Free radical generation and post-anoxic injury in rice grown in an iron-toxic soil

The effects of previous submergence and added antioxidant (ascorbate) on free radical generation and internal iron oxidation in leaves of rice grown in an iron-toxic soil have been investigated by electron paramagnetic resonance (EPR) spectroscopy for three varieties with distinctly different tolerance levels. The survival rate of plants was increased appreciably by addition of ascorbate 24 hours before they were returned to aerobic conditions. EPR spectra of frozen leaf tissue from surviving plants recorded at 110K showed distinct features corresponding to Fe(III), Mn(II) and free radical (FR) species. Compared to unsubmerged controls, submergence treatment of the sensitive varieties resulted in larger increases than with the tolerant variety in total Fe and the intensities of the Fe(III)(g=4.27)/Mn(II) EPR peaks, indicating that the tolerant plants were more effective at excluding Fe(II). This increase in the EPR signal was halved in the Fe-tolerant variety by ascorbate treatment, but unchanged in the Fe-sensitive varieties, whereas small reductions in total Fe contents were seen with all three varieties. The intensity ratio of the FR/Mn(II) peaks was lowest for the submergence-tolerant variety, in which they were only slightly increased by the submergence treatments; in contrast higher ratios were found in the controls of submergence-sensitive varieties, and they increased appreciably with the submergence treatment. In the Fe tolerant varieties more than half of this increase was removed by ascorbate treatment prior to returning to aerobic conditions, whereas the addition of ascorbate had negligible effect on FR generation in the Fe sensitive variety. These results suggest that, in rice plants growing in Fe-toxic soils, free radicals are responsible for both post-anoxic injury and damage caused by Fe toxicity; free radical scavengers, such as ascorbate, appear to play a role in ameliorating the former, but not the latter, of these effects.