Optimizing sweet fennel growth and quality: the impact of cobalt supplement on vegetative growth, yield, and chemical composition

This study investigated the effects of cobalt treatments on the growth, yield, and chemical composition of sweet fennel (Foeniculum vulgare cv. Dulce) in both pot and field experiments conducted over two growing seasons. The experimental design was structured as a completely randomized block design. In the pot experiment, cobalt was applied at concentrations of 0.0, 2, 4, 6, 8, 10, 12, 14, 16, 18, and 20 mg L⁻¹. For the field experiment, the concentrations tested were refined to 0.0, 4, 8, 12, 16, and 20 mg L⁻¹ based on preliminary results to better assess the impact on sweet fennel growth and yield. Vegetative growth, yield parameters, and chemical constituents were evaluated 60 and 120 days after sowing. The results indicated that a cobalt concentration of 16 mg L⁻¹ significantly enhanced plant growth, yield, and nutritional quality. Specifically, plant height, leaf number, fresh and dry weights, bulb dimensions, and total yield per feddan showed substantial increases compared to the control. Chemical analysis revealed improvements in macronutrients, micronutrients, total proteins, carbohydrates, soluble sugars, phenolic content, vitamins C and A, and oil content at this optimal cobalt level. However, higher concentrations (20 mg L⁻¹) led to a slight decline in some metrics, suggesting a potential toxicity threshold. The composition of essential oils also improved with cobalt treatment, with significant increases in key components such as α-pinene, camphene, and fenchone. These findings demonstrate that cobalt supplementation at 16 mg L⁻¹ can significantly enhance the growth, yield, and quality of sweet fennel, although excessive levels may have adverse effects.