Protein hydrolysates are mixtures of free amino acids and short peptides produced by breaking down protein-rich raw materials, either chemically or with enzymes. In plant science they belong to a fast-growing class of products known as biostimulants: substances that, applied in small quantities, enhance crop performance without acting as conventional fertilizers, pesticides, or growth regulators. The mechanisms are thought to include a signaling role for specific amino acids, stimulation of root development, improved nutrient uptake, and priming of the plant’s antioxidant machinery. What distinguishes the new work is the raw material, fish waste from aquaculture, and the production route, enzymatic hydrolysis, which is gentler than acid hydrolysis and better at preserving bioactive peptides.

The research team prepared the hydrolysate from rainbow trout (Oncorhynchus mykiss) by-products using an enzymatic method and then tested it on tomatoes cultivated in a soilless greenhouse system. Soilless, or hydroponic, cultivation is prized for its water efficiency and precise control of the root zone, but it also deprives plants of the complex microbial and organic chemistry of living soil. Growers therefore compensate with carefully balanced nutrient solutions, and biostimulants have emerged as a possible third lever. The researchers applied the hydrolysate in two ways, to the roots and to the leaves, at four concentrations: zero as a control, 0.2 grams per liter, 0.4 grams per liter, and 0.8 grams per liter.

The dose mattered, and the pattern that emerged was far from a simple more-is-better relationship. Plants receiving the 0.2 grams per liter treatment posted the strongest photosynthetic performance, with a photosynthesis rate of 39.3 micromoles per square meter per second and stomatal conductance of 797.2 millimoles per square meter per second, alongside the highest chlorophyll content index and the greatest accumulation of mineral nutrients in plant tissue. Moderate stomatal opening with high chlorophyll is exactly the profile a grower wants: the leaves are pulling in carbon dioxide efficiently while retaining water, converting light into sugars at close to peak capacity.

At the 0.4 grams per liter dose, the plants’ stress physiology told a subtler story. Leaves showed the highest values for the antioxidant enzyme ascorbate peroxidase, at 50.4 millimoles, and for malondialdehyde, a marker of oxidative damage to cell membranes, at 0.010 nanomoles per gram, together with a proline concentration of 0.21 milligrams per gram. Proline is an osmoprotectant that cells accumulate under pressure, while malondialdehyde indicates lipid peroxidation, so its elevation suggests that this dose pushed the plants mildly out of their comfort zone even as it triggered a stronger defensive response. The antioxidant enzyme ascorbate peroxidase converts hydrogen peroxide, a reactive oxygen species, into water, and its induction is a classic sign of biostimulant priming, in which a mild challenge leaves the plant better prepared for stress.

The strongest yield and quality gains appeared at the highest dose tested. Plants treated with 0.8 grams per liter produced the heaviest average fruit weight, 114.3 grams per fruit, along with the highest leaf catalase activity, 0.71 units per gram per minute, and the highest lycopene value recorded in the trial, 34.4. Lycopene is the red carotenoid pigment prized both for its contribution to tomato color and for its documented health benefits as a dietary antioxidant, so elevating it is commercially and nutritionally significant. The authors concluded that yield and profitability were increased with hydrolysate doses of 0.4 and 0.2 grams per liter respectively, indicating that the economically optimal dose depends on whether a grower prioritizes total output or input efficiency.

Not every quality parameter moved in the same direction. The highest soluble solids content, 3.98 percent, and the highest anthocyanin concentration, 0.19 grams of cyanidin-3-glucoside equivalents per kilogram, were both found in the untreated control plants. This is a recurring phenomenon in horticulture: when plants are well supplied with water and nutrients, sugars and pigments get diluted in larger fruits, whereas slight stress concentrates flavor compounds. For growers the finding implies a genuine trade-off between size and taste chemistry, one that a carefully chosen dose of biostimulant may help navigate rather than eliminate.

The study’s framing of foliar application as a supporter of physiological homeostasis deserves attention. The authors suggest that spraying the hydrolysate onto leaves helps the plant maintain its internal balance under standard greenhouse conditions, essentially acting as a biochemical stabilizer rather than a dramatic growth driver. In a soilless system, where the root environment is already precisely controlled, foliar feeding of amino acids and peptides may smooth out the small daily fluctuations in water status and nutrient supply that plants would otherwise buffer themselves. This positions protein hydrolysates less as miracle inputs and more as a fine-tuning tool for high-technology growing systems.

The broader significance lies in the circular economy logic of the approach. Aquaculture is one of the fastest growing food industries in the world, and fish processing generates large volumes of protein-rich waste, including heads, skin, bones, and viscera, that often becomes low-value feed or landfill. Enzymatic hydrolysis converts this material into a product with clear agricultural value, closing a loop between marine food production and protected horticulture. Compared with synthetic plant growth regulators, hydrolysates are perceived as more sustainable and more acceptable to consumers, and compared with mineral fertilizers they require far smaller application rates because they work through biological signaling rather than bulk nutrient supply.

There are caveats, as with any single-crop, single-season greenhouse trial. The results were obtained in one soilless system with one hydrolysate preparation, and the interplay between application method, dose, and outcome is complex enough that recommendations for commercial growers will need replication across seasons, cultivars, and growing conditions. The biochemical markers measured, from malondialdehyde to catalase, indicate that the doses used were not physiologically inert and that the highest dose imposed some oxidative signaling. Still, the study provides a quantified, dose-resolved picture of how a waste-derived biostimulant moves the needle on tomato photosynthesis, fruit weight, and pigment content. As growers worldwide face pressure to cut synthetic inputs while raising yields on shrinking land and water budgets, the idea that tomorrow’s tomatoes could be fattened, in part, by yesterday’s fish fillets is exactly the kind of practical, evidence-based circularity that sustainable agriculture has been searching for.

Subject of Research: Use of fish-waste-derived protein hydrolysate as a biostimulant to improve tomato growth and productivity in soilless cultivation

Article Title: Utilization of protein hydrolysates to enhance growth and productivity of tomato under soilless cultivation systems

Article References: Tütüncü, A. Ç., Özer, H., Korkmaz, K., Hazneci, E., Öztürk, B., & Gülser, C. (2026). Utilization of protein hydrolysates to enhance growth and productivity of tomato under soilless cultivation systems. BMC Plant Biology. https://doi.org/10.1186/s12870-026-09959-9

Image Credits: AI Generated

DOI: 10.1186/s12870-026-09959-9

Keywords: protein hydrolysate, biostimulant, soilless cultivation, tomato, rainbow trout, circular economy, foliar application, lycopene, antioxidant enzymes, greenhouse horticulture, aquaculture waste, plant physiology