Grain availability is determined on a global level by a balance between grain production and use (Tsujii, 2000). The potential for grain production is a result of productivity of grain crops and agricultural area. Over the last century (Mann, 1999), conventional plant breeding has developed crop productivity to a level that closely approaches the maximum potential, while the global arable area reached its ceiling by the mid-1970s and is now decreasing slowly due to increasing urbanization. It is feared that the negative trend in grain production will be exacerbated by three tightly correlated factors, namely water shortage, deterioration of soils, and global warming (Vörösmarty et al., 2000).

Such negative factors will severely affect photosynthesis, the primary step in grain production. Plant leaves are organs that are optimized for photosynthetic performance, this efficiency being maximal when sufficient water and nitrogen are available for the plants at moderate temperatures (Boyer, 1982). Thus, we have entered a time when we need to develop technology to maintain or increase the present productivity of crop plants to overcome grain shortage within the near future to satisfy increasing demands (Mann, 1999).

This chapter deals with challenges and initiatives for improving metabolic reactions in photosynthetic pathways, including the photosynthetic carbon reduction (PCR) cycle and other reactions in primary metabolism. The basic reaction mechanism of ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO) and regulation of the PCR cycle are not included in this chapter as they have been addressed in several scholarly reviews (Andersson and Taylor, 2003; Cleland et al., 1998; Fridyand and Scheibe, 2000; Hartman and Harpel, 1994; Martin et al., 2000; Roy and Andrews, 2000).