Analysis and experimentation on ecosystems

In the context of increasing population and decreasing acreages, the reduction in crop yields induced by tropospheric ozone should be better evaluated. Therefore, ozone-resistant and ˗sensitive tobacco (cv. Bel-B and Bel-W3), barley and rapeseed plants were submitted to ozone-enriched air, during pre-flowering period. Six ozone concentrations (ranging from 30 to 130 ppb for six hours during the photoperiod) and two treatment durations (1 or 2 weeks) were applied. These treatments correspond to low, moderate and high exposure levels that occurs in France. Several leaf parameters were measured: necrosis development, chlorophyll content, chlorophyll fluorescence (Fv/Fm, ΦPSII), gas exchange (carbon assimilation and leaf conductance), seed yield, and fatty acid composition in cell membranes. The fatty acid composition was used to calculate the "Omega-3 index", a biomarker based on stress-induced changes in leaf 18:3 content, developed to assess soil quality. To estimate the oxidative load in leaf tissues, quantification of targets of oxidative stress (such as intracellular proteins carbonylation) was rather used instead poorly reproductible ROS quantification. Results show that ozone treatments decrease chlorophyll content and omega-3 index. The extent of the declines and the threshold levels of ozone exposure triggering the declines observed in the different plant species studied will be discussed. Direct continuation of this study, will involve a water stress applied to the plants subsequent to the ozone treatments. The same physiological approach will then be used to characterize the plant responses to the combined stress treatments, to assess their resilience to drought after an ozone pollution episode.