Yield response of some italian and spanish cultivars of durum wheat to elevated ozone: a varietal screening

Durum wheat is the main wheat species grown in Southern Europe countries. Hence, the evaluation of the response of the main cultivars of durum wheat to tropospheric ozone is a priority for Southern Europe. The present work investigated the O3 effects on five cultivars of durum wheat, three of them from Italy (Colombo, Sculptur, Faraon) and two from Spain (Gallareta and Vitron) in terms of yield and related physiological drivers. The experiment was performed in the open-top chambers (OTC) facility of C.R.IN.E.S at Curno, Italy, from 09th March to 15th June 2013. Each cv was sown in 12 pots (three plants per pot) and pots were randomly assigned to 4 OTC (three pots per OTC for each cv), two of them supplied with charcoal filtered air (-50% of ambient air ozone = CF) and two with ozone enriched air (+50% of ambient air ozone = EN). During the experiment the ozonated plants were exposed to a daylight AOT40 value of 11.561 ppm•h, well above the UN/ECE critical level of 3 ppmh, while the control (CF) plants experienced an exposure of 0.432 ppm•h. Periodical measurements of stomatal conductance, chlorophyll-a fluorescence and growth were performed during the growing season. Furthermore, macroscopic (chlorotic/necrotic spots) and microscopic (cell death and H2O2 deposits in mesophyll tissue) symptoms development were followed on the flag leaves. At the end of the season plants were harvested and the stems and the ears of each plant were separately dried and weighted. Then the grain yield was assessed per each OTC as total grain weight and hectoliter weight. The Italian cv Sculptur, the most productive one without ozone, together with the cv Colombo resulted the most ozone sensitive cvs, with a grain yield decrease of 16% (p<0.05) and 10% (ns) respectively in the EN treatment. Nevertheless very small differences were observed in the hectoliter weight between the two ozone treatments. The remaining cvs did not show any negative effect of ozone but rather a slight yield stimulation (between 1 and 5%, ns). The Spanish Gallareta resulted the best performing cv when both CF and EN yield are considered. Ozone affected the earing and the plants growth: the number of ears was quite uniformly reduced by ozone, with a remarkable low for Sculptur, and the losses of the aboveground biomass confirm the ozone sensitivity of Sculpur and Colombo (-20% and -25% respectively, both p<0.01). The stomatal conductance was greatly decreased by ozone in all cvs (between 11% and 37%, p<0.01), but it is notable that the most sensitive cvs showed the highest gs values, even under the ozone treatment, which implies a highest ozone dose. This would confirm that the stomatal dose is an important driver of the plants’ response to this pollutant. On the contrary the overall performance of the photosynthesis (PI) - assessed by the OJIP test on the chlorophyll fluorescence measurements – was not affected by ozone. Thus, the observed reduction of productivity in the sensitive cvs is not to be ascribed to a damage of the photosystyems but rather to a dissipation of the available energy for the metabolic defense against the oxidative stress. Finally, the observation of the the Sculpure cv behaviour suggests that the breeding strategy of selecting a greater gs to increase the yield may not be appropriate under increasing ozone conditions.