Systemic root-zone melatonin application balances vegetative biomass with postharvest biochemical antioxidant capacity in soilless lettuce production

Intensive soilless culture lettuce (Lactuca sativa L.) production is frequently associated with succulent tissue formation, which is prone to rapid postharvest deterioration, creating a trade-off between biomass maximization and supply chain resilience. In this study, we investigated the potential of using melatonin applied via fertigation to overcome this trade-off in cv. 'Lollo Bionda'. The factorial experiment was carried out in which plants were treated with five concentrations of melatonin (0, 25, 50, 75, and 100 μM) throughout the growth cycle and stored at 4°C for 15 days. Results indicated a hormetic dose-response, with 50 μM serving as a biostimulant that enhanced fresh weight by 3.3% and increased canopy area. Conversely, treatment with 100 μM led to a minor growth penalty, which significantly improved postharvest quality. Root-zone application at the highest dose effectively arrested the degradation of chlorophyll, maintained higher hue angles and reduced weight loss by 50% compared to the controls during early storage. At the biochemical level, the 100 μM treatment resulted in a delayed increase in total phenolic content (maximum at day 9) and a significant upregulation of the enzymatic antioxidant system (superoxide dismutase and catalase and peroxidase). Importantly, melatonin treatment prevented the linear increase in Polyphenol Oxidase (PPO) activity, the main cause of enzymatic browning, in controls to near basal levels (0.14 U mg^-1 protein). These findings suggest that root-zone melatonin application establishes a systemic "enhanced biochemical antioxidant capacity," where a specific threshold (100 μM) shifts resource allocation from vegetative expansion to defense activation, offering a practical physiological intervention to extend the shelf life of soilless greens.