Enhancing lutein content in yellow-fleshed watermelon through potassium-driven regulation of lycopene ?-cyclase (LCYB) expression

Watermelon (Citrullus lanatus) is notably rich in carotenoids, with yellow-fleshed varieties containing high levels of lutein. Lutein accumulation is influenced by genetic, environmental, and cultivation factors. Potassium, a critical macronutrient for photosynthesis and metabolism, plays an unclear role in regulating lutein accumulation in watermelon. To investigate this, foliar sprays of varying potassium chloride (KCl) concentrations were applied and agronomic trait changes during fruit development were monitored. Ultra-performance convergence chromatography (UPC2) was used to quantify changes in lutein content, while RNA-seq was employed to analyze the regulatory mechanisms behind potassium-induced changes in carotenoids. It was found that foliar application of 50 mgL-1 KCl significantly increased fruit weight, soluble solid content, chroma, and lutein levels. qRT-PCR revealed that lycopene β-cyclase (LCYB) expression was upregulated with this treatment, which positively correlated with lutein accumulation. Transcriptome analysis suggested that potassium mediates lutein content through chloroplast-related pathways. In conclusion, optimal potassium concentrations enhance lutein biosynthesis and accumulation in watermelon by regulating key genes involved in carotenoid synthesis and activating chloroplast metabolic pathways.