Impact of Drought and Biostimulant in Greenhouse Tomato: Agronomic and Metabolomic Insights

Widespread drought conditions have increasingly affected agricultural productivity, requiring the exploration of alternative approaches for improving crop tolerance, yield and quality, since plants adopt many physiological strategies to cope with challenging environments. This study evaluated the effects of a vegetal-derived protein hydrolysate (PH), applied via foliar spray or root drench at a concentration of 3 mL L−1, on tomato plants (n = 96) under well-watered and drought-stressed conditions over a 136-day greenhouse experiment. Overall, sub-optimal irrigation significantly decreased plant dry biomass (−55.3%) and fruit production (−68.8% marketable yield), and enhanced fruit quality in terms of sugar concentration and antioxidant levels. PH treatments, regardless of the application method, did not notably influence above-ground dry biomass, yield, or fruit quality, suggesting that the intensity of drought might have limited PH effectiveness. Metabolomic analysis showed higher concentrations of stress- and quality-related metabolites in tomato fruits from plants under stress, with PH not exerting significant metabolic changes in the fruits. These findings revealed the diminished effectiveness of PHs under severe drought conditions, suggesting that drought stress level needs to be taken into consideration for optimizing biostimulant efficacy.