Carbon Dioxide Exchanges in Leaves. I. Discrimination between 14CO2 and ^{12}\text{CO}{}_{2}$ in Photosynthesis

In order to measure CO2 exchange reactions by leaves using isotopes of CO2, it is necessary to know precisely the discrimination against 14CO2 by leaves. Earlier determinations of discrimination are at variance, and may be inaccurate because of assumptions made about the rate of photorespiration. Maize leaves evolve little or no CO2 in light, and so provide suitable material for this measurement. Discrimination against 14CO2 in photosynthesis by maize leaves is almost precisely the same as in CO2 absorption by NaOH solution, amounting to 2.1 and 2.0% respectively. The agreement between these values and their close approximation to the relative rates of diffusion of ^{12}\text{CO}{}_{2}$ and 14CO2, calculated from Graham's law, shows that diffusion into the leaf is primarily responsible for discrimination against 14CO2 in photosynthesis.