Ozone impact on maize

Ozone (O3) is the air pollutant that currently represents one of the greatest environmental concerns in the troposphere, with particles and nitrogen oxides. Ground levels of O3have increased continuously over the three decades before 2000, at a rate of 0.5-2% per year1. Although a stagnation or slight decrease over Europe and North America was observed since 2000, O3concentration is still increasing in Asia and others world parts2. Based on dose-effect curves, Mills et al. (2007)3identified three crops groups according to ozone sensibility: ozone sensitive crops (such as wheat), moderately sensitive crops (such as maize) and ozone resistant crops (such as barley)4. For a given AOT40, the decrease in leaf photosynthesis (established using the A/Ci curves) is similar in maize and wheat whereas the decrease in grain dry matter is about doubled in wheat compared to maize. To understand this discrepancy, we investigated changes in maize (Zea mays L.) Leaf Area Index (LAI), leaf gas exchanges and harvest index, due to ozone using anew linear field fumigation device. When fumigation occurred before flowering, leaf area decreased up to 15%4. If the LAI remained over 4, the intercepted radiation by the maize plants was maximal. Therefore, in this case, ozone impact on maize intercepted radiation was limited. When fumigation occurred after flowering, the decrease in leaf photosynthesis in the upper leaves of the plants negatively impacted their dry matter production. In response to an AOT40 of 22 mmol mol-1h, leaf photosynthesis decreased by 30% whereas the grain dry matter was reduced by 5% only, as previously observed by Mills et al.(2007)3. In addition, the shoot dry matter production was reduced by more than 11% and the harvest indexes were 51.6 % in control plants and 59 % in fumigated plants. Moreover, ozone-induced moderation of stomatal conductance resulted in a limitation of gase fluxes (CO2, O3) entering in the leaves. This moderation of stomatal conductance was similar to that observed for leaf photosynthesis. Since the stomatal conductance of maize leaves was approximately half that of wheat leaves, the phytotoxic ozone dose (POD)5in maize was half that of wheat POD, for a given AOT40.In conclusion, maize grain production was less impacted by ozone than wheat one due in part to lower maize gs and a grain sink priority.