3 The Biochemistry of C 4 Photosynthesis

C 4 photosynthesis consists of the coordinated function of two cell types in the leaves, usually designated mesophyll cells (MC) and bundle sheath cells (BSC), because enzymes of the C4 pathway are located separately in these morphologically distinct cell types. In C4 leaves, atmospheric CO2 enters through stomata and is first accessible to MC, where it is fixed by phosphoenolpyruvate (PEP) carboxylase to form oxaloacetate, and then malate and aspartate. These Ca dicarboxylic acids are transported to BSC where they are decarboxylated, and the released CO2 refixed by ribulose1,5-bisphosphate (RuBP) carboxylase (Rubisco) and assimilated through the enzymes of the photosynthetic carbon reduction (PCR) cycle to form sucrose and starch. Although anatomic differentiation is apparent in BSC, they are functionally similar to C3 MC in carbon assimilation except for the presence of enzymes concerned with decarboxylation of C4 acids. The physiological significance of separate but coordinate function of the two cell types in Ca photosynthesis is the specialization of MC for generation of a high concentration of CO2 in BSC in order to reduce the oxygenase activity of Rubisco and consequential reduction of photorespiration. Without consideration of a possible positive function of photorespiration in C3 plants (cf., Osmond and Grace, 1995), it is clear that Ca plants have the capacity to perform effective photosynthesis under conditions in which RuBP oxygenase activity is restricted. Ca photosynthesis can be visualized as a mechanism to provide Rubisco with near saturating CO2 when C4 plants can afford a high stomatal conductance, or to provide sufficient CO2 for survival and growth when stomatal conductance is low.