Effects of Three Organic Fertilizers and Biostimulants on the Morphological Traits and Secondary Metabolite Content of Lettuce

The aim of the work was to find out the effects of three types of organic fertilizers (S-horse and cow, P-chicken, B-compost) and three types of biostimulants (BC-plants, BO-microorganisms, BS-microorganisms) on the growth of lettuce (Lactuca sativa cv. Maradona) and the secondary metabolites accumulation. The combination of manure (P) and biostimulant (BC) produced the highest yield, exceeding the control by 50% and 66% in two successive growing cycles (p < 0.05). Across all fertilizer treatments, BC application consistently improved plant performance and metabolic activity. Among secondary metabolities, the highest total phenolic content was observed in S and S + BC (≈2.56 mg g−1), phenolic acids in S + BC (≈2.54 mg g−1), antioxidant activity in P + BC (≈1.24 mg g−1), flavonoids in control with S (≈14.09 µmol/g), and proline in P + BO (≈2.45 µmol/g). These findings highlight the synergistic effects of organic fertilizers and biostimulants, suggesting their potential to improve both productivity and nutritional quality in sustainable horticultural systems.