Warming will generally increase evaporation, total precipitation, and the spatial variability of precipitation, leading to less rainfall in the tropics and more rainfall at higher latitudes. However, the spatial and temporal boundaries between areas of projected increasing and decreasing precipitation are uncertain. Globally, the intensity of precipitation events is projected to increase, even in areas with a mean reduction in precipitation, and the time between precipitation events is also projected to increase, thereby increasing both the risk of flooding and drought (IPCC, 2007).

Unlike [CO₂], tropospheric [O₃] is spatially and temporally heterogeneous because O₃ is short lived and its synthesis is tied to the abundance of its pollutant precursors, and water vapor and sunlight. In industrialized countries of the northern hemisphere, daily 8-h tropospheric [O₃] is estimated to have increased from approximately 10 nmol mol⁻¹ prior to the industrial revolution to a current level of approximately 60 nmol mol⁻¹ during summer months, and is predicted to increase 20% more by 2050 (IPCC, 2007). This is particularly relevant to agriculture because sensitive crops show a reduction in yield once the [O₃] exceeds 40 nmol mol⁻¹ for extended periods (Heagle, 1989).

As [CO₂] rises, photosynthesis in C₃ crops shifts from being limited by Rubisco to being limited by the capacity for regeneration of the CO₂ acceptor, ribulose-1,5-bisphosphate (RuBP; Ainsworth and Rogers, 2007). An alternative to reducing excess Rubisco content at elevated [CO₂] is to increase the capacity for regeneration of RuBP and thereby match the increased carboxylation rates. Recent modeling analysis suggests that manipulating the partitioning of N resources among enzymes of C metabolism could greatly increase photosynthesis without any increase in the total N requirement (Zhu et al., 2007). Greater capacity for the regenerative phase of the Calvin cycle was identified as a key target to improve C acquisition. There is already some evidence that crops increase RuBP regeneration capacity at elevated [CO₂]. The level of Fru-1,6-bisphosphatase was increased in a forage crop (Lolium perenne) grown at elevated [CO₂] with a high N supply (Rogers et al., 1998) and in tobacco (Nicotiana tabacum), Rubisco activity decreased and other Calvin cycle enzymes increased at elevated [CO₂] (Geiger et al., 1999). In addition, antisense technology coupled with flux analysis has shown that sedoheptulose-1,7-bisphosphatase exerts considerable control over photosynthesis, and plants overexpressing sedoheptulose-1,7-bisphosphatase had a marked increase in photosynthesis and growth (Raines, 2003; Lefebvre et al., 2005).

Accumulation of foliar carbohydrates is one of the most pronounced and universally observed responses of C₃ plants to elevated [CO₂], even in field-grown plants where rooting volume is unrestricted (Long et al., 2004). Knowledge of the relationship between C status and growth is advancing (Raines and Paul, 2006; Smith and Stitt, 2007), but a clear picture of how carbohydrates are sensed by the plant and how they regulate the flux of C through different pathways, impacting growth and partitioning, remains a major challenge to developing crops that will respond maximally to rising [CO₂]. Large and sustained increases in Suc, starch, or fructan content that persist over a number of days are usually indicative of replete sinks (Rogers and Ainsworth, 2006). This has two important implications. First, crops could be manipulated to use the additionally available C to increase growth, seed yield, or stress tolerance (see below). Second, because foliar carbohydrates are known to have negative feedback on photosynthetic capacity (for review, see Long et al., 2004), ensuring adequate sink strength in crops will be an essential part of maximally exploiting rising [CO₂]. Evidence from a Free Air Concentration Enrichment (FACE) experiment supports this view. There was no reduction in maximum Rubisco activity at elevated [CO₂] in an indeterminate soybean (Glycine max) cultivar, but in an otherwise identical isoline that had a single gene mutation resulting in a determinate growth habit and lower sink strength, carbohydrates accumulated and photosynthetic capacity was significantly reduced (Ainsworth et al., 2004). This illustrates that the capacity for utilization of photosynthate is critical to a crop's ability to sustain increased C acquisition at elevated [CO₂]. Further, a recent review concluded that at current [CO₂], yields of maize (Zea mays), wheat (Triticum aestivum), and soybean were limited more by sink capacity than photosynthetic capacity (Borras et al., 2004). In contrast, results from elevated [CO₂] research clearly show that increased photosynthesis does stimulate yield at elevated [CO₂] (Kimball et al., 2002; Ainsworth and Long, 2005; Long et al., 2006), suggesting that increases in yield can also be source driven. However, it is likely that increases in photosynthesis will need to be balanced by commensurate increases in sink capacity if maximum yield potential is to be realized.

What are the components of yield that might be limiting the response of crops to elevated [CO₂]? Yields of soybean and rice (Oryza sativa) grown under elevated [CO₂] in FACE experiments were 15% and 13% higher than yields of crops grown at ambient [CO₂], despite significant declines in harvest index (HI; seed yield/total aboveground biological yield; Kim et al., 2003; Morgan et al., 2005). Genetic improvements over the past century in grain yield in a number of crops, including soybean, wheat, and rice, have been closely associated with improvements in HI (Gifford et al., 1984); however, it has been argued that the scope for further increasing yield through increasing HI is limited (Austin, 1999). Still, at least maintaining current levels of HI at elevated [CO₂] would improve productivity. Regardless of improvements to HI, a second way to increase yield is to increase total biomass at maturity. More aboveground biomass per area has been the major factor in yield improvements at elevated [CO₂] in FACE experiments. In rice, elevated [CO₂] decreased the productive tiller ratio (Kim et al., 2003) and reduced the percentage of fertile spikelets (Yang et al., 2006). However, because the tiller number (and panicle number) per area was significantly higher at elevated [CO₂], yields were increased despite the negative effects on productive tiller ratio and spikelet fertility (Kim et al., 2003; Yang et al., 2006). Elevated [CO₂] also has little effect on individual grain or seed mass (Morgan et al., 2005; Yang et al., 2006), which may not be surprising because individual grain weight has not substantially changed with genetic improvement in wheat, maize, or soybean over much of the last century (Morrison et al., 2000; Fischer, 2007). Therefore, efforts to target grain or seed mass may not be fruitful, but there appears to be the opportunity to improve biomass production, the number of reproductive sinks, and therefore yield.

In their research and development pipelines, the major biotechnology companies (Monsanto, Syngenta, and Pioneer-DuPont) all have investments aimed at improving N use efficiency. Although growth at elevated [CO₂] improves N use efficiency (Drake et al., 1997), improved nutrition is essential to maximally exploit rising [CO₂]. The greater efficiency of carboxylation at elevated [CO₂] makes it theoretically possible for C₃ crops in 2050 to reduce their Rubisco content by approximately 25% and maintain the same photosynthetic rate as they have today (Drake et al., 1997). Consistent with this theory, C₃ crops grown at 567 μmol mol⁻¹ [CO₂] in FACE studies reduced their maximum Rubisco activity by approximately 20% (Ainsworth and Rogers, 2007). This reduction in Rubisco activity is most likely due to a reduction in Rubisco content (Long et al., 2004). Crops with the C₃ pathway invest up to 25% of their leaf N in Rubisco to compensate for its poor catalytic rate (Long et al., 2004). The large investment in Rubisco, coupled with a potential 25% reduction in Rubisco content, could release approximately 6% of leaf N at elevated [CO₂]. However, there is only benefit in reducing the amount of N in Rubisco when the resources can be usefully deployed elsewhere, either within the leaf, or to support sink development in the plant (Parry et al., 2003). When this 6% theoretical savings in N is compared to the predicted 10% to 40% increase in C acquisition (Long et al., 2004), the N acquisition gap at elevated [CO₂] is readily apparent. Despite increased yields in C₃ crops at elevated [CO₂], a recent meta-analysis of the protein concentration in food crops shows that the N acquisition gap at elevated [CO₂] manifests itself as a 10% to 15% reduction in grain protein content, even in field-grown crops (Taub et al., 2008). This is notable because the major biotechnology companies are attempting to increase protein content in grain crops and will have to work against the effects of rising [CO₂] to achieve this goal (Stafford, 2007). Legumes might be expected to avoid the N acquisition gap at elevated [CO₂] because they can trade their excess C for N with their bacterial symbionts. Evidence from soybean supports this hypothesis because grain protein content showed only a 1.4% decrease at elevated [CO₂] (Taub et al., 2008) and soybean grown at elevated [CO₂] using FACE technology avoided N limitation (Rogers et al., 2006). Introducing the capacity for N fixation into nonleguminous plants to improve nutrition and reduce demand for fertilizer has always been desirable, but closing the N acquisition gap at elevated [CO₂] and maximizing increases in yield will make this goal even more attractive in 2050.

These targets for biotechnological improvement of crop responses to elevated [CO₂] should be selected with consideration for the projected increase in temperature, drought stress, and [O₃] that will accompany rising [CO₂]. Because C supply is increased at elevated [CO₂], it may be possible to partition a greater fraction of photosynthate into C-rich metabolites associated with stress resistance. For instance, pinitol, mannitol, and raffinose are examples of osmolytes that support maintenance of turgor under drought and high temperatures, whereas antioxidants such as ascorbate have been implicated in tolerance of crops to drought, heat, salt, and oxidative stresses (Streeter et al., 2001; Bartels and Sunkar, 2005; Busch et al., 2005). Larger pool sizes of these metabolites could extend the benefits of growth at elevated [CO₂] in germplasm marketable to farmers in poor growing areas, not just prime agricultural zones.

Recent reviews of the effects of elevated [O₃] on plants have identified distinct mechanisms of response to chronic and acute O₃ exposure (Long and Naidu, 2002; Fiscus et al., 2005; Kangasjärvi et al., 2005). Exposures to daily peak [O₃] of <120 nmol mol⁻¹ for days, weeks, or months are typically considered to be chronic, whereas exposures to daily peak [O₃] of >120 to 150 nmol mol⁻¹ for as little as a few hours are considered acute (Long and Naidu, 2002; Fiscus et al., 2005). Distinguishing between chronic and acute responses has helped fair comparison of experiments imposing shock treatments versus those simulating current or future field conditions. However, the threshold concentrations characterizing acute versus chronic experiments are arbitrary and the distinction between plant responses is subjective. With monitoring stations in locations such as China reporting hourly maximum [O₃] >200 nmol mol⁻¹ and average annual concentrations of 74 nmol mol⁻¹ (Wang et al., 2007), plants in the field will likely experience both acute and chronic O₃ conditions simultaneously. This situation is more likely to occur in the future and determining the threshold [O₃] required for induction of various molecular, biochemical, and physiological responses will be increasingly important.

The process of conventional breeding, integrating the results of thousands of plots across years and environments, would be expected to enable exploitation of some of the potential benefits of elevated [CO₂], be it with a significant time lag. In contrast, the variability in [O₃] will not lead to the consistent selection pressure that is needed to make breeding advances. To date, discussions of improving tolerance to elevated [O₃] have focused on three general strategies: (1) controlling O₃ entry into the leaf, (2) improving detoxification within cells, and (3) altering the signal transduction pathway. Stomata regulate the flux of O₃ into leaves, and the protective value of stomatal closure in response to O₃ has long been recognized (Hill and Littlefield, 1969). Therefore, reducing stomatal conductance is an obvious biotechnological strategy to increase O₃ tolerance (Lin et al., 2001). However, the fundamental trade-off of lower stomatal conductance leading to lower photosynthesis must be recognized, and the balance between reducing stress versus reducing productivity evaluated. Elevated [CO₂] will reduce stomatal conductance and O₃ uptake in the future, while increasing rather than decreasing photosynthesis. This will reduce the negative impacts of elevated [O₃] (Booker and Fiscus, 2005). In many growing regions experiencing moderate increases in [O₃], the gain from targeting reduced stomatal conductance to ameliorate stress would likely be small, as the decrease in productivity would outweigh any reduction of O₃-induced stress. The wheat genotypes released in China between 1945 and today provide an example of net gain in yield, despite increased sensitivity to elevated [O₃]. Breeding for higher C uptake, shorter stalks, and improved HI has improved the yield potential from approximately 1,000 kg ha⁻¹ to approximately 8,000 kg ha⁻¹, despite greater O₃ uptake from greater stomatal conductance (Biswas et al., 2008). Although there is not always a simple relationship between stomatal conductance and O₃ tolerance, targeting stomatal conductance as a means to improve crop production under elevated [O₃] seems unlikely to be an effective strategy for maintaining productivity, other than in regions with the highest [O₃]. Nonetheless, it should be noted that the relative concentrations of CO₂ necessary to protect crops from O₃ damage are very poorly defined, and variable for species and genotypes. Furthermore, any direct effect of elevated [O₃] on reproductive structures will probably not be mitigated by elevated [CO₂], and loss of reproductive sinks may constrain the response to elevated [CO₂] (McKee et al., 1997; Ashmore, 2005). In addition, elevated [CO₂] did not prevent acceleration of senescence that is characteristic of O₃ fumigation at ambient [CO₂] (Fiscus et al., 2005). Without further experimentation, it is not possible to precisely predict where and when the stress of elevated [O₃] will be matched or exceeded by the benefits of elevated [CO₂].

A second strategy for improving crop tolerance to O₃ has focused on improving detoxification of O₃-induced reactive oxygen species (ROS; Fiscus et al., 2005). Ozone passes through the stomata, and is rapidly degraded in the apoplast forming various ROS. The ROS burst following O₃ exposure is similar to the oxidative burst associated with the hypersensitive response, which leads to programmed cell death (Overmyer et al., 2003). Ascorbate is one of the major antioxidants thought to govern O₃ tolerance in plants, and total ascorbate content has been correlated with O₃ tolerance in a wide range of species (for review, see Conklin and Barth, 2004). Localization of ascorbate or other antioxidants to the apoplastic space, where interception and detoxification occurs, appears to be important in protection from O₃ damage, as is recycling of ascorbate back to the reduced “anti-oxidant” state (Burkey et al., 2003; Conklin and Barth, 2004). However, not all studies have demonstrated that O₃ tolerance correlates with ascorbate content (e.g. D'Haese et al., 2005), and apoplastic ascorborate concentrations in some plants may be too low to play a significant role in detoxification. Still, increasing the apoplastic antioxidant capacity of crops may be a fruitful target for limiting O₃-induced ROS damage.