Abstract
Plant-based biostimulants are sustainable tools widely applied in agriculture to enhance plant physiology and stress resilience. Shelf-life extension has a pivotal role in postharvest as low-temperature storage of lettuce ( Lactuca sativa L.) often results in weight loss, surface damage, and a decline in quality and nutritional value. This study investigated the metabolic responses of lettuce to three commercial biostimulants, Ascophyllum nodosum extracts (Algae), plant-derived protein hydrolysate (PHs), and humic substances (Hum), applied via foliar spray (-S) and drip irrigation (-D). All treatments improved agronomic traits, with foliar-applied PHs (PHs-S) showing the most pronounced effects. Metabolomic profiling revealed treatment-specific shifts, particularly in secondary metabolites such as alkylphenols, flavonoids, and flavonoid glucosides, along with increased levels of sugars, polyols, vitamins, and phytohormones. In parallel, biostimulants positively influenced key agronomic parameters during lettuce growth, including plant diameter, chlorophyll content, and nitrogen balance index, with PHs applied via foliar spray showing the most pronounced effects. At harvest, these metabolic changes reflected a broader metabolic reprogramming, which appeared to contribute to improved postharvest traits, including reduced weight loss, enhanced leaf firmness, increased antioxidant capacity, and higher total phenolic content, particularly under Algae-D and PHs-S treatments. These biochemical changes suggest that biostimulants, particularly via foliar application, enhance antioxidant capacity, improve nutritional quality, and may contribute to extended shelf life in lettuce.