FIGURE 6.

APH CO₂ Drawdown Experiment. (A) Stepwise changes in GC light intensity (blue line) and (B) CO₂ concentration (black line) during the two consecutive CO₂ Drawdown experiments. The CO₂ control switch [(A,B), gray line] enabled CO₂ injections [(A), orange line] to raise chamber CO₂ to 2,000 μmol mol^{– 1} before each drawdown. CO₂ concentration decreased when CO₂ control was disabled due to photosynthetic CO₂ uptake. The CO₂ injection rate during the second and third drawdowns was not sufficient to overcome canopy P_net at higher light levels and chamber CO₂ concentration [(B), black line] did not reach the CO₂ setpoint [(B), green line] in 1 h. As PPFD decreased, chamber CO₂ reached the setpoint and even increased above it due to respiration.