Optimizing Target Metabolites Production in Coleus blumei Indoor Cultivation: Combined Effects of LED Light and Salinity Stress

Light quality is a recognized driver of plant growth and secondary metabolism in Coleus blumei, a valuable source of rosmarinic acid (RA) and quercetin (QU), whereas its combination with salinity stress represents a potential strategy that still requires further investigation. We evaluated four LED spectra, red–blue (RB) (6:1, control), blue (B), red (R), and RB + Far-Red, under both control (0 mM NaCl) and moderate salt stress (120 mM NaCl), measuring biomass (dry weight) and RA/QU in leaves and roots after three (T1) and five weeks (T2). Blue light produced the greatest root biomass, while the leaf dry weight under B did not differ significantly from RB or RBfr. RA peaked at T2 under B in leaves and under R in roots; QU was maximal under B in leaves and under RB in roots. Extending exposure from T1 to T2 markedly increased both metabolites’ yield. Salinity had little effect on biomass, increased the total QU yield, and did not enhance the total RA yield. These results indicate that targeted LED regimes and longer exposure can raise the yields of bioactive compounds, and that combining specific spectra with moderate salinity is an effective strategy for selectively increasing quercetin accumulation in indoor-grown C. blumei.