ABSTRACT
Introduction e majority of terrestrial plant species use the C3 photosynthetic pathway. However, the e ciency of this process is compromised by photorespiration, and its rate is strongly limited by CO2 diusion from the atmosphere. Photorespiration increases at low CO2 concentrations and high temperatures, and CO2 limitation is accentuated by the reduction of stomatal aperture under arid conditions (Björkman 1971; Osmond et al. 1982). e evolution of C4 photosynthesis has solved each of these problems via a suite of physiological and anatomical adaptations that concentrate CO2 at the site of carbon xation, minimize photorespiration and raise the a nity of photosynthesis for CO2 at low mesophyll concentrations (Björkman 1971; Osmond et al. 1982). As a consequence, C4 plants have the potential to achieve higher rates of photosynthesis than their C3 counterparts, particularly at high irradiance (Black et al. 1969). Since C4 photosynthesis draws mesophyll CO2 down to lower concentrations than the C3 type, it also allows stomatal conductance to be reduced, leading to greater water-use e ciency than the C3 pathway under the same environmental conditions (Downes 1969). e C4 pathway is therefore classically viewed as an adaptation to declining levels of atmospheric CO2 (Ehleringer et al. 1991), and hot, open, arid environments (Björkman 1971; Loomis et al. 1971).
