Crop Responses to Elevated Ozone

Tropospheric ozone concentrations ([O3]) have more than doubled over land in the Northern Hemisphere since pre-industrial times (Akimoto 2003; Vingarzan 2004). Ozone is a dynamic secondary pollutant formed from the photochemical oxidation of methane, carbon monoxide, and volatile organic compounds in the presence of nitrogen oxides. Hot, sunny weather favors formation of ozone in the troposphere, and high concentrations can occur across large areas, far from industrial sources (Ashmore 2005). Between 1876 and 1910, background O3 concentrations were estimated to range from 5 to 16 ppb (Volz and Kley 1988), while modern day global annual mean O3 concentrations range from approximately 28 ppb in South America to 45 ppb in Southern Asia (Dentener et al. 2006). Unlike CO2 which is relatively well mixed in the atmosphere, there is significant variability in [O3] depending on geographic location, elevation and the extent of anthropogenic sources (Vingarzan 2004; Ashmore 2005). In the major crop growing regions of the United States in 2005, the daytime surface O3 concentrations during summer months ranged from 50 to 65 ppb (Tong et al. 2007). The future [O3] will depend upon anthropogenic emissions, trends in temperature, humidity and solar radiation, and implementation of air quality legislation. Only with a global implementation of O3 precursor control measures will background [O3] decrease in the future. Without rapid and global implementation of legislation, by 2030 average [O3] over the Northern Hemisphere could increase by 2 to 7 ppb, and by 2100, extreme emission scenarios project a baseline increase of more than 20 ppb (Prather et al. 2003).

Ozone enters plants through the stomata, where it reacts to form other reactive oxygen species, which in turn alter a number of physiological processes (Fiscus et al. 2005; Fuhrer 2009). Ozone decreases photosynthetic carbon gain by impairing Rubisco activity and reducing stomatal conductance (e.g., Morgan et al. 2004), inhibits reproduction by affecting pollen germination, fertilization and abortion of flowers (Black et al. 2000), impairs phloem loading and assimilate partitioning to roots and grains (Fuhrer and Booker, 2003), and decreases aboveground biomass, individual grain number and mass, and final harvestable yield (Morgan et al. 2003; Ainsworth 2008; Feng et al. 2008).

A number of different exposure indicators are used to calculate dose response functions, including seasonal 7 and 12 h mean [O3] during daylight, and seasonal cumulative exposure over a threshold of 40 ppb (AOT40) or 60 ppb (SUM06) (Mauzerall and Wang 2001). Crop-specific O3-exposure functions, which relate a quantifiable O3-exposure indicator to reductions in crop yield, have been developed from extensive OTC studies in the United States (National Crop Loss Assessment Network - NCLAN) and Europe (European Open Top Chamber Program - EOTCP)

(Heck et al. 1987; Fuhrer et al. 1997), and are used to assess both current and future levels of crop loss to O3. Mills et al. (2007) synthesized linear AOT40-based response functions for different crops from over 700 studies, and found that there were three significantly different groups of responses (Fig. 7.4). Wheat, watermelon, pulses, cotton, turnip, tomato, onion, soybean and lettuce were O3-sensitive; sugar beet, potato, oilseed rape, tobacco, rice, maize, grape and broccoli were moderately sensitive; and barley, plum and strawberry were O3-resistant (Fig. 7.4).

Ozone-crop yield response functions can be used with different emissions scenarios and global chemistry transport models to estimate current and future relative yield losses to [O3] (e.g., Wang and Mauzerall 2004; Tong et al. 2007; Van Dingenen et al. 2009). Using the IPCC B2 scenario of moderate population growth, intermediate levels of economic development and increased concern for environmental and social sustainability, Wang and Mauzerall (2004) projected that between 1990 and 2020, grain yield loss to [O3] would increase by 35, 65 and 85% in Japan, Korea and China, respectively. In a global analysis with the optimistic scenario that all current emissions legislation will be fully implemented by 2030, Van Dingenen et al. (2009) project that the global relative yield losses to O3 will increase by 4% for wheat, 0.5% for soybean, 0.2% for maize and 1.7% for rice by 2030. Clearly, estimates of the effects of O3 on future crop production depend upon trends in

Resistant Crops Moderately Sensitive Crops Sensitive Crops

10 20 30 40 50 60

Fig. 7.4 The combined response of O3-resistant crops (barley, plum and strawberry), moderately O3-sensitive crops (sugar beet, potato, oilseed rape, tobacco, rice, maize, grape and broccoli) and O3-sensitive (wheat, water melon, pulses, cotton, turnip, tomato, onion, soybean and lettuce) to O3 dosage, measured as the accumulation over a threshold of 40 ppb (AOT 40) (figure is redrawn with permission from Mills et al. (2007)

emissions and legislation, and they also have a number of other limitations. First, they are based on crop response functions derived for European and North American crops that were grown under well-fertilized and well-watered conditions (Van Dingenen et al. 2009). Second, they do not take into account the interaction of rising [O3] with changes in temperature, atmospheric [CO2] and the hydrological cycle, which would affect O3 uptake into the leaves (Wang and Mauzerall 2004). In most studies where crops have been grown in elevated [CO2] and elevated [O3], yield loss is less than with [O3] alone (Morgan et al. 2003; Fuhrer 2009). Still, with a current cost of crop losses to O3 in the range of $14-26 billion (Van Dingenen et al. 2009), further research on understanding the mechanisms of response and breeding for tolerance is critical.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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