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

Fe excess is believed to generate oxidative stress. To contribute to the understanding of Fe metabolism, Fe excess was induced in Nicotiana plumbaginifolia grown in hydroponic culture upon root cutting. Toxicity symptoms leading to brown spots covering the leaf surface became visible after 6 h. Photosynthesis was greatly affected within 12 h; the photosynthetic 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 cellular damage, respiration rate increased and cytosolic glucose-6-phosphate dehydrogenase activity more than doubled. Simultaneously, the content of free hexoses was reduced. Indicative of generation of oxidative stress was doubling of ascorbate peroxidase activity within 12 h. Contents of the antioxidants ascorbate and glutathione were reduced by 30%, resulting in equivalent increases of dehydroascorbate and oxidized glutathione. 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.