Light Regime and Exposure Duration Shape Essential Oil and Biochemical Responses in Rosemary (Salvia rosmarinus Spenn.)

The spectral composition and duration of light exposure can influence plant growth, secondary metabolism, and essential oil composition in medicinal and aromatic plants. Here, we evaluated the effects of lighting regime and exposure duration on the biochemical traits and essential oil composition of rosemary (Salvia rosmarinus Spenn.) under controlled conditions. Plants were exposed to white, blue, red, red/blue LED, or natural sunlight for 2, 3, or 4 weeks. The artificial light treatments were maintained at 200 ± 10 μmol m−2 s−1 PPFD, whereas the sunlight treatment was not PPFD-matched and therefore served as a greenhouse reference rather than a spectrum-matched control. Four biological replicates were used for each treatment. Chlorophyll, carotenoids, soluble sugars, total phenolics and flavonoids, DPPH radical scavenging activity, and essential oil composition were assessed. Essential oils were analyzed by gas chromatography–mass spectrometry (GC-MS). Lighting regime and exposure duration significantly affected several biochemical traits, whereas total phenolic content was not significantly affected by lighting regime. Red/blue LED treatment was associated with the highest mean values of total chlorophyll, carotenoids, total flavonoids, soluble sugars, and DPPH radical scavenging activity, although statistical significance varied among traits. Approximately 37 volatile compounds were identified across the treatments. The major constituents included α-pinene, 1,8-cineole, camphor, verbenone, borneol, bornyl acetate, and camphene and their relative abundances varied among lighting regimes and exposure durations. Principal component analysis (PCA) described differences in multivariate essential oil profiles among treatment combinations, while correlation analysis revealed associations among the relative abundances of volatile constituents. The highest essential oil yield was obtained under blue light at week 2 (1.53 ± 0.02% dry weight), whereas red/blue lighting at week 3 was associated with favorable responses in several biochemical traits. Thus, no single lighting regime and exposure duration was optimal for all measured traits. These findings indicate that lighting regime and exposure duration may be useful factors for modulating specific biochemical and essential oil characteristics of rosemary under controlled environment conditions.