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Recent studies have shown that global atmospheric carbon dioxide (CO2) has increased markedly due to human activities, including burning of fossil fuels and deforestation, and its current level (383 ppm) has far exceeded the natural range (180-300ppm) seen over 6500 centuries (IPCC 2007). Rising of atmospheric CO2 has caused the globally averaged surface temperatures to increase by 0.6 ± 0.2°C over the 20th century, while surface air temperature is estimated by models to warm 1.1-2.9°C "low scenario" or 2.4-6.4°C "high scenario" by the end of the 21st century relative to 1990 (IPCC 2007). A new global climate model predicts that in the coming decade, the surface air temperature is likely to exceed existing records (Smith et al. 2007). Global warming can be accompanied by shifts in precipitation patterns around the world (IPCC 2007).

Both, natural and cultivated plants are affected by different components of global climate change, including elevated CO2 (Kimball 1983; Bazzaz 1990; Jablonski et al. 2002; Norby and Luo 2004; Ziska and Bunce 2006) and high temperature (Ferris et al. 1998; Challinor et al. 2005; Korner 2006; Morecroft and Pater-son 2006). Elevated atmospheric CO2 has positive effects on crop growth and productivity, both in terms of quantity and quality, by increasing photosynthesis and water use efficiency and decreasing transpiration through reducing stomatal conductance (Morison 1998; Long et al. 2004). Crop species are directly affected by increased atmospheric CO2, which changes the plant physical structures and carbon: nitrogen balance (Torbert et al. 2004), and, in turn, affects growth, yield (Kimball et al. 2002a), tolerance to drought stress (Robredo et al. 2007) and susceptibility to pests and herbivores (Heagle 2003). Responses of crops to climate change are closely related to the local climate variability rather than to the global climate patterns and, therefore, crop responses to climate change vary with region and plant species (IPCC 2007). For instance, in the western Canadian province of Alberta,

Department of Biological Sciences, University of Calgary, Calgary, Alberta T2N 1N4, Canada e-mail: [email protected]

S.N. Singh (ed.), Climate Change and Crops, Environmental Science and Engineering, DOI 10.1007/978-3-540-88246-6.1. © Springer-Verlag Berlin Heidelberg 2009

it is predicted that yields of canola (Brassica napus L.), corn (Zea mays L.) and wheat (Triticum aestivum L.) would increase, while in the province of Quebec, yields of corn and sorghum [Sorghum bicolor (L.) Moench] might increase, but yields of wheat and soybean [Glycine max (L.) Merr.] would decrease. Similarly, in the Atlantic Provinces, yields of grain corn and soybean might increase, but no changes are expected to occur in barley (Hordeum vulgare L.) yield (Lemmen and Warren 2004). Also, there is an expectation that yields of temperate crops would increase under elevated CO2 levels, but high temperature may offset this positive effect in determinate crops (Ferris et al. 1998).

A shift in growing areas of the world will probably occur due to global warming and as a result, crop productivity would increase in some areas, especially in temperate regions with extended growing season, but drastically decrease in others, such as equatorial regions (Hardy 2003). Also, some regions that currently support growth of a particular crop may in the future become unsuitable for the growth of that species because of warmer and drier conditions. In such conditions, a shift will have to be made for the production of another crop (Krupa and Groth 1999) as well as changes to human diet (Reddy et al. 1996).

The potential for crop productivity should increase with increased local average temperature over a range of 1-3°C, but decrease above this range (IPCC 2007) likely because of poor vernalization (Trnka et al. 2004), shortened phenological stages (Mitchell et al. 1993), decreased photosynthesis and increased transpiration and stomatal conductance (Nobel 2005). As indicated, increases in the frequency of both droughts and floods are projected to negatively affect local crop production (IPCC 2007), which may have a great impact on local, regional and global food supplies (Hardy 2003; Brouwer and McCarl 2006). Obviously, predictions based on the assumption that enhancement of crop productivity would be greater in drier ecosystems or in drier years have limited support (Nowak et al. 2004).

The separate and combined effects of elevated CO2 and high temperature on plants have been studied, either in growth chambers, in greenhouses or in the field (Nijs et al. 1997; Morison and Lawlor 1999; Tuba et al. 2003; Norby and Luo 2004; Lawlor 2005). Over the past decade, there have been increasing interests in studying the effects of CO2 on plants under field conditions, using free-air CO2 enrichment (FACE) facilities (Kimball et al. 2002a; Nowak et al. 2004; Ainsworth and Long 2005). The main reason for using FACE has been that the results obtained from enclosure studies cannot always accurately portray the response of plants to the natural environment because of size limitation on these systems (e.g., using pots, which constrain root growth) and focus on the early stages of plant growth rather than on the whole life-cycle of plants (Long et al. 2004). However, Amthor (2001) reviewed 156 experiments on winter wheat and showed similarities in results between field and controlled-environment experiments. For example, yield increased in the controlled-environment-grown plants and field-grown plants by 12-14% and 8.0-8.6%, respectively, per 100 ppm increase in CO2 concentration. Also, Kimball et al. (2002a) have shown that results from the earlier chamber-based studies are consistent with those from the FACE studies, and conclusions are accurate on the basis of both types of experimental approaches.

An ongoing argument exists among plant scientists who predict that the concomitant elevation of atmospheric CO2 and air temperature will improve crop production and increase food supply for the increasing human population worldwide (Wittwer 1995) and those who are concerned about the negative impacts of high temperature, which offsets the positive effects of elevated CO2 on crops in the future and, in turn, may lead to food shortage (Rosenzweig and Hillel 1998). The pros and cons of these two different opinions will be discussed later.

The purpose of this paper is to discuss the results of some of the experiments conducted to evaluate the separate and combined effects of elevated CO2 and temperature on crop growth and physiology. The implications of such studies in crop productivity and food supply in the future will also be discussed. In this review, relevant data from various studies considering CO2, temperature or other environmental factors have been used, regardless of the experimental systems, including controlled-environment growth chambers, greenhouses, open-top field chambers and free-air CO2 enrichmment (FACE) facilities.

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