PHYSIOLOGICAL AND ECOLOGICAL DRIVERS AND AGRONOMICAL CONSEQUENCES OF THE OZONE-LIKE SYNDROME IN WHEAT

Tropospheric ozone (O3) is known to adversely affect the productivity of a wide range of crops including wheat. However different species, can exhibit different responses to ozone exposure. Since the Triticum genus (wheat) is one of the most cultivated and consumed cereal on the global scale and also considered an O3-sensitive crop, research on its protection against ozone damages can contribute to the improvement of its productivity and thus the worldwide food security. The first part of this research was focused on a varietal screening experiment carried out in 2013 to assess the ozone sensitivity of 3 Italian and 2 Spanish cultivars of durum wheat (Triticum durum), applying two different levels of ozone (50% increment and 50% decrement of the ambient ozone concentration ) to plants grown in Open-Top Chambers. The durum wheat sensitivity to ozone was based on the assessment of leaf visible injuries, histochemical observations, physiological parameters, yield and yield quality analysis. Two Italian cultivars (Colombo and Sculptur) resulted more sensitive to ozone than the others, according to the physiological parameters tested and to the grain yield and quality analysis. However, they also showed different levels of leaf visible and microscopic injuries. The flag-leaves of cv Colombo resulted clearly more damaged by ozone exposure than the flag-leaves of cv Sculptur at both visible and microscopic levels. The second part of the research tried to analyze to a deeper extent the response to ozone exposure of the two most sensitive cultivars selected from the previous experiment. A second experiment was performed in 2014 using four ozone levels: -5% and -50% of ambient ozone concentration in non-filtered and charcoal-filtered OTCs, respectively; +30 and +60% of ambient ozone concentration in ozone-enriched OTCs (OZ+ and OZ++ OTC). In order to test the effectiveness of an antitranspirant compound in protecting durum wheat from ozone oxidative stress, a chitosan solution was weekly applied as leaf spraying during the growing season in 2014. The chitosan treatments were applied at 3 levels: tap-water (CTRL, no chitosan), 40kDa chitosan solution (CHI40) and 300kDa chitosan solution (CHI300). Both durum wheat cultivars confirmed their sensitivity to ozone as observed in the previous experiment. Grain yield losses observed in ozonated treatments were related to a decrease of stomatal conductance that is due to damages to the Rubisco and Calvin cycle. No protective effect due to chitosan treatments was observed in both cultivars. However, chitosan improved slightly the grain yield and the aboveground biomass production in plants grown in charcoal-filtered and non-filtered OTCs. Biomass data were also used for the definition of dose-effect relationships based on the ozone exposure (AOT40), the phytotoxic ozone dose (POD6) and the yield losses. The grain yield losses were plotted against AOT40 and POD6 in order to test the linear regression of these two indices. Each increase of AOT40 3000 ppb.h caused a grain yield loss of about 1.8%, while for the POD6, an increase of 1mmol O3 m-2 caused 1.3% reduction. Both AOT40 and POD6 resulted appropriated for assessment of durum wheat yield losses. However, the dose-effect relationship based on POD6 showed a better fit compared to the AOT40. During the 2014 experiment an important part of the research regarded the ultrastructural analysis of ozone-like symptoms on flag-leaves carried out by transmission electron microscopy (TEM), and the assessment of the levels of some antioxidant molecules (ascorbate and glutathione) involved in the ozone-detoxifying process, to understand the mechanisms underlying the different ozone sensitivity of Colombo and Sculptur in terms of visible and microscopic symptoms. Results from TEM demonstrated that visible symptoms in Colombo are due to the presence of damaged stomata and plasmolyzed mesophyll cells around the sub-stomatal cavity. On other hand, no damage on stomata, mesophyll cells and chloroplasts were observed in Sculptur cultivar explaining the absence of the visible symptoms. In general Sculptur showed higher levels of ascorbate content than Colombo, suggesting a higher capacity ascorbate biosynthesis. No significant difference in ascorbate content was found between plants exposed and not exposed to elevated ozone. The total and the oxidized glutathione content increased in the Colombo cultivar grown in elevated ozone conditions indicating that plant ability to maintain glutathione in the reduced form was decreased by the ozone stress.