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.
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).