The projected increase in atmospheric CO2 is expected to enhance growth and production of agricultural terrestrial plants (Easterling et al., 2007). Studies have also shown that the effects of elevated CO2 on plant growth and yield may depend on photosynthetic pathway, plant species, growth stage, and management practices, such as water and nitrogen applications (Jablonski et al., 2002; Kimball et al., 2002; Ainsworth and Long, 2005). Crops with C3 photosynthetic pathway respond markedly to increasing CO2 concentrations compared to C4 crops. Common C3 crops are small grain cereals (wheat, rice, barley, oat (Avena sativa L.), and rye); grain legume or pulses (soybean, peanut (Arachis hypogaea L.), various beans (Phaseolus sp.) and peas), root and tuber crops (potato (Solanum tuberosum L.), cassava (Manihot esculenta L.), sugar beet (Beta vulgaris L.), and yams (Dioscorea sp.), and most oil, fruit, nut, vegetable, and fiber crops. Common C4 crops are maize, sugarcane (Saccharum officinarum L.), sorghum (Sorghum bicolor L. Moench), millet, and many tropical and subtropical zone (warm-climate) grass species. Elevated CO2 generally increases both above- and belowground biomass, volume and length of roots, and biomass allocation to roots (increased root-shoot ratio). Root and tuber crops tend to have a greater yield response to elevated CO2 than seed or forage crops. Increased photosynthesis also favors symbiotic nitrogen fixation in legumes. Since legumes can supply nitrogen via symbiotic nitrogen fixation, legumes (both seed and forage) respond relatively more to increased CO2 than non-legumes. Seed yields generally increase in a nonlinear fashion in response to increased CO2; however, this increase in not as much as photosynthesis because part of the fixed carbon goes to increased vegetative biomass. Averaged across several species and under unstressed conditions, analysis shows that, compared with the current CO2, crop yields increased at 550 ppm, CO2 was 10% -12% for C3 crops and 0% -10% for C4 crops (Ainsworth et al., 2004; Gifford, 2004; Long et al., 2004). However, in a recent analysis of the FACE (free-air-carbon-dioxide enrichment) experimental results, Long et al. (2005, 2006) argued that crop responses to elevated CO2 might be lower than previously thought because of overestimation of responses using crop models. Others have suggested that these new analyses are in fact consistent with previous findings from both FACE and other experimental settings (Tubiello et al., 2007). It is recognized that the models may overestimate the actual field-level responses because of many limiting factors including disease and insects, weeds, soil, water, and nutrient quality, which are neither well understood at large scales nor well implemented into the models (Easterling et al., 2007). In addition, the increase of CO2 is subjected to a considerable interaction with other climatic factors; therefore, the rising CO2 can not be assumed to be a single factor because of the associated changes in the temperature and other climatic factors (Giorgi et al., 1998; Prasad et al., 2002; Prasad et al., 2003a, b; Prasad et al., 2005; Zoltan, 2005) that directly affect crop growth and development. The beneficial affects of elevated CO2 on seed yields are decreased for several crops under elevated temperatures (Prasad et al., 2002; Prasad et al., 2003a; Prasad et al., 2006a).
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