EFFECTS OF IRON EXCESS ON NICOTIANA-PLUMBAGINIFOLIA PLANTS - IMPLICATIONS TO OXIDATIVE STRESS

Fe excess is believed to generate oxidative stress. To contribute to t he understanding of Fe metabolism, Fe excess was induced in Nicotiana plumbaginifolia grown in hydroponic culture upon root cutting. Toxicit y symptoms leading to brown spots covering the leaf surface became vis ible after 6 h. Photosynthesis was greatly affected within 12 h; the p hotosynthetic rate was decreased by 40%. Inhibition of photosynthesis was accompanied by photoinhibition, increased reduction of photosystem II, and higher thylakoid energization. Fe excess seemed to stimulate photorespiration because catalase activity doubled. To cope with cellu lar damage, respiration rate increased and cytosolic glucose-6-phospha te dehydrogenase activity more than doubled. Simultaneously, the conte nt of free hexoses was reduced. Indicative of generation of oxidative stress was doubling of ascorbate peroxidase activity within 12 h. Cont ents of the antioxidants ascorbate and glutathione were reduced by 30% , resulting in equivalent increases of dehydroascorbate and oxidized g lutathione. Taken together, moderate changes in leaf Fe content have a dramatic effect on plant metabolism. This indicates that cellular Fe concentrations must be finely regulated to avoid cellular damage most probably because of oxidative stress induced by Fe.