Formation of paramagnetic products in leaves of wheat (Triticum aestivum) as a result of ozone-induced stress

Electron paramagnetic resonance (EPR) measurements on light-harvested samples of wheat leaves that had been exposed to toxic ozone levels showed the production of a single-peak free radical signal and a large increase in intensity of the Fe(III) (g = 4.3) signal compared with control samples. These changes were accompanied by a decrease in the intensity of the more complex free radical signal that is associated with a tyrosine-based radical centre in photosystem II (PSII) protein. No qualitative changes in the Mn(II) signal were seen as a result of ozone exposure. Identification of the stress-induced free radical is discussed in the context of previous speculations in the literature, and it is concluded that there is still insufficient evidence for a positive identification. A similar situation exists with the Fe(III) component, where the ligand(s) remain(s) unidentified, although the complex is confirmed as a mononuclear species.