An open gas exchange system that uses an infrared gas analyzer and an ionization chamber to measure CO 2 and 14 CO 2 is described. The system will continuously measure the CO 2 and 14 CO 2 exchange from a leaf within 30 s after the 14 CO 2 gas mixture is supplied to the leaf. Measurements are obtained under steady rate conditions since the 14 CO 2 gas mixture is supplied at a constant composition from pressurized cylinders. Methods of determining true photosynthesis, apparent photosynthesis, CO 2 evolution, and the specific activity of the 14 CO 2 evolved are clearly described. The rate of CO 2 evolution in the light was not affected by the CO 2 concentration and the evolved CO 2 was derived from the early products of photosynthesis.
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Ludwig et al. (1971) studied this question.