On the Nature of Carbon Isotope Discrimination in C4 Species

Theory is developed to explain the carbon isotopic composition of plants with the dicarboxylic acid (C4) pathway of carbon fixation. It is shown how leakage of inorganic carbon from the bundle sheath cells to the mesophyll cells can affect carbon isotopic discrimination. It is suggested that different types of C4 pathways will show variation in the degree of leakiness, reflecting variations in anatomy, and in the requirements for oxygen egress from the bundle sheath cells. It is also suggested that quantum yields may reflect these variations. A simplified expression is developed relating discrimination to leakiness and the ratio of intercellular and atmospheric partial pressures of CO2.