Processing Tomato Responses to Plant-Based Biostimulants Are Modulated by Environmental Conditions.

Tomatoes (Solanum lycopersicum L.), rich in health-promoting bioactive compounds, are vital to the economy and culture of Mediterranean regions such as southern Italy. However, open-field cultivation in these areas faces increasing challenges due to fluctuating environmental conditions, which intensify oxidative stress, accelerate ripening, and compromise yield and quality. Plant-based protein hydrolysates (PHs) and optimized irrigation practices have emerged as promising strategies to enhance crop resilience. This study assessed the effects of two foliar-applied biostimulants: MU, a seaweed and plant amino acid-based formulation, and SR, a potassium-rich botanical extract. Both were tested on tomatoes (cv. "H1534") under open-field conditions in southern Italy (Foggia) over two seasons (2019-2020). Both biostimulants had no significant impact on yield traits or technological quality, whereas year-to-year variability markedly influenced outcomes. In 2020, compared with 2019, total yield declined by 45%, and defective fruits rose by 311%.°Brix, polyphenols, lycopene, and sweetness index also decreased significantly (-41%, -18%, -58%, and -14%, respectively), indicating stress conditions. Under these circumstances, MU increased polyphenols (+27%) and enhanced essential (+42%) and branched-chain amino acids (BCAAs, +63%), while SR also stimulated BCAAs accumulation (+30%). These findings suggest that, under variable open-field conditions, biostimulants mainly influenced fruit metabolic profiles rather than directly enhancing growth or yield. Their performance appears closely tied to environmental factors, reinforcing the need for realistic, context-specific evaluations to guide their effective integration into sustainable cropping systems.