Ozone biomonitoring with clover clones: yield loss and carryover effect under high ambient ozone levels in northern Italy

Two clones of white clover (Trifolium repens L. cv. Regal), NC-R (O3-resistant) and NC-S (O3sensitive) were used to assess the effects of ambient ozone at two sites located in the Po plain of Italy. The experiments were carried out as part of the UNECE ICP Vegetation, an international programme within the convention on long-range transboundary air pollution. Clover clones were exposed at Redecesio di Segrate (an urban site near Milan) during 1996 and at Isola Serafini (a rural site near Piacenza) from 1997 to 1999. Each year, the plants were grown in pots according to a standard protocol and exposed to ambient air from May to September. The plants were harvested five times each season by cutting leaves, stolons and flowers, and the dry weight per pot was determined. Ozone concentration was continuously monitored and the total AOT40 (in ppb h) for the 3 months from harvest 1 to harvest 4 was 13985 in 1996, 18275 in 1997, 27815 in 1998 and 18378 in 1999. These ozone levels always affected the growth of the NC-S clone, with biomass reduction of up to 60% (relative to NC-R) recorded at harvests. Linear regression was conducted after pooling the AOT40 and NC-S/NC-R values for harvests 2-5 and year (r2=0.026). Moreover, the same analysis was carried out by correlating the AOT40 value of a certain growth period to NC-S/NC-R ratio of the following growth period (r2=0.61). These results suggest a carryover effect of ozone doses during one growth period to the subsequent growth period. Finally, data processing was refined by considering only harvests 2-4 of the Isola Serafini experiments and including the carryover effect. The function thus obtained explained an even bigger part of the statistical variability, having an r2 of 0.91. Carryover effect and site characteristics seem to be important factors to understanding the ozone dose-response relationships.