Nitrogen Fertilization Coupled with Zinc Foliar Applications Modulate the Production, Quality, and Stress Response of...

The demand for medicinal and aromatic plants (MAPs) has grown significantly in recent years, due to their therapeutic value. Among these, Sideritis cypria Post is a promising yet under-evaluated species. Existing research assessing the effects of nitrogen (N) fertilization, zinc (Zn) foliar applications, and toxic copper (Cu) concentrations often overlooks MAPs such as S. cypria. Additionally, the interactions among these parameters, as well as their combined roles in MAPs plant physiology and secondary metabolite biosynthesis, have yet to be fully elucidated. In this study, hydroponically grown S. cypria plants were cultivated using nutrient solutions (NSs) with different N (75, 150, and 300 mg L^-1) and Cu (5 and 100 μM) levels, combined with foliar spraying (0 and 1.74 mM Zn), to evaluate the growth, mineral uptake, secondary metabolites production and stress response. N levels at 75 and 150 mg L^-1 resulted in increased dry matter content, whereas fresh biomass production was preserved. Foliar Zn applications enhanced chlorophylls and antioxidants, contingent upon N and Cu in the NS. Increased N accumulation was observed via the increase in N in the NS, while foliar Zn enhanced its uptake at moderate N levels. Excess Cu stimulated its accumulation, while a reduction was observed with foliar Zn at low and high N levels. Excess Cu increased lipid peroxidation (MDA) at low and moderate N in the NS, while foliar Zn decreased both MDA and hydrogen peroxide, contingent upon Cu and N levels. Low-to-moderate N in the NS can be applied under excess Cu without compromising the yield, quality, and safety of S. cypria plants, while foliar Zn can modulate the stress response of plants under excess Cu and the production of secondary metabolites. These results may be utilized for optimizing nutrient management strategies for the cultivation of MAPs, contributing to conservation efforts by supporting the cultivation of endemic species like S. cypria, considering the potential benefits of Zn foliar applications under Cu-contaminated conditions.