Fig. 5.

A/C_i responses for C₃ (A–C) and C₄ (D–F) Panicoid grasses during three watering treatments: well-watered, moderate drought and severe drought, as indicated. Photosynthetic rates are expressed as a percentage of the rates measured at C_a = 38 Pa for well-watered plants (X). During the severe drought treatment, photosynthetic rates were only measured at a C_a of 38 Pa, or at a C_a corresponding to a C_i of 38 Pa. Also shown are the average photosynthetic rates at an atmospheric CO₂ concentration (C_a) of 38 Pa assuming no stomatal limitation (i.e. C_a = C_i = 38 Pa; A, B and C) and supply functions representing the limitation on A imposed by CO₂ diffusion through the stomata (X, Y and Z) for plants treated with three levels of drought. Each of these functions has a slope, which is set by the stomatal conductance and intercepts the C_i axis at C_a. n ≥ 4 and vertical bars represent standard errors. Shading indicates relative stomatal limitations (R_SL) and brackets the relative metabolic limitations (R_ML).