The combined and single effect of salinity and copper stress on growth and quality of Mentha spicata plants.

Copper is essential for plant growth, but in excess may cause adverse effects on plant physiology. Harmful effects are also caused by plant exposure to salinity (NaCl) due to the excessive use of fertilizers, soil degradation and/or the quality of the water used for irrigation. The impact of single and combined salinity (Sal) and copper (Cu) stress on spearmint metabolism were studied in hydroponics. Spearmint plants (Mentha spicata L.) were subjected to salinity stress (150 mM NaCl) and/or excessive Cu concentration (60 μM Cu) via the nutrient solution. Not only Sal and Cu, but also their combination suppressed plant growth by decreasing plant biomass, root fresh weight and plant height. Chlorophyll content decreased mainly for the combined stress treatment (Sal + Cu). Polyphenols and antioxidants (FRAP, DPPH, ABTS) increased in single stress treatments (Sal or Cu), but decreased in the combined stress (Sal + Cu). The application of Sal or Cu stress decreased Zn, N and K (leaves), K, Ca, P and Mg (roots) content. Copper application increased Ca and Mg in leaves. In conclusion, salinity stress and Cu exposure may change the primary metabolic pathways in favor of major volatile oil components biosynthesis, resulting in significant changes of essential oil yield and composition.