Impact of salinity stress on growth, aroma, and gene expression of hydroponic fennel (Foeniculum vulgare Mill.)

Water salinization poses a major threat to the agricultural and food systems. Previous research has shown that fennel can be utilized as a salt-tolerant plant to grow in arid regions. However, the impact of salinity stress on fennel aroma and genes underlying these changes is still unknown. The objective of this study was to investigate the effects of salinity stress on the growth, volatile aroma compounds, and gene expression of fennel (Foeniculum vulgare Mill., cultivar Grosfruchtiger) cultivated with nutrient film technique (NFT) hydroponic systems in a greenhouse environment. The experiment followed a randomized block design with four salinity treatments. Fennel was grown under varying salinity levels, with electrical conductivity controlled at 1.4, 3.4, 5.4, 7.4 dS/m, representing 0, 20, 40, and 60 mM additional NaCl. After 3 and 6 weeks of exposure to salinity stress, fennel samples were harvested. Plant growth parameters were measured upon harvest, and aroma characterization of fennel leaves was performed using headspace solid phase microextraction – gas chromatography – mass spectrometry – olfactometry (SPME-GC-MS-O). Quantitation of aroma compounds was performed using standard addition methods. RNA sequencing (RNA-Seq) and bioinformatics analysis were performed to study the changes in gene expression levels. The results showed that the salinity stress caused a significant decrease in plant height, plant width, number of branches, and fresh biomass (p<0.05) in both week 3 and week 6 samples. Based on preliminary GC-O, the most representative fennel aroma compounds, including 3 monoterpenes, 4 alcohols, 1 ketone, and 5 phenylpropanoids, were selected and quantified. Two-way ANOVA showed that salinity induced significant changes in the concentrations of α-pinene, α-phellandrene, cis-3-hexenal, and apiol (p<0.05). However, the overall content of aroma-active compounds was not significantly discriminated by either salinity level or harvest time. RNA-Seq and gene expression analysis will help understand how salt stress regulates corresponding genes. The results are useful for breeding work and decision-making in sustainable agriculture. The present study demonstrates the utilization of indoor farming techniques and chemistry tools to evaluate the influence of abiotic factors on crop flavor quality.