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Ozone (O3) in ambient air was investigated as a possible cause of visible damage to leaves of white clover (Trifolium repens L.) in agricultural areas of Switzerland. Differences in the sensitivity of five commonly grown cultivars were studied, namely Ladino Sacramento, Ladino California, Alban, Sonja and Milkanova. Plants were grown in pots and exposed to O3 using three different experimental systems: (i) exposure to ambient air, (ii) exposure in opentop chambers to filtered air, unfiltered air, or unfiltered air to which O3 was added from 09.00 until 17.00 h during a four-day period, (iii) exposure under controlled environmental condition to filtered air with or without O3 added daily. Occurrence of injury symptoms on field-grown plants was recorded during the summers of 1987 and 1988. Ozone exposure produced visible injury expressed as small necrotic flecks on the upper leaf surface. Irrespective of the experimental system used, the following order in O3 sensitivity was observed: L. Sacramento > L. California ≫ Alban > Sonja > Milkanova. Injury symptoms were seen when plants were exposed to ambient O3 or to unfiltered air in open-top chambers, but not in charcoal-filtered air. In ambient air, symptoms in the most sensitive cultivars appeared after episodes with 1-h maximum concentrations of above 120-140 μg m-3 occurring on several consecutive days. The symptoms of injury could be reproduced in controlled-environment fumigation experiments using concentrations comparable to those observed in ambient air. Plants of the different cultivars grown under controlled environmental conditions did not differ in superoxide dismutase or peroxidase activity, or in the content of fructose, sucrose or glucose. Differences in O3-sensitivity between cultivars were related to differences in stomatal density and to some degree to the length of the stomatal pore. Stomatal densities were related to leaf conductance, transpiration, and rate of water loss by excised leaves. Leaves of Milkanova (resistant) had higher δ13C values than L. California (sensitive). Compared with filtered air, exposure to unfiltered ambient air or O3-amended air in open-top chambers caused a change in δ13C value in leaves of L. California (sensitive), but not in leaves of Milkanova (resistant). It is concluded that ambient levels of O3 in Switzerland can be sufficient to produce visible leaf injury in sensitive white clover cultivars characterized by a high gas-phase conductance of the leaves, that the isotopic composition of leaf tissue could be used as an indicator of O3 sensitivity in different cultivars, and that ambient levels of O3 can change the isotopic composition of the leaf tissue in sensitive species.
The New Phytologist © 1989 New Phytologist Trust