Grafting has been reported as a factor regulating the metabolome of a plant. Therefore, a comprehensive metabolic profile and comparative analysis of metabolites were conducted from fully mature fruit of pumpkin-grafted watermelon (PGW) and a self-rooted watermelon (SRW). Widely targeted LC-ESI-MS/MS metabolomics approach facilitated the simultaneous identification and quantification of 339 metabolites across PGW and SRW. Regardless of grafting, delta-aminolevulinic acid hydrochloride, sucrose, mannose-6-phosphate (carbohydrates), homocystine, 2-phenylglycine, s-adenosyl-L-homocysteine (amino acids and derivatives), malic, azelaic, H-butanoic acid ethyl ester-hexoside isomer 1, (organic acids), MAG (18:3) isomer1, LysoPC 16:0, LysoPC 18:2 2n isomer (lipids) p-coumaric acid, piperidine, and salicylic acid-o-glycoside (secondary metabolites) were among the dominant metabolite. Dulcitol, mono-, and disaccharide sugars were higher in PGW, while polysaccharides showed complex behavior. In PGW, most aromatic and nitrogen-rich amino acids accumulated greater than 1.5- and 1-fold, respectively. Intermediates of the tricarboxylic acid cycle (TCA), stress-related metabolites, vitamin B5, and several flavonoids were significantly more abundant in PGW. Most lipids were also significantly higher in grafted watermelon. This is the first report providing a comprehensive picture of watermelon metabolic profile and changes induced by grafting. Hence, the untargeted high-throughput LC-ESI-MS/MS metabolomics approach could be suitable to provide significant differences in metabolite contents between grafted and ungrafted plants.

Watermelon fruits of different ploidy levels exhibit significant variations in texture. This study aimed to investigate the primary factors that influence texture differences. To achieve this, we conducted an investigation into the differences in fruit texture development using homozygous autogamous diploid (2×) lines and their autotriploid (3×) and autotetraploid (4×) lines ‘Yixuan’. The study investigated fruit development, flesh texture profile analysis (TPA), flesh cell wall polysaccharide content, and flesh cell microstructure analysis in 2×, 3×, and 4× watermelon fruits. The study found that as watermelon fruits matured, several characteristics increased, including fruit weight, TSS (total soluble solids) content, rind hardness, flesh cell size, and cell wall polysaccharide contents such as crude fiber, cellulose, hemicellulose, lignin, and protopectin. In contrast, the flesh texture parameters and soluble pectin content of the flesh cell wall decreased over time. Significant differences in fruit flesh texture and flesh cell structure were observed among fruits of different ploidy levels at the 32 DAP. Specifically, the 2× fruits displayed considerably lower rind hardness, reduced flesh texture (Hardness, Fracturability, Chewiness, Gumminess), and cell density compared to the 3× and 4× fruits. Additionally, the 2× fruits had larger cell sizes than the 3× and 4× fruits. However, there were no significant differences observed in the flesh cell wall polysaccharide contents across various ploidy levels. These findings suggest that the variation in texture among watermelon fruits of different ploidy levels can be attributed to the size and arrangement of the flesh cells. This research provides a foundation for the further exploration of the intrinsic regulatory factors and molecular mechanisms contributing to texture variation in polyploid watermelon fruits.

Watermelon (Citrullus lanatus) is one of the most nutritional fruits that is widely distributed in the whole world. The nutritional compositions are mainly influenced by the genotype and environment. However, the metabolomics of different domestication status and different flesh colors watermelon types is not fully understood. In this study, we reported an extensive assessment of metabolomic divergence in the fruit flesh among Citrullus sp. and within Citrullus sp. We demonstrate that metabolic profiling was significantly different between the wild and cultivated watermelons, the apigenin 6-C-glucoside, luteolin 6-C-glucoside, chrysoeriol C-hexoside, naringenin C-hexoside, C-pentosyl-chrysoeriol O-hexoside, and sucrose are the main divergent metabolites. Correlation analysis results revealed that flavonoids were present in one tight metabolite cluster. The main divergent metabolites in different flesh-colored cultivated watermelon fruits are p-coumaric acid, 2,3-dihydroflavone, catechin, N-(3-indolylacetyl)-l-alanine, 3,4-dihydroxycinnamic acid, and pelargonidin o-hexoside. A total of 431 differentially accumulated metabolites were identified from pairwise comparative analyses. C. lanatus edible-seed watermelon (cultivars) and C. mucosospermus (wild) have similar fruit metabolic profiles and phenotypic traits, indicating that edible-seed watermelon may be a relative of wild species and a relatively primitive differentiation type of cultivated watermelon. Our data provide extensive knowledge for metabolomics-based watermelon improvement of Citrullus fruits meet their enhanced nutritive properties or upgraded germplasm utility values.