Respiratory Oxygen Uptake Is Not Decreased by an Instantaneous Elevation of [CO2], But Is Increased with Long-Term Growth in the Field at Elevated [CO2]

Averaged across many previous investigations, doubling the CO 2 concentration ([CO 2 ]) has frequently been reported to cause an instantaneous reduction of leaf dark respiration measured as CO 2 efflux. No known mechanism accounts for this effect, and four recent studies have shown that the measurement of respiratory CO 2 efflux is prone to experimental artifacts that could account for the reported response. Here, these artifacts are avoided by use of a high-resolution dual channel oxygen analyzer within an open gas exchange system to measure respiratory O 2 uptake in normal air. Leaf O 2 uptake was determined in response to instantaneous elevation of [CO 2 ] in nine contrasting species and to long-term elevation in seven species from four field experiments. Over six hundred separate measurements of respiration failed to reveal any decrease in respiratory O 2 uptake with an instantaneous increase in [CO 2 ]. Respiration was found insensitive not only to doubling [CO 2 ], but also to a 5-fold increase and to decrease to zero. Using a wide range of species and conditions, we confirm earlier reports that inhibition of respiration by instantaneous elevation of [CO 2 ] is likely an experimental artifact. Instead of the expected decrease in respiration per unit leaf area in response to long-term growth in the field at elevated [CO 2 ], there was a significant increase of 11% and 7% on an area and mass basis, respectively, averaged across all experiments. The findings suggest that leaf dark respiration will increase not decrease as atmospheric [CO 2 ] rises.