Alternatively, experimental results such as those in Table 1 could be used to identify candidate traits that confer superior e[CO2] response. A subset of the most promising traits could be tested in field scale FACE facilities, and then become selection targets for breeding efforts (Ainsworth et al., 2008a). Time spans to release new cultivars are sometimes 10–20 years, even if target traits are known (Semenov and Halford, 2009). It is therefore urgent to better understand the fundamental physiology of intra-specific variability in e[CO2] response and identify target traits. Perhaps most importantly, the environmental circumstances where these traits are of practical importance as selection targets must be assessed.