Growth and visible injuries of four Centaurea jacea L. ecotypes exposed to elevated ozone and carbon dioxide

We studied the variation in the responses of brown knapweed (Centaurea jacea) ecotypes from Finland and Switzerland to elevated O3 and CO2 and, the possible role of CO2 as a factor modifying O3 sensitivity. Individuals of four C. jacea ecotypes were exposed to elevated O3 (40 ppb) and CO2 (450 ppm) alone or in combination in open-top chambers (OTC) in the summer 2003. Open-field plots served as controls for the chamber effect. Ozone effects were mainly manifested as visible injuries: O3-specific light brown flecks and non-specific purple pigmentation. The proportion of O3-specific visible injuries was highest in the Swiss plants, where they correlated positively with early flowering, high dry matter production and high stem dry weight. CO2 ameliorated the severity of O3-specific visible injuries only in the most O3-sensitive ecotype, but diminished the differences between the different ecotypes. Ozone exposure also accelerated plant senescence, which was seen as enhanced development of purple pigmentation. The lack of growth responses may be explained by the low exposure concentrations. The intraspecific differences in O3 sensitivity may limit the use of C. jacea as a bioindicator for intact vegetation.