Climate change, ozone and plant life

Ozone (O₃) is a secondary gaseous pollutant since it is formed, under solar radiation and high temperature, by reactions among precursors, primarily nitrogen oxides and volatile organic compounds (VOCs). Concentrations of ground-level O₃ have been increasing over the last century and, despite significant control efforts and legislation to reduce O₃ precursor emissions, ground-level O₃ is still a major air quality issue over large regions worldwide. The current tropospheric O₃ levels (35-50 ppb in the northern hemisphere) are high enough to damage both forests and crops by reducing growth rates and productivity. Plants are essential players in the context of air pollution scenarios due to O₃. They are primary target of O₃ toxicity, but – at the same time – they remove pollutants from the atmosphere (mainly during their physiological gas exchange activity), so doing protecting human health. However, many species themselves emit biogenic VOCs, which enter the troposphere to take part to the whirlwind of reactions which bring to photochemical smog and O₃ formation. The real impact of O₃ on plants is however related to their capacity to respond by detoxification and repair mechanisms. The production of O₃ is controlled by temperature, sunlight and humidity, and by the longrange transport of precursors all of which are sensitive to changes in climate. Many of the processes creating or destroying O₃ or delivering it to ground level are influenced by synoptic and local weather patterns. Climate change is thus inherently coupled to changes in regional and local meteorology, which may affect the potential for build-up of plant (and human) harmful levels of O₃ in regionally polluted areas. Globally, an increase in the frequency of high O₃ concentrations due to changes in weather and rainfall patterns is expected due to climate change. During the 21st century over Europe, for example, an increased frequency of summer droughts, heat-wave events and associated high O₃ episodes is anticipated. Stomatal closure in vegetation under these expected dry conditions will also reduce the absorption of pollutants, one of the dominant processes controlling ground-level O₃. On the other hand, increases in O₃ will also have indirect effects on global warming, by reducing plant growth and consequently its role in carbon sink for carbon dioxide (CO₂), so representing a driver for an increase in the rate of CO₂ rise in the atmosphere. Ozone is also an important greenhouse gas, already ranked third behind CO₂ and methane, with a direct radiative forcing on climate of 0.35-0.37 W m-2. Feed-back reactions are a never ending story when complex processes are involved. A full comprehension of the prediction of the overall continual impact of O₃ on plant life under changing climate – able of quantifying effects on ecological processes and provisioning services, but also considering the implications for ecosystem services, and based on a mechanistic and biologically-relevant understanding of the single processes involved across time and space – is so far missing. The road is still long and winding.