Biostimulation of Hydroponic Lettuce With Milk‐Derived Protein Hydrolysate Under Seasonal Nutrient Limitation: Enhancement of Physicochemical and Flavor Attributes

Protein hydrolysates (PHs), including milk‐derived PH (MPH), are sustainable biostimulants that can enhance crop yield, quality, and stress tolerance in hydroponic systems while reducing chemical fertilizer dependance. The MPH tested was produced by acid hydrolysis, giving a high degree of hydrolysis and a profile dominated by free amino acids, principally glutamic acid, aspartic acid, proline, leucine, and valine, with organically bound nitrogen and minerals. Cos lettuce ( Lactuca sativa L.) was grown across cool, summer, and rainy seasons under a half‐strength control (HS), HS with 1 mL L −1 MPH (HS + MPH1), and full‐strength with 1 mL L −1 MPH (FS + MPH1). HS + MPH1 increased fresh yield over HS in all seasons (+13% cool, +25% summer, +46% rainy); the largest relative gain occurred in the rainy season, whereas the highest absolute yield and strongest overall response occurred in the cool season. In the cool season, HS + MPH1 raised total phenolics and flavonoids (+61%–62%), enriched hydroxycinnamic and hydroxybenzoic acid derivatives and flavonoids, and markedly increased catalase (CAT, +496%), peroxidase (POD, both +137%), and superoxide dismutase (SOD, +60%). All minerals increased, particularly calcium, magnesium, iron, phosphorus, and potassium (+19%–56%), along with the dominant amino acids aspartic, glutamic, and leucine (+52%–59%). HS + MPH1 also lowered nitrate (−15% to −22%) and bitterness‐associated sesquiterpene lactones (SL, −48% to −62%), while FS + MPH1 added minerals but did not exceed HS + MPH1 for growth or quality. The summer and rainy seasons retained quality under suboptimal conditions, with HS + MPH1 showing the greatest relative gain in protective metabolites in summer (flavonoids + 135%). Our findings support MPH as a nitrogen‐centered biostimulant, with its readily assimilable amino acids serving as organic nitrogen reservoirs and protein‐synthesis substrates, supporting nitrogen assimilation, osmoprotection, and phenylpropanoid pathways. Multivariate analysis supported HS + MPH1 as the most effective treatment, an environmentally friendly strategy to optimize Cos lettuce yield, quality, and seasonal adaptability under reduced nutrient input.