Drought stress is one of the major abiotic factors limiting crop growth and yield production. Protein hydrolysates
have been used as plant biostimulants in agriculture due to their positive impacts on plant productivity under
abiotic stress; however, little is known about their roles in inducing drought tolerance and the underlying
mechanisms. Therefore, we investigated the effects of a new pig blood-derived protein hydrolysate (PP) in increasing tomato tolerance to drought stress. We found that foliar PP application reduced the inhibited impacts of drought stress (10% PEG-6000) on tomato growth, as indicated by improved plant growth parameters. Exogenous PP application inhibited the degradation of chlorophyll, maintained chloroplast structures, increased stomatal aperture, and thereby improved photosynthetic rate under drought stress. The higher accumulation of proline, soluble sugars, soluble proteins, inorganic ions including K+, Ca2+, and Mg2+, and subsequently higher
relative water content were observed in PP-treated tomato leaves and roots. Moreover, PP application mitigated oxidative damages of drought affected-tomato plants by increasing the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX) and the accumulation of total phenolic, total flavonoid, ascorbic acid, and glutathione. These findings indicated that foliar PP application could obviously alleviate drought stress by regulating stomatal aperture, chloroplast ultrastructure, osmotic alteration, and antioxidant systems. Therefore, protein hydrolysate derived from animal protein is an effective, economical, and environmental plant biostimulant for improving plant performances under drought stress.