Ozone dose-response relationships for durum wheat in Mediterranean conditions

The aim of this study was to define an ozone (O3) dose-response function and identify a critical level for the protection of Triticum durum in Mediterranean conditions. ‘Colombo’ and ‘Sculptur’ are two modern durum wheat cultivars that in previous studies proved to be very sensitive to O3 stress at both eco-physiological and agronomical level. Two consecutive experiments on these cultivars were carried out in 2013 and 2014 at the Open-Top Chambers facility of Curno (Northern Italy). Plants of durum wheat were exposed to 2 and 4 different levels of O3 in 2013 and 2014, respectively. The seasonal accumulation of Phytotoxic Ozone Dose (as POD6) and O3 exposure (as AOT40) were correlated with the reduction of grain yield, total aboveground biomass, stems, number of spikes and hectolitre weight. ‘Colombo’ resulted more affected by O3 than ‘Sculptur’ in both years of the experiments, with a significant decrease in yield and growth parameters. ‘Sculptur’ showed significant negative effects only in the highest O3 level treatments. Regression analysis on grain yield were performed using both the AOT40 and the POD6, and the relative effects were calculated on the basis of the mean values of plants grown in Charcoal-Filtered OTC (-50% of ambient ozone). According to this study the POD6 value causing a 5% of decrease in relative grain yield was around 3 mmol O3 m-2 for cv ‘Colombo’ and 4 mmol O3 m-2 for cv ‘Sculptur’. Considering the two cultivars together, we can propose a critical level of POD6 of 3.5 mmol O3 m-2 for a 5% reduction of grain yield to be used in the Mediterranean countries for Triticum durum. Analogously the AOT40 critical level could be set to 8’000 ppb.h. The critical level based on POD6 is 3.5 times higher than that proposed in the Mapping Manual for Triticum aestivum (1 mmol O3 m-2). Results of this study demonstrate clearly that both relationships based on the O3 exposure and POD6 proposed in the Mapping Manual could overestimate the O3 effects on durum wheat under Mediterranean conditions.