Valorization of Pea (Pisum sativum L.) Hull as a Source of Protein- and Phenolic-Rich Extracts with Biostimulant Potential: Extraction, Characterization and Evaluation in Tomato Seedlings

Pea (Pisum sativum L.) processing generates large amounts of by-products, including hull, which remain largely undervalued despite their protein and phenolic content. We hypothesized that dried pea hull can be valorized into liquid extracts with biostimulant potential, and that their composition and bioactivity can be modulated by the extraction conditions applied. Sixteen extracts were obtained by combining solvent (water or 80% ethanol), pH (neutral, acidic, or alkaline), temperature (room temperature or 60 °C) and extraction mode (agitation, ultrasound, or 72 h maceration), and were characterized by protein and total phenolic content. Protein content ranged from 0.012 to 2.368 mg/mL and total phenolic content from 9.78 to 299.99 µg GAE/mL, with alkaline extraction at pH 9.0 combined with heating and/or ultrasound maximizing both parameters. Twelve extracts were further evaluated for potential biostimulant activity on tomato seed germination and seedling growth. None of the extracts significantly affected seed germination (70–100% versus 88% in the control). However, several extracts significantly promoted root elongation, increasing root length from 2.19 cm in the control to up to 3.52 cm (+61%) at 1% concentration. In seedling assays, root fresh biomass increased from 0.74 g in the control to up to 1.49 g (+101%), while shoot dry biomass increased from 158 mg to up to 244 mg (+54%) following treatment with selected extracts at 5–10% concentration. Biostimulant efficacy did not directly correlate with protein or phenolic content. These results indicate that pea hull can be converted into extracts with promising biostimulant potential, supporting circular economy strategies in agriculture.