Oxygen nanobubble drip irrigation differentially regulates yield, ascorbate accumulation, and flavor quality in watermelon

The response of greenhouse watermelon to oxygen nanobubble drip irrigation may depend on the dilution level applied, but the treatment level that optimizes individual yield and fruit-quality traits remains unclear. Here, we evaluated a groundwater control (CK) and three oxygen nanobubble dilution treatments (OB 1 , 1:0; OB 2 , 1:4; OB 3 , 1:16) in greenhouse watermelon by combining yield and fruit-quality measurements with fruit transcriptome analysis. Oxygen nanobubble irrigation increased yield by 8.36%-23.65% and improved single-fruit weight and fruit quality relative to CK, but the optimum treatment differed among target traits. OB 1 produced the highest yield and single-fruit weight; in the autumn crop cycle, it also produced the highest ascorbic acid content, 188.71% higher than CK and 14.15%-83.99% higher than the other oxygen treatments. In contrast, OB 2 gave the best flavor-quality response, producing the highest soluble sugar content and sugar-acid ratio; soluble sugar increased by 6.32%-28.44% relative to CK, and OB 2 exceeded OB 1 and OB 3 by 5.38%-20.64%. Fruit RNA-seq identified treatment-associated expression differences among genes annotated to carbon metabolism, transport and signaling, starch and sucrose metabolism, and ascorbate-related redox processes. OB 1 showed expression differences among genes annotated to AsA biosynthesis and recycling, glutathione metabolism, peroxide detoxification, and NADPH supply, whereas OB 2 showed a distinct carbohydrate-related expression profile alongside the highest soluble-sugar content and sugar-acid ratio. Together, the contrasting yield and fruit-quality outcomes were accompanied by distinct fruit-transcriptome profiles across the oxygen nanobubble-water dilution treatments.