Acclimation of beans to oxidative stress by treatment with sublethal iron levels

The relationship between Fe²⁺-overloading enhanced antioxidative mechanism and protection from successive oxidative stress in plant cells was studied. The involvement of Fe²⁺ in the reduction of super-oxide (O₂^-) and peroxide (H₂O₂) to hydroxyl radical (OH) suggests that excess Fe leads to oxidative stress. An excess of free Fe was induced in Phaseolus vulgaris (cv. Pinto) plants by soaking the roots in a 900 μmol/L Fe(III)-EDTA solution for 24 h. As a result, Fe content in bean leaves increased from 160 mg Fe kg^-1 dry weight to 530 mg Fe kg^-1 dry weight. Oxidative stress responses were detected by monitoring changes in the activities and contents of a few components of the antioxidative mechanism. The Fe treatment resulted in increased activity of ascorbate peroxidase (APX, EC 1.11.1.11) from 5.3 to 18.2 μmol ascorbate g^-1 fresh weight min^-1 increased ascorbate content from 6.6 to 10 mg g^-1 fresh weight, and increased catalase (EC 1.11.1.6) and glutathione reductase (GR, EC 1.6.4.2) activities by 380 % and 55 %, respectively. Fe treatment induced the activity of Fe-SOD (SOD, EC 1.15.11) isozyme, which was not detected in the control bean leaves. The increase in the antioxidative mechanism resulted in acclimation of the Fe treated beans to subsequent methyl viologen treatment relative to control bean plants.