Valorization of Tagetes erecta L. Leaves to Obtain Polyphenol-Rich Extracts: Impact of Fertilization Practice, Phenological Plant Stage, and Extraction Strategy

Tagetes erecta L. is an ornamental crop known for its medicinal qualities. Large amounts of waste are produced in the commercial usage of T. erecta flowers, including leaves that could be used to develop new eco-friendly phenolic extracts with additional value for the food industry. To maximize the phenol content in the leaf extracts, this study used a Box–Behnken design with Response Surface Methodology, considering three extraction methods (Soxhlet distillation, heat, and vacuum-assisted extraction), three cropping practices (without fertilizer, chemical fertilizer, and vermicompost), and three phenological stages (plants without buds, with buds, and in flower). Extracts from plants fertilized with vermicompost (Eisenia foetida, 10 t ha−1), collected during the blossoming stage and extracted via Soxhlet distillation, exhibited the highest phenol content (25.66 mg GAE/g). Further chemical characterization of the optimized extract (UV-Vis, UV-fluorescence, FTIR, GC-MS, HPLC) confirmed the occurrence of polyphenols in the extract, including quercetin, chlorogenic, gallic, p-coumaric, 3-hydroxycinnamic, and caffeic acids. This underscores the significance of T. erecta leaf residues as a valuable source of bioactive molecules, highlighting the importance of integrating agricultural practices and chemical extraction methods to enhance the phenolic content in leaf extracts from this species.