Improvement of the yield and essential oil of fennel (Foeniculum vulgare Mill.) using external proline, uniconazole and methyl jasmonate under drought stress conditions

Drought stress is a significant environmental constraint that seriously limits crop productivity. A two-year field study was carried out as a split plot based on a randomized complete block design (RCBD) with three replicates in the experimental station of the agricultural faculty of Azarbaijan Shahid Madani University, Tabriz, Iran. This study evaluated the impacts of various irrigation regimes and plant growth regulators/proline on yield, its components, and essential oil content. Water shortage led to considerable decreases in umbels per plant, umbellets per umbel, fruits per umbellet, 1000-fruit weight, biological yield, and finally, fruit yield, despite increases in fruit essential oil content under moderate drought stress conditions. External methyl jasmonate, uniconazole, and proline significantly increased these parameters measured in fennel plants compared with the control. Improved effects of external plant growth regulators/proline tended to be greater in drought-stress plants, as it enhanced yield, its components, and fruit essential oil content compared with control. Foliar spraying, particularly methyl jasmonate, and uniconazole, enhanced e-anethole content under drought stress. Fenchone and methyl chavicol contents increased in plant growth regulators/proline-treated plants when they were kept under moderate drought stress conditions; however, their contents decreased by the severity of drought stress apart from the application of methyl jasmonate. Hence, applying these PGRs or proline could be critical in cultivating fennel plants under water shortage conditions to achieve higher essential oils yield and its constituents, particularly anethole, compared with control.