The CO₂ Concentrating Mechanism of C₄ Photosynthesis: Bundle Sheath Cell CO₂ Concentration and Leakage

Abstract

The C₄ acid cycle functions in C₄ photosynthesis to concentrate CO₂ in bundle sheath (BS) cells, thereby minimising oxygenase activity of Rubisco and associated photorespiration. Some key features of this process are reviewed here. During steady-state photosynthesis, the CO₂ concentration maintained in BS cells must be a compromise since, at the higher levels required to prevent photorespiration, the potential for CO₂ leakage from BS cells would be increased. Despite its central importance in C₄ photosynthesis, it has not been possible to precisely determine the BS CO₂ concentration. Leakage requires that the C₄ cycle rate exceeds the net photosynthesis rate and lowers the efficiency of the overall process. Leakage of CO₂ from the BS has been estimated by a number of indirect and, recently, by more direct methods. In a simulation, simple relationships between Rubisco activity, photorespiration, and leakage were calculated at increasing BS CO₂ concentrations. From this, and determined values for leakage, the likely concentration of CO₂ in BS cells may be 10–20-fold greater than in mesophyll cells. Higher concentrations would have little further effect on oxygenase activity.