Effects of Shading on the Yield, Chemical Profile and Antioxidant Activity of Rosemary (Rosmarinus officinalis L.) Essential Oil

The effects of shading with pearl- and red-colored nets on the essential oil content, yield per hectare, chemical composition, and antioxidant potential of rosemary (Rosmarinus officinalis L.) were evaluated in comparison with plants grown under non-shaded conditions. Pearl (1.69%) and red shade nets (1.73%) significantly reduced essential oil content compared to the control (2.50%). The number of individual compounds identified in rosemary essential oil (REO) varied slightly among the shading treatments, ranging from 35 to 36 constituents. The most abundant components were eucalyptol/1,8-cineole (31.5–32.5%), α-pinene (12.9–21.1%), camphor (13.4–18.4%) and borneol (4.6–5.9%). The content of the undesirable REO component, such as camphor, was the lower in plants covered with shading nets (13.4–13.7%) than in non-shading plants (18.4%). Ferric reducing antioxidant power assays (FRAP) ranged from 0.651 mg Fe2+/g in plant cover by pearl nets and 1.509 mg Fe2+/g in red nets to 1.905 mg Fe2+/g from non-shading, control plants. The DPPH (2,2-diphenyl-1-picrylhydrazyl radical-scavenging assay), expressed as EC50 (half maximal effective concentration—the lowest EC50 value, corresponding to the highest activity), ranged from 0.819 mg/mL in non-shaded plants to 1.722 mg/mL in red and 16.456 mg/mL in plant cover by pearl nets. Shading modified EO composition, mainly by increasing the relative content of α-pinene and decreasing that of camphor, but it reduced antioxidant activity, as indicated by lower FRAP values and higher DPPH EC50 values, particularly under pearl nets. Both shade nets reduced EO content compared with the unshaded control; however, owing to higher dry leaf biomass, the estimated EO yield per hectare under pearl nets was similar to or slightly higher than that of the control. These results suggest that reduced light availability lowers EO accumulation per unit of plant mass in rosemary, a species adapted to high-light environments.