Effects of Elevated CO 2 on Growth of the Industrial Sweetpotato Cultivar CX-1

Atmospheric CO2 concentrations have been steadily rising each year, from approximately 315 ppm in 1958 to 385 ppm in 2009 (Keeling et al., 2009), and are continuing to rise, with some estimates showing an increase to 700 ppm by the end of this century (Meehl et al., 2007). Generally, increases in CO2 are largely attributed to anthropogenic causes, including fossil fuel combustion and land use changes such as deforestation and urbanization (Hegerl et al., 2007). It is well established that elevated CO2 increases growth and yield of most plant species (Kimball, 1983). This added growth and yield is primarily attributed to increased rates of photosynthesis and water use efficiency (Rogers and Dahlman, 1993; Amthor, 1995). Growth in elevated CO2 induces a partial closure of leaf stomatal guard cells resulting in reduced transpiration and water loss (Jones and Mansfield, 1970) which increases water use efficiency for plants with both C3 and C4 photosynthetic pathways (Prior et al., 2011). However, research has shown that biomass response to atmospheric CO2 enrichment is generally greater for plants with a C3 (33 40% increase) vs. a C4 (10 15% increase) photosynthetic pathway (Kimball, 1983; Poorter, 1993; Prior et al., 2003; 2005). Plants with a C3 photosynthetic pathway show both increased water use efficiency and increased photosynthesis, while the CO2concentrating mechanism used by C4 plants limits their photosynthetic response to CO2 enrichment (Amthor and Loomis, 1996). Sweetpotatoes [Ipomoea batatas (L.) Lam.] have a C3 photosynthetic pathway and, like most plants, have a positive growth response to elevated CO2 (Bhattacharya et al., 1985; Biswas et al., 1996). In fact, total storage root dry weight response to CO2 enrichment can exceed the general range for C3 plants. Bhattacharya et al. (1985) reported increases of 87% at 675 mol mol 1 and 172.6% at 1,000 mol mol 1 for dry weight of ‘Georgia Jet’ storage roots. Biswas et al. (1996) reported total storage root dry weight increases of 44% and 75% at 665 mol mol 1 for two growing seasons. These large increases are not surprising since it is known that plants with a strong sink for photosynthate, such as sweetpotato storage roots, can respond to a greater degree than plants with other growth habits (Idso et al., 1988). In addition to use as a food crop, sweetpotato storage roots have been shown to be a good source material for bioethanol production (Qiu et al., 2010). In fact, Ziska et al. (2009) reported that sweetpotatoes (cv. Beauregard) have the ability to out-produce other sources of crop plant bioethanol (e.g., corn, potatoes, sugar cane, and sugar beets) in both Maryland and Alabama. Recently, several “industrial cultivars” of sweetpotatoes have been bred specifically for bioethanol production. For example, storage roots of the industrial sweetpotato cultivar CX-1 (Ryan-