For example, high carbohydrate storage capacity may be an important trait for wheat cultivars adapted to hot and dry environments (Rattey et al., 2009). The accumulation of stem reserves is thought to improve adaptation to post-anthesis stress, such as drought, by maintaining grain size or percentage of grains filled. Under e[CO2], such an effect may be amplified: Assimilate gain during vegetative growth is greater at e[CO2] and the larger accumulated carbohydrate store would then allow better grain filling during post-anthesis stress. Any target traits will have to be considered under the particular cropping environment in question. For example, the importance of stem reserves under post-anthesis water deficit suggests that a strong interaction between e[CO2] and water deficit can be expected, and that different traits may be suited to increase sink capacity in the absence of water deficit (e.g. tillering) and in presence of water deficit (e.g. high stem reserves maintaining grain size). Improvement of the C source, i.e. photosynthesis, is currently heavily researched, but not yet available for field testing (Reynolds et al., 2011). Strategies in wheat and rice include the quest for
Rubisco with better suited properties, apparently available in other species, or engineering of Rubisco or the Ribulose-1,5-diphosphate (RuBP) regeneration pathway for better efficiency. Increased yield and photosynthetic C assimilation rate under e[CO2] are cited as feasibility proof for improving crop yield through improved photosynthesis (C source; Kirschbaum, 2011). However, as [CO2] increases, many C3 plants may approach CO2 saturation of photosynthesis
by the middle of this century, thus strategies will have to be re-evaluated. For example, limitations on photosynthesis under
e[CO2] may shift from Rubisco to RuBP regeneration, and improvements in Rubisco properties may have less effect under e[CO2] (Zhu et al., 2010).