Watermelon is an important fruit that is widely distributed around the world. In particular, the production and consumption of watermelon in China ranks first in the world. Watermelon production is severely affected by a variety of biotic and abiotic stresses during cultivation, and polyploidization can promote stress resistance and yield. However, the morphological and physiological characteristics of tetraploid watermelon and the underlying molecular mechanisms are still poorly understood. In this study, we revealed that the leaves, fruits, and seeds of tetraploid watermelon were significantly larger than those of the diploid genotype. Some physiological characteristics, including photosynthetic rate (Pn) and stomatal conductance (Gs), were greater, whereas the intercellular CO₂ concentration (Ci) and transpiration rate (Tr) were lower in tetraploid than in diploid watermelon. Two-dimensional gel electrophoresis combined with tandem mass spectrometry (MALDI-TOF/TOF) was performed to compare proteomic changes between tetraploid and diploid watermelon. A total of 21 differentially expressed proteins were identified; excluding the identical proteins, 8 proteins remained. Among them, four proteins were upregulated and four were downregulated in tetraploid versus diploid genotypes. qRT-PCR results showed inconsistencies in gene expression and protein accumulation, indicating a low correlation between gene expression and protein abundance. Generally, this study extends our understanding of the traits and molecular mechanisms of tetraploid watermelon and provides a theoretical basis for watermelon polyploid breeding.

Watermelon is an important fruit widely cultivated in tropical and temperate regions across the world. Particularly in China, both the production and consumption of watermelon lead the world. Watermelon is popular all over the world because it is rich in a variety of nutrients. Despite its widespread appeal, watermelon cultivation is frequently challenged by biological and abiotic stresses, which can significantly impair both yield and fruit quality. Grafting is a technique known to enhance the yield and quality of watermelons, yet the identification of suitable rootstocks and a comprehensive understanding of the cytological, physiological, and biochemical aspects of grafting remain elusive. In this study, we had identified the ‘Heiniu’ pumpkin variety as the optimal rootstock for watermelon grafting. Furthermore, we had determined that the most appropriate stages for scion and rootstock grafting are the one-true-leaf stage and the two-true-leaf stage, respectively. During the grafting process, the necrotic layer of the graft junction gradually dissipated, facilitating a tight connection between the scion and rootstock. Concurrently, there was a notable fluctuation in the levels of key plant hormones and protease activity throughout the healing process of the graft union, suggesting that these factors were pivotal to the success of watermelon grafting. Additionally, our findings indicated that grafting can significantly increase the sugar and titratable acid content within watermelons. This research contributes to a deeper comprehension of the histological and physiological mechanisms underlying watermelon grafting. It offers both theoretical insights and practical guidance essential for the advancement of watermelon grafting techniques.