Contrasting responses by respiration to elevated CO₂ in intact tissue and isolated mitochondria

The question of whether elevated concentrations of CO₂ directly inhibit mitochondrial respiration in plants has received considerable attention. Although there is a growing consensus that elevated [CO₂] rarely inhibits respiration of intact tissues, past studies have reported that elevated [CO₂] does impact on O₂ uptake in isolated mitochondria; what remains unclear, however, is the site(s) where elevated [CO₂] impacts on mitochondrial electron transport (ETC). Here we investigated direct effects of [CO₂] on respiratory activity of ETC enzymes, intact mitochondria and whole tissues using potato tubers (Solanum tuberosum L. cv. Desiree). Plots of O₂ uptake against the redox poise of the ubiquinone (UQ) pool in isolated mitochondria were used to determine whether elevated [CO₂] inhibits UQ-reducing and UQ-oxidising pathways differentially. Our results show that mitochondrial respiration was more inhibited via [CO₂]/[HCO₃^-] effects on cytochrome c oxidase (COX) than on succinate dehydrogenase, with [HCO₃^-] rather than [CO₂] inhibiting COX. However, the inhibitory effects at the mitochondrial level did not translate into inhibitory effects at the tissue level. Alternative oxidase (AOX) activity is normally absent in young potato tubers, as was the case in the present study. Thus, the lack of CO₂-mediated inhibition at the tissue level was not the result of increases in AOX activity masking the effects of CO₂ elsewhere in the respiratory system. We discuss whether the direct impact of elevated [CO₂] on respiration is dependent on the rate of metabolic activity and flux control coefficients in individual tissues.