It is believed that narrow-wavelength red LED is sensed by plant phytochromes and can regulate the accumulation of secondary metabolites. However, plant responses are species-specific and the influences of lighting on fennel remains unclear. Fennel (Foeniculum vulgare Mill. cv. Grosfruchtiger) is a culinary herb that is cultivated worldwide for its unique aroma profile. Our objective is to investigate the effect of supplemental red LED lighting on plant growth, aroma compounds, and gene expression in hydroponically grown fennel. Fennel was cultivated with nutrient film technology (NFT) hydroponic systems under controlled environment. After fennel seedlings were transplanted to hydroponic systems, two treatments were applied: a natural (greenhouse) light control and a supplemental red LED lighting (PPFD 100 μmol m^-2s^-1, 14h d^-1). Whole plants were harvested six weeks after the transplanting. Results showed that supplemental red LED was beneficial for fennel growth and led to increased plant height and fresh weight. Headspace solid phase microextraction (SPME) gas chromatography-mass-spectrometry-olfactometry (GC-MS-O) was used for the identification and quantitation of aroma compounds in fennel leaves, and supplemental red LED increased the total concentration of aroma-active compounds in fennel. Specifically, the concentration of several most potent aroma compounds, such as trans-anethole (sweet, anise), estragole (anise, herbal), and p-anisaldehyde (floral, sweet) showed significant increase in fennel grown under supplemental red LED (t-test p<0.05). Preliminary results from RNA-seq gene expression analysis suggested that supplemental red LED also caused substantial changes to plant transcriptome. This study demonstrated the influence of narrow-wavelength LED lighting on the flavor and quality of culinary herbs and laid the foundation for future aroma expression research at molecular levels.

David Haak

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