The CO2 Concentrating Function of C4 Photosynthesis

Abstract

C4 plants suppress photorespiration by concentrating CO2 at the site of ribulose-l,5-bisphosphate carboxylase (Rubisco). During photosynthesis in C4 leaves an inorganic carbon pool develops which is up to 10 x that expected by simple equilibration with external CO2 (1). The effective concentration of CO2 in bundle sheath cells requires that the mesophyll-bundle sheath cell interface be resistant to CO2 diffusion. The more “leaky” this interface is to CO2 the more energy must be expended by “overcycling” of the C4 cycle relative to net CO2 assimilation to maintain a high bundle sheath CO2 concentration. The work presented here quantitatively examines the permeability of the bundle sheath-mesophyll interface to CO2 and its implications for C4 photosynthesis.