Drainage Recycling Ratio and Fruit Developmental Stages Affect Metabolite Profiles and Mineral Accumulation in Strawberries Under Hydroponics Cultivation

Drainage recycling management in recycling hydroponic systems affects root-zone conditions and fruit metabolism. However, its interaction with fruit developmental stages on strawberry biochemical quality and mineral content is unclear. Therefore, this study investigated how developmental stages and drainage recycling ratio influence antioxidant properties, pigment contents, carbohydrate accumulation, and mineral composition in hydroponically grown 'Seolhyang' strawberry. The experiment was conducted from 20 September 2024 to 31 March 2025 in a smart greenhouse. Plants were grown in coconut coir in a closed hydroponic system with 30%, 50%, 70% and 100% (open-system) drainage recycling ratios. Fruits were harvested at green, white, turning, and red stages on March 11, 2025. Fruit developmental stages primarily determined biochemical composition, while drainage recycling ratio modified traits in a stages-dependent way. Antioxidant properties including total phenolic content (TPC), total flavonoid content (TFC), ferric reducing antioxidant power (FRAP), and 2,2-diphenyl-1-picrylhydrazy (DPPH) radical-scavenging activity were highest in green fruit, especially under 30% drainage, reaching 45.8 mg GAE g⁻¹ FW for TPC and 5.5 mg QE g⁻¹ FW for TFC. Conversely, anthocyanin and soluble sugar accumulation increased during ripening, with 50% drainage recycling yielding the highest concentrations in red fruit (30.0 and 17.8 mg g⁻¹ DW, respectively) and lower starch accumulation. Chlorophyll a, chlorophyll b, and total chlorophyll were also highest in green fruit and enhanced by 30% drainage. Mineral responses were less pronounced and primarily driven by developmental stages. Correlation analysis showed associations among phenolic compounds, antioxidant capacity, pigments, Ca, and Mg, while soluble sugars were negatively associated with early-development traits. Partial least squares analysis identified DPPH, TPC, TFC, FRAP, and anthocyanins as key variables distinguishing developmental stages and drainage recycling treatments. Drainage recycling management does not have a single optimum for strawberry fruit quality; its effects depend on the developmental target. A 30% drainage recycling regime favored phenolic-antioxidant characteristics in early-stages fruit, while 50% drainage recycling enhanced sugar accumulation and pigmentation at full ripeness. In contrast, a 70% drainage recycling ratio promotes the accumulation of Na⁺, Cl⁻, and SO₄²⁻, disrupts nutrient balance, whereas an open system showed lower phytochemical accumulation. This stages-specific response helps guide drainage recycling management based on the harvest goals and shows that immature strawberries may be a useful source of phytochemicals.