Effects of plant density and nitrogen fertilization on purple basil growth, biomass allocation, and organ-specific essential oil content and yield

This study evaluated plant density and nitrogen fertilization effects on growth, biomass partitioning, herbage yield, and organ-specific essential oil content of purple basil ( Ocimum basilicum L.) under Mediterranean conditions. A split-plot experiment with three replications was conducted in 2021 and 2022 using three plant densities (40,000, 50,000, and 66,667 plants ha −1 ) and four nitrogen doses (0, 50, 100, and 150 kg N ha −1 ). Herbage denotes the harvested aboveground biomass comprising leaves, stems, and flowers. Averaged across seasons and plant densities, fresh herbage yield increased from 4.98 to 8.97 t ha −1 and dry herbage yield from 2.15 to 4.76 t ha −1 between the control and 150 kg N ha −1 treatments (nitrogen main effects: P < 0.0001). Conversely, leaf essential oil content decreased from 0.47 to 0.35%, and flower essential oil content from 2.43 to 1.84% ( P < 0.0001). Fresh leaf ratio was not significantly affected by nitrogen dose ( P = 0.6233), although dry leaf and flower ratios increased. Significant season × density × nitrogen interactions for herbage yield indicate that treatment responses depended on the combination of these factors. The 2022 season generally produced greater biomass and higher organ-specific essential oil contents than 2021. The results indicate contrasting responses of biomass production and organ-specific oil content to nitrogen supply within the tested treatments; the underlying physiological mechanisms were not measured.