Comparative Evaluation of Plant-Derived Protein Hydrolysates as Biostimulants for Enhancing Growth and Mitigating Fe-Deficiency Stress in Tomato

Sustainable agriculture increasingly relies on biostimulants like protein hydrolysates (PHs) to enhance crop resilience. This study characterized and compared three plant-derived PHs (PH1, PH2, and PH3) from the Malvaceae, Brassicaceae, and Fabaceae families, respectively, under optimal (40 µM Fe3+-EDTA) and iron (Fe)-deficient (4 µM Fe3+-EDTA) conditions. Initial assays demonstrated that the PHs possessed significant antioxidant capacity and influenced biological activity: PH2 and PH3 promoted pollen germination, while PH1 exhibited a weaker stimulatory effect. In vivo experiments on tomato plants revealed that PH application effectively modulated root architecture and biomass accumulation. Moreover, PH2 and PH3 significantly mitigated Fe deficiency’s impact, by maintaining biomass and preventing chlorosis. Interestingly, while Fe deficiency typically triggers massive root Fe3+-chelate reductase activity, PH treatments, particularly PH2, significantly down-regulated this response. This suggests that PHs may improve internal Fe use efficiency or facilitate alternative uptake pathways. Overall, these findings establish a link between the intrinsic bioactive properties of PHs and their biostimulant action, highlighting their potential as innovative tools for improving nutrient use efficiency and crop resilience in sustainable farming systems.