Protein Hydrolysates Modulate Quality Traits of Tomato Fruit Under Salt Stress by Regulating the Expression Patterns of Genes Related to Sugar Metabolism

Salinity is a major stress factor that limits tomato yield and fruit quality. The aim of this study was to evaluate whether vegetal-derived protein hydrolysates (PHs) can alleviate salt stress in tomato plants and how they affect sugar metabolism at the molecular level. A greenhouse experiment was carried out to test three PHs, containing mainly peptides and aminoacids and derived from the enzymatic hydrolysis of protein sources belonging to Leguminosae (PH1), Malvaceae (PH2), and Solanaceae (PH3) plants under non-saline (1 mM NaCl) and saline (50 mM NaCl) conditions. PH1 and PH3 increased marketable yield under non-saline conditions, while no yield improvement was observed under salinity. Nevertheless, all PHs reduced leaf Cl− accumulation and improved fruit nutritional quality by increasing antioxidant activity and total phenol content. Under salt stress, PH1 and PH2 raised the content of total soluble solids, whereas PH3 enhanced titratable acidity. Gene expression analysis revealed that PHs modulated sugar metabolism, shifting it towards starch synthesis and accumulation in fruits, consistent with the observed increase in soluble solids. These results demonstrate that PHs exert family-specific effects on tomato fruit quality and provide molecular evidence of their role in metabolic adjustment under salinity. Practically, vegetal-derived PHs can represent a sustainable agronomic strategy to enhance fruit quality traits and improve tomato marketability in salt-affected cultivation systems.