Nano-SiO2 and Light Quality Synergistically Regulate External Morphology, Postharvest Coloration, Endogenous Hormonal Metabolism, and Nutritional Quality in Mature-Green Tomatoes

Mature-green tomatoes are prone to rapid ripening and quality deterioration during the postharvest stage, highlighting the urgent need for environmentally friendly and efficient preservation technologies. This study investigated the synergistic regulatory effects of nano-SiO2 and light quality (white light, W; blue light, B; red/blue mixed light, RB, 1:1) on postharvest appearance, physiological processes, and quality attributes in ‘Yu Zhu’ (Solanum lycopersicum L.), a tasty tomato cultivar with light-yellow fruit color. Mature-green fruits were treated with light quality in combination with nano-SiO2 (pre-immersion in 1 mL/L nano-SiO2 for 1 h, followed by periodic spraying with 0.5 mL/L nano-SiO2 every two days). Key indicators—including ripening traits, flavor attributes, antioxidant capacity, and endogenous hormone metabolites—were monitored on their respective sampling days. The results revealed distinct light quality-dependent responses: (1) B-Si (B + nano-SiO2) significantly delayed the breaker stage compared to W, maintained the lowest water loss, and exhibited the slowest softening rate. W-Si showed a significantly higher dry weight-to-fresh weight ratio than W. (2) RB-Si achieved superior flavor quality, with 11.47% soluble solids, 1.62% titratable acidity, and a sugar-to-acid ratio of 7.2—values markedly higher than those in RB. (3) RB-Si increased total phenolic (TP), flavonoids, and ascorbic acid (AsA) levels relative to RB, while enhancing total antioxidant capacity (T-AOC) and the activities of superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD), with only slight suppression of ascorbate peroxidase (APX) activity. (4) Nano-SiO2 differentially regulated hormonal metabolism depending on light quality: it activated the jasmonic acid (JA)–gibberellin (GA) pathway under W light, fine-tuned cytokinin (CK) metabolism under B light, and upregulated JA, GA, CK, and auxin under RB light. Moreover, RB-Si significantly reduced ACC accumulation compared to W, thereby delaying senescence. Collectively, RB-Si synergistically regulates endogenous hormone metabolism to simultaneously delay ripening, reduce water loss, maintain firmness, optimize flavor, and enhance antioxidant capacity. This study elucidates the interaction mechanism between nano-SiO2 and light quality, providing theoretical and technical support for the green preservation of horticultural crops.