Impact of growth stage, supplemental red LED, and salinity stress on the quality and aroma attributes of hydroponic fennel (Foeniculum vulgare Mill.)

Abstract

Fennel (Foeniculum vulgare Mill.) is a widely used culinary herb valued for its distinct flavor, rich essential oil content, and health-promoting secondary metabolites. Due to its diverse culinary, medicinal, and industrial applications, optimizing fennel aroma, the key quality characteristic of fennel and its products, is of significant interest. The production of aroma compounds, which arise from secondary metabolism, is influenced by factors such as growth stage and environmental conditions. Understanding how secondary metabolite biosynthesis are affected by these factors is crucial for optimizing the quality and flavor of fennel. In particular, supplemental red light and salinity are known to modulate the production of aroma compounds in herbs, but the molecular mechanisms underlying these effects remain largely unexplored. Controlled environment agriculture (CEA) provides an ideal platform for studying plant responses to environmental stimuli. Accordingly, this dissertation aims to investigate the influences of growth stage, supplemental red LED light, and salinity stress on the quality and aroma compounds of fennel cultivated by CEA. Fennel was cultivated with nutrient film technique (NFT) hydroponic systems under controlled conditions. Solid phase microextraction (SPME) - gas chromatography - mass spectrometry olfactometry (GC-MS-O) was utilized for aroma characterization. RNA sequencing was used to generate transcriptome profiles of fennel under different environmental treatments. A total of 32 aroma-active compounds were identified in fennel microgreens, compared to 28 in mature fennel. Compared to mature plants, fennel microgreens contained a significantly higher level of monoterpenes, showing an 81.4-98.1% increase when compared to mature fennel. Supplemental red LED significantly increased both fennel yield and aroma compounds accumulation, particularly phenylpropanoids such as (E)-anethole ("sweet", "anise"), estragole ("anise", "herbal"), and p-anisaldehyde ("floral", "sweet"). Transcriptome analysis showed upregulation of key genes involved in phenylpropanoid biosynthesis, including eugenol synthase and isoeugenol synthase, which likely contributed to the increased phenylpropanoid concentrations under red LED light. Salinity stress, while significantly reducing plant growth, did not notably affect the overall content of aroma-active compounds. However, salinity triggered defense mechanisms in fennel, particularly through the activation of plant hormone signal transduction and mitogen-activated protein kinase (MAPK) signaling pathways. The findings of this study enhanced understanding of aroma formation of fennel in response to environmental factors at molecular and transcriptomic levels. These results also offer opportunities for growers to optimize fennel flavor through precise control of environmental conditions.