Loading beans with sublethal levels of copper enhances conditioning to oxidative stress

The relationship between Cu²⁺-overloading and oxidative stress in plant cells was studied. The involvement of Cu²⁺ in the reduction of superoxide O₂^- and hydrogen peroxide (H₂O₂) to hydroxyl radical (OH), suggest that excess Cu in plants leads to oxidative stress. Cu excess was induced in Phaseolus vulgaris (cv. Pinto) plants by soaking the roots in a 1 mmol/L Cu(NO₃)₂ solution for 24 hours. As a result, Cu content in bean leaves increased from 12 to 23 mg Cukg^-1 DW. Indications for oxidative stress response were detected by monitoring changes in the activities and contents of some components of the antioxidative mechanism. The Cu treatment resulted in doubled activity of ascorbate peroxidase (APX, EC 1.11.1.11) and catalase (EC 1.11.1.6). Excess of copper inhibited glutathione reductase (GR, EC 1.6.4.2) to 60% of control activity. No differences were found in CuZn-SOD (SOD, EC 1.15.11) and Mn-SOD activity, and glutathione and ascorbate content. Copper overloading affected also, primary reactions of photosynthesis and electron transport as probed by the photochemical energy storage in far-red light. Cu treatment of bean plants resulted in enhanced conditioning to successive methyl viologen and SO₂ treatments as compared to non-treated control.