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An Open-Top Chamber experiment on two tomato cultivars (cv. Oxheart and cv. San Marzano) was carried out in Curno (Northern Italy) between June and September 2007. Two ozone treatments were applied for a 3.5 months period: Non-Filtered OTC (NF-OTC, 95% of ambient ozone) and Charcoal-Filtered OTC (CF-OTC, 50% of ambient ozone). Diurnal cycles of porometry measurements were performed during the season and allowed to draw a stomatal conductance model for each cultivar in order to calculate the ozone stomatal fluxes taken up by plants. Assessments on fruits yield were performed during the season, taking into account the number of fruits, their fresh weight and their marketability. In addition, measurements on the chlorophyll fluorescence of photosystems were carried out to assess possible negative effects on photosynthetic efficiency. Despite the two cultivars absorbed a similar ozone stomatal dose during the season (with an 8% difference), their responses to ozone treatments were totally divergent in relation to both fruits yield and photosynthetic efficiency. Plants of cv. Oxheart grown in NF-OTCs showed significant yield loss in the total weight of fruits (-35.9%) which is exclusively related to a decrease in the number of fruits produced (-35.7% of total fruits; -30.6% of marketable fruits), since mean fresh weight of fruits remained unaffected. Moreover the same plants displayed low values (in comparison to CF-OTCs plants) of the photosynthetic efficiency index (PIabs) during the most intense period of ozone stress (July) occurred in the flowering stage of plants and at the beginning of fructification. Plants of the cv. San Marzano had an opposite response behaviour with an increase of the mean fresh weight of fruits in plants grown in NF-OTC (even if not statistically significant) and no difference in the number of fruits produced and in the values of photosynthetic efficiency.
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