Fig. 1.

Modelled responses of C₄ photosynthesis to changes in C_i at 25^oC and saturating irradiance according to von Caemmerer et al. (2000). The modelling depicts three hypothetical genotypes. G1 (continuous line) and G3 (broken line) possess different photosynthetic capacity. G2 (continuous line) has similar photosynthetic capacity to G1, but operates with lower g_s.

Within each scenario, reduced g_s (due to low-g_s genotype or dry soil or air) increases TE_i at the expense of reduced A. Accordingly, TE_i increases by 73% while A decreases by 13%, when moving from points 2 to 1 (filled circles) in G1 and from 3 to 4 (filled circles) in G3; g_s decreases by 50%.

Greater photosynthetic capacity in G3, relative to G1 and G2 genotypes, leads to increased TE_i at any given g_s. G3 is the desirable genotype because it can potentially fix more CO₂ in wet (e.g. high g_s^W) or dry (e.g. low g_s^D) conditions.

The shaded area represents the ideal (C_i) conditions under which genotypic screening gives the best population estimates of TE_i according to Jackson et al. (2016). At higher C_i, A is no longer sensitive to changes in g_s. At lower C_i, A is highly sensitive to C_i giving erroneous estimates of TE_i; or reduced g_s may be due to water stress, in which case C_i rises due to photosynthetic inhibition (Ghannoum, 2009), rendering TE_i estimates unreliable.