Exploring Hydroxytyrosol as a Novel Biostimulant: Effects on Tomato Yield and Quality Under Greenhouse Conditions

Agro-food sector byproducts are valuable sources of biostimulants. Alperujo, the primary by-product generated during the two-phase centrifugation process of olive oil extraction, is rich in bioactive compounds, particularly phenolic compounds, which could enhance its valorization potential. Among these, hydroxytyrosol-4-glucoside and hydroxytyrosol were the predominant phenolic compounds detected. Despite the well-documented antioxidant and health-promoting properties of hydroxytyrosol, there is a notable lack of studies evaluating its use as a biostimulant in agricultural applications. To address this gap, the present study evaluated the agronomic and qualitative responses of tomato plants to the application of two commercial hydroxytyrosol-rich formulations (formulas A and B) derived from olive oil extraction by-products. Tomato plants were cultivated under greenhouse conditions using a soilless coco peat substrate, and the formulations were applied via the root system at different doses and application times, following the manufacturer's recommendations. Formulae A and B contained hydroxytyrosol at concentrations of 2.8 mg mL−1 and 6.4 mg mL−1, respectively. Key production parameters, including yield, fruit size, and number of fruits, both marketable and nonmarketable, were recorded. Additionally, fruit quality attributes were assessed, including shelf life, firmness, total soluble solids (TSS), antioxidant capacity, and concentrations of lycopene, vitamin C, total polyphenols, and anthocyanins. Formula B also increased fruit Ca, K and N concentrations, suggesting enhanced nutrient partitioning towards fruits. Metabolomic analysis of fruit tissues was performed using NMR spectroscopy to quantify primary metabolites, including sugars, organic acids, and amino acids, in the fruit. The results demonstrated that the application of these biostimulant formulations enhanced crop yield. Formula B increased the total and marketable yields by 15.7% and 21%, respectively, and formula A by 9.6% and 14.5%, respectively. Fruit firmness improved by up to 30%, and shelf life was extended by 37% and 54% with the use of formulas A and B, respectively. In addition, the key antioxidant attributes were improved. Lycopene increased by 36% and 21% under formula A and B, respectively. Total vitamin C increased by 18% with Formula B. Sugar content (glucose and fructose) and malate concentrations were significantly higher in treated fruits, whereas glutamine levels increased by 67% with Formula B. These findings highlight the potential of hydroxytyrosol-rich biostimulants to improve tomato yield, fruit quality, and postharvest longevity while promoting sustainable agriculture and the valorization of agro-industrial waste.