Stable free radicals in ozone-damaged wheat leaves

Chlorophyll fluorescence measurements were performed on attached leaves of wheat plants (Triticum aestivum L. cv. Nandu) that were exposed to ambient air and to air supplemented with 80 and 120 nmol mol-1 ozone. Decreases in the "current photochemical capacity" were observed that were dependent on both the ozone concentration and duration of exposure. Electron paramagnetic resonance (EPR) spectra on freeze-dried samples from the same batches of plants showed the presence of an unidentified stable free radical, whose spectra had similarities to that of the ubisemiquinone radical. The intensity of this radical signal increased with the duration of ozone exposure in leaves that received an additional 120 nmol mol-1 ozone. In contrast, with exposure to air with 80 nmol mol-1 added ozone, there was little if any change in free radical signal intensity over the 4 week period of the experiment. The increase in intensity of the EPR signal occurred later than the chlorophyll fluorescence changes, which suggests that it is associated with permanent leaf damage.