits exposed to natural light compared with those under shade. Furthermore, expression of the ribulose-1,5-bisphosphate carboxylase/oxygenase large subunit gene—a key component of the photosynthetic pathway was downregulated under shading, particularly under full shading, confirming the negative impact of reduced light on fruit photosynthesis. Consequently, fruit diameter, weight, sweetness, and red color decreased, and the final yield was reduced by 28 % under half shading and by 39 % under full shading, compared to the control. Thus, photosynthetic parameters showed significant positive correlations with yield-related traits, highlighting the important role of fruit photosynthesis in sustaining watermelon productivity and quality. These findings emphasize the need to consider fruit photosynthesis in both cultivation practices and breeding strategies aimed at enhancing overall yield and fruit quality in watermelon.