Metabolomic and physiological insights into the biostimulant activity of Chlorella vulgaris extract in hydroponic lettuce under salinity stress

• C. vulgaris extract enhanced lettuce growth and biomass production under both normal and salinity stress conditions, restoring stress-affected parameters to control levels. • The extract application improved nitrate balance under both normal and salinity stress conditions. • The extract mitigates salinity-induced oxidative stress, reflected by reduced MDA accumulation together with elevated phenolic, flavonoid, and overall antioxidant capacity. • Metabolomic profiling revealed extract-driven accumulation of osmolytes and central energy metabolites, indicative of improved osmotic adjustment and stress resilience. The application of microalgal biostimulants has emerged as a sustainable strategy to enhance crop performance, improve stress tolerance, and reduce the environmental footprint of agricultural practices. The present study investigates the biostimulant potential of Chlorella vulgaris extract on lettuce ( Lactuca sativa L.) grown hydroponically under both normal and salinity stress conditions. Two extract concentrations (CV10 and CV50) were foliarly applied, and their effects on plant growth, nutrient balance, stress tolerance, and metabolomic responses were assessed. Both concentrations significantly promoted lettuce growth under normal conditions and effectively mitigated the negative impacts of salinity stress. Application of C. vulgaris extract improved nitrate balance and reduced nitrate accumulation in both normal and stressed plants, thereby enhancing nutritional quality. The extract also alleviated oxidative stress induced by salinity, boosting antioxidant capacity by increasing phenolic and flavonoid content. Metabolomic analysis revealed that C. vulgaris triggered distinct metabolic reprogramming, particularly at the higher concentration (CV50), influencing processes related to osmotic adjustment, energy metabolism, and secondary metabolite biosynthesis. Salinity-stressed plants treated with CV50 exhibited significant accumulation of key sugars, amino acids, organic acids, and phenylpropanoids associated with stress resilience. These findings underscore the multifaceted role of C. vulgaris extract in improving lettuce yield, physiological performance, stress tolerance, and nutritional quality, while also reducing nitrate content.