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 i
n the "current photochemical capacity" were observed that were dependent on
both the ozone concentration and duration of exposure. Electron paramagnet
ic resonance (EPR) spectra on freeze-dried samples from the same batches of
plants showed the presence of an unidentified stable free radical, whose s
pectra had similarities to that of the ubisemiquinone radical. The intensit
y of this radical signal increased with the duration of ozone exposure in l
eaves 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 exper
iment. 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.