There are two direct, instantaneous effects of elevated [CO2] on C3 plants: an increase in photosynthetic carbon gain and a decrease in stomatal conductance of CO2 and water vapor. Any stimulation of crop yield by elevated [CO2] is principally determined by those two fundamental responses (Farquhar et al. 1978; Drake et al. 1997; Long 1999; Long et al. 2004; Ainsworth and Rogers 2007). An immediate rise in [CO2] increases the net photosynthetic carbon gain in C3 plants because ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco), the enzyme that initially fixes CO2, is not saturated in today's atmosphere. Therefore, the velocity of Rubisco carboxyla-tion reactions increases with rising [CO2]. Rising [CO2] also competitively inhibits the oxygenation reaction, which improves the efficiency of net carbon gain by decreasing photorespiratory CO2 loss (Bowes 1991; Long et al. 2004).
The immediate gains in photosynthesis are not always maintained at the same magnitude when plants are grown at elevated [CO2] for longer durations. Growth at elevated [CO2] results in altered photosynthetic capacity in C3 crops, namely decreased maximum Rubisco activity (Drake et al. 1997; Long et al. 2004; Ainsworth and Long 2005). This mechanism is thought to operate to optimize utilization of nitrogen, and on average, C3 crops show a 17% decrease in maximum Rubisco activity when grown at 567 ppm, based on the results of recent FACE experiments (Ainsworth and Rogers 2007). Environmental and genetic factors that limit sink strength (e.g., grain number) and lead to accumulation of carbohydrate content in leaves are associated with down-regulation of photosynthetic capacity (Long et al. 2004; Ainsworth and Rogers 2007; Leakey et al. 2009a). For example, when isogenic lines of soybean (Glycine max) were exposed to elevated [CO2], only the non-nodu-lating line showed decreased photosynthetic capacity (Ainsworth et al. 2004). Low nitrogen fertilization exacerbates any shortage of nitrogen relative to carbon and results in significant and pronounced decreases in photosynthetic capacity of C3 crops (Ainsworth and Long 2005).
However, despite the changes in photosynthetic capacity, carbon gain is significantly greater in C3 plants grown at elevated [CO2] anticipated for the middle to end of this century. On average, daily photosynthetic carbon gain increased by 9% for rice (Oryza sativa), 13% for wheat (Triticum aestivum) and 19% for soybean grown at elevated [CO2] in FACE experiments (Long et al. 2006). The increase in carbon gain in C3 crops feeds forward to increased vegetative and reproductive growth, and harvestable yield (Ainsworth and Long 2005; Long et al. 2006).
The second direct effect of elevated [CO2] on plants is decreased stomatal conductance of CO2 and water vapor (Long et al. 2004; Ainsworth and Rogers 2007). Decreased stomatal conductance is common to both C3 and C4 species, unlike the direct stimulation of photosynthesis, which is only observed in C3 species. C4 species concentrate CO2 in bundle sheath cells where Rubisco is located, which essentially saturates the carboxylation reaction and eliminates photorespiration in C4 species (von Caemmerer and Furbank 2003). However, both C3 and C4 species show decreased stomatal conductance at elevated [CO2]. While a change in stomatal conductance does not always translate into an equivalent change in canopy water use, recent FACE experiments with both C3 and C4 crops reported 5-20% reductions in canopy transpiration (reviewed in Leakey et al. 2009a). Changes in canopy transpiration at elevated [CO2] were also associated with improvements in soil moisture content (Conley et al. 2001; Hunsaker et al. 2000; Leakey et al. 2006), and maintenance of canopy carbon gain during dry periods (Leakey et al. 2004; Bernacchi et al. 2007). The direct effect of elevated [CO2] on stomatal conductance provides a second means for improvement of both C3 and C4 crop yield at elevated [CO2] in times and places of drought (Leakey 2009).
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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.