Ten cultivars of spring wheat, bred and introduced in Greece between 1932 and 1980, were exposed to ozone (180 μg m−3) or to charcoal-filtered air (< 4 μg O3 m−3) for 21 d. Ozone sensitivity was assessed by recording the extent of visible injury, effects on mean relative growth rate (R̄) and changes in fast fluorescence kinetics. Ozone significantly (P < 0.01) depressed the mean relative growth rate (R̄) and there were significant (P < 0.01) differences in response between cultivars. Moreover, there was a highly significant (P < 0.002) negative relationship between the reduction in R̄ and the year of introduction; the more modern the cultivar the greater its sensitivity to ozone. Ozone reduced root growth relative to the shoot, but the effect varied with cultivar. Although all varieties developed typical visible symptoms of ozone damage, there was little relationship between the extent of this damage and effects on growth, or changes in fluorescence.
After 18 d fumigation there were significant (P < 0.001) changes in fast fluorescence kinetics in leaves that had not developed visible symptoms of injury. The ozone-induced reduction in t1/2 was closely related (r= 0.92, P < 0.002) to the depression in R̄, indicating that a fluorescence assay based upon changes in the rise in fluorescence between Fo and Fm can be used to distinguish between genotypes of contrasting ozone sensitivity. It is concluded from this study that inadvertent selection by plant breeders may have resulted in modern varieties of some crop plants that are considerably more sensitive to ozone than their predecessors. Possible explanations for this phenomenon are discussed.