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Plant proteolytic system includes proteases, mainly localized inside the organelles, and the ubiquitin–proteasome pathway in both, the cytoplasm and the nucleus. It was recently demonstrated that under severe Cd stress sunflower (Helianthus annuus L.) proteasome activity is reduced and this results in accumulation of oxidized proteins. In order to test if under other heavy metal stresses sunflower proteolytic system undergoes similar changes, an hydroponic experiment was carried out. Ten days old sunflower plants were transferred to hydroponic culture solutions devoid (control) or containing 100μM of AlCl₃, CoCl₂, CuCl₂, CrCl₃, HgCl₂, NiCl₂, PbCl₂ or ZnCl₂ and analyzed for protein oxidative damage and proteolytic activities. After 4 days of metal treatment, only Co²⁺, Cu²⁺, Hg²⁺, and Ni²⁺ were found to increase carbonyl groups content. Except for Al³⁺ and Zn²⁺, all metals tested significantly reduced all …