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

The relationship between Cu2+-overloading and oxidative stress in plant cel ls was studied. The involvement of Cu2+ in the reduction of superoxide O-2( -) and hydrogen peroxide (H2O2) to hydroxyl radical (OH), suggest that exce ss Cu in plants leads to oxidative stress. Cu excess was induced in Phaseol us 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 f rom 12 to 23 mg Cu kg(-1) DW. Indications for oxidative stress response wer e detected by monitoring changes in the activities and contents of some com ponents of the antioxidative mechanism. The Cu treatment resulted in double d activity of ascorbate peroxidase (APX, EC 1.11.1.11) and catalase (EC 1.1 1.1.6). Excess of copper inhibited glutathione reductase (GR, EC 1.6.4.2) t o 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.