Copper-stress tolerance induced in Phlomis tuberosa depends on nitric oxide accumulation

In this study, we compared the impact of high Cu concentrations on the photosynthetic apparatus and antioxidant capacity of the Phlomis tuberosa. Plants were grown in perlite culture for 5 weeks, and then treated with 0, 100, 200, 300 and 400 µM Cu for 21 days. Results indicated that Phlomis tuberosa plants showed tolerance to 100 and 200 µM Cu. This increased tolerance was achieved through enhancement in antioxidant system activity. However, exposure to high Cu (300 and 400 µM) showed the highest level of stress leading to higher hydrogen peroxide (H2O2) and malondialdehyde (MDA) levels and lower inferred oxygen evolving complex activity (Fv/Fo) and calculated Performance Index (PIabs) values. Based on effective concentration (EC50 – substrate Cu concentration resulting in 50% biomass reduction) parameter, Cu toxicity thresholds for Phlomis tuberosa plants were determined at substrate Cu levels up to 300 µM. Interestingly, plants exposed to medium Cu levels for 21 days exhibited higher nitric oxide (NO) accumulation in the leaves, which was associated with enhanced levels of antioxidative enzyme activities, whilst maintaining photosynthesis.