Figure 1.

Mass-spectrometric signal curves of ¹²CO₂ and ¹³CO₂ after switching to the gas stream containing ¹³CO₂ and no ¹²CO₂ with an empty cuvette (I, IV) and with a leaf in the cuvette in the dark (II, V) or during photosynthetic steady state (III, VI) under a light intensity of 850 μmol photons m⁻² s⁻¹. The system was provided with a gas stream containing ¹²CO₂ prior to application of ¹³CO₂ (gas flow 50 L h⁻¹, 350 μL L⁻¹ CO₂, 210 mL L⁻¹ O₂, 70% relative humidity, 23°C). The assimilation of ¹³CO₂ by the leaf (A_13C) and the release of ¹²CO₂ from the leaf in the light (R_12C) are used to determine the intercellular ¹³CO₂ and ¹²CO₂ concentration, respectively, and to determine true CO₂ assimilation. The curves were smoothed with a quadratic Savitzky-Golay function using an appropriate software (HP ChemStationDataAnalysis, Hewlett-Packard) and transferred to the same time axis (switching to ¹³CO₂ at t = 0 including 2 s of response time). For further details see “Results” and “Discussion.”