LED Light-Quality Optimization to Enhance Shoot and Essential Oil Yield of Tagetes erecta L. in Controlled Environment

This study evaluated the effects of light spectral quality on shoot yield and essential oil of Tagetes erecta L. cultivated in controlled growth chambers under three LED lighting treatments with different red, blue, and white wavelength ratios and a constant 16 h photoperiod for up to 101 days. The F2 treatment (5 red:1 blue) produced yields of fresh shoots, early blooming flowers, and oils of 1586 ± 164 g/m2, 569.77 ± 76.81 g/m2, and 307 ± 31.7 mg/m2, respectively. These values were significantly higher (p < 0.05) than those of the F1 treatment (white:red-phosphor), and represented increases of 1.37-, 1.26-, and 1.38-fold, respectively. Gas chromatography identified 30–31 compounds in the oil with three major constituents—(E)-β-ocimene (22.9–28.8%, highest under F3), (E)-myroxide (13.9–20.6%, highest under F1), and piperitone (7.3–9.6%, highest under F3). Essential oils inhibited from four to five of the seven tested microbial strains, with the notable activity against Escherichia coli and Candida albicans recorded in F2 and F1, respectively. These findings confirm that light spectral quality is a critical factor regulating flower, essential oil, and antimicrobial efficacy in T. erecta, demonstrating that optimized LED spectra offer a practical strategy to improve plant yield and phytochemical quality.