Effects of excess copper on sunflower seedling growth, mineral nutrition, and cellular redox state

In order to assess the impact of copper (Cu) stress on plant metabolism, sunflower seedlings were cultivated for 15 days on a nutrient medium and then transferred to the same medium enriched with increasing concentrations of Cu (0, 2.5, 5, 10, 50 and 100 μM). When copper doses in the culture medium exceeded 10 µM, a reduction in biomass production, more pronounced in roots than in leaves and stems, was noticed. Excess Cu disrupted the mineral composition of the plant tissues. This effect was reflected in decreased contents of K, Mg, Zn and Mn and enhanced amounts of Fe in roots at the highest doses of Cu (50 and 100 µM). However, Ca content was not affected by Cu stress in any plant organ. These findings were further confirmed by a principal component analysis (PCA) of the mineral composition of sunflower tissues and the applied treatments. A Cu excess enhanced the accumulation of oxidative stress biomarkers in plant tissues, namely malondialdehyde and hydrogen peroxide (H2O2), testifying to the disruption of the cellular redox state. This impact was aggravated by the inhibition of antioxidative enzymes such as superoxide dismutase, catalase and ascorbate peroxidase. However, the stimulation of guaiacol peroxidase activity could be correlated with its role in membrane lignification. Based on the current findings, we suggest that the tolerance threshold of a sunflower plant to Cu stress is 10 µM. When Cu exceeds this concentration in the culture medium, toxicity symptoms may occur in the plant.