Fennel (Foeniculum vulgare Mill) is a valuable medicinal plant from the Apiaceae family, cultivated for its essential oil, seeds, and leaves, which are used in the pharmaceutical, food, and cosmetics industries. This study tested the hypothesis that plant growth regulators (PGRs) positively modulate the morphological, physiological, and phytochemical properties of fennel. To test this hypothesis, a factorial experiment was carried out with foliar applications of four plant growth regulators, including salicylic acid (SA: 1 and 2 mM), gibberellic acid (GA: 0.25 and 0.5 mM), gamma-aminobutyric acid (GABA: 5 and 10 mM), and sodium nitroprusside (SNP: 1.25 and 2.5 mM). The results showed that SNP2.5 and GA0.5 treatments achieved the greatest improvement in growth and morphological traits. For the SNP2.5 treatment, morphological traits exhibited percentage changes ranging from 36.5 to 198.6%. SNP2.5 and SA2 treatments resulted in the highest values for physiological traits. The SNP2.5 treatment caused significant increases in flavonoid and phenolic contents, DPPH radical scavenging activity, and chlorophyll and carotenoid levels, with increases ranging from about 39% to 231% compared to the control. Meanwhile, SA (1 and 2 mM) and GA0.25 had superior effects on phytochemical traits, including essential oil yield and secondary metabolites. GABA10 also positively influenced all traits, though to a lesser extent than other treatments. Control plants consistently exhibited the lowest values across all traits. Metabolite analysis using GC-MS identified a total of 150 compounds, among which 41 showed significant differences among treatments. Notably, six major metabolites accounted for about 70% of the total metabolite content, including apiol (0.742-1.299), camphene hydrate (0.407-0.708), cis-anethole (0.328-0.975), estragole (0.916-1.059), phenyl acetate (5.005-9.705), and limonene (3.902-1.647) respectively for SA1 and SA2. Among these, cis-anethole (40–75%) and limonene (6.5–19%) were identified as the predominant components in the essential oil of the leaves. These findings suggest that certain plant growth regulators, especially SNP and SA, can be used as effective agricultural tools for improving the growth and phytochemical quality of fennel.
