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months, were classified as moderately sensitive to O3. In contrast, important cereal crops, such as maize and barley, can be considered to be moderately resistant and insensitive to O3, respectively.

The reasons for inter- and intra-specific differences in O3 sensitivity are not yet fully understood. It is clear that stomatal conductance, which determines O3 uptake, is often related to sensitivity (Reich 1987), although other factors, such as antioxidant levels or the evolution of stress ethylene, can also be related to O3 sensitivity. There is an evidence for a genetic basis of such difference in sensitivity and inadvertent selection in breeding processes may have affected O3 tolerance. There are, however, no indications that modern crop cultivars, bred under higher O3 concentrations than decades ago, are more tolerant to O3. Indeed, modern Greek cultivars of wheat are actually more sensitive to O3 than older cultivars (Barnes et al. 1990), suggesting that in wheat, selection for higher yield led to selection for characteristics associated with lower O3 resistance.

O3 may also have direct or indirect (through reduced carbon allocation) effects on the reproductive capacity, such as pollen germination, tube growth, fertilization, and the abscission or abortion of flowers, pods, ovules or seeds (Black et al. 2000). Due to changes in metabolite pools, increasing O3 concentrations can have significant impact on crop quality. Soja et al. (1997) showed that O3 exposure over 2 years caused a large decrease in sugar content of grape (Vitis vinifera). Juice quality was more sensitive to ozone exposure than grape yield (Soja et al. 2004). In oil seed rape (Brassica napus), both seed yield and oil content were reduced, which represents an additional economic loss (Ollerenshaw et al. 1999). The CHIP study (Changing Climate and Potential Impacts on Potato Yield and Quality) on potato at seven different sites across Europe, reported both beneficial and detrimental effects of season-long O3 exposure on tuber quality (Vandermeiren et al. 2005). Vitamin C content was increased, whereas reducing sugar and starch content were reduced (Vorne et al. 2002). Decrease in forage quality of grasslands has been demonstrated both in North America (Powell et al. 2003) and Europe (Fuhrer et al. 1994), which has economic implications for their use by ruminant herbivores.

Such qualitative and nutritional characteristics may become increasingly important from a commercial and industrial viewpoint, and hence more interesting for future research, especially in those countries where demand for food is stable or increasing slowly. On the other hand, in regions where there are problems in maintaining food supplies, the economic and social implications of widespread loss of yield could be very serious in the face of rapidly increasing populations and loss of productive land (Ashmore and Marshall 1999). A 20% increase in surface O3 by 2050 would result in yield losses relative to today's yields of 5, 4, 9 and 22% for maize, rice, wheat and soybean, respectively, and approximately double these losses by the end of the century (Long et al. 2005). Wang and Mauzerall (2004) project that with the very large increases in surface O3 projected for east-central China, crop losses for maize, rice and soybean will each exceed 30% by 2020. Although the actual economic costs of O3-induced crop losses are difficult to assess, the total benefits resulting from various regulatory scenarios, mostly involving reductions of current ambient levels, ranged from about 0.1-2.5 billion US $ in 1980 in the United States (Adams and Horst 2003). Additionally, in 1996, the US Environmental Protection Agency estimated annual national level losses to major crops to be in excess of 1 billion US $ in 1990 (US EPA 1996). Recently, Holland et al. (2006) quantified the range of O3-induced yield losses for 23 crops in 47 countries in Europe to be C4.4 to 9.3 billion per year for year 2000 emissions, with a best estimate of C6.7 billion per year. The core estimate represents losses equal to 2% of arable agricultural production in Europe. These estimates, however, do not account for damage via visible injury, changes in crop quality, or interactions with pests.

10.5 Modelling Ozone Uptake and Crop Yield Responses

Effects-based research has resulted in the establishment of critical levels2 of O3 for vegetation (LRTAP Convention 2007). Historically, critical levels of O3 for vegetation were based on the concentration of O3 in the atmosphere, but it has long been recognised that plant responses to O3 are more closely related to the internal O3 dose in the leaf, or the instantaneous flux of O3 through the stomata, than the ambient O3 exposure (Lefohn and Runeckles 1987; Fuhrer 2000). This approach requires mathematical modelling of the pathway of O3 into the leaf including atmospheric, boundary layer and stomatal resistances. The influence of plant phenology, irradi-ance, temperature, vapour pressure deficit and soil moisture on stomatal O3 uptake is incorporated in the models through their interference with stomatal conductance (Jarvis 1976). The differences between a concentration or flux-based approach can have very important implications for risk assessments across Europe (Simpson et al. 2007). While the highest AOT40 exposures are in central and southern Europe, considerable O3 fluxes have also been predicted in parts of northern and western Europe. Calculations of cumulative stomatal O3 uptake over the growing season for wheat and beech for four grid squares (in Sweden, UK, Czech Republik and Spain), which experience quite different AOT40 values, showed very little difference in modelled cumulative stomatal dose, primarily because of the effects differences in phenology and of modelled vapour pressure deficit (Emberson et al. 2000).

2 Critical level = concentration above which direct adverse effects on receptors, such as plants, ecosystems or materials, may occur according to current knowledge (UNECE 1988).

Crops and cultivars with equal stomatal conductance may have different tolerances to the same O3 concentrations. Therefore, Massman et al. (2000) and Tausz et al. (2007) proposed extending the flux approach linking estimates of O3 dose through stomata to the capacity of defence mechanisms to detoxify the incoming O3 flux. Both the absolute level of antioxidant enzymes (mainly superoxide dismutase, ascorbate peroxidase, glutathione reductase) and non-enzymatic low-molecular weight antioxidant molecules (ascorbate, glutathione, a-tocopherol), as

Fig. 10.2 The relationship between the relative yield of wheat and AFst6 for the wheat flag leaf based on five wheat cultivars and for sunlit leaves of potato cv Bintje from four European countries. The effective temperature sum was used to describe phenology (BE -Belgium, FI - Finland, IT -Italy, GE - Germany, SE -Sweden) (LRTAP Convention 2007)

Fig. 10.2 The relationship between the relative yield of wheat and AFst6 for the wheat flag leaf based on five wheat cultivars and for sunlit leaves of potato cv Bintje from four European countries. The effective temperature sum was used to describe phenology (BE -Belgium, FI - Finland, IT -Italy, GE - Germany, SE -Sweden) (LRTAP Convention 2007)

well as the capacity to enhance the anti-oxidative potential in response to O3, might contribute to protection of photosynthetic machinery and membrane functions from oxidative stress. Thus, it has been suggested that O3 flux models should include a coefficient for O3 detoxification capacity, such as a threshold O3 flux (see Fig. 10.2), which clearly improved the performance of flux-response relationships (Pleijel et al. 2004). In another approach, the defence response is regarded as an indirect function of photosynthesis, since photosynthate provides the raw material and energy for the defence and repair processes. Musselman and Massman (1999) defined the "effective flux" as the balance between O3 uptake into the leaf at a given time and the defence response at that time. The defence response factor is proportional to the effect of O3 on gross photosynthesis. Martin et al. (2000) proposed to determine a threshold coefficient based on the effects of O3 on the maximum carboxylation rate of Rubisco. More detailed models are available to describe the extent to which incoming O3 flux can be detoxified by reactions with ascorbate in the apoplast (Plochl et al. 2000).

Recently, stomatal flux-based critical levels of O3 were developed for crops (Pleijel et al. 2007; LRTAP Convention 2007) based on the relationship between the relative yield and modelled stomatal O3 flux for wheat and potato (Fig. 10.2). The flux-response relationships with the highest correlation were obtained using the exposure index AFst63.

In combination with the dry deposition model DO3SE (Deposition of Ozone and Stomatal Exchange) that allows the calculation of both stomatal and non-stomatal O3 deposition for a variety of land covers across Europe (Simpson et al. 2003), such flux-response models allow an estimation of the risk of O3 impact to vegetation across Europe under different policy scenarios. Further, cost-benefit analysis then allows policy-makers to devise cost-effective emission control programmes.

10.6 Influence of Global Change on Stomatal Flux and Detoxification of Ozone

The flux of O3 into the stomata and its subsequent detoxification are key determinants of the ultimate response of the plant to the pollutant. Since both are highly dependant on climatic conditions, there is a significant potential for the predicted changes in the climate to influence the response to O3 through an effect on rates of flux and detoxification. Effects can be direct - e.g. temperature, CO2 and humidity effects on stomatal conductance or indirect via an influence on soil water potential (SWP) and plant development.

3 AFst6 = accumulated stomatal flux of O3 above a threshold of 6 nmol O3 m 2 projected sunlit leaf area, based on hourly values of O3 flux.

10.7 Direct Impacts on Stomatal Flux of Ozone 10.7.1 Increased Temperature

The stomatal response to leaf temperature is described by a parabolic function (Fig. 10.3) with a minimum and maximum temperature at which stomatal opening occurs and an optimum temperature for stomatal conductance (gs) (Emberson et al. 2000). Each plant species will have its own optimum temperature for stomatal conductance and the impact of climate warming on gs will depend on which part of the temperature response function corresponds with the current ambient temperature. In temperate moist climates, an increase in temperature is likely to result in an increase of gs and therefore, an increase in the stomatal uptake of O3, possibly resulting in enhancement of leaf damage. For those plants already at their optimum temperature for gs, warming is likely to result in a decrease of gs and stomatal uptake of O3, possibly resulting in a reduction of leaf damage. The changes described can be predicted for sunlit leaves at the top of the canopy, however, the consequences for the stomatal flux into the whole canopy are yet unknown.

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