Effect of Lead and Copper on Photosynthetic Apparatus in Citrus (<i>Citrus aurantium</i> L.) Plants. The Role of Antioxidants in Oxidative Damage as a Response to Heavy Metal Stress

Photosynthetic changes and antioxidant activity to oxidative stress were evaluated in sour orange (<i>Citrus aurantium</i> L.) leaves subjected to lead (Pb), copper (Cu) and also Pb + Cu toxicity treatments, in order to elucidate the mechanisms involved in heavy metal tolerance. The simultaneous effect of Pb⁻ and Cu on growth, concentration of malondialdehyde (MDA), hydrogen peroxide (H₂O₂), chlorophylls, flavonoids, carotenoids, phenolics, chlorophyll fluorescence and photosynthetic parameters were examined in leaves of <i>Citrus aurantium</i> L. plants. Exogenous application of Pb and Cu resulted in an increase in leaf H₂O₂ and lipid peroxidation (MDA). Toxicity symptoms of both Pb and Cu treated plants were stunted growth and decreased pigments concentration. Furthermore, photosynthetic activity of treated plants exhibited a significant decline. The inhibition of growth in Pb and Cu-treated plants was accompanied by oxidative stress, as indicated by the enhanced lipid peroxidation and the high H₂O₂ concentration. Furthermore, antioxidants in citrus plants after exposure to high Pb and Cu concentrations were significantly increased compared to control and low Pb and Cu treatments. In conclusion, this study indicates that Pb and Cu promote lipid peroxidation, disrupt membrane integrity, reduces growth and photosynthesis and inhibit mineral nutrition. Considering the potential for adverse human health effects associated with high concentrations of Pb and Cu contained in edible parts of citrus plants the study signals that it is important to conduct further research into the accessibility and uptake of the tested heavy metals in the soil and whether they pose risks to humans.