Figure 2.

Canopy gross CO₂ assimilation rate as a function of irradiance. Data measured with a wheat crop (Evans and Farquhar, 1991) are as follows: leaf area index, 7.1; leaf temperature, 22°C; ambient CO₂ partial pressure, 340 µbar. Lines are as follows: 1, maximum photon yield for C₃ plants in the absence of photorespiration (0.088 mol of CO₂ per mol of PAR photons); 2, maximum photon yield for C₄ plants (0.069); 3, photon yield for C₃ plants in normal atmosphere (0.058); 4, response curve for gross canopy CO₂ assimilation; 5, response curve for a single wheat leaf. Curve parameters are as follows: light-saturated gross CO₂ assimilation rate, 135 or 30 µmol CO₂ m⁻² s⁻¹ for the canopy and leaf, respectively; Θ (the convexity term), 0.7 using Equation 1a from Ögren and Evans (1993). On the right side, four regions for improvement are indicated: A, reducing photorespiration; B, increasing photosynthetic capacity; C, reducing losses due to non-steady-state conditions and sink limitations; D, increasing the PAR waveband.