Melatonin Dose-Mediated Increment in Growth, Floral Production, Essential Oils Yield and Composition in Marigold

Globally, African marigold (MG) derived essential oils (EOs) have attained an immense economic pertinence in the flavor, fragrance, food, medicinal, and floricultural industries, which necessitate boosting its production on a commercial scale. Therefore, we aimed to assess varying levels (0, 50, 100, 150, and 200 mg L-1) of exogenously applied melatonin (MT) as a growth hormone to trigger growth, flower yield, and EOs of MG. The MT was applied as a foliar spray after thirty days of transplantation of MG plants and repeated thrice at fifteen-day intervals. The results depicted that exogenous MT (150 mg L-1) recorded the maximum plant height and leaves number along with fresh and dry weights of leaves and roots. The same treatment exhibited 66%, 64%, and 18% higher flower fresh and dry weights and flower yield respectively, than the control. Additionally, MT remained effective in reducing days taken to bud emergence and flowering, while flower retention duration increased by 11 days. Following the trend of vegetative growth traits, foliar-applied MT (150 mg L-1) remained unmatched in terms of physiological attributes (transpiration rate, stomatal conductance, chlorophyll a, and b contents) of MG. Moreover, for EOs extracted from fresh and dry flowers and leaves, this treatment remained effective by producing 77%, 73%, 53%, and 85% higher content, respectively than the control. Finally, the chemical profiling analyses detected eighty-seven chemical constituents (Caryophyllene oxide was the most dominant compound, and cis-Z-alpha-Bisabolene epoxide followed it) in MG flowers. In terms of the retention time of different chemical compounds in MG flowers, Calarene epoxide had the highest retention time of 19.75 minutes among major compounds. Based on these results, 150 mg L-1 dose of MT may be recommended to growers for boosting MG growth, floral yield and EOs content sustainably.