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.