Direct inhibition of mitochondrial respiratory enzymes by elevated CO2: does it matter at the tissue or whole-plant level?

On average, a doubling in current atmospheric [CO2] results in a 15 to 20% direct inhibition of respiration, although the variability associated with this value is large within and among species. Direct effects of CO2 on resp iration may also be relevant to tree canopies because of dynamic fluctuatio ns between nighttime and daytime [CO2] throughout the growing season. The m echanism by which CO2 inhibits respiration is not known. A doubling of ambi ent [CO2] inhibits the activity of the mitochondrial enzymes, cytochrome c oxidase and succinate dehydrogenase. If inhibition of these enzymes is the only factor involved in the direct inhibition of respiration, the overall i nhibition of specific respiration will be proportional to the control that such enzymes exert on the overall respiratory rate. We analyzed the effects of [CO2] on respiration in an attempt to scale the direct effects of CO2 o n respiratory enzymes to the whole-plant level. Sensitivity analysis showed that inhibition of mitochondrial enzymes by doubling current atmospheric [ CO2] does not explain entirely the CO2 inhibition of tissue or whole-plant respiration. We conclude that CO2-dependent suppression of respiratory enzy matic activity will be minimal when cytochrome c oxidase inhibition is scal ed up from the mitochondria to the whole tree and that the primary mechanis m for the direct inhibitory effect remains to be identified.