Ozone stress in rust resistant and susceptible Helianthus annuus cultivars as measured with chlorophyll fluorescence

Rust resistant and susceptible cultivars of Helianthus annuus were exposed for short periods to 230ppb ozone, a concentration often measured in central Europe and elsewhere during summer months. Chlorophyll fluorescence induction was used to measure a number of photosynthetic parameters. The stress caused was detectible within minutes following exposure to ozone. Measurements of phi(max) and phi(ss) showed that only the susceptible cultivar was stressed. However, following F-s and F-m. fluorescence over long periods, it was clear that both cultivars were stressed by ozone. Subsequent to the fumigation with ozone, the resistant cultivar restored to levels prior to exposure to ozone, indicating repair to the damage caused. Both cultivars showed repair during the dark, while the damage caused to the susceptible cultiva appeared to be permanent. Both the utilisation of excitons as well as the number of closed versus open reaction centres were influenced by the ozone. Resistance to a stressor (in this case to rust) was beneficial to the plants, indicating a common response to stress and also adaptation to stressors.