Fig. 4.

Representative light–response curve of net photosynthesis (CO₂ assimilation) rate, measured on an upper, fully expanded leaf of a sunflower plant (see Fig. 1), as described by Kutschera et al. (2010). The light compensation point (CP) is indicated (approx. 40 μmol photons m^–2s^–1) (A). The calculated ratio (photons per CO₂ assimilated) indicates the growing inefficiency of photosynthesis with increasing irradiance (B).