In summary, although a C4 rice might confer some benefit, particularly in drought-prone upland settings, an important alternative to C4 rice would be a C, rice variety that is better-designed for a high CO, environment. It would probably be easier to engineer a high-CO,-adapted C, rice variety than C4 rice, as this would involve simple changes in regulatory set points, rather than engineering major changes in leaf anatomy and physiology. Substantial variation already exists in the response of rice to elevated CO, (Ziska et al 1996), and many of these traits could be co-opted to engineer superior cultivars for future high CO, atmospheres. In addition, the genetics of CO, responsiveness is rapidly being elucidated, providing a blueprint for the types of changes that can be exploited to design a rice plant that is optimized for high CO,. Although the strategy of genetically adapting C, rice to high CO, is an important alternative, one large uncertainty is when atmospheric CO, levels will reach the point at which the high-CO,-adapted rice is more efficient than C4 rice. It is commonly stated that atmospheric CO, will double within a century, but this estimate is quite uncertain given the problems of predicting human economic and social trends. Currently, CO, levels are rising at slightly less than 2 ppm per year. If we assume they will increase over the next 50 years by an average of 3 ppm per year, then the atmospheric CO, level will be 520 ppm in 2050. This may not be high enough to allow for C, plants to yield more than C4 plants, even with C, plants that are adapted to higher CO,. Given this, it is reasonable to argue that engineering a C4 rice may be critical to meeting production goals within the next half-century.

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