Vegetal-derived biostimulants distinctively command the physiological and metabolomic signatures of lettuce grown in depleted nitrogen conditions

Biostimulants are sustainable inputs that can be used to reduce chemical fertilizers dependency while reinforcing nutrient uptake, yield and quality of crops, and modulate plant metabolic processes. Protein hydrolysates (PHs) are prominent biostimulants that guarantee a reduction in yield loss under sub-optimal nitrogen (N) conditions. On these bases, a new Malvaceae-derived PH product was tested along a commercial legume-derived PH on soilless greenhouse-grown lettuce, to comparatively assess their activity under N depletion conditions (1 mM NO3−). Both PHs increased biometric parameters under optimal but to a lesser extent under depleted N conditions. Legume-derived PH promoted greater Fv/Fm, lutein and β-carotene under optimal N conditions and higher catalase and total phenolic acids. In contrast, Malvaceae-derived PH did not affect phenolic acids but increased leaf concentration of Ca, Mg and catalase while reducing H2O2. Biochemical changes were then evaluated through untargeted metabolomics. Metabolomics showed a hierarchically prevalent effect of the N level, with the PHs showing distinctive reprogramming under optimal and depleted N conditions. Among others, phenylpropanoids were mainly down-accumulated in stressed plants, while (polyunsaturated fatty acids) PUFA accumulated following the application of PHs. Notwithstanding, the severe depletion of N cannot be compensated by PHs treatment since biostimulants are used to complement fertilizers use and not to replace it.