Dose-Dependent Effects of the Protein Hydrolysate-Based Biostimulant Terrativa® on Growth, Photochemical Performance, and Quality of Theobroma cacao L. Seedlings

Protein hydrolysate-based biostimulants have been increasingly investigated due to their potential to improve seedling growth and physiological responses; however, integrated interpretations of dose-dependent morphophysiological and photochemical responses in Theobroma cacao L. during nursery production remain limited. This study evaluated the effects of increasing concentrations of Terrativa®, an organomineral biostimulant formulated with animal-derived protein hydrolysate, on growth, chlorophyll indices, and photosystem II performance in cacao seedlings of the Catongo and TSH1188 genotypes. The experiment was conducted in a randomized block design in a 2 × 6 factorial arrangement, corresponding to two genotypes and six Terrativa® concentrations (0, 1, 2, 3, 4, and 6 mL L−1). Most morphological and physiological variables exhibited quadratic responses. Intermediate concentrations promoted greater vegetative growth and improved photochemical responses, with maximum values observed for number of leaves (13.98 at 2.50 mL L−1), leaf area (994.10 cm2 at 2.91 mL L−1), stem length (42.79 cm at 3.44 mL L−1), root volume (9.21 cm3 at 3.70 mL L−1), and total chlorophyll (34.58 at 3.08 mL L−1). Catongo seedlings showed greater chlorophyll accumulation, whereas TSH1188 exhibited higher ABS/RC, TRO/RC, and PIabs values, indicating genotype-dependent differences in photochemical energy utilization. Higher concentrations were associated with reduced performance in several variables, suggesting lower physiological efficiency under elevated biostimulant doses. Overall, Terrativa® concentrations between 2.50 and 3.70 mL L−1 were the most effective for promoting balanced seedling development in both cacao genotypes, with Catongo responding more strongly in chlorophyll accumulation and TSH1188 showing greater photochemical responsiveness.