Effect of Copper Stress and Foliar Iron Application on the Growth and Quality of Sideritis cypria Plants Grown in Cocopeat Under Different Potassium Levels

Abstract The integration of medicinal and aromatic plants (MAP) in conventional cultivation is hindered by a limited understanding of appropriate cultivation practices and nutritional requirements, particularly for under-evaluated MAP such as the Sideritis cypria Post. species. Furthermore, the development of comprehensive nutrient management is essential, with potassium and iron to have a fundamental role in plant growth and mitigate potential adverse effects of heavy metal toxicity. The current study assessed the effect of nutrient solutions (NS) with different potassium (K; 150, 350, 550 mg L − 1 ) and copper (Cu; 5, 100 µM) levels, in combination with foliar iron (Fe) applications, in the soilless cultivation of S. cypria plants. The lowest K application (150 mg L − 1 ) was associated with increased fresh biomass production, while the highest K application (550 mg L − 1 ) revealed improved chlorophyll and carotenoid contents. Foliar Fe modulated these responses, and was associated with growth inhibition, oxidative stress and Fe accumulation in leaves. It also increased the accumulation of N and Cu. However, copper accumulation was well below the maximum residue levels of Cu in herbal infusions. S. cypria antioxidant activity and stress response to excess Cu was altered under the different K levels and foliar Fe application, highlighting the interplay between heavy metal toxicity and K levels. Moreover, treatments with intermediate K levels produced plants with high antioxidant activity and moderate antibacterial activity. Low to moderate K levels in the NS can be deployed when in excess Cu, without compromising S. cypria production and safety. Moreover, foliar Fe applications may be applied for biofortification, upon consideration of potential toxicity risks.