Carbon Dioxide Exchanges in Leaves. I. Discrimination Between CO(2) and 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 ¹²CO₂ and ¹⁴CO₂, calculated from Graham's law, shows that diffusion into the leaf is primarily responsible for discrimination against ¹⁴CO₂ in photosynthesis.