Unraveling effects of foliar protein hydrolysates on growth, physiology and water-use efficiency of greenhouse tomato and common bean

Abstract The growing demand for sustainable crop management has intensified interest in protein hydrolysates (PHs) as bioactive biostimulants. While the efficacy of foliar PH applications has been documented in tomato at concentrations of 2.5 mL/L and above, or through combined foliar-drench programs, evidence for significant agronomic responses at low foliar rates- particularly in legumes- remains limited. The present study evaluated the effects of two novel plant- and microbial-derived PH formulations, AM2025 and MP2025, applied twice as foliar sprays at 0.05% and 0.1% (v/v) (equivalent to 0.5 and 1.0 mL/L) on vegetative growth, photosynthesis and water-use efficiency in Phaseolus vulgaris L. and Solanum lycopersicum L. under controlled greenhouse conditions. In S. lycopersicum , several morphological, biomass, and physiological traits, including leaf area, shoot biomass, leaf chlorophyll content (SPAD), net photosynthetic rate, and water-use efficiency, were significantly affected by PH treatments. A strong interaction between formulation and application rate was observed, indicating that treatment effects depended on their specific combination. Notably, these responses were achieved with only two foliar applications at concentrations lower than those typically reported in the literature. In P. vulgaris , responses were more limited and variable, with fewer statistically significant effects. However, some parameters, including shoot dry weight, leaf area, and intrinsic water-use efficiency, showed treatment-related increases. These findings demonstrate species-specific responses to PH application and highlight the importance of formulation-dose interactions, while also suggesting that these products exhibit considerable biological activity at very low application rates. Further research is needed to validate these effects across a wider range of species, application regimes, and field conditions.